Canadian Grain Commission
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Spoilage and heating of stored agricultural products



Chapter 4 – Prevention of spoilage and heating

Spoilage and heating problems in stored commodities are prevented by a knowledge of commodity storage behavior, prior planning, and application of appropriate management practices. Ways to prevent spoilage and heating problems during storage are outlined in Table 4.

Table 4 – Prevention of spoilage and heating problems in stored products
Grain crop in fields
  • Air-dry small grains in swaths to safe moisture content levels.
  • Provide special binning or artificial drying for moist or immature grains
Binning
  • Obtain advice on suitability of bin system for required purpose.
  • Provide adequate site drainage.
  • Clean interior and surroundings to remove pest harborages.
  • Keep in good repair and inspect regularly for leaks.
  • Spray with insecticide, and fumigate if required.
  • Refuse loads of doubtful keeping quality on entry.
  • Know history of material.
  • Get pre-binning samples and test for spoilage mold invasion.
  • Remove debris before binning.
  • Use a properly adjusted grain spreader or stirring device in the bin to evenly distribute fines; some spreaders may worsen fines distribution.
  • Sample and determine range of moisture content of material throughout binning.
  • Turn over stocks periodically. This procedure, however, is expensive, labor intensive, and may create more broken material, e.g., in corn. It is better to aerate
Aeration
  • Know principles of aeration and likely problems.
  • Obtain advice on floor design and fan size.
  • Remove debris before aerating stocks.
  • Aerate to cool or warm the product (see Friesen and Huminicki 1986).
Drying
  • Obtain advice on most suitable system.
  • Clean stocks except corn before drying; clean corn after drying because broken corn and foreign material (BCFM) must be dried for storage and feed use.
  • Remove accumulations of dust and fuzz from walls and burner area.
  • Use wind deflectors to keep airborne material and moisture from entering burner.
  • Check for leaking propane tanks and lines.
  • Inspect electrical wiring and circuit breakers.
  • Check for uneven drying.
  • Use proper air-to-grain ratios on stirring devices to prevent stagnating drying fronts.
  • Be careful of excess drying temperatures.
  • Cool after drying.
  • Check electric moisture meters for accuracy.
High moisture grains
Chemical preservatives
  • Clean material before treatment.
  • Use correct dosage rate for a particular grain moisture content.
  • Use sufficient chemical on all or part of the bulk.
  • Aerate material to prevent moisture migration.
  • Protect concrete or steel surfaces with plastic or acid-resistant paint when using propionic acid preparations.
Ensiling green material
  • Cut green material at correct stage of development and pay special attention to any stock that is wilting, the percentage of dry matter, and the length of the cuts.
  • Fill silos quickly to ensure good compression for air exclusion.
  • Use distributor to pack material along silo wall.
  • Ensure doors and walls are tight (top unloading).
  • Keep top and bottom hatches closed to prevent chimney effect (bottom unloading).
  • Know the minimum percentage of moisture content for safe storage of material.
  • Put a green plug of moist material at top of filled silo (unsealed silo).
  • Remove material quickly.
  • Partially unload immediately after filling to prevent bridging of auger (oxygen-Iimiting silos).
Processed products
  • Remove metal fragments to prevent foci for hot spots.
  • Cool off ground material or artificially dried material in small quantities before binning.
  • Vibrate bin walls to prevent bridging.
  • Avoid contamination with liquids liable to self-heat.
  • Avoid storage in close proximity to heat sources, for example hot ducts, engines, or bin lights.
  • Avoid overdrying.
  • Ensure that bin conveyors do not become sources of frictional heat or electrical sparks.
Education
  • Educate staff on storage characteristics of different commodities.
  • Ensure staff are aware of moisture content ranges and moisture migration.
  • Stress the importance of regular inspections and a proper reporting system

Storage structures

Obtain advice from a registered professional engineer on the most suitable types of storage structures for the region and their intended use. Once selected, locate the structures on a well-drained site on properly designed foundations, thus avoiding ingress of drainage water and cracked floors. Locate vertical silos that are to be used for storing large amounts of combustible animal feeds in the open, well clear of buildings, and away from any other combustible material (Fig. 4) (Fire Protection Association 1968).

Location of farm silos in relation to buildings and other combustible material

Figure 4 - Location of farm silos in relation to buildings and other combustible material (after Fire Protection Association 1968).

Empty the bins completely at regular intervals and examine within to detect adherent material (hang-ups) sticking to the walls. Clean empty bins thoroughly, then spray floor and walls with an appropriate insecticide to kill any remaining insects which might infest the new grain. Remove old grain and debris near bin doors, under aeration floors, and under drier floors. Remove any vegetation growing near the bins as it could harbor grain pests.

Some silos are available (for example, Carter-Day All Flow®, Minn.) with inflatable membrane liners, designed to fit against the inner surface wall and floor, which prevent hang-ups, rat holes, and bridges from forming in the stored product. When a sensor senses demand for product discharge the membrane liner automatically inflates, changing the angle of repose of the product and pushing it to the discharge opening.

Keep structures in a good state of repair and weathertight to keep out wind-driven rain or snow which, on melting, provides moisture for mold development. Repair cracks in concrete walls and floors, and seal gaps at the metal wall/concrete base interface to keep out moisture and to reduce the number of hiding places for insects.

Management before storage

Decisions made while the crop is still in the field can prevent potential storage problems. Such decisions might be to air-dry small grains sufficiently in the field to ensure safe moisture levels during binning; to determine the seed moisture content and temperature of samples obtained from the combine to predict, by means of charts, the keeping quality of the crop (Kreyger 1972; Mills and Sinha 1980; Wallace et al. 1983); and to separately combine wetter outer or lower areas of the field, with immature seeds of high respiratory activity, then bin the grain in smaller observation bins in the farmyard until it is aerated.

Rejection at entry

Elevator managers receiving loads for storage in a facility have the right to refuse loads that have doubtful keeping quality or that are in poor condition, thus preventing later handling and storage problems.

If time permits, before elevator entry, managers should determine the degree of invasion by spoilage fungi of suspect seed lots by plating out representative samples and reject if found unsuitable. A railcar load of soybeans, for example, already lightly or moderately invaded by spoilage fungi, is a poorer risk for continued storage than a railcar load of sound beans. The molds may or may not be visually apparent. Such loads stored under conditions that are favorable to mold development progress toward advanced spoilage more rapidly than sound beans (Christensen and Kaufmann 1972). Plating of samples before elevator entry is likely impractical in most busy elevators but could be useful in particular situations.

Management during storage

General handling

Cleaning harvested material to remove high-risk debris, broken seeds, immature weed seeds, chaff, dust, and other fines can improve the efficiency of aerators and bin driers by increased airflow. With corn, remove fines from the screens after drying, because BCFM (broken corn foreign material) has to be dried for storage and feed use. Cleaning units incorporated into standard grain augers now permit grain cleaning during augering (Anonymous 1982). If it is not possible to clean the seed, then use a properly adjusted grain spreader or stirring device (Gebhardt 1983) inside the bin to prevent the accumulation of fines and other material in the bin core. Withdraw one or more loads, on completion of binning, through the bottom of the bin to remove fines and to make an inverted cone suitable for ambient air-cooling rather than a peaked surface. In Saskatchewan, bins are often filled as full as possible to provide less room for snow to accumulate, but in high-humidity areas such as Manitoba the overfilling of bins can reduce ventilation and increase the likelihood of spoilage.

Sample and determine the range of moisture content of the harvested material as it is placed in the bin. The highest moisture level is the one of greatest concern. Aerate the binned material. In non-aerated bins, after binning turn the material into another bin or truck to even-out any moisture pockets and temperature differences. Turning is costly and increases FM (foreign material), especially in corn, and is therefore done sparingly.

Inspect regularly the storage structure and its contents for leaks in bin roofs or in hatch covers on ships, for snow blown in through cracks, for open doors, and for spoilage. Install permanent metal ladders on the sides of storage structures to facilitate such inspections. Probe the material regularly for signs of spoilage and heating, particularly in the top bin centre, or install remote CO2 (carbon dioxide) and temperature sensors. The detection of such problems is described in a later section (Chapter 5).

The probable development of spoilage molds in a lot of seeds during storage is predicted by the amount of existing spoilage mold present and a knowledge of the history, condition, and type of seeds involved. Decisions are then made regarding how long to keep the particular lot, whether it should be disposed of, or which preventive measures need to be taken.

Aeration

Aeration is the practice of forcing unheated air, by means of a fan, through grain to maintain its condition and reduce the chances of spoilage and heating. This system lowers the temperature of the bulk if the grain is above the ambient air temperature, maintains a uniform temperature throughout the grain mass, which reduces or eliminates moisture migration, removes hot spots, reduces mold and insect growth, and removes storage odors. Aeration is commonly used on the Canadian prairies at airflow rates of 1-2 (L/s)/m3. There are many aeration systems available. Proper floor design and fan size are important and requirements need to be checked with a professional agricultural engineer. Aeration principles and the proper operation of aeration systems are described by Friesen and Huminicki (1986).

Localized spoilage may occur if the airflow is too low in some regions of the bin, if there is excessive debris, or if the fan is shut off before all the grain has been cooled in the fall or warmed in spring, as condensation may occur between cooler and warmer parts of the grain mass. In extreme situations, if heating is far advanced, aeration may result in self-ignition and loss of the crop.

Drying

Drying grain prevents spoilage and subsequent heating of grain in storage. Other advantages of drying grain include a longer harvest season and the option for earlier harvesting with reduced field losses (Friesen 1981). To obtain maximum benefits from a grain dryer it is necessary to set up a well-organized system for grain handling. The following methods, each involving the movement of air through grain, are available for drying grain:

  • High temperature or heated air drying method involves blowing heated air through the grain in a separate dryer or in a bin.
  • Low temperature air drying method uses the potential of atmospheric air for drying grain.
  • Combination air drying method involves heated air drying, followed by in-storage cooling and atmospheric air drying.
  • Dryeration method involves heated air drying, followed by a tempering period and cooling and drying in a separate bin.
  • In-storage cooling method (an alternative to in-dryer cooling) transfers hot grain directly to the storage bin for cooling.

The subject of grain drying is too complex to describe here. The reader is referred to publications by Friesen (1981), Friesen and Huminicki (1986), and Moysey (1973) for detailed information. General recommendations for the design, installation, and utilization of driers are published by the American Society of Agricultural Engineers (1986), but these may differ from Canadian recommendations.

Fires in dryers

Fires can occur in both farm and commercial dryers, and are detected by a sudden rise in exhaust air temperature. Fires can occur in heated air dryers if dirt and residues accumulate in the burner area. Sunflower seeds often have fuzz attached to them which is released in the drying process and if drawn through the fan and burner may ignite. Corn kernel debris composed of red dog and other fines may ignite. Canola seeds may also ignite when passed through a burner.

Suggested ways to reduce the risk of fires in dryers (Broadhurst 1985; Friesen 1981) are as follows:

  • Check propane gas tanks and lines for leaks.
  • Inspect electrical wiring for soundness. Wire insulation can be cracked, dried out, or rubbed bare, especially on connections to equipment which is in constant movement. Circuit breakers can deteriorate through lack of use to the point where they do not break when overloaded.
  • Repair and do not bypass controls such as air switches provided as safety measures by the manufacturer.
  • Clean the seed to remove light or fine material before drying.
  • Use wind deflectors to prevent drawing of airborne material through the burner.
  • Remove accumulations of dust and fuzz from the walls and other areas of the drier.
  • Avoid excess drying of seed.
  • Keep temperature of drying air within recommended safe limits.
  • Be alert at all times during the drying cycle.
  • Put sunflower seed and canola through the dryer on warm dry days without starting the burner.
  • If a fire occurs, shut off the heat and fan.
  • Have water and/or fire extinguisher ready for use.

Commercial dryers are also vulnerable to fires. Detection and control of fires in commercial dryers are discussed in a later section.

High moisture grain storage

Mold growth on high moisture grains of above 22-25% M.C. is prevented by limiting the available oxygen supply, as in sealed silos. Another method is by the application of mold inhibitory chemicals to the grains.

Sealed storages When high moisture grain such as corn is placed in a sealed silo, the grain undergoes fermentation, oxygen is depleted, and carbon dioxide is increased by the respiration of the grains, yeasts, and bacteria. Aerobic mold growth is halted but germination is impaired, making the seed only suitable for animal feed. On removal from the silo, mold growth recommences; therefore exposed grains must be fed rapidly to keep ahead of mold growth. The high moisture grains may be stored in glass-Iined, oxygen-limiting steel silos or in other kinds of airtight bins that have a breathing system to prevent structural failure because of differential pressures and to limit the exchange of in-storage gas with outdoor air. For details on the selection and use of oxygen-limiting silos see Bellman (1982) and Pos (1980). High moisture corn is also stored in sealed concrete silos and in bunkers covered with plastic. The corn is usually cracked and packed tightly to quickly develop anaerobic conditions (Tuite and Foster 1979).

The following problems may be encountered during storage of high moisture grains in sealed silos: spoilage of corn near walls of concrete silos caused by the failure to spread the material in uniform horizontal layers; and surface spoilage caused by removing the grains too slowly, allowing oxygen levels to increase. In oxygen-limiting silos, bridging may occur over stationary unloading augers. Where only one fixed auger position is available some unloading should be done immediately after filling is completed to set up a flow pattern, thus preventing bridging (Pos 1980).

Chemical preservatives When high moisture grain is treated with the recommended dosage of a registered preservative chemical the grain can be removed from storage without concern for spoilage. Propionic acid is the most common material used, applied either as 100% propionic acid or in mixtures with acetic acid, isobutyric acid, or formaldehyde, but the latter materials do not enhance the efficacy of propionic acid significantly. Other materials under investigation are sulfur dioxide (SO2) and ammonia (NH3) (Tuite and Foster 1979). The acid dosage rate depends on the moisture content of the grain (higher moisture contents requiring more acid (Table 5)), temperature, and length of storage period. Acid-treated grain does not need any particular type of storage structure, but when galvanized steel sheets are used severe corrosion can result. This problem can be corrected by prior coating with chlorinated rubber paint (Theakston 1972).

Table 5 – Amounts of propionic acid required for preventing mold growth in high moisture grain*
Moisture content (%) Propionic acid required
Percentage
**    ***
Kilograms/Tonnes
**    ***
* Source: University of Kentucky (1984).
** Rate for short-term storage in cool weather over winter.
*** Rate for 1 year’s storage, beginning late fall.
18 0.3 - 0.6 2.5 - 4.9
22 0.5 - 0.8 4.1 - 6.6
26 0.6 - 1.0 4.9 - 8.3
30 0.8 - 1.2 6.6 - 9.9

Grain that has been treated with a preservative can also be subject to mold spoilage and should be inspected regularly during storage (University of Kentucky 1984). Mold growth releases moisture, enabling the molds to spread to the treated grains. This occurs under the following conditions:

  • When an incorrect dosage rate of preservative for the particular grain moisture content is used.
  • When insufficient acid is used on all or part of the bulk.
  • When wet spots develop through moisture migration. To prevent wet spots, aerate acid-treated grain to break up temperature gradients that cause moisture migration, clean the grain before storage, and use correct fan and aeration flow rates. If aeration is improperly done, readsorption could occur in the upper grain layers, and free moisture could drop back from the roof onto the grain, diluting the acid, thus permitting mold growth.
  • When treated grains are in contact with unprotected concrete or steel. Such surfaces should be covered with plastic or acid-resistant paint (University of Kentucky 1984).

Ensiling green material

Chopped green plants and chopped green hay, stored as silage and haylage (medium moisture forages) in vertical and horizontal silos, are subject to both spoilage and heating problems. Spoilage commonly results in a loss of 10% dry matter in a properly managed, conventional concrete silo. New concrete designs now provide walls of greater density with lower porosity, allowing a more airtight container and ensuring less spoilage. Losses have been further reduced by proper filling techniques and the use of silage distributors to ensure uniform loading. Oxygen-limiting silos with an effective gas seal can control dry matter losses to 2-4%. Traditionally, losses in uncovered horizontal silos have been as high as 32%, but they can be reduced by as much as 50% by sealing the silage with an airtight polyethylene cover, properly weighted down and protected from puncturing (Pos 1980), and by utilizing the material at a sufficient rate to minimize exposed surfaces to the air.

Heating problems and fires can occur in both top- and bottom-unloading vertical silos. With top-unloading types, ensure that the doors and walls are tight with no gaps. With bottom-unloading types, keep the top and bottom hatches closed to prevent the creation of a chimney effect by air being drawn through the silo. The moisture content of the ensiled material is critical in preventing heat-damage and fires. If haylage, for example, is stored below 40% M.C., fires may result. The advised moisture contents of haylage in bottom-unloading silos are 40-55% (R. Nelles, pers. com. 1986), and in top-unloading silos they are 50-65% (CampbeIl 1973). The key to preventing outbreaks of fires is good filling management. This includes cutting at the right stage, proper attention to wilting, short chop length, and fast filling rates with decreasing dry matter levels and, in the case of “unsealed” silos, a “plug” of unwilted material at the top (Institution of Fire Engineers 1970).

Processed products

Handle processed products more carefully than unprocessed ones because the natural defenses of the seeds have been mechanically destroyed, they are mixtures of finely divided materials and additives, and they are often subjected to heat during processing. Take the following precautions to prevent spoilage and heating problems:

  • Replace damaged pellet dies every 4-5 months to prevent formation of burnt pellets and metal fragments.
  • Remove metal fragments that could act as focal points for hot spots in the processed product and cause damage to the pellet die.
  • Avoid excessive drying of meals. Tung nut meals, for example, are susceptible to overheating if dried too much (National Fire Protection Association 1949).
  • Cool the product properly before storing it.
  • Install appropriate vibrating devices to feed bin walls to prevent bridging and hang-ups (Fig. 5).
  • Check loading and unloading conveyors to ensure that fires cannot be caused by frictional heat or electrical sparks (Fig. 6).
  • Avoid contaminating the product with liquids liable to self-heat.
  • Store the product away from heat sources (Fire Protection Association 1968, 1978).
  • Clean feed distribution equipment (Hamilton 1985).

Note: Vibrating devices added to bin walls require proper installation and use, or structural problems may result.

Magnetic vibration device installed at base of feed bin to prevent hang-ups and bridging

Figure 5 – Magnetic vibration device installed at base of feed bin to prevent hang-ups and bridging (Seedburo Equipment Co., Chicago).

Fire on belt conveyor caused by frictional heat or electrical sparks as a result of poor maintenance

Figure 6 – Fire on belt conveyor caused by frictional heat or electrical sparks as a result of poor maintenance (after Fire Protection Association 1968).

Education

A well-informed staff is the first defense against spoilage and heating problems in storages. Ensure that staff are aware of the storage characteristics of different commodities; have a knowledge of the importance of moisture content ranges and moisture migration patterns in bins, silos, and ships; know the probable causes of spoilage and heating problems; and know the importance of regular inspections and a good reporting system. This knowledge can be obtained from training courses and on-the-job instruction.