Canadian Grain Commission
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Spoilage and heating of stored agricultural products



Chapter 10 – Commodity characteristics

In Part I, the principles involved in spoilage and heating of stored products were dealt with. In Part II, the storage behavior and problems associated with specific commodities are described. For convenience, the characteristics of each commodity are described in the following order: relative storage risk; moisture content standards for dry, tough, damp, moist, and wet categories, established by the regulations of the Canada Grain Act and subjected to periodic revision; moisture content limits by the United States Department of Agriculture (1978); safe storage guidelines; drying guidelines, mainly as described by Friesen (1981) and Hall (1980); spoilage and heating degrading factors such as heated, bin-burnt, fire-burnt, and rotted; appearance of damaged kernels as described by the Canadian Grain Commission (1987); and known storage and/or drying problems. Problem situations encountered during storage of the commodity are described, including details of case histories and management practices used.

For additional information on the characteristics of a wide range of commodities and associated problems encountered during storage, stowage, and carriage, the reader is referred to Lloyd’s Survey Handbook (Knight 1985).

Relative storage risk

Five risk levels of spoilage and/or self-heating in stored commodities are given in Table 14 for a range of commodities. The risk level for each commodity was determined according to an overall assessment of seed/particle size, the need for an inert storage atmosphere, the total oil content, the presence of residual oil, and the known history of storage problems. Table 14 is an expanded and updated version of Table 2 (National Fire Protection Association 1949), to include soybean, canola/rapeseed, and other products.

Table 14 – Relative risk of spoilage and heating in stored commodities
  Risk level
Very High
(Class 1)
High
(Class 2)
Moderate
(Class 3)
Moderate-low
(Class 4)
Low
(Class 5)
* Essential
Size of particles/seeds very small small small to large moderate moderate
Type of product oilseeds; grass products meals with oil, small fibers oilseeds cereals with high oil content cereals pulses
Problem frequency very many numerous numerous some occasional
Inert gas requirement essential to preferred none none none none
Examples alfalfa brewers’ grains canola/ rapeseed cattle, swine, poultry feeds barley
poppyseed* corn meal cottonseed   domestic buckwheat
  cotton domestic mustard seed corn/maize fababeans
  fishmeal flaxseed canola/ rapeseed meal field beans
  hay seed   lentils
  rice bran soybean   millet
    sunflower seed   oats
        peanuts peas rice rye screenings sorghum triticale wheat
        wheat bran, shorts, middlings

Safe storage guidelines

Safe storage of a commodity depends largely on its moisture content (M.C.) (more strictly, the relative humidity of the intergranular atmosphere), its temperature, the period of storage, and other factors. Whenever possible, information on these key factors, together with the commodity moisture content in equilibrium with 70% relative humidity (R.H.), about which level molds begin to develop, are provided for each of the 35 commodities described in Part II. For convenience, the moisture content-relative humidity data are summarized in Table 15.

Table 15 – Equilibrium moisture content and percentage wet basis of grains, and other materials (after Hall 1980; Henderson 1985; Kreyger 1972; Löwe and Friedrich 1982)
Material Temperature
(°C)
Relative humidity (%)
40 50 60 70 80 90 100
Grains
* Unreliable because of mold growth
Barley 25 9.7 10.8 12.1 13.5 15.8 19.5 26.8
Buckwheat 25 10.2 11.4 12.7 14.2 16.1 19.1 24.5
Cottonseed 25 6.9 7.8 9.1 10.1 12.9 19.6
Field beans - flat small white 25 9.6 11.0 12.6 15.0 18.1*
Field beans - dark red kidney 25 9.6 10.7 12.5 15.0 18.6*
Flaxseed 25 6.1 6.8 7.9 9.3 11.4 15.2 21.4
Oats 25 9.1 10.3 11.8 13.0 14.9 18.5 24.1
Peas (green) 25-35 9.7 11.3 13.1 15.3 19.3 27.2
Poppy (opium) 25-35 5.9 6.9 8.0 9.5 11.7 17.0
Rice (whole grain) 25 10.9 12.2 13.3 14.1 15.2 19.1
Rye 25 9.9 10.9 12.2 13.5 15.7 20.6 26.7
Shelled corn 25 9.8 11.2 12.9 14.0 15.6 19.6 23.8
Sorghum 25 9.8 11.0 12.0 13.8 15.8 18.8 21.9
Soybean 25 7.1 8.0 9.3 11.5 14.8 18.8
Wheat - soft red winter 25 9.7 10.9 11.9 13.6 15.7 19.7 25.6
Wheat - hard red winter 25 9.7 10.9 12.5 13.9 15.8 19.7 25.0
Wheat - hard red spring 25 9.8 11.1 12.5 13.9 15.9 19.7 25.0
Wheat - durum 25 9.4 10.5 11.8 13.7 16.0 19.7 26.3
Other materials
Alfalfa hay 25 6.6 8.3 10.0 13.0 14.5
Bran 21-27 14.0 18.0 22.7 38.0
Linseed cake 21-27 13.5 17.5 23.5 40.5
Oat straw 29 7.6 8.5 10.9 11.5 14.5
Pig feed pellets 25 9.4 10.6 12.2 14.0 17.0 22.7
Broiler pellets 25 13.0
Dairy cattle pellets 25 13.0

Drying guidelines

As the temperature of the drying air is raised the rate of grain drying is increased. However, grain damage occurs if the temperature is too high. To prevent grain damage it is important that the maximum air temperature does not exceed the maximum allowable temperature of the grain being handled. The maximum drying temperatures cited for each commodity are conditional on drying to not more than 1% below the moisture content standards for straight grade seeds (except canola/rapeseed) (Canadian Grain Commission 1987), and on the removal of not more than 6% moisture in one pass through a high-speed dryer. With dryers where the grain is exposed to heat for long periods (such as in non-recirculating bin dryers) it is advisable, particularly with canola/rapeseed, to use temperatures 5-10°C lower than those listed for commercial use (Friesen 1981). The consequences of dryer damage are more serious with some crops than with others. It may reduce the value of a given crop more for some uses than for others. Indirect effects of dryer damage may be more important than direct effects. Reduction in viability makes the grain more susceptible to invasion by molds and subsequent deterioration. Brittleness caused by the effects of high drying temperatures leads to more breakage in handling (Freeman 1980).

Definitions of degrading terms

According to the Canadian Grain Commission (1987) definitions of degrading terms are as follows:

Bin-burnt kernels closely resemble fire-burnt kernels in color. However, unlike fire-burnt kernels, a cross section of bin-burnt kernels appears smooth and glassy. The weight of a bin-burnt kernel is similar to a comparable-sized sound kernel.

Fire-burnt refers to kernels charred or scorched by fire. In cross section such kernels resemble charcoal with numerous air holes. Unlike a bin-burnt kernel, a fire-burnt kernel weighs much less than a normal kernel of comparable size.

Heated refers to kernels having the typical color, taste, or odor of grain that has heated in storage, including kernels discolored from artificial drying, but it does not include charred kernels.

Rotted refers to the decomposition or decay of kernels caused by bacteria or fungi indicated by a blackening, discoloring, and softening of all or part of the kernel.

Alfalfa pellets (Medicago sativa L.)

Relative storage risk: Very high

Moisture content standards: None in Canada but manufacturers are required to set down the maximum moisture content present.

Safe storage guidelines: Generally avoid moisture extremes. The safe moisture content is regarded as 9-10%; however, after processing, pellet moisture content may be as low as 6.6-8.5%. Pellets are sieved before binning, and the resultant fines are pelleted to improve ventilation, avoid waste, and reduce fire risk. After cooling, pellets are stored in large bins of up to 810 t. Commonly, a carlot is withdrawn from filled bins to remove any out-of-condition pellets near the surface. Later in the season the bin is topped up with more pellets for long-term (9 months) storage. Aeration is sometimes used to cool stored pellets. A pipe is installed at the top of sealed bins for the addition of nitrogen gas to maintain pellet quality and to extinguish fires. For transportation, it is essential to use tight railcars (National Fire Protection Association 1981).

Appearance: Pellets made from first-cut alfalfa are usually much greener and contain more weed seeds than those made from second-cut-alfalfa.

Storage problems: Dehydrated alfalfa pellets have a history of heating problems and are difficult to handle once severe heating or fire occurs. In several recent cases in Canada the following procedures were used after heating was discovered:

  • The bin was opened up but oxygen gas present in the air stimulated the incipient fire, which produced smoke and heat and resulted in a severe loss of product.
  • The bin was aerated but the pellets were in an advanced stage of heating and the plant burnt down.
  • The bin was flushed with nitrogen gas but fire restarted when the product was moved.
  • The bin could not be unloaded because the internal auger was clogged. A hole was cut at the base of the bin for placement of an external auger. The external auger then clogged, and a workman who tried to clear it with a stick was thrown back and injured.

Anticipate problems with stored alfalfa pellets by constantly monitoring pellet condition, using sealed bins with inlet pipes for carbon dioxide or nitrogen gas addition, in case of fires or, ideally, storing the pellets under nitrogen gas or some other inert gas. Once the fire is out, removal of heated pellets from bins is achieved by using a front-end loader to lift the metal sheets at the base of the bin, then augering the product from the resulting pile.

Case history: In September 1987 smoke and steam was observed coming from the roof ventilator of an 810-t bin of alfalfa pellets at a plant in western Canada. The pellets had been in storage from 1 to 2 months, during which time bin temperatures were not monitored. A decision was made to remove any undamaged pellets by cutting two ground level apertures in the walls at opposite sides of the bin. After 100 t of pellets were removed an explosion occurred, displacing the ventilator and pellet inlet pipe and damaging the roof strakes (Fig. 19a, 19b). The accompanying fire blackened the walls and roof above the level of the pellets (Fig. 19c) .The fire was controlled by adding water through the top opening and by removing hot material through the larger (225 x 130 cm) aperture. Later, another hole was made in the wall to allow access by front-end loader. All pellets were damaged by smoke or fire.

Effects of a fire in binned alfalfa pellets

Effects of a fire in binned alfalfa pellets; a, explosion damage to roof apex, fire damage above wall opening , and brown distillate on wall

Figure 19a – Explosion damage to roof apex, fire damage above wall opening (arrow), and brown distillate on walls (arrow)

Explosion damage with displacement of roof cap, ventilator, inlet pellet pipe, and distortion of roof strakes

Figure 19b – Explosion damage with displacement of roof cap, ventilator, inlet pellet pipe, and distortion of roof strakes

Fire damage to interior walls directly above removed wall panel

Figure 19c – Fire damage to interior walls directly above removed wall panel.

Management practices used

  • Pellet moisture content determinations were taken after drying and were allegedly about 10%.
  • Pellets were removed periodically through a vacuum pipe attached to the lower bin wall and added through the roof inlet pipe.
  • Pellets were removed by cutting apertures in bin walls with an oxyacetylene torch.
  • Fire was controlled by adding water under pressure through the top vent from an aerial ladder and by removing burnt and smoldering material through the wall apertures. Correct procedure
  • Pellets should have been screened before storage in order to remove fines and waste, to improve airflow, and to reduce fire risk.
  • Moisture contents should have been determined on periodic loading samples to obtain information on maximum moisture content and likely hazards.
  • Moisture content and temperature of the binned stocks should have been carefully monitored at intervals and data recorded for future reference.
  • Thermocouples should have been installed in the bin.
  • Stocks should have been aerated to even out temperature and moisture gradients.
  • Inlet pipes for CO2 or N2 gas addition should have been installed on the bin for pellet storage or fire control.
  • Professional advice should have been sought when smoke was first noticed.
  • Fire should have been extinguished by blanketing with CO2 from above through the 15-cm diameter inlet pipe and through other small holes drilled in the walls.
  • The bin should have been sealed to keep out air and provision made for gas pressure release.
  • Water should not have been added to the burning stocks, as oxygen may be introduced creating explosive conditions (National Institute for Occupational Safety and Health 1985).
  • Bin contents should have been left to cool off before removal.
  • Holes should not be cut in the wall when the material is smoldering because of oxygen introduction, favoring fires and explosions (Fig. 18c).
  • Holes should be cut with a metal cutting saw, not a cutting torch unless the structure is empty (Harvestore® Products 1982).

Canola oil made from severely heat-damaged canola seeds (dark bottle), from the same seeds but later decolorized, and undamaged canola seeds (clear bottle), respectively

Figure 18c – Canola oil made from severely heat-damaged canola seeds (dark bottle), from the same seeds but later decolorized, and undamaged canola seeds (clear bottle), respectively.

Barley (Hordeum vulgare L.)

Relative storage risk: Low

Moisture content standards:

  • Dry: up to 14.8%
  • Tough: 14.9-17.0%
  • Damp: over 17.0%

Safe storage guidelines: The maximum moisture content for safe storage of barley is 13% for 1 year and 11% for 5 years (Hall 1980). Barley containing 10.3-12.1% initial moisture content and having a temperature of 22-35°C was kept in good condition with no increase in free fatty acids in Manitoba farm bins for 3 years (Sinha and Wallace 1977). Barley seeds stored for 18 months in a laboratory at and below 13.2% M.C. were not invaded by fungi, according to Tuite and Christensen (1955) .Seeds just above and below 14% M.C., however, were invaded by Aspergillus restrictus, a slow-growing member of the A. glaucus group of spoilage molds, after several months. Burrell (1970) has delimited the moisture content-temperature combinations at which mold spoilage and mite problems may be expected in barley over a 32-week period under UK farm conditions. He showed that high-value malting barley needed to be dried down to 12% M.C. and cooled to avoid risk of mite infestation.

Drying guidelines: Maximum safe drying temperatures are 45°C for barley required for seeding or malting purposes, 55°C for commercial use, and 80-100°C for feed (Friesen 1981). However, the preference of maltsters in Canada is that barley intended for malting should not be dried by the producer.

Degrading factors: Barley seed is degraded when it contains fire-burnt, heated, or rotted kernels or has a heated or fire-burnt odor. Barley is graded Sample if it contains over 0.5% fire-burnt seed or has a fire-burnt odor, if it contains over 10% heated seed and has a distinctly heated odor, or if it contains more than 10% pure rotted kernels. When both heated and rotted kernels are present they are considered in combination.

Appearance of heated kernels: The hull over the germ is discolored often to a golden brown color. When the hull is removed by pearling (mechanical dehulling), the germ appears red or brown. As the degree of heat damage increases, a greater portion of the pearled kernels shows the mahogany-red to brown coloration.

Storage problems: Freshly harvested grain with a moisture content above 14% may heat and go out of condition. Only a moderate development of spoilage molds is needed to destroy the germination ability of barley and give it a musty odor. Barley that is to be used for seed or malting purposes requires close watching and special care in storage (Dickson 1959). Any detectable rise in temperature of malting barley is regarded as an indicator of trouble (Christensen and Kaufmann 1972). Table 16 indicates the estimated number of weeks for decreased germination to occur in 11-23% M.C. barley stored at 5-25°C (Kreyger 1972).

Table 16 – Estimated number of weeks for decreased germination to occur in stored barley (after Kreyger 1972)
Moisture content
(wet basis)
(%)
11 12 13 14 15 16 17 19 23
Storage
temperature
(°C)
Maximum safe storage (weeks)
25 54 39 25 16 9 5 2.5 1
20 110 80 50 32 19 10 5 2 0.5
15 240 170 100 65 40 20 10 4 1
10 600 400 260 160 90 50 21 8.5 2
5 >1000 1000 600 400 200 120 50 17 4

Spoilage of moist barley (23-40% M.C.) may occur in sealed and unsealed silos and in structures containing acid-treated grains. Air may enter airtight tower silos during top reloading as grain is removed from below, resulting in molding and heating (Nichols and Leaver 1966). In unsealed concrete-staved silos spoilage occurs in the uppermost grain when the top seal of wilted grass and plastic sheeting is inadequate or when less than 7.5 cm of feed is removed each day (Lacey 1971). Spoilage can also occur in high moisture barley treated with propionic and other acids when inadequate acid is used, and when condensation occurs, diluting the acid treatment.

Case histories:

  1. In Greece, barley with an average of 13.5-14.5% M.C. was stored in silos at a brewery. The contents of one silo heated to 40°C and seed germination was reduced due to the activity of spoilage fungi in the Aspergillus glaucus group. The problem was how to prevent spoilage, heating, and loss of seed germination in barley received from farms at variable moisture contents and temperatures. The plant was equipped with aeration equipment, but when it was used condensation occurred in the bins, aggravating the situation. The problem was solved by turning the barley to evenly distribute seed moisture and temperature (De Vries, pers. com. 1985).
  2. During the prolonged wet fall of 1977 in western Canada considerable amounts of grain were piled on the ground prior to artificial drying and binning. The ecological changes occurring within the piles were studied over a period of time. One 6-week-old sprouted barley pile was found to have ecological habitats favoring development of particular fungi. Samples from the south and west of the pile, warmed by the sun, had the most Alternaria (a field fungus), a very low Penicillium level, and low carbon dioxide levels; samples from the north and east of the pile had the highest levels of Aspergillus glaucus group species; and samples from the centre of the pile had a low level of Alternaria, a high level of Penicillium, a trace of A. glaucus, higher seed moisture, and lower germination (Mills and Wallace 1979).
  3. A self-unloading lake ship filled with barley was moored overwinter in Montreal harbor. In the spring, the contents of one hold were severely spoiled and heated, with strong off-odors and steam rising from the cargo. Red hot fused chunks and free water were present in the lower bulk. The problem was traced to a continuously lit lamp in the hold.

Barley malt (Hordeum vulgare L.)

Malt is germinated, kilned, and aged barley.

Relative storage risk: Low

Safe storage guidelines: Malt is kilned in a current of hot dry air and is stored with or without the removal of dry and brittle culms. Dry malt is stable in storage because it has a low moisture content varying, according to type, from 1.5 to 7%. Unlike barley, it is readily crushed (Briggs 1978). Considerable volumes of malt are shipped from Europe and North America to breweries abroad. Some of the strongly hygroscopic malt spoils in transit when exposed to moist conditions. Caking of malt by spoilage fungi on arrival at a brewery in Nigeria was investigated by Okagbue (1986). Problems associated with occurrence of the field fungus Fusarium and the storage fungus Aspergillus versicolor on malting barley are described by Christensen and Meronuck (1986).

Case history: In March 1983, prepared malt of 4.2% M.C. was binned in a concrete silo at a malting plant in western Canada. About 5 weeks later three carlots of the prepared malt were removed from the silo, sampled, and found to contain traces of burnt malt. The affected bin was partially unloaded and both heated and unheated malt was discharged. The heated malt varied in moisture content from 7.0 to 10.4%. Hot spots were also present. The problem was traced to a heavy rain about the time of loading when the bin was one-third full. Water had drained from a large open area into the basement and entered the bin via a bucket elevator and a conveyor belt. Subsequently, the bin was loaded with more malt until it was filled.

Brewers’ and distillers’ grains

Brewers’ grains, or spent brewing grains, are the insoluble residues from brewed malt; distillers’ grains are the residues from the manufacture of alcoholic beverages distilled from grains.

Relative storage risk: High risk of fire with overdried brewers’ and distillers’ grains.

Moisture content standards: In Canada no moisture content standards are delimited for dried or undried material, but the manufacturer is required by the Feeds Act to state the maximum moisture content present.

Safe storage guidelines: Brewers’ grains normally contain about 11% water by weight and are either sold wet and used directly as feed for stock, or dried. The dried grains are either used directly as fodder or incorporated into feed mixes. When dried brewers’ or distillers’ grains are stored in large quantities, spontaneous fires sometimes occur. Dry brewers’ grains react exothermally with dry oxygen or air. The generation of heat is caused by oxidation of the natural oil present in the grains (Walker 1961). The National Fire Protection Association (1981) states that dried brewers’ and distillers’ grains need to be maintained between 7 and 10% M.C. and require cooling below 38°C before storage. The Association also states that it is very dangerous to dry the grains below 5% M.C. According to Snow et al. (1944), the safe moisture content levels for distillers’ grains are 11% (equivalent to 72% R.H.) for 3 months storage, and 9.8% (65% R.H.) for 2-3 years storage, at 15.5-21°C.

Canola/rapeseed (Brassica campestris L.; B. napus L.)

Canola is the term used in Canada for low erucic and low glucosinolate content Brassica campestris and B. napus cultivars. The term rapeseed is used outside Canada to describe all B. campestris and B. napus cultivars but refers in Canada only to high erucic acid types. The storage behavior of canola and rapeseed is similar.

Relative storage risk: Moderate

Moisture content standards:

  • Dry: up to 10%
  • Tough: 10.1-12.5
  • Damp: over 12.5%

Safe storage guidelines: Extreme care is needed to safely store canola/rapeseed seed because the upper moisture content limit of so-called dry seed is currently 10.0%. This is too high for long-term safe storage, as growth of spoilage fungi in the Aspergillus glaucus group occurs at 70% R.H., which is equivalent to 8.3% M.C. at 25°C. If the seed is binned at above 25°C, or if pockets of immature seeds or green weed seeds are present, a seed moisture level of even 8.3% is too high for long-term safe storage. As a rule of thumb, bin canola at a maximum of 8.0% M.C. for storage longer than 5 months. The chart shown in Fig. 1 predicts the keeping quality of canola/rapeseed seeds for a 5-month period, given varying temperature-moisture relationships (Canola Council of Canada 1981; Mills and Sinha 1980). If the temperature or the moisture content of the harvested seed falls within the spoilage area of the chart, the grower must take steps to reduce one factor or both. To ensure safe storage, the following steps are recommended:

  • Bin at least 1.5 percentage points below the 10.0% cut off.
  • Use an efficient deflector under the auger to spread the heavier, moister, green material and fines away from the core.
  • Clean the seeds as soon as possible.
  • Use an aeration unit to quickly cool the seed to 5°C or 0°C for the winter-holding period.
  • Monitor the seed temperature every few days during the fall and every 2 weeks during the winter.

Re-auger non-aerated seed after it has been stored for 3-5 days to break up any pockets of green weed seeds and dockage that might facilitate heating, and to create an inverted cone to allow air to penetrate. Canola/rapeseed is more vulnerable than barley to pest infestation when stored in farm bins (Sinha and Wallace 1977).

Drying guidelines: The maximum drying temperatures are 45°C for canola/rapeseed intended for seeding purposes and 65°C for commercial use, providing the seed is not dried below 7.5% M.C. When canola/rapeseed is exposed to heat for a long period, as in non-recirculating bin dryers, it is advisable to use temperatures 5° to 10°C lower than those listed for commercial use. This is because the oil quality is affected by long exposure to high temperatures (Friesen 1981). Damaged seeds undergo a reduction in oil quality because of the marked rise in level of free fatty acids (Nash 1978).

Degrading factors: Canola/rapeseed is degraded when it contains heated, bin-burnt, or fire-burnt seed and/or has a heated or fire-burnt odor. Canola/rapeseed is graded Sample if it contains over 2.0% heated seed and/or has a distinctly heated odor, or if it has a fire-burnt odor.

Appearance of heated seeds: Seeds are crushed in strips of 100 seeds (Canola Council of Canada 1974) to determine the extent of heating. Heating is classified into three categories: charcoal black (badly bin-burnt), dark chocolate brown (distinctly heated), and light tan (slightly damaged from oxidation). Limits of heat-damaged seeds specified in statutory grades apply to charcoal black and/or dark chocolate brown seeds. Samples containing light tan seeds are carefully checked for odor, including both the bulk of the sample and the freshly crushed seed strips. If an odor is present, or if in combination with black or brown crushed seeds, the light tan seeds are considered as heated. In the absence of these symptoms, the light tan seeds are classed as damaged.

Charcoal black (Fig. 17c) and dark chocolate brown canola seeds when crushed produce a dark canola oil (Fig. 18c).

Badly bin-burnt (charcoal black) and undamaged (brown) canola seeds

Figure 17c – Badly bin-burnt (charcoal black) and undamaged (brown) canola seeds.

Storage problems: Canola/rapeseed goes through a period of active respiration after binning. If the heat and moisture of respiration is not quickly removed, mold growth and respiration soon occurs. To counteract the situation, aerate or turn the stocks as soon as possible. lf some or all of the stocks are of higher seed moisture content, the seeds need to be dried, then aerated. Do not consider stored canola to be similar to stored wheat because, unlike wheat, adverse changes can occur very rapidly.

Drying problems: A mass of stored canola/rapeseed is much denser than that of a stored cereal grain, and has a higher resistance to air movement. Two to three times more static pressure is required to force drying air through canola than through wheat. Since the designs of most dryer fans do not allow for changes in the air pressure produced, the result is lower airflow when drying canola. Less airflow means less energy required to heat the air to the selected drying temperature. Readjust the temperature when going from a cereal grain to canola during drying operations, because the lower airflow means longer drying times and the subsequent possibility of a temperature buildup (Canola Council of Canada 1981).

Molding and heating can occur exceedingly quickly in moist canola/rapeseed, and where this happens the seeds are likely to stick together (Fig. 17b). The result is that the value of the stored product for processing is greatly reduced because of a marked increase in the level of free fatty acids, probably associated with mold growth (Nash 1978). Burrell at al. (1980) determined the amount of time available for drying rapeseed before the appearance of surface molds at five temperatures and seven moisture levels. They found that seed clumping preceded the appearance of visible fungal colonies, but that germination was affected much later. For example, seeds at 25°C and 10.6% M.C. clumped after 11 days. Visible fungal colonies appeared after 21 days but germination was still unaffected after 40 days.

Aggregation or clumping of canola seeds by mold mycelium with two seeds visible in cross section

Figure 17b – Aggregation or clumping of canola seeds by mold mycelium with two seeds visible in cross section

Case histories:

  1. In August 1976 a farmer in the interlake area of Manitoba filled a 68-t bin with rapeseed. The rapeseed, combined on an extremely hot day, went into storage at 8.5-9.0% M.C. In late October, a hot spot was discovered at the centre of the bin, extending down about 120 cm from the top surface. The hot weather threshing produced a lot of fines, which had accumulated at the bin centre. The heating from the already warm rapeseed was compounded by the large amount of fines. On discovery, the heated material was immediately removed; the remainder of the contents was stored without incident. To prevent similar problems from reoccurring the farmer installed an aeration system, activated by a humidistat at relative humidities of 40% or less. Aeration took place from the moment the first seed entered the bin until winter, when it was switched off. A digital temperature probe was also purchased to check the bin contents every few days in the fall and 2 weeks in winter. No subsequent storage problems were encountered (Lyster 1978).
  2. In the fall of 1985, a farmer near Winnipeg, Man., filled a 64-t bin with canola. During the winter of 1985 spoilage and heating occurred. The stocks could not be unloaded by the inbuilt auger at the base of the bin due to clogging by aggregated seeds. The problem was solved by removing the dry, free-flowing seed above the aggregated material by vacuum aspiration (Fig. 11) through the top vent, and then removing the aggregated material in the same manner. The farmer used a 12.5-cm diameter flexible hose and a portable 52 220-W 70-hp vacuum unit. It took 6 hours to totally unload the bin.
  3. In 1975 a wooden boxcar filled with tough and damp rapeseed was sent to the drying plant at one of the terminal elevators. Unfortunately, the car was mislaid in a siding for 3 months and when it was opened the contents were a very light gray color and disintegrated when touched. The contents of the car had heated to the ashing point and had no salvage value, as nothing could be saved. The wooden walls of the car were unharmed. (NB. The presence of blackened seeds would have indicated incomplete combustion of the car contents.)

Canola/rapeseed meal (see definitions and usage or terms under Canola/ rapeseed)

Relative storage risk: Moderate to low

Moisture content standards: Pellets are guaranteed not to have more than 11% moisture content by the manufacturer.

Safe storage guidelines: The safe moisture content levels for meal storage are 7% at 30°C or 9.5% below 25°C, for 1 year (White and Jayas 1989). Discoloration of the meal from yellow-green to brown occurred at 50°C, 10% M.C., in 1 month. Meal stored at 10.4% and 11.5% M.C. at 40°C, and at various moisture contents from 6.3 to 11.5% at 50°C, discolored after 3 months.

Storage problems: Oil, hexane, and other solvents that remain in meals and pellets after oil extraction are fire and explosion hazards during ship transport. Permissible levels of oils and residual solvents are regulated by the Canadian Coast Guard (1984). Solvent- extracted canola/rapeseed meal and pellets that contain not more than 4% oil and 15% oil and moisture combined, and that are substantially free from flammable solvent are exempt from the regulations on provision of a certificate from a recognized authority.

Cattle, swine, and poultry feeds

Formulations for complete feeds and for custom feeds are used for meals, pellets, and crumbles. Formulations are complex with many ingredients, including corn and/or other cereals, oilseed meals such as soybean, fats such as tallow, mixed vitamins, minerals, and other additives.

Relative storage risk: Moderate to low

Moisture content standards: There is no labeled moisture content requirement in Canada for cattle, swine, or chicken feeds containing more than one ingredient.

Safe storage guidelines: Avoid extremely low or high moisture content (National Fire Protection Association 1981); keep moisture content levels between 10 and 14%. This range is considered safe for storage of animal feeds by industry. Moisture content is of concern to manufacturers because of its effects on the efficiency of the pelleting process and how it affects storage of pellets. Look for excessive heating occurring during grinding of cereals prior to pelleting and during pelleting of certain feed concentrates containing high levels of proteins and animal fats.

Precautionary practices for storing and utilizing meals and pellets include watching for uneven flowability of feed indicating moisture uptake and incipient spoilage; looking for moisture migration and mold development affecting feed at the top and sides of the bin; checking for leaks through missing bolt holes, and poorly welded joints, and so forth; purchasing feeds according to usage, that is, not letting feeds sit for long periods; never putting new feed on top of old feed; and always cleaning out bins thoroughly when empty, preferably followed by a wash down in dilute Chlorox® solution to kill mold spores. Henderson (1985) examined the moisture content-equilibrium relative humidity (M.C.-E.R.H.) relationships at 5, 15, and 25°C of three pig meals and pellets made mostly from barley meal and wheatings with various proportions of soybean meals, and found that they were similar. The moisture content in equilibrium with 70% R.H. at 25°C was about 14%, similar to that for cereal grains. For most practical purposes, the M.C. - E.R.H. relationships of barley, wheat, and animal feeds containing 80% or more of cereal products, such as the pig feeds, can be regarded as similar. However, if the animal meal contains more than 18% of an oilseed such as soymeal it may be expected that the E.R.H. would be higher and a lower safe storage moisture content would be required.

Clancy (1979a, 1979b) gives the storage characteristics (moisture content, flowability, compaction properties, bulk density, and hygroscopicity) of some feedstuffs under UK conditions. The products described include cottonseed and groundnut pellets, coconut flakes, soybean meal, beet pulp pellets, grass cubes, fats, and molasses. Feedstuffs containing high amounts of molasses have poor flowability. Hamilton (1985) describes the problem of molding, and factors influencing the activity of fungi and antifungal agents, in poultry feed.

Case history: As a result of self-heating, a smoldering fire developed within a large silo in West Germany that contained animal feed pellets (Dinglinger 1981). Initially, fire fighters attempted to fill the headspace above the feed with carbon dioxide (CO2) foam, but CO2 concentrations in the workrooms at the silo base reached levels far above maximum allowable levels for safety. When attempts were made to empty the silo only the lower part could be cleared. A bridge had formed at a height of 15 m in the vicinity of the fire pocket, leaving about 150 t of feed pellets suspended in the upper part of the silo. In order to collapse the bridge, a hole about 50 cm in diameter was drilled. Because the discharge chute at the base of the silo couId not be closed off completely, a strong convection current developed inside the silo, sucking enough air from leaks to keep the fire burning. A nozzle was mounted on the discharge chute (see Fig. 14) through which nitrogen gas (N2) was fed into the silo from a supply unit and a normal fire-fighting hose. After N2 purging, the oxygen gas (02) content in the silo at the source of the fire was reduced to 7%, thus suffocating and slowly cooling down the fire and reducing risks to fire fighters. Because of the low 02 content in the silo, there was never a danger of a dust explosion occurring if the bridge had collapsed while the silo was being emptied. A total of 18 000 M3 of N2 were used during the entire process, which lasted 10 days.

Corn/maize (Zea mays L.)

Relative storage risk: Moderate to low

Moisture content standards:

  • Dry: up to 15.5%
  • Tough: 15.6-17.5%
  • Damp: 17.6-21.0%
  • Moist: 21.1-25.0%
  • Wet: over 25.0%

The maximum moisture content limits for grades (US) l, 2, 3, 4, and 5 yellow, white, or mixed corn are 14.0, 15.5, 17.5, 20.0, and 23.0%, respectively (United States Department of Agriculture 1978).

Safe storage guidelines: The effect of moisture and temperature on allowable storage time for corn is given by Friesen and Huminicki (1986). Corn stored at 22% M.C., for example, will keep at 27°C for about 5 days, at 19°C for 10 days, at 13°C for 20 days, at 7°C for 40 days, and at 4°C for 60 days. The maximum moisture content for safe storage of corn is 13% for 1 year and 11% for 5 years. In shelled corn an intergranular relative humidity of 70% equilibrates with 14.0% M.C. at 25°C (Table 15) (Hall 1980). In Ontario, because of low winter temperatures, corn can be safely stored at 15% M.C. over the winter and spring if aerated properly. A moisture content of 13-14% is required for storage to late summer, and 11-13% for storage over several years. High moisture corn for animal feed is stored at moisture contents between 22 and 32% M.C. (Morris et al. 1981).

Drying guidelines: The maximum drying temperatures are 45°C for seed, 60°C for commercial use, and 90-100°C for feed (Friesen 1981). According to Morris et al. (1981) the critical maximum temperatures for drying of grain corn harvested at 28% M.C. in Ontario are 45°C for seed, 70°C for starch milling, 90°C for other industrial uses and non-ruminant feed, and 120°C for cattle feed. Drying corn is essential when harvested above 18% M.C. unless it is placed in airtight storage, preserved in propionic or other acids, or frozen (Campbell et al. 1977). Dryer damage to protein (case hardening) in corn diminishes its value for wet milling by rendering the separation of starch and protein more difficult (Freeman 1980).

Degrading factors: Corn is degraded when it contains fire-burnt, heated, or rotted kernels or has a fire-burnt, smoke, or heated odor. Corn is graded Sample if it contains fire-burnt kernels and/or has a fire-burnt or smoke odor, if it contains over 3% heated kernels or has a heated odor, or if it contains over 3% rotted kernels.

Appearance of heated, rotted, and blue-eye-molded kernels: Heated kernels include whole kernels or partial kernels that are either discolored by natural fermentation or severely scorched by artificial drying, and display a color range of amber to dark brown over the entire kernel. The germs are brown and severely damaged kernels have a puffed appearance, especially in the germ area. Rotted kernels are whole kernels or pieces of kernels that show advanced stages of decomposition and feel spongy under pressure. Samples are degraded according to established grade tolerances for heat- and rot-damaged kernels. Blue-eye-molded kernels have dark germs and when peeled show mold development. Corn that contains over 15% of kernels affected by blue-eye mold is graded Sample.

Storage problems: Corn is prone to breakage during handling and after drying at too high a temperature and drying rate (Tuite and Foster 1979; HohenadeI1984). As a result, broken corn foreign material (BCFM) is increased and constitutes a storage hazard. When corn is loaded into bins from above, the BCFM) collects in the central core or spout line. Spoilage may begin in the spout line partly because the fines, which consist mainly of fragments of corn endosperm, are more susceptible than whole kernels to invasion by storage fungi, and partly because insects and mites thrive in the fines and promote the growth of such fungi (Christensen and Sauer 1982).

Corn is prone to spoilage during transport by river barge and by ocean freighter from North and South American ports (Christensen and Kaufmann 1978; Milton and Jarrett 1969). In the United States, substantial amounts of corn are shipped from the midwest during cold weather to warmer southern states either for local consumption as animal feed or for export. Spoilage occurs on board ship because of a combination of factors (Tuite and Foster 1979). On arrival at New Orleans and other Gulf ports the corn is usually already infected by storage fungi, mainly Aspergillus glaucus. It picks up at least 0.1-0.2% M.C. from the humid air, and has a substantially increased level of BCFM) due to the numerous handlings en route. The likelihood of spoilage is further enhanced by shipment at 15.5 to 16.0% M.C. and a lack of effective aeration on board ship. Corn shipped to the tropics or subtropics is particularly vulnerable. The storage life of corn is rapidly decreased with increased temperatures and increased breakage levels (Fig. 22) (Calverley and Hallam 1982). Spoilage and heating occur also in corn shipped from Argentina to Europe; the factors involved have been studied in detail by Milton and Jarrett (1969).

Storage life of maize in different environments

Figure 22 – Storage life of maize in different environments (after Calverley and Hallam 1982).

Aggregation of high moisture corn due to mold activity may cause blockage of augers and other components of grain handling systems. Uneven pressure effects can be created, resulting in total or partial bin or system collapse.

Drying problems: There is less trouble with drying corn than with small grains or oilseeds, because of its lower resistance, which reduces temperature variations in hot-air plenums. Stress cracks and “moisture rebound” may cause problems because of the large amount of water removed. These problems can be reduced by slowing down the drying rate after the corn reaches 18% M.C., using a slower cooling rate, or using dryeration (Campbell et al. 1977).

Case histories:

  1. In March 1981, 10 000 t of yellow corn was shipped from North America via the St. Lawrence Seaway to Mali through the West African ports of Dakar and Abidjan. About 5000 t was unloaded at each port and transported inland by road and rail. Of the original consignment, only 5000 t was fit for human consumption by the time it reached its final destination. The changes that occurred in transit have been followed in detail by Calverley and Hallam (1982). According to the export certificate, the corn had 15.3% M.C. and 4.6% BCFM) at loading. At Dakar the corn, now at E.R.H. 70% at 22°C (monthly daily mean temperature at Dakar), was shipped by rail in early June and was received inland without problems. At Abidjan the corn, now at E.R.H. 77% at 28°C, was mostly spoiled. Although some was transported inland, none was used for human consumption. The spoilage was aggravated by changes in temperature, breakage at loading, breakage at off-loading, and rough handling during inland transport. The situation was further compounded at Abidjan because natural ventilation in the bagged stacks was reduced by the high levels of BCFM), and a thick covering of dust on the bags.
  2. In November 1982, a cargo of 9000 t of white dent maize was shipped by freighter from East London, South Africa to Liverpool, England. En route, fire was discovered in the cargo and the ventilators of the affected hold were sealed. On arrival in Britain the hatch was further sealed, gas extraction equipment temporarily employed, the unaffected cargo in the other holds removed, and 17 t of carbon dioxide (CO2) added at intervals. The CO2 contained the fire but the temperature continued to increase, indicating that the fire was spreading and increasing in intensity. Another 1.5 t of CO2 was added and the cargo was unloaded by mechanical grabs. The fire only became visible later and was extinguished by 79.3 m3 medium expansion foam and 140 t of water. The fire was attributed to heat from an unlagged section of main engine exhaust ducting, causing corn in the adjacent hold to overheat (Darby 1973).

For other case histories see Christensen and Meronuck (1986).

Corn meal

Relative storage risk: High

Safe storage guidelines: Corn meal usually contains an appreciable quantity of oil, which has a rather severe tendency to heat. Material should be processed carefully to maintain safe moisture content and to cure before storage (National Fire Protection Association 1949). The advised maximum moisture content for safe storage of corn meal is 11.5% at temperatures up to 27°C (Muckle and Stirling 1971).

Cotton bales (Gossypium hirsutum L.)

Relative storage risk: High Relative storage risk: High

Safe storage guidelines: The myriad of small fibers that make up the cotton bale and cover its surface make it particularly vulnerable to sources of ignition as well as to rapid combustion. Fire in baled cotton has its own peculiarities, which demand respect and consideration if a large loss is to be avoided. Fires have been known to start in cotton that has been stored for months in inaccessible parts of warehouses. This cotton is often referred to as cold cotton. Fire has also been packed into a bale of cotton, at the gin, and the bale has been received, weighed, and placed into storage without the fire having been detected. This type of bale is known as a fire-packed gin bale. Bales involved in warehouse and yard fires have even flared up several days after it was thought the fires had been extinguished. Fire is considered to be extinguished in a bale 5 days after proper techniques have been used to put out the original fire. The best defense against fires is a well-maintained sprinkler system and good housekeeping, which includes setting up clean, wide aisles between the stacked bales.

Recommended procedures for fighting fires are as follows:

Fires in baled cotton in warehouses:

  • Close all doors and cut off all drafts in the compartment concerned. This should be done whether the warehouse is sprinklered or not. Drafts not only provide fresh air to the fire, they also blow heat away from the fire, thus setting off more sprinkler heads where they can do no good.
  • Give the sprinklers a chance to operate by not using the private hose system until the fire has been knocked down unless the system is not working or not controlling the fire.
  • Let the sprinkler system do its job and vacate the premises if the fire cannot be controlled with inside equipment. This is because fire in baled cotton can flash over the storage with almost explosive violence.
  • When the fire is under control by the sprinklers open the compartment door only enough to use the hose or to remove the cotton. The smoldering bales should be moved outside as soon as possible, where they can be given individual attention.
  • Use a spray or fog nozzle, not a solid hose stream. The force from a solid stream can scatter the burning wads of cotton over a wide area, thus spreading the fire.

Fire-packed gin bales:

  • Understand how fires occur. During the ginning operation, any stones, pieces of metal, or other foreign objects in the seedcotton that strike the metal parts of the gin can cause sparks and ignite the fibers. Sometimes a fire immediately bursts forth, but often the smoldering cotton becomes incorporated into bales. Usually, the fire burns through to the outside of the bale within a few hours or days, and its presence is detected by smoke or smell.
  • Store all suspicious bales in the open at least 1 m from other such bales and keep them under constant surveillance for a minimum of 5 days.
  • Wet hot areas immediately they are detected, using water containing a wetting agent.
  • Remove burned cotton by hand but do not remove bands from the bale, as this exposes more cotton fibers to ignition.

Fires in cotton yards:

  • Apply water ahead and downwind of the fire, then work toward it.
  • Look for fire under the bales.
  • Be alert for flying sparks.
  • Remove uninvolved cotton nearby and make a fire break with it.
  • Remove burned cotton to segregated area.

The foregoing recommendations are abstracted from an excellent report by Baker (1963).

Cottonseed (Gossypium hirsutum L.)

Relative storage risk: Moderate

Moisture content standard: For the U.S. Quality Index, prime cottonseed is prescribed as containing no more than 12% moisture (Whitten 1981). Maximum moisture labeling is required.

Safe storage guidelines: In cottonseed, an intergranular R.H. of 70% at which molds could be expected to appear, equilibrates with 10.1% M.C. at 25°C (Table 15) (Hall 1980). For storage over several months, cottonseed should be below 10.0% M.C. and not above 10-15°C. Deterioration of cottonseed may begin in the field, particularly 10-15 days after the bolls have opened. Under ideal weather conditions the moisture content decreases at this time from 50% to 10% but if wet weather conditions prevail, delaying harvest, free fatty acid (FFA) levels increase. Cottonseed entering storage with a FFA content above 2.5% deteriorates much more rapidly than seed with 1% FFA. The rate of deterioration increases considerably at higher temperatures (Gustafson 1978).

In the United States, cottonseed is stored for 120 to 130 days (Whitten 1981), usually in flat metal-type warehouses equipped with aeration and temperature detection systems (Gustafson 1978). Cottonseed of questionable keeping quality is processed soon after arrival at the mill. Seed containing over 12% M.C. may heat unless steps are taken to cool the seed. With close control it is possible to store seed containing 10-11% M.C and 2.5-5% FFA. Take extreme care when storing high fatty acid seed above 10% M.C. This seed heats in pockets and seed temperatures must be observed daily to ensure that the seed cools selectively; otherwise the entire mass may char (Whitten 1981).

High moisture content has a decisive effect on the respiration of cottonseed and mold development which results in self-heating and charring. Self-heating may reach elevated levels, for instance, 95°C (Navarro and Paster 1978).

Case histories:

  1. In Israel, cottonseed harvested during the 1978 season was stored in a Muskogee-type storage structure that contained 500 t of seed. The temperature of the cottonseed rose to 270°C due to self-heating. Tests were done on samples of cottonseed taken from different locations in the bulk. The moisture content of undamaged cottonseed was 7.2%, that of seed in the first stage of the heating process was 13.6%, and that of seed found in the hottest spot was 2.8%. The loss in weight of damaged cottonseed in the hot spot was 55.4% of that of undamaged seed. Free fatty acids of cottonseed in an advanced stage of self-heating reached 21.9%, whereas an increase in percentage of oil content was recorded. The observations indicated that the spontaneous heating was caused by movement of moisture within the bulk and failure of the aeration system in the storage structure (Navarro and Friedlander 1979).
  2. Cargoes of cottonseed pellets on arrival in northwestern Europe from West Africa commonly have as much as 20 t of congealed and moldy pellets covering the surface due to condensation. This crust, sometimes 30 cm thick, has to be removed and discarded at considerable expense. The problem could be minimized if pellets were shipped in ventilated containers in which the air is changed five times per hour (Clancy 1979a). Similar problems occur with groundnut (peanut) pellets.

Domestic buckwheat seed (Fagopyrum esculentum Moench)

Relative storage risk: Low

Moisture content standard:

  • Dry: up to 16.0%
  • Tough: 16.1-18.0%
  • Damp: over 18.0%

Safe storage guidelines: A moisture content of 16% is considered safe for storage. Buckwheat is usually swathed when 75% of the seeds have turned brown. However, since ripening is rarely uniform, some green seeds are usually present at harvest. The influence of storage regime and cultivar on lipid content, fatty acid composition, and sensory quality of buckwheat seed has been investigated by Mazza (1988).

Drying guidelines: The maximum drying temperature is 45°C for buckwheat intended for either seeding purposes or commercial use (Friesen 1981).

Degrading factors: Buckwheat is degraded to Sample if it contains distinctly heated or fire-burnt kernels and/or has a distinct fire-burnt or heated odor.

Domestic mustard seed

  • Yellow (Sinapis alba L.)
  • Brown (B. juncea (L.) Cosson)
  • Oriental (B. juncea (L.) Cosson)

Relative storage risk: Moderate

Moisture content standards:

  • Dry: up to 10.5%
  • Tough: 10.6-12.5%
  • Damp: over 12.5%

Safe storage guidelines: Mustard seed requires careful storage in a tightly sealed bin. Growers are advised to store seed at levels under 10% to minimize the risk of spoilage, and to use a deflector inside the bin to spread heavier, immature material, which is most prone to heating, away from the centre of the bin (Campbell et al. 1977).

Drying guidelines: The maximum safe drying temperature is 45°C for seed required for either seeding purposes or commercial use (Friesen 1981).

Degrading factors: Domestic mustard seed is degraded when it contains heated or bin-burnt seed and/or has a distinctly heated odor. Domestic mustard seed is graded Sample if it contains over 1.0% heated seed and/or has a distinctly heated odor.

Appearance of heated seeds: Seeds are crushed in strips of 100 seeds (Canola Council of Canada 1974) to determine the extent of heating. Heating is classified into three categories: charcoal black (badly bin-burnt), dark chocolate brown (distinctly heated), and light tan (slightly damaged from oxidation). Limits of heat-damaged seeds specified in statutory grades apply to charcoal black and/or dark chocolate brown seeds. Samples containing light tan seeds are carefully checked for odor, including both the bulk of the sample and the freshly crushed seed strips. If an odor is present, or if in combination with black or brown crushed seeds, the light tan seeds are considered as heated. In the absence of these symptoms, the light tan seeds are classed as damaged.

Fababeans (Vicia faba L. var. minor)

Relative storage risk: Low

Moisture content standards:

  • Dry: up to 16.0%
  • Tough: 16.1-18.0%
  • Damp: over 18.0%

Safe storage guidelines: The maximum recommended moisture content for storing sound fababeans is 16% in Canada (Evans and Rogalsky 1974) and 15% in Britain (United Kingdom Ministry of Agriculture, Fisheries and Food 1970). Fababeans of 14.2% M.C. that had not undergone frost damage were safely stored for 2 years in Manitoba by Wallace et al. (1979). Low-quality, frost-damaged beans that had been overwintered and had a moisture content above 15% often heated during the following summer.

Drying guidelines: Drying at a maximum of 32°C is recommended. Drying should be done in two stages if more than 5% M.C. is to be removed to attain a 16% storage M.C. Allow a few days between each stage to permit internal moisture to move to the surface. Do not dry beans rapidly at high temperatures because this cracks the seed and reduces viability (Campbell et al. 1977). The beans may also become over-dried on the outside and under-dried within. Under-dried beans result in a pasty meal, which on prolonged storage becomes rancid and heated. At drying temperatures above 40°C, the skin wrinkles or splits, particularly with high moisture beans. Avoid cracking the testa, as microorganisms can then gain entrance and cause rotting (Nash 1978).

Degrading factors: Fababeans are degraded when they contain heated and/or rotted beans, or have a distinctly heated or musty odor. Entire beans and pieces of beans are considered in the grading. Fababeans are graded Sample if they contain over 1% heated and/or rotted beans, or have a distinctly heated or musty odor.

Appearance of heated and rotted beans: Heated and/or rotted fababeans are those which are materially discolored as a result of heating or rotting. Seed coats are dark brown to black, and the cotyledon tissue on dissected beans is either tan or brown.

Storage problems: Beans are normally combined when the pods are black and the haulms have shriveled. Because water loss is slow from the thick fleshy pods and large seeds, a prolonged period of ripening and drying may be required before combining, particularly in cool climates. If the crop is harvested too soon, the beans in the topmost pods will be immature. They will also be higher in moisture content than those in lower pods. Because of problems associated with prolonged ripening, late harvesting, frost damage (Wallace et al. 1979), and prolonged drying, fababeans are frequently binned in a nonuniform state and consequently need to be carefully monitored during storage.

Case history: In October 1979 a farmer in western Manitoba noticed steam coming from the top of one of his 270-t bins containing fababeans that had been harvested in 1978. The bin was fitted with a perforated floor and a detachable aeration unit. The beans had been initially stored at an average of 15.5% M.C. during cold weather the previous November. Heating in the bin was observed for the first time on 28 September 1979. After noticing the steam, the farmer switched on the aeration unit to suck cold night air through the beans to reduce their temperature. During the night, flames were seen coming from the housing that connected the aeration unit with the bin. The fan was then turned off. Five days after the fire the fababeans within the bin were partially cooked. Steam was coming out of the open roof hatch and a brown liquid was running out of the rivet holes at the join between the roof and bin wall. Two days later at least 90 L of the liquid had collected on the ground (Fig. 20a). The air temperature in the air space (Fig. 20b) under the perforated floor, determined by a thermocouple probe, was 260°C at a distance of 180 cm from the bin wall. The temperature within the bean mass above must have been higher but this could not be determined because of the difficulty of access. The outside bin wall was cool to the touch. Ominous bubbling and hissing sounds could be heard from within the bin and a strong, burnt, organic smell was noticeable at a distance of several hundred metres from the bin. Since no salvage was possible, the bin contents were left to smolder and eventually become converted to ash (Fig. 20c) (Mills 1980). Heating of the fababeans occurred in two stages:

  1. a slow biological heating associated with deterioration, and
  2. a rapid chemical heating, which was accelerated by aeration.

Case history: In October 1979 a farmer in western Manitoba noticed steam coming from the top of one of his 270-t bins containing fababeans that had been harvested in 1978. The bin was fitted with a perforated floor and a detachable aeration unit. The beans had been initially stored at an average of 15.5% M.C. during cold weather the previous November. Heating in the bin was observed for the first time on 28 September 1979. After noticing the steam, the farmer switched on the aeration unit to suck cold night air through the beans to reduce their temperature. During the night, flames were seen coming from the housing that connected the aeration unit with the bin. The fan was then turned off. Five days after the fire the fababeans within the bin were partially cooked. Steam was coming out of the open roof hatch and a brown liquid was running out of the rivet holes at the join between the roof and bin wall. Two days later at least 90 L of the liquid had collected on the ground (Fig. 20a). The air temperature in the air space (Fig. 20b) under the perforated floor, determined by a thermocouple probe, was 260°C at a distance of 180 cm from the bin wall. The temperature within the bean mass above must have been higher but this could not be determined because of the difficulty of access. The outside bin wall was cool to the touch. Ominous bubbling and hissing sounds could be heard from within the bin and a strong, burnt, organic smell was noticeable at a distance of several hundred metres from the bin. Since no salvage was possible, the bin contents were left to smolder and eventually become converted to ash (Fig. 20c) (Mills 1980). Heating of the fababeans occurred in two stages:

  1. a slow biological heating associated with deterioration, and
  2. a rapid chemical heating, which was accelerated by aeration.
Effects of a fire in binned fababeans

Brown distillate leaking through seams onto ground

Figure 20a – Brown distillate leaking through seams onto ground

Soot deposit within aeration duct

Figure 20b – Soot deposit within aeration duct

Ash residues and undamaged beans as comparison

Figure 20c – Ash residues and undamaged beans as comparison

Management practices used

  • Samples for moisture content determinations were taken from the trucks by probe.
  • The Halross 919 moisture meter that was used was checked against a similar machine in a local elevator and found to register 0.04 higher.
  • Beans were turned three or four times during the winter of 1978-1979 but not subsequently.
  • Beans were aerated only on the night of the fire, 4 October 1979
  • Fire was extinguished by switching off the fan.
  • Bin contents were left to smolder. Correct procedure
  • Beans should have been cleaned, aerated, or dried to several percentage points below 16.0% M.C. to provide a safety margin for moisture increases by translocation during winter months.
  • Moisture content and temperature of the binned stocks should have been carefully monitored at intervals and data recorded for future reference.
  • Moisture contents should have been determined on several samples from each load to obtain information on maximum moisture content and likely hazards.
  • Thermocouples should have been installed in a bin this large (270 t) to indicate any abnormal rise in temperature caused by molds and bacteria.
  • Stocks should have been aerated and/or turned at regular intervals to even out moisture and temperature gradients and reduce biological heating.
  • Material known to be in an advanced stage of heating should not have been aerated.
  • Farmer should have obtained professional advice on how to handle the heating problem, which was detected 7 days prior to the fire.

Field beans (Phaseolus vulgaris L.)

This heading includes white pea beans, also known as white beans or navy beans (most important), light and dark red kidney beans, black beans, pinto beans, pink beans, small red beans, Great Northern white beans, yellow eye beans, and cranberry beans.

Relative storage risk: Low

Moisture content standards:

  • Dry: none
  • Tough: none
  • Damp: over 18.0%

Safe storage guidelines: A moisture content of 18% or less is recommended for safe storage of field beans (Campbell et al. 1977). For long-term storage, a moisture content of 18% is too high, even at 5°C for beans required for seeding purposes (Table 17) (Kreyger 1972). The maximum moisture content for safe storage of pea beans for up to 1 year is 17.0% (Hall 1980). Beans should be harvested when most of the pods are dry and the beans have hardened but before the seeds begin to shatter. The optimum moisture content for combining beans is 16-18%. At moisture content levels lower than this, damage can be severe and costly, as broken or cracked beans can only be used for livestock feed (Campbell et al. 1977).

Table 17 – Estimated number of weeks for decreased germination to occur in brown beans (after Kreyger 1972)
Moisture content
(wet basis)
(%)
11 12 13 14 16 18 20.5 23
Storage
temperature
(°C)
Maximum safe storage (weeks)
25 31 22 16 11 7 4 2 0.5
20 55 40 28 19 13 7 3.5 1.5
15 100 75 50 30 20 12 6 3
10 200 140 95 60 38 20 11 4.5
5 370 270 170 110 70 39 20 9

Drying guidelines: Drying is necessary when beans are harvested damp because of poor weather or because of excessive harvesting losses due to shattering. Maximum drying temperatures for beans are 27-32°C. Dry beans slowly and, if necessary, remove excess moisture in two stages (see section on fababeans). Great care must be taken during drying; otherwise splits develop, even at relatively low temperatures, and hairline cracks, a degrading factor, increase at elevated temperatures. During drying keep the relative humidity of the heated air above 40% (Campbell et al. 1977; R. Stow, pers. com. 1986).

Degrading factors: Beans are degraded when they contain heated or moldy beans (Fig. 17a), or have a heated or distinctly musty odor. Beans are graded Sample if they contain over 1% heated beans or have a heated or distinctly musty odor, or if they contain over 1% moldy beans. Moldy beans are characterized by the presence of dark blue exterior molds that have developed in crevices on machine-damaged beans, and yellow to black Interior molds that have developed in the concave centre area common to light and dark red kidney beans.

Discoloration due to visible molds on surface of red kidney bean

Figure 17a – Discoloration due to visible molds on surface of red kidney bean

Appearance of heated kernels: Heated pea beans have a dull-colored seed coat varying from cream to mahogany. The color is more intense in the hilum area. Cotyledons vary in color from tan to dark brown when viewed in cross section. Very light cotyledons are classed as damaged rather than as heated. Heated light and dark red kidney beans have a dull, dark red to black seed coat. Beans must be split to determine the degree and intensity of heat damage.

Storage problems: Mechanical handling damage is a problem which becomes more severe at low temperature and moisture levels. To reduce damage, wherever possible use belt conveyors or front-end loaders rather than augers when handling beans. Avoid dropping beans from excessive heights, particularly onto concrete floors (Campbell et al. 1977).

Fishmeal

Relative storage risk: There is high risk with cargoes from Chile, Peru, and South Africa but less risk with cargoes from the Northern Hemisphere.

Moisture content standards: The moisture content standard is set between 6 and 12% in South Africa (Anonymous 1983a). In Canada, no levels are delimited but the manufacturer is required by the Feeds Act to state the maximum moisture content present in the product.

Safe storage guidelines: According to Snow et al. (1944), at 15.5-21°C, the safe moisture content levels for fishmeal are 11.5% (equivalent to 72% R.H.) for 3 months storage and 9.9% (65% R.H.) for 2-3 years storage. In 1983, South Africa produced a set of guidelines for carriage of fishmeal in ships’ holds. The fat content should not exceed 11%, the product should be stored for at least 21 days before loading, and at loading the moisture content should be between 6% and 12%. Finally, the bags should have sufficient space around the rows in the hold to permit dispersal of heat generated within the stow. Fishmeal has recently been successfully carried in bulk in ships’ holds under an inert gas blanket and as pellets treated with an antioxidant agent; however, there are problems with both methods and most of the fishmeal in international trade is still carried in bags (Anonymous 1983a).

Storage problems: Fishmeal carried in bags tends to heat when subjected to pressure stacked in ships’ holds. Heating causes damage to the bags, a reduction in protein value, and self-ignition of cargo. It can also cause damage to the ship. In the early 1960s there were a number of incidents involving self-ignition of Peruvian and Chilean fishmeal cargoes. The high fat content of anchovies in the meal rendered the commodity particularly susceptible to self-ignition. Introduction of the new guidelines has reduced the number of such incidents in recent years.

Case history: Persistent fires occurred in a cargo of Chilean fishmeal held in the lower holds of the M .V. Luise Bornhofen in December 1982. The vessel, en route to China, was diverted to Honolulu, and the crew spent 6 weeks discharging damaged and heated cargo. The self-heating of Chilean fishmeal cargoes occurred in three other vessels en route to China or Japan in January 1983. Tests on bagged fishmeal in the M.V. Luise Bornhofen’s holds revealed that the fat content was below the accepted maximum for carriage in bags. To date, it is not known whether these latest incidents happened because of a relaxation in stowage standards or because of some other entirely new set of circumstances (Anonymous 1983a).

Flaxseed (Linum usitatissimum L.)

Relative storage risk: Moderate

Moisture content standards:

  • Dry: up to 10.0% (from 1 August 1988)
  • Tough: 10.6%-13.5%
  • Damp: over 13.5%

Safe storage guidelines: The maximum recommended moisture content for storage of flaxseed is 10.0%. However, for long-term storage, flaxseed for seed purposes at 10.5% M.C. requires cooling to 10°C or lower (Kreyger 1972). For more than 6 months storage at above 20-25°C the maximum moisture content must not exceed 10% anywhere in the bulk (Christensen and Kaufmann 1969). Harvested seed must be stored under dry conditions, because flaxseed is coated with a mucilaginous substance that becomes very sticky when wet. If flaxseed is stored in a tough or damp condition or is exposed to rain or snow, severe caking of the seed can occur, rendering it unfit for sale (Daun 1982). Flaxseed respires much more vigorously than cereals do in the 11-17% M.C. range (Bailey 1940), and when binned in a moist condition it will heat very quickly within a few days. For example, at 14% M.C. and 25°C, there was a marked production of carbon dioxide after 8 days and the presence of molds after 10 days (Larmour et al. 1944).

Drying guidelines: The maximum drying temperatures are 45°C for flaxseed required for seeding purposes, 80°C for commercial use, and 80-100°C for feed (Friesen 1981).

Degrading factors: Flaxseed is degraded when it contains fire-burnt or heated kernels, or has a fire-burnt or heated odor. Flaxseed is graded Sample if it contains over 10% heated seed or has a heated or fire-burnt odor.

Appearance of heated kernels: Heated kernels are usually shiny brown or black in appearance. When they are cut open, the color of the pulp is dark tan, orange, or dark brown, depending upon the severity of the damage. Severely heated kernels often have a heated odor.

Storage problems: Flaxseed harvested damp can produce deadly hydrogen cyanide (hydrocyanic acid, prussic acid) in the bin. Hydrogen cyanide is a very fast-acting poison, which can be absorbed through the skin (Bond 1984). Before entering a bin containing flax stored at high moisture levels or with a large proportion of sprouted or damaged seed make sure that the bin is thoroughly ventilated and that other persons are available if help is needed. In 1977, a Minnesota elevator worker died from hydrogen cyanide poisoning when he jumped into a bin of flaxseed. The hydrogen cyanide was generated by the seed, which had sprouted in the field before threshing and had been binned at a high moisture content (Western Producer 1977). In 1941, again in Minnesota, levels of up to 0.03% carbon monoxide (300 parts per million (ppm)) were found in the interseed air of Sample grade flaxseed in commercial storage (Ramstad and Geddes 1942). A useful account of spoilage problems occurring in stored flaxseed in Minnesota and North Dakota is given in Christensen and Kaufmann (1969).

Case history: Near Winnipeg, Man., in October 1985, an unaerated, wooden-floored 127 t metal bin on wooden skids was filled with flaxseed. Seeds from the same field were also put into an adjacent aerated 254 t metal bin built on a concrete base. The unaerated bin was 6 m from ground level to eave and strengthened by vertical angle irons; at binning, the seeds in the upper 1.8 m of the bin were at 11.3% M.C. Five months later, in March 1986, seed was removed by auger from the lower porthole of the unaerated bin. After 5 t of seed had been removed, the auger became clogged, and the lower part of the bin adjacent to the porthole collapsed (Figs. 21a and 21b). After flaxseed had been removed from the top 1.8 m of the bin, a solid crust, hot to the touch and with visible steam, was uncovered. Much adherent fines and pod debris was present on the bin walls. Beneath the crust was a large volume of fused, charcoal black, and aggregated moldy seeds. The producer was able to save 24% of the binned seeds in good condition; of the remainder, 40% was less than 25% heated, 6% was 50% heated, and 30% was totally unusable and discarded (Fig. 21c). All the aerated flaxseed was disposed of in good condition (Wilkins 1986). The structural problem here is denting, a condition caused by eccentric unloading (Jenike 1967); it is not a floor problem (G. Henry, pers. com. 1986).

Effects of spoilage and heating in binned flax

Collapse of bin after partial removal of contents by auger - View 1

Figure 21a – Collapse of bin after partial removal of contents by auger

Collapse of bin after partial removal of contents by auger - View 2

Figure 21b – Collapse of bin after partial removal of contents by auger

Fused lumps of severely heated and moldy seeds with ashes of totally burnt seeds from inside bin.

Figure 21c – Fused lumps of severely heated and moldy seeds with ashes of totally burnt seeds from inside bin.

Management practices used

  • Probe samples for moisture content determinations at the elevator were obtained but only from the upper 1.8 m of the bin.
  • Samples taken at similar locations during the winter failed to detect adverse smells or heating.
  • After the bin had collapsed, the wall near the lower porthole was supported with a front-end loader bucket.
  • A hole was cut in the upper bin wall to permit removal of good flaxseed at the top of the bin.
  • Flaxseed was shoveled to the hole by the producer.
  • Initially, no ventilation was provided, but later, roof sheets were removed and a 50-cm diameter fan was installed at the apex to provide cross ventilation.
  • Crusted material was broken up with a rototiller and the bin was cut in half and lifted off to permit removal of fused material. Mistakes made
  • Moisture content determinations were not done on representative samples during loading of bin.
  • Only the top 30% of the contents were monitored.
  • The air space above the binned flaxseed was inadequate, yet unaccompanied entry was made without safety ropes and without adequate ventilation on entrance.
  • The producer reacted immediately; he should have asked for professional advice before taking any action. Correct procedures
  • The bin should have been erected with provision for aeration.
  • A spreader could have been used to disseminate plant debris and fines; however, spreaders are of low capacity and tend to densify the grain, thus they may be a disadvantage when aerating.
  • Thermocouples should have been installed vertically through the bin centre.
  • Several loads should have been augered into trucks, then re-added to the top of the bin; removed material should have been examined for heating.
  • Deep probe samples should have been taken from the lower part of the bin.
  • Persons entering the bin should have been wearing protective clothing and a safety harness, and should have been under strong cross ventilation.
  • At least one other person should have been in attendance.

Hay

Relative storage risk: High

Safe storage guidelines: The maximum moisture content for safe storage of hay is 20-25% for 1 year and 15 - 20% for 5 years (Hall 1980). Wet or improperly cooled hay is almost certain to heat in hot weather. Baled hay seldom heats to a dangerous level, and it should be kept dry and cool for safe storage (National Fire Protection Association 1949). Quality changes that occur in hay during storage, including heating and non-enzymatic browning, are summarized by Moser (1980). Wet hay or dry silage (30 - 50% M.C.) may heat and result in considerable nutrient loss. In normal situations, a rise in temperature to about 50°C should not be alarming, since a temperature rise occurs in the sweating process. If the temperature reaches 60°C, the drop in feed value is of concern. The higher the temperature, the higher the oxidative losses and the most easily digested nutrients are oxidized first. Table 18 outlines the heating process in wet hay or dry silage (Moser 1980).

Table 18 – Steps In the heating of stored wet hay or dry silage
Temperature (°C) Processes Nutrient changes
Source: Moser, L.E. 1980. Crop quality, storage, and utilization. Reproduced by permission of the American Society of Agronomy and the Crop Science Society of America.
Ambient - 40 Normal “sweating,” possible cell respiration, limited microbial action. Some fermentation may take place. Excess moisture driven off. Very little respiration loss.
40 - 50 Normal “sweating,” microbial action, plant processes stop at 45°C. Some fermentation may take place. Excess moisture continues to be driven off, very little respiration loss.
50 - 60 Thermophilic microorganism activity. Non-enzymatic browning begins. Lowered digestibility and protein availability.
60 - 70 Thermophilic microorganism activity, increased oxidative reactions. Non-enzymatic browning continues. Further lowering of digestibility and protein availability.
70 - 80 Biological activity ceases. Strictly chemical oxidative reactions. Above 80°C, temperatures may rise very rapidly. Severe non-enzymatic browning, caramelization of sugars. Very high losses in digestibility and protein availability.
80 - 280 Oxidative reactions occur rapidly due to high temperature. Charring of forage. Large dry matter loss.
280 - 300 Oxidative reactions continue Possible ignition if ample oxygen is present.

Lentils (Lens esculenta Moench)

Relative storage risk: Low

Moisture content standards:

  • Dry: up to 14.0%
  • Tough: 14.1-16.0%
  • Damp: over 16.0%

Safe storage guidelines: Once lentils are swathed they require 7-10 days to dry, depending on the weather, but they should not remain in swaths for a long period or they will become discolored. The acceptable moisture content for stored lentils is 13.5% or less. Efforts are under way to raise the upper limit of dry seeds to 16.0% M.C. similar to that of other pulses.

Drying guidelines: Natural air drying of lentils has several advantages over heated air drying, including the elimination of stress cracks, a reduction in augering resulting in less cracking and chipping, and less supervision. Proper design of a natural air drying system is critical; spoilage and heating may result if inadequate airflow is used (Manitoba Agriculture 1986). In heated air dryers, the maximum recommended drying temperatures for seed required for seeding and feeding purposes are 38-40°C. Corresponding maximum plenum temperatures are 60-65°C. Higher temperatures can harm germination and give off a roasted odor and taste (F. Beaudette, pers. com. 1986).

Degrading factors: Lentils are degraded when they contain heated seeds, or have a heated or musty odor. Lentils are graded Sample if they contain over 1% heated seeds, or have a heated or distinctly musty odor. Samples that contain distinctly heated seeds with brown to dark brown meats are degraded according to established tolerances. Samples that contain lightly damaged seeds with tan-colored meats are classified as heated if a distinct odor is present; otherwise they are classified as damaged.

Storage problems: If lentils are stored until the spring their thin seed coating may peel when handled. This results in the product being downgraded; consequently the crop is usually moved by Christmas.

Meals, pellets, and cakes

Relative storage risk: Very high to low, depending on the product.

Moisture content standards: In Canada, manufacturers are required to state the maximum moisture content on bags containing certain single-ingredient feeds, for example soybean meal. Labeling the maximum moisture content on bags containing mixed feeds and pellets is not a requirement. In Germany, the Feed Laws prescribe a moisture content of 14.0% for feed pellets (Lowe and Friedrich 1982).

Safe storage guidelines: The safe moisture content for processed feeds is usually taken as that in equilibrium with a maximum of 70% R.H., the level at which molds begin to grow. Safe moisture contents for selected feeds are shown in boldface in Table 15, assuming a storage period of up to 1 year and an absence of mites. If mites are present, then the safe moisture content is that in equilibrium with 60-65% R.H., as mite populations tend to die out or remain static in numbers at this relative humidity (Henderson 1985). Table 19 lists the major pellet, meal, and cake exports from Canada in 1985 (Statistics Canada 1985). In order of importance these are dehydrated alfalfa; canola/rapeseed oil cake and meal; dairy and cattle feeds; wheat bran, shorts, and middlings; pelleted screenings; brewers’ and distillers’ grains; and fishmeal. The storage behavior of these products is described under separate topic headings in the text.

Table 19 – Summary of pellet, meal, and cake exports from Canada in 1985 (Statistics Canada 1985)
Commodity Main importing countries in order of their trading importance
Japan USA South Korea Ire-
land
Taiwan Indo-
nesia
Norway Britain Dollar value per tonne Total dollar value
(all countries)
1 very small (5000 t)
2 small (5000-25 000 t)
3 moderate (25 000-100 000 t)
4 large (100 000-200 000 t)
5 very large (>200 000 t)
Alfalfa, dehydrated 15 32     22       147.0 43 197 000
Canola/rapeseed, oil cake, and meal 33 14 43   63 23 53   128.7 39 620 000
Dairy and cattle feeds 23 14             217.4 24 204 000
Wheat bran, shorts, middlings 23 14 32           118.1 16 044 000
Pelleted screenings 14 22 32 42         77.0 13 567 000
Brewers’ & distillers’ grains and other solubles   13             116.1 11 447 000
Fish meal   22     12     31 220.0 4 463 000
Total                   152 542 000

Storage problems: Moisture and heat migration caused by extremes of temperature occur within pelleted and other granular materials, particularly when silos are not insulated. Temperature changes in a silo can occur because of fluctuating daytime and nighttime temperatures or because of the effect of the sun shining on one wall in conjunction with lower temperatures on the shaded side. They can also occur when inadequately cooled, recently made pellets are stored or shipped under cold prairie conditions. Moisture migration leads to pockets of moisture accumulation, mold development, and eventually spoilage problems. Lowe and Friedrich (1983) in West Germany simulated the effect of the sun shining on one side of a large silo filled with pig pellets. Condensed water accumulated at the surface and at the outer walls, and spoilage occurred after 1 week. The molds clumped the pellets, impeding their flow and later their transportation.

Even before any visible molds were evident, the flow characteristics declined because the pellets picked up moisture and became swollen and softened. A decline in flowability of pellets is indicative of moisture changes, and possibly also of mold and mycotoxin development.

Long-term hang-up, rat hole, and bridging problems frequently occur in silos that contain meals, screenings, pellets, and other finely divided materials. Examples of hang-ups recently removed from silos in the USA and Canada include corn screenings, corn gluten pellets, wheat middlings, bran, ground milo, soybean meal, cracked bean hulls, oat husks, and multiple screenings (B. Cartright, pers. com. 1986).

Heating problems in stored meals and pellets are caused by hot metal fragments and/or molasses being present in the meal (Fire Protection Association 1978), moisture migration, and the poor compressibility of hot (exceeding 150°C) outer layers of pellets (Friedrich 1980).

Oats (Avena sativa L.)

Relative storage risk: Low

Moisture content standard:

  • Dry: up to 14.0%
  • Tough: 14.1-17.0%
  • Damp: over 17.0%

Safe storage guidelines: The maximum moisture content for safe storage of oats is 13% for 1 year and 11% for 5 years (Hall 1980). A moisture content of 13% equilibrates with a relative humidity of 70% at 25°C (see Table 15). At this relative humidity level, mold development slowly begins; therefore for long-term storage the moisture content should be below 13% M.C. to allow for anticipated moisture and temperature changes. If aeration is used, then 13% M.C. is safe for long-term storage. If oats contain more than 14% M.C. when binned they tend to become musty or heat-damaged due to mold activity. This markedly reduces their feed value and may make them unfit for use as food (Stanton 1959). At moisture contents between 15 and 17%, oats should be cooled to 15 and 5°C, respectively, to prevent mold development during medium-term (45 weeks) seed storage (Kreyger 1972).

Safe storage limits are similar for both hulled and hull-less oats at relative humidity levels below 90% (equivalent to 18.5% M.C. at 25°C), although the hull-less oats are more susceptible to mite infestation. At moisture contents in equilibrium with 90% R.H. or higher, hull-less oats are more vulnerable to infection by spoilage molds and decreased seed viability than hulled oats (Sinha et al. 1979). Levels of hydrolytic rancidity occurring in stored hulled and hull-less oats were investigated by Welch (1977). The level of hydrolytic rancidity was found to increase at higher moisture contents and with longer storage periods, but the level in hull-Iess oats only exceeded that in hulled oats if the grain was severely bruised.

Drying guidelines: The maximum drying temperatures are 50°C for oats required for seeding purposes, 60°C for commercial use, and 80-100°C for feed (Friesen 1981).

Degrading factors: Oats are degraded when they contain heated, fire-burnt, or rotted kernels, or have a heated, distinctly musty, or fire-burnt odor. Samples containing heated and rotted kernels are degraded numerically up to a combined maximum of 10%. Oats are graded Sample if they contain over 10% heated kernels, over 10% purely rotted kernels, over 0.5% fire-burnt kernels, or if they have a heated, distinctly musty, or fire-burnt odor.

Appearance of heated and rotted kernels: Heated oats that have been dehulled have a discolored germ or an orange or brown groat. Severely heated oats have a heated odor and/or a distinct brown or orange hull. Rotted oats are dark gray or black, and are spongy to the touch.

Storage problems: Instances of self-heating in oats are rarely reported. Self-ignition occurred in an elevator bin that contained oats (Grain Dealers Mutual Insurance Company 1961), and in a bin that contained wet oats (Bowes 1984). Levels of temperature, carbon dioxide, microflora, and so forth in stored oats harvested under wet fall conditions were monitored by Mills and Wallace (1979). Maximum levels attained in outdoor piles were 32°C (ambient 12°C) and 15.5% CO2 and in bins 37°C (ambient -4°C) and 2.0% CO2.

Peanut/groundnut (Arachis hypogaea L.)

Relative storage risk: Low

Safe storage guidelines: The advised maximum moisture content is 9% for unshelled peanuts and 7% for shelled peanuts at temperatures up to 27°C (Muckle and Stirling 1971). In shelled peanuts (groundnuts) an intergranular relative humidity of 70% equilibrates with a moisture content of 7% at 25°C (Pixton 1982). Peanuts differ from cereals, pulses, and oilseeds in that the flowers are fertilized above the ground and the developing fruit bend down and develop in the soil. Peanut kernels can thus be invaded by aerial molds, terrestrial molds, and intermediate molds, including Aspergillus flavus, both above and below ground (Martin 1976).

Drying guidelines: The maximum temperature for safe drying according to Hall (1980) is 32°C for peanuts intended for either seeding or commercial use, but Muckle and Stirling (1971) recommend a maximum temperature of 37°C for seed.

Storage problems: Careful harvesting and storage procedures are required to reduce fungal infection by Aspergillus flavus and the development of aflatoxins (Martin 1976). The degree of toxin production has been reduced by artificial drying (Jackson 1967).

Self-heating, promoted by the presence of damaged nuts and moisture, sometimes occurs when peanuts are stored in large stacks. It is often detected by an unpleasant smell given off by the decomposing stacks. To prevent heating, stacks should be limited to 2.4-3.0 m in height and 6.0 m in width. Lanes should be left between the stacks to allow for access in the event of fire. All wet nuts should be thoroughly dried before being stacked. The nuts should be kept dry and the maximum amount of ventilation provided to the storage. In the open, the stacks should be sited in a well-drained position and be protected against ingress of moisture. Where possible, damaged nuts should be kept separate in smaller stacks. Stack temperatures should be monitored at regular intervals. Once heating has reached 80°C, the temperature will likely continue to increase until ignition occurs. Because of this, affected stacks should be opened only after arrival of the fire brigade. Fires are extinguished with water, which should be applied to the seat of the fire and kept away from unaffected stacks (Fire Protection Association 1954).

Peanuts may be damaged by water that condenses on the roof of containers as a result of temperature gradients during shipment. The condensed water drips onto the upper layers, causing spoilage. To prevent such damage, calcium chloride was incorporated into the upper layers of 8.5% M.C. unshelled peanuts held in shipping containers in Israel (Navarro et al. 1982). The moisture content of peanuts in the upper layer of the control (untreated) container increased to 10.2% but dropped to 8.0% M.C. in the container treated with 60 kg calcium chloride. Considerable mold damage occurred in the control containers.

Drying problems: The drying of peanuts presents a special problem because the flavor of the dried product is of major importance (Freeman 1980). For reduction of aflatoxins, fast rather than slow drying has been recommended (Jackson 1967).

Case history: In April 1985, near Bombay, India, a 30-40 t stack of bagged unshelled peanuts was built on the ground and covered with a waterproof tarpaulin. The peanuts, harvested 6 months previously, contained an average moisture content of 8% and the 35-kg jute bags were piled 20-25 high. No spaces were left between the bags and no temperature monitoring of the bags was done. During May, 4 weeks after storage began, the pile was consumed by fire, which originated from within the stack. During storage, the average maximum daily temperature for the area was 38°C and the maximum temperature was 42°C (L.R. Sutar, pers. com. 1986).

Mistakes made

  • The moisture content was too high for safe storage at a temperature of 38°C or more, particularly as the moisture content of some of the peanuts was above the average figure of 8%.
  • No ventilation was provided to reduce effects of temperature and moisture migration.
  • The stack was too large.
  • The stack was not monitored for temperature and other changes.

Peas (Pisum sativum var. arvense (L.) Poir.)

Relative storage risk: Low

Moisture content standards:

  • Dry: up to 16.0%
  • Tough: 16.1-18.0%
  • Damp: over 18.0%

Safe storage guidelines: Peas are harvested when they are mature and hard in the pod. Yellow-seeded cultivars are harvested beginning at 16% M.C. Green-seeded cultivars are harvested at 18% M.C., or higher, to maintain good color, then dried down to 16%, or lower, for safe storage (Manitoba Agriculture 1986).

Drying guidelines: The maximum drying temperatures cited by Friesen (1981) are 45°C for seed required for seeding purposes, 70°C for commercial use, and 80-100°C for feed, whereas Campbell et al. (1977) cite 43°C and 71°C for seed and commercial peas. Temperatures higher than 45°C will harm germination of seed peas, especially green peas.

Degrading factors: Peas are graded Sample if they contain over 0.2% heated seeds, or have a heated, fire-burnt, or distinctly musty odor.

Appearance of heated seeds: Heated peas have dull seed coats and discolored cotyledons, ranging in color from light tan to dark brown.

Storage problems: Peas of about 15% M.C. may develop a surface crust during the winter as a result of moisture migration and snow seepage, particularly when they are stored warm without aeration. The seeds tend to clump and if left undisturbed become blackened as a result of mold activity. To prevent clumping, periodically walk across the top of the bin or move the top 30 cm of stocks with a shovel.

Before moving the first load in the spring, examine the top surface of the stocks. If there is any black crust remove it with a shovel; otherwise the first load will be ruined by admixture. Crusting is a particular problem in overfilled steel bins, and it also occurs in stocks stored in Quonset huts. It can be prevented by using a front-end loader to divide the stocks and disturb the surface layers (F. Beaudette, pers. com. 1986). Because of their size and shape peas exert a greater lateral pressure than wheat; therefore if grain bins are also used for storing peas they may require reinforcement (Winnipeg Free Press 1978).

Case history: In late October 1985 a seed plant operator in southern Manitoba unloaded split peas from a 4.3 x 8.2-m bin that contained 54 t of stock. The peas were warm to the touch and about 10% of them had turned brown. They had been harvested 2 months earlier and binned together with pod debris, volunteer material, and weed seeds. Management expressed concern that harmful toxins and molds might be present on the heated peas.

Management practices used

  • Practically no management of the pea stocks occurred over the 2-month period.
  • Samples were obtained through the bottom of the bin and sent for mold, toxin, and nutritional analyses. Mistakes made
  • The bin was filled to the top, making it difficult to monitor stocks.
  • Debris was left with the peas for a prolonged period.
  • Moisture content determinations were not made on incoming material and monitoring of pea stocks was not done. Correct procedures.
  • Samples of each load should have been taken at binning to determine the range of moisture content present.
  • Material should have been cleaned soon after binning, then aerated.
  • Peas should have been monitored at regular intervals by probing, checking seed temperature, and running a quantity out and examining it for signs of deterioration.

Poppyseed (Papaver somniferum L.)

Relative storage risk: Very high

Safe storage guidelines: Poppyseed is extremely difficult to store, using regular equipment, because it has high levels of lipids and unsaturated oils. Autoxidation occurs very quickly. For example, 2-3 t of poppyseed placed in a truck can self-ignite within 2-3 hours of loading. Because of this problem, poppyseed must be stored under nitrogen in specially designed facilities. Poppyseed is grown in France for the pharmaceutical industry and is stored in a 4400-t facility at La Grande Paroisse, near Paris. On arrival at the storage plant, the seed is at a temperature of about 40°C. It is then cooled for 25 hours to 15°C by air at 0°C. If the seed is not cooled, the temperature can rise from 40° to 75°C within 24 hours. For long-term storage, 350 t sealed empty metal silos, 8 m high, are filled with 100% nitrogen before the poppy seed is added. The O2 level in each silo when filled is about 2.8%. This level is reduced to a safe 0.8-0.4% O2 during storage, which is from about 20 August to 10 June each year (F. Benit, pers. com. 1985).

Rapeseed (see Canola/rapeseed)

Rice (Oryza sativa L.)

Relative storage risk: Low

Safe storage guidelines: Rice is usually combined at a moisture content above safe storage levels; therefore additional drying is required after harvest. In the United States, a moisture content of 12.5% or below is generally considered suitable for rice storage (Kunze and Calderwood 1980). The maximum moisture content for safe storage of rice for 1 year is 13% (Hall 1980). For rough paddy rice the maximum moisture content is 14.0% and for milled rice it is 12.0% at 27°C (Muckle and Stirling 1971). For whole grain rice an intergranular relative humidity of 70%, at which molds can be expected to appear, equilibrates with 14.1% M.C. at 25°C (Hall 1980). A commercial bulk storage system designed for long-term safe storage of rough rice must provide proper aeration to prevent self-heating and maintain the rice grain at a low moisture content (around 13.5% wet basis) to protect it from fungi and insects. Mold growth is inhibited below 21.1°C for rice at 13% M.C. wet basis, and insect activity is considerably reduced below 15.6°C. The operation of aeration systems for bulk storages in high humidity environments calls for the operator’s constant attention (Steffe et al. 1980). For information on physical changes occurring in bulk stored rice see Gough et al. (1987).

Drying guidelines: The maximum drying temperature for rice intended for either seeding purposes or commercial use is 43°C (Hall 1980). The maximum drying temperature for paddy rice containing up to 20% M.C. is 44°C. If the moisture content is above 20%, the temperature should be reduced to 40°C (Muckle and Stirling 1971). Grain type affects the drying characteristics: long grain varieties dry the fastest, short grain varieties dry the slowest (Kunze and Calderwood 1980).

Rice bran

Relative storage risk: High

Safe storage guidelines: At 15°C, 12% M.C. equilibrates with 70% R.H. in adsorbing rice bran (Pixton 1982).

Storage problems: Rice bran is particularly susceptible to self-heating because it has a high content of oxidizable oils (National Fire Protection Association 1981). Fires resulting from rice bran self-heating have occurred in at least three ships. In one ship, fires occurred in two separate holds, one fire broke out while the vessel was at sea and the other when the ship was berthed at Avonmouth Docks, England. Steam injection was used to contain the fire at sea. Water was used to extinguish both fires soon after berthing (Anonymous 1966).

Twenty seven to 36 million tonnes of rice bran containing 5 to 7 million tonnes of bran oil are produced worldwide each year. Until recently, rice bran was only used for animal feed, but it is now made in extruded form for human consumption. In the past, millers did not know how to prevent bran enzymes from mixing with bran oil, and this mixing caused the oil to break down rapidly and render the bran inedible for humans. The extrusion process stabilizes the bran by using friction to create heat, destroys the bran enzymes that break down the oil, and allows the oil to be extracted economically. Removal of the oxidizable oils makes the rice bran safer to transport.

Rye (Secale cereale L.)

Relative storage risk: Low

Moisture content standards:

  • Dry: up to 14.0%
  • Tough: 14.1-17.0%
  • Damp: over 17.0%

Safe storage guidelines: Because rye matures early in the summer, the moisture content more quickly reaches a safe storage level when compared to wheat or other grains (Shands 1959). To avoid spoilage, the moisture content of rye should not be over 13% (Rozsa 1976). Kreyger (1972) recommends 14% as the maximum moisture content for storage of rye seed. Long-term storage of rye seed at this moisture content requires cooling to 15°C, or less. Mold development occurs rapidly on seeds stored at above 14.0% M.C. For example, visible molds occurred on 15% M.C. seed stored at 25°C after only 4 weeks.

Drying guidelines: The maximum drying temperatures are 45°C for seed required for seeding purposes, 60°C for commercial use, and 80-100°C for feed (Friesen 1981).

Degrading factors: Rye seed is degraded when it contains fire-burnt or heated kernels and/or has a fire-burnt or heated odor. Rye is graded Sample if it contains fire-burnt kernels, if it contains over 5% heated seeds, or if it has a fire-burnt or heated odor.

Appearance of fire-burnt and heated kernels: Fire-burnt kernels are charred or scorched. Heated kernels are orange to dark brown, somewhat similar to heated barley, but they are difficult to detect because of color variations among rye samples. Heated rye often has a heated odor and/or other heated cereal grains in the sample.

Safflower seed (Carthamus tinctorius L.)

Relative storage risk: Moderate

Moisture content standards:

  • Dry: up to 9.5%
  • Tough: 9.6-13.5%
  • Damp: 13.6-17.0%
  • Moist: 17.1-22.0%
  • Wet: over 22.0%

Safe storage guidelines: Safflower seed, grown in the drier areas of western Canada and USA, is either crushed and used for oil or used for bird feed. Direct combining is preferred over swathing, and shelling out is not a problem if the crop is harvested at or above 10% M.C. Safflower seed is stored at 9-10% M.C. (Wilkins 1985b). In California, Heaton et al. (1978) showed that increased free fatty acid (FFA) levels occurred in damaged and intact safflower seeds when stored at above 8% M.C. for 2 months. The increased FFA levels were largely due to field fungi. According to Christensen and Sauer (1982), an intergranular relative humidity of 65-70% equilibrates with 5-6% M.C., and a relative humidity of 70-75% equilibrates with 6-7% M.C. in safflower. Growth of Aspergillus glaucus spp. occurs at 6-7% seed M.C., and growth of Penicillium spp. and other fungi occurs at 10-12% M.C. (equivalent to 85-90% R.H.).

Degrading factors: Safflower seed is degraded when it contains heat-damaged kernels or has a heated odor, or rotted kernels, which are considered in combination with heat-damaged kernels. Safflower seed is graded Sample if it contains over 1% heat-damaged kernels or 1% rotted kernels, or if it has a heated odor.

Screenings

Pelleted screenings have a variable composition including, for example, No. 1 and No. 2 screenings from elevators (odd kernels, flax or barley screenings, weed seeds, chaff, and so forth) and/or refuse screenings including dust, molasses, steam, vitamins plus a binder, often ground barley, and sometimes fire-burnt salvaged grains.

Relative storage risk: Low

Moisture content standards: There is no labeled moisture content requirement for pelleted screenings.

Safe storage guidelines: Moisture content levels considered safe for pelleted screenings are 8-10%.

Sorghum (Sorghum bicolor (L.) Moench)

Relative storage risk: Low

Moisture content standards: The maximum moisture content limits for grades (US) 1, 2, 3, and 4 for all classes of sorghum are 13, 14, 15, and 18%, respectively (United States Department of Agriculture 1978).

Safe storage guidelines: Sorghum grain, also known as milo, is a cereal grain. Although the sorghum kernels are smaller and more rounded than corn, they have more protein and less fat than corn, and about the same amount of carbohydrates. Sorghum is grown mainly in semiarid regions and is used for human food and for animal feed. In the United States, it is grown for animal feed, mostly in Texas and Kansas. The maximum moisture content for safe storage of grain sorghum is 13% for 1 year and 10-11% for 5 years (HaIl 1980). According to Muckle and Stirling (1971), the maximum moisture content for safe storage of sorghum at 27°C is 13.5%, but this figure varies considerably between varieties. In sorghum, an intergranular relative humidity of 70% equilibrates with 13.8% M.C. at 25°C (Table 15) (HaIl 1980). In the United States, grain sorghum is harvested from standing stalks with a combine. The grain is physiologically mature when the greenest seeds drop to 35% M.C., but it should not be harvested until the grain has dried to 13% or less moisture unless the grain is to be dried artificially (Kramer 1959). Sorghum is readily stored if the usual management practices for cereals are employed. For more information on sorghum storage see Doggett (1970) and Sorensen and Person (1970).

Drying guidelines: The maximum safe drying temperatures are 43°C for grain sorghum intended for seeding purposes; 60°C for commercial use, and 82°C for feed (HalI1980).

Soybean (Glycine max (L.) Merrill)

Relative storage risk: Moderate

Moisture content standards:

  • Dry: up to 14.0%
  • Tough: 14.1-16.0%
  • Damp: 16.1-18.0%
  • Moist: 18.1-20.0%
  • Wet: over 20%

The maximum permissible moisture content limits for soybean grades (U.S.) 1, 2, 3, and 4 are 13, 14, 16, and 18%, respectively (United States Department of Agriculture 1978).

Safe storage guidelines: In dry fall weather, mature soybeans dry in the field from about 15% M.C. in the early morning to 10% at noon (Holman and Carter 1952). They absorb moisture again during the following night to repeat the cycle the next day. Soybeans can be harvested at a low moisture but only at the expense of added field losses and excessive mechanical damage. These effects can be minimized if beans are harvested at a higher moisture content before pods are completely mature, then dried to a safe moisture content for storage.

The safe moisture content for commercial seed is 13% for up to 1 year (Hall 1980), 10-11% for up to 4 years (Table 20) (Holman and Carter 1952), and 10% for up to 5 years (Hall 1980). These guidelines do not take into consideration such things as accumulation of fines under the spout lines. Soybeans are more difficult to store than shelled corn at the same moisture content and temperature. This is because the equilibrium moisture content of soybeans at a relative humidity of 65% and 25°C is almost 11%, or 2% less than for shelled corn (Barre 1976).

Table 20 – Safe storage periods for soybean at several moisture levels (after Holman and Carter 1952)
Moisture content (%) Market stock Seed stock
10-11 4 years 1 year
10-12.5 1-3 years 6 months
13-14 6-9 months Questionable, check germination
14-15 6 months Questionable, check germination

Storage fungi can slowly invade soybeans stored at 12-12.5% M.C. with the rate of invasion increasing above this moisture content level. Invasion of soybeans of 12.5-13.0% M.C. is unlikely to result in any loss of processing quality within a year even if the temperature is favorable for the growth of fungi, although it may cause some loss of germinability. The slow invasion of soybeans at moisture content levels of up to 13.0% by storage fungi can, however, be dangerous because it may result in a sudden, unexpected, and perhaps uncontrollable increase in fungus growth and heating (Christensen and Kaufmann 1972).

For continued silo storage, soybeans that are already lightly or moderately invaded by storage fungi are a poorer risk than sound beans, and progress toward advanced spoilage more rapidly. Once the seeds have been moderately invaded by storage fungi, the fungi may continue to grow and cause damage at slightly lower moisture contents and temperatures than they would in sound beans (Christensen and Kaufmann 1972).

Drying guidelines: The maximum safe drying temperatures, according to Hall (1980), are 43°C for soybeans intended for seeding purposes, and 49°C for commercial use, whereas Muckle and Stirling (1971) recommend maximum safe drying temperatures of 38 and 48°C, respectively.

Degrading factors: Soybeans are degraded when they contain heat-damaged, moldy, or rancid beans, or have a heated, distinctly musty or unpleasant odor. Heated beans are degraded numerically according to established grade specifications. Moldy and rancid beans are considered in combination with heated beans for grading purposes. Soybeans are graded Sample if they contain over 5% heated beans or have a distinctly heated or musty odor.

Appearance of heated, moldy, and rancid soybeans: Heated soybeans have an olive to dark brown seed coat and, when bisected, have tan to dark brown cotyledons. Moldy soybeans are wrinkled, misshapen, medium to dark brown, and often have a superficial covering of gray mold. They may also have a spongy texture and an unpleasant odor. Rancid soybeans have a deep pink discoloration.

Storage problems: Most cases of serious loss of quality in stored soybeans occur because those in charge of the beans do not know precisely the conditions prevailing in different portions of the bulk (Christensen 1976). The seed moisture contents and temperatures within the bulk must be known at all times and maintained at low levels to prevent mold development for safe storage. The condition of the stocks at the beginning of storage has an important bearing on their future keeping quality. Storage problems are aggravated by binning beans already lightly or moderately invaded by storage molds, the presence of significant amounts of cracked and split beans, and the presence of fines in the bin spout lines. The cracked and split beans and fines (mainly weed seeds), form focuses for heating and subsequent deterioration. Spoilage commonly begins in soybeans in the spout line because the high moisture weed seeds pack densely, preventing air penetration during aeration. Even if the beans at binning contain only 2-5% fines, the spout line may consist of 50-80% fines (Christensen and Kaufmann 1972).

Sweating, which occurs when cold grain is removed from storage and exposed to air that has a high relative humidity and is more than 8-10°C warmer, is also of concern. Under these conditions, moisture from the air actually condenses on the beans, and when rebinned the cumulative effect of this sweat, or moisture, can cause heating problems in storage (Gustafson 1978).

There is a genuine danger of self-ignition in soybeans because, unlike temperatures during heating of cereals, which do not usually exceed 55°C, temperatures during heating of soybeans can exceed 200°C (Christensen and Kaufmann 1972). The heat-damaged seeds lose at least 30% of their dry weight when the temperature reaches 200°C (Christensen and Kaufmann 1977). The differences between soybeans that have been subjected to microbiological heating (bin-burnt) and those that have been exposed to fire, or which have ignited (fire-burnt) are described by Christensen et al. (1973) and Christensen and Meronuck (1986). The distinction is important because insurance companies pay for loss due to fire, but they do not pay for loss due to microbiological heating.

Production of carbon monoxide (CO) was demonstrated during heating of soybeans by Ramstad and Geddes (1942). Several samples drawn 6-15 m below the surface of a heating soybean bulk gave lethal CO values between 0.005 and 0.02% (50-200 ppm).

Case histories:

  1. In a Kentucky elevator in December 1950, vapor or smoke was observed coming from several bins containing soybeans. Columns of extremely hot, compressed grain in the centre of the bins, extending almost to 32 m, the full height of the bins, were uncovered as the sound grain at the periphery was withdrawn. The columns had to be broken up mechanically to permit removal through the unloading spouts. Maximum temperatures in the centre of one heating mass were 145-170°C. No free ash was observed on even the most deteriorated samples, indicating that combustion temperatures were not attained. Elevator records revealed that many of the soybean lots received 6-8 weeks previously had contained more than 15% M.C., a value higher than that considered safe (Milner and Thompson 1954).
  2. A United States cargo of 6000 t soybeans was sent by freighter from New Orleans to a country in the Caribbean. The cargo was off-loaded by clamshell into trucks and then delivered to the processing plant. On arrival at the plant, discharge of the cargo was halted because the beans smelled bad and some had already sprouted. Since the general manager of the plant knew that conflicts would arise over who was responsible for the damaged beans, he immediately called his lawyer and insurance agent and later the shipper and importer of the beans. The Food and Drug Authority sent inspectors to examine the cargo and they declared that it was rotten and could not be off-loaded. Conversations with the captain and crew revealed that two hatches had leaked during the stormy voyage from New Orleans. Also, by the time the ship had reached its destination, some of the beans tested at 48% M.C. instead of the specified 12% (Anonymous 1983b).
  3. In Israel, a bin of 2000 t (US) Grade 2 soybeans that contained up to 20% cracked and split beans, up to 3% damaged beans, and up to 2% foreign material was stored safely for 5 months. Temperatures were recorded weekly by means of three thermocouple cables, each with seven junctions (Fig. 23a). Following a rise in temperature to 35°C in the upper part of the bin (Fig. 23b), it was decided to unload the bin contents. The presence of 30-50 t of heat-damaged beans in the upper central part of the bin (Fig. 23c), an area where no thermocouples were directly located, was only detected during unloading operations. Probably the low thermal conductivity of the soybeans prevented heat from dissipating rapidly enough for the thermocouples to detect the problem at an earlier stage. At the heated core, which consisted of a large amount of dockage and split beans, the moisture content was 22.4% and the free fatty acid (FFA) level was more than 35%. Corresponding figures for intact, undamaged beans around the heated core were 12.8% M.C. and 0.56% FFA. The maximum temperature recorded was 98°C at a depth of 1 m within the mass of heated beans (Ben-Efraim et al. 1985).

Diagram of bin containing soybeans, which shows location of heat-damage

Figure 23 – Diagram of bin containing soybeans, which shows location of heat-damage: A, cross-section, showing location of thermocouple cables; B, longitudinal section, showing temperatures In degrees Celsius before unloading; C, longitudinal section, showing the area (X) where heat-damaged beans were detected (after Ben-Efraim et al. 1985).

For other case histories see Christensen and Meronuck (1986) and Hesseltine (1982).

Sunflower seed (Helianthus annuus L.)

Relative storage risk: Moderate

Moisture content standards:

  • Dry: up to 9.5%
  • Tough: 9.6-13.5%
  • Damp: 13.6-17.0%
  • Moist: 17.1-22.0%
  • Wet: over 22.0%

Safe storage guidelines: Manitoba provincial recommendations state that sunflower seed can be stored at up to 10% M.C., but that a moisture level of 8.5% or lower is more desirable. Even at this moisture content level, spoilage can occur if the temperature is not reduced at the time the seed is put into storage (Manitoba Agriculture 1986). In North Dakota a maximum of 9% M.C. is suggested for safe storage (Cobia and Zimmer 1975). At or below 7.0% M.C. sunflower seed can be stored without aeration in the long term, but at 9.5% M.C., and above, only in the short term (Gustafson 1978). Robertson et al. (1984, 1985a) investigated the effect of seed moisture content on fungal growth and seed quality in seed stored at 10°C and 20°C for up to 60 weeks. No deterioration occurred in either the 7.5% M.C. seed stored at 10°C or the 6.7% M.C. seed stored at 20°C. Significant deterioration occurred in 9.8% and higher moisture content seed stored at 20°C, and this was likely caused by storage fungi in the A. glaucus group.

When sunflowers reach maturity, usually in mid-September, their heads turn yellow at the back and the bracts around each head turn brown. At this stage the seed moisture content is about 50%, but harvesting is usually delayed until the seed has dried to 12% M.C., or less (Daun 1982). In most areas of western Canada, there is no need to dry the seed (Durksen 1975). Since sunflowers can be threshed cleanly at 20% seed M.C., some growers prefer to harvest at this level and then dry the seed artificially to a safe moisture level for storage (Daun 1982).

Drying guidelines: Sunflower seed is easily dried and, because of its bulkiness, with relatively little cost (Durksen 1975). The maximum safe drying temperatures cited by Friesen (1981) are 45°C for sunflowers required for seeding and 50°C for commercial use. Durksen (1975) cites maximum safe drying temperatures of 43°C, 49°C, and 60°C, respectively, for sunflowers dried in batch-type non-recirculating, continuous flow, and batch-type recirculating dryers. Reduce temperatures of the batch type dryers during the last half hour of drying, and dry sunflower seed to 8.5% M.C. to allow for any recovery in moisture during storage.

Degrading factors: Sunflower seeds are degraded when they contain heated or fire-burnt kernels and/or have a heated, fire-burnt, or musty odor. When both heated and rotted kernels are present they are considered in combination. Sunflower seeds are graded Sample if they contain over 2% heated or rotted kernels, or have a distinctly heated, musty, or fire-burnt odor.

Appearance of heated seeds: When cut lengthwise, heated seeds have brown-colored meats.

Storage problems: Sunflower seed, as received from the field, normally contains from 3 to 20% trash, which should be removed, along with fine material and large blank seeds, before storage. Removing large, blank seeds allows for maximum utilization of storage space, and eliminating fines prevents hot spot development and allows for proper aeration.

Drying problems: The drying process should be carefully monitored to avoid two commonly encountered problems over-drying and dryer fires (Cobia and Zimmer 1975).

Over-drying occurs when operators forget or are unaware that sunflower seeds dry more rapidly than higher bushel weight seeds such as corn. Over-drying may result in heat-damaged kernels with dark-colored meats that are indistinguishable from those caused by post-harvest fungal invasion during storage. Robertson et al. (1985b) studied overheated Sample grade sunflower seed and found that heat-damage scores did not always accurately reflect sunflower seed and oil quality as determined by chemical analyses.

A dryer fire occurs when very fine hairs or fibers from the seed are rubbed loose during handling, float in the air, and ignite when drawn through the drying fan and open burner. The hazard is increased when seed is dried above 60°C and for this reason many farmers prefer to dry the seed at lower temperatures. The fire hazard is decreased when the fan can draw in clear air that does not contain fine hairs or fibers. This may be accomplished by using a portable dryer, by turning the fans into the wind, or by attaching long snorkel tubes to the drying fan (Cobia and Zimmer 1975).

Guidelines for drying sunflowers are as follows:

  • Maintain good housekeeping practices. Clean around the dryer and in the plenum chamber.
  • Do not over-dry.
  • Ensure even flow for all sections of batch-type recirculating dryers and continuous flow dryers. Uneven flow causes over-dried spots and increases fire hazards.
  • Do not leave drying equipment unattended.

Triticale (a hybrid of wheat and rye)

Relative storage risk: Low

Moisture content standards:

  • Dry: up to 14.0%
  • Tough: 14.1-17.0%
  • Damp: over 17.0%

Safe storage guidelines: In triticale an intergranular relative humidity of 70% equilibrates with 15.1% M.C. at 22°C. The moisture content-relative humidity equilibrium values for triticale at 22°C are higher than those for rye at 25°C or wheat at 20°C or 25°C. Triticale has a density about 20% less than that of wheat and 15% less than that of rye, and this may have some bearing on its higher moisture content-relative humidity values (Sinha and White 1982). Information on storage behavior of triticale is lacking, but from the foregoing it appears that triticale is less likely to spoil than wheat when stored at the same moisture content and temperature.

Wheat (Triticum aestivum L.)

Relative storage risk: Low

Moisture content standards:

  • Dry: up to 14.5%
  • Tough: 14.6-17.0%
  • Damp: over 17.0%

Safe storage guidelines: In soft red winter, hard red winter, hard red spring, and durum wheats an intergranular relative humidity of 70% equilibrates with 13.5%, 13.9%,13.9%, and 13.7% M.C., respectively, at 25°C (see Table 15). At 10°C, 70% R.H. equilibrates with a wheat moisture content of 15% (Friesen and Huminicki 1986). According to Hall (1980), the maximum moisture content for safe storage in a tight structure is 13% for commercial wheat and 12% for seed wheat. For long-term storage of commercial wheat the maximums are 13-14% M.C. for up to 1 year and 11-12% for up to 5 years. Safe storage guidelines for hard red spring wheat have been developed by Wallace et al. (1983) and summarized by Wilkins (1983). The periods of time during which wheat can be safety stored at various seed moisture content-temperature combinations are shown in Fig. 24. Compared to many other crops, wheat is readily stored but on occasion hot spots may develop.

Wheat storage time chart showing zones in which spoilage occurs in less than 10 days

Figure 24 – Wheat storage time chart showing zones in which spoilage occurs in less than 10 days, within 10-30 days, within 1-3 months, and no spoilage for at least 6 months (after Wilkins 1983).

Drying guidelines: The maximum safe drying temperatures are 60°C for seed required for seeding purposes, 65°C for commercial use, and 80-100°C for feed (Friesen 1981). Excessive heat during drying of wheat can damage the endosperm protein, impairing the suitability of the flour for bread-making (Freeman 1980). Maximum recommended air temperatures for drying milling wheats are 60°C for non-recirculating batch-type dryers and cross-flow continuous dryers, 60-70°C for recirculating batch-type dryers, and 70°C for parallel-flow continuous dryers. The grain temperature in any part of the dryer should never exceed 60°C.

Degrading factors: Wheat seeds are degraded when they contain heated, bin-burnt, fire-burnt, severely mildewed, or moldy kernels, or have a fire-burnt odor. Wheat seeds are graded Sample if they contain over 2% fire-burnt kernels, or over 10% heated, bin- burnt, rotted, severely mildewed, or rotted kernels, or if they have a distinctly fire-burnt odor.

Appearance of seeds: Fire-burnt kernels are charred or scorched. Distinctly heated kernels are pale brown to very dark brown but not black. Bin-burnt, rotted, severely mildewed, and moldy kernels are blackened and swollen, and have a puffed-up appearance as a result of severe heating or exposure to high moisture conditions. Such kernels may be discolored throughout and be spongy to the touch.

Storage problems: Hot spots, originating from either fungal or insect activity, may develop during the late fall, particularly in non-aerated grains. The ecology of an artificially induced hot spot was studied, using samples collected from two 13-t wheat bulks stored at Winnipeg, Man., during 1959-1961 (Sinha and Wallace 1965). Heating by fungi was initiated in winter primarily by the activity of low temperature Penicillium species growing in a 4-month-old grain pocket of -5°C to +8°C and 18.5% to 21.8% M.C. The hot spot reached a maximum of 64°C, and cooled in 2 weeks.

Case histories:

  1. A large bin at Cairo, III., was filled with wheat, which had been harvested at 27-32°C. According to the records, the average moisture content was 13.2%; however, some grain was binned at or near 14.0%, and even at 16.0%, because of inaccurate readings taken from a faulty moisture meter. During the subsequent cool autumn, rapid moisture transfer likely occurred in the bulk. First slow, then rapid heating occurred, resulting in 40% germ damage, reduction to Sample grade, and considerable monetary loss. The spoilage was due to development of post-harvest fungi (storage fungi), and the warehouse manager was judged responsible (Christensen and Kaufmann 1969).
  2. In a Middle Eastern country, a 30-m high concrete silo was filled with 5000 t of 13% M.C. wheat. The silo was instrumented with seven thermocouple cables, each with 10 equidistant sensors. The cables were located 1 m from the silo wall. After 3 months storage without aeration, the temperatures along six of the cables ranged from 24 to 36°C, but along the seventh cable they ranged from 89 to 96°C. The smoldering grains at 89-96°C were located on the sunny side of the silo. Probably the heating was caused by moisture transfer within the bulk, aggravated by diurnal changes occurring on the sunny side of the silo.

Wheat bran, shorts, middlings

Bran pellets contain about 50% large flake bran, 35% shorts (intermediate in size), and 15% wheat middlings (fine size). Mill run pellets contain 80-85% shorts from the milling process, 10% reground bran, and 5-10% ground screenings, consisting of buckwheat, barley, oats, broken wheat, weed seeds, and filter and flour dusts.

Relative storage risk: Low

Moisture content standards: There are no delimiting standards in Canada but ground wheat germ is required to have maximum moisture content labeling.

Safe storage guidelines: Moisture content levels considered safe by industry are below 10% M.C. for bran pellets and below 13.5% M.C. for mill run pellets for 0-3 weeks storage. According to Snow et al. (1944), the safe moisture content level below which mold growth does not normally take place for bran and middlings is 14.4% (equivalent to 72% R.H.) for 3 months storage at 15.5-21°C. For 2-3 years storage at 15.5-21°C, the safe moisture content level for bran is 12.8% (65% R.H.) and for middlings it is 13.1% (65% R.H.). In practice, freshly made bran pellets after cooling are at about 9.5% M.C. and mill run pellets are at about 13.2-13.6% M.C. To prevent condensation and subsequent mold problems from occurring in pellets in winter, bin the cooled pellets and examine them for residual heat, turn if necessary, then load into railcars. In summer, load the cooled pellets directly into the cars.

Appearance: Bran pellets have a pinkish tinge; mill run pellets are pink but less so than bran pellets.