Canadian Grain Commission
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Spoilage and heating of stored agricultural products

Chapter 5 – Detection of spoilage and heating

Spoilage and heating in stored commodities are detected by the presence of certain distinguishing features, by regular environmental monitoring, and by detailed examination of samples. Some distinguishing features, for example, melted snow on roofs and putrefactive odors, permit recognition of very advanced stages of spoilage and heating, whereas others, such as carbon dioxide levels slightly above those present in ambient air, permit recognition of the early stages or even incipient spoilage. A list of distinguishing features useful for detecting early (E), intermediate (I), or advanced (A) stages of spoilage and heating is given in Table 6. The features are grouped according to the following detection viewpoints: (a) from the exterior of the storage structure; (b) from the interior of the storage structure above the stocks; (c) during the movement of stocks; (d) during monitoring of the stocks in storage; and (e) during detailed examination of samples in the laboratory.

Table 6 – Detection of spoilage and heating in stored products
Detection viewpoint Distinguishing features Indicative of spoilage Indicative of heating
E = early or incipient spoilage and/or heating.
I = intermediate spoilage and/or heating.
A = advanced stage of spoilage and/or heating.
  • Odor (putrefactive)
  • Odor (burnt)
  • A
  • A
Exterior: walls and roof of silo
  • Snow melting on the roof; space existing between the structure and surrounding snow
  • Brown liquid flowing through wall seams onto the ground
  • Smoke, vapors, steam, or fire
  • Hot areas near bin wall visible by thermography
  • Change in the color of thermal paint or label

  • A

  • I
  • A

  • I, A
  • A

  • A

Interior: above stocks
  • Heat haze above the surface
  • Steam column rising from the surface
  • Odor (musty)
  • Sprouting grains, bridging, visible mold (green, blue, yellow, or white)
  • Sampling probe difficult to push into contents

  • A
  • I
  • A

  • I, A
  • A

  • A

During movement of stocks
  • Auger ceases to operate
  • Smoldering fire bursts into flame
  • Presence of black fused hot or cold materials in stocks
  • Presence of insects, middle bridge
  • A, I

  • I
  • A
  • A, I

  • I
During monitoring of stocks in storage
  • Temperature higher (I,E) or much higher (A) than expected product temperature, as detected by rods, thermometers, thermocouples, or cables
  • Moisture content levels increase, particularly near surface
  • Elevating CO2 levels
  • Sampling
  • A, I, E

  • I, E

  • I, E
  • A, I, E
  • A, I, E

  • A, I, E
Detailed examination of samples in laboratory
  • Presence of brown or black material, sometimes fused
  • Tobacco-like odor when crushed
  • Presence of spoilage molds (green, blue, yellow, or white)
  • Presence of the fungi Monascus and Paecilomyces is indicative of failure of acid treatment to grain

  • I, E

  • I, E
  • A, I

  • I
  • I, E

Exterior of storages


Spoilage and heating can occasionally be detected from outside the storage structure by a recognizable change in the normal odor of the stored product. The presence of putrefactive or burnt odors means that much of the product in store is likely in an advanced stage of spoilage or heating. Putrefactive odors were evident outside flood-damaged bins of cereals after the Manitoba Red River flood in the spring of 1979 (see Fig. 18a), and burnt odors were detected outside a severely heat-damaged bin of faba beans in the fall of 1979. The odors were associated with advanced spoilage or heating and detectable from a distance of several hundred metres.

Melted snow

The absence of snow from the roof of a bin when snow is present on other bin roofs indicates advanced heating, and the melting of snow around the bin for several centimetres indicates severe heating.

Free liquid

In some instances of advanced heating, particularly of moist seeds, the bin or silo contents are heated to such an extent that distillation occurs. A brown liquid is produced, which may flow through the seams of metal bins or the joints or cracks of silos, and may collect in pools on the ground outside the structure. This phenomenon occurs in stored soybeans and stored faba beans (Mills 1980).

Steam, smoke, and flames

Steam may emerge from roof hatch openings as part of the distillation process described in the previous section. Smoke and flames may be visible from outside the structure when severely heated contents are in contact with air at the point of attachment of aerators, under drying floors, or in the upper parts of the storage structure. Smoke and ionic products produced during early stages of fire hidden within structures, for example in a ship’s hold, are detectable by smoke and other detectors.


Thermography is the science of producing pictures from invisible thermal radiation (Wishna 1979). Temperature variations in stored products within storages are converted into images and viewed and recorded by photography (Boumans 1985; Rispin 1978; Wishna 1979). The technique is particularly useful for early detection and for determination of the extent of grain fires in large concrete silos. Devices are available with or without temperature measurement. The equipment is expensive but may be obtained on a fee-paying basis from local energy conservation companies, or services may be contracted.

Interior of storages above stocks

Heat haze

When viewed horizontally the air space above the surface of the stocks appears to shimmer. This “heat haze” is caused by the release of heat into the air from a heat source in the product and indicates advanced heating within the bulk.


When the stored commodity contains high amounts of moisture or immature material, steam may be released from the surface, indicating advanced heating. During a 1982 Manitoba survey of bins containing frost-damaged canola, advanced heating was detected in one wooden bin by the presence of a column of steam rising from the centre surface. The canola was between 9.1 and 14.1% M.C. and reached 102°C after only 10 days storage (Mills et al. 1984).

Sprouting, bridging

The presence of sprouted grains, often with green vertical shoots, on the centre surface of bulks indicates that the seed moisture content levels in the uppermost layers, apart from being high enough for seed germination, are more than sufficient to support mold spoilage. Sprouted grains also indicate poor air circulation in the bin and leaking roofs, and are often associated with the development of an upper bridge across the bin. The presence of such a bridge can be detected by probing with a grain probe.

Probe resistance

The degree of resistance experienced when pushing downward with a grain probe can determine the vertical extent of the upper bridge and the degree of aggregation among the grains below. If it is difficult or impossible to push downward, it is likely that aggregation and compaction due to activities of spoilage molds are the causes.


The presence of blue, green, yellow, orange, or white spoilage molds on the surface of the bulk or within the upper bridge indicates that moisture and temperature conditions suitable for development of such molds exist. Musty odors are often associated with the development of spoilage molds on the grain.

Movement of stocks

Adverse changes within bulk-stored products are frequently detected by removing 5-10 t of product through the bottom of the bin to see whether it flows freely or has any sour, musty (indicative of mold spoilage), or tobacco-Iike (heating) odors, or other abnormalities.

Auger blockage

During the process of bin unloading, augers sometimes stop operating. The cause is frequently due to blockage of the augers by loosely aggregated or densely packed material resulting from either localized or more extensive mold activity, indicating intermediate or advanced levels of spoilage. If the auger under the floor has multiple openings and they are all open (a common practice), then a blocked centre opening will result in off-centre emptying and potential structural problems.

Heat fusion

Sometimes black, fused chunks of product are present in the unloaded stocks. The fused chunks are likely to be first noticed in clogged augers or on gratings, for example when railcars are unloaded. These fused chunks can be very hot and are the result of advanced biological and chemical heating. If they are very hot, the chunks may spontaneously ignite when exposed to air during unloading operations. For this reason, fused chunks are a serious potential cause of elevator fires and explosions and should be handled with extreme care.


When advanced heating occurs deep down in large concrete silos, affected stocks may smolder undetected for many months. The presence of such heating problems are frequently first detected during movement of stocks, when hot, smoldering material from deep within a silo is exposed to air. The exposed smoldering material may be accompanied by considerable smoke, burnt odors, and even flames. Black fused chunks may also be present.


Movement of grains sometimes reveals unsuspected insect infestations. At a terminal elevator in British Columbia, a large silo was used for wheat and barley cleanings for several years, during which time it was not emptied. A severe insect infestation, later controlled by fumigation, was discovered during partial unloading of the silo but the problem reoccurred. On emptying the bin a well-developed middle bridge consisting of high-moisture material, some of which was at an intermediate stage of spoilage, was present. The bridge provided an ideal habitat for ongoing insect development.

Monitoring of stocks

Most of the detection methods described previously have been concerned with ways of recognizing the intermediate or advanced stages of spoilage and/or heating in stored commodities. Monitoring of stocks in situ, however, provides the main means of recognizing the early stages of these problems. Four major monitoring methods are employed: temperature, moisture/relative humidity, and carbon dioxide measurements, and sample removal and examination.


Temperatures within stocks much higher than ambient air temperatures usually indicate heating, but in some instances, they indicate retained field heat. In winter, temperatures at the centre of unaerated bins, especially those of large diameter, are higher than that of the surrounding grain or ambient air because the product will be at the temperature it entered storage. Temperatures of various parts of the stocks and of the ambient air need to be monitored from initial storage on a regular schedule to determine whether the stocks are actually heating. As an illustration, the temperature of ambient air and of stored grain 1 m and 2 m from the walls was determined in two 4-m-diameter bins. The temperatures were -5°C, 4°C, and 14°C and -5°C, 6°C, and 31°C, respectively. The grain temperature when the bins were filled was 18°C; therefore, the 14°C temperature was considered the result of residual field heat, and the 31°C temperature was considered the result of probable biological heating.

Monitoring of temperature changes and detection of heating within stocks is achieved by using equipment such as bin thermometers, thermocouples, thermistors, temperature-sensing cables, thermography, temperature sensitive paint and labels, and vertically inserted steel rods.

Bin thermometers are designed for grain but can be used for other products. They consist of a mercury-in-glass thermometer inserted within and near the tip of a pointed steel pipe into which other sections of pipe and a T-shaped handle are threaded to push the thermometer to the desired depth. The advantage of bin thermometers is that temperature checks can be conducted at a variety of locations. Disadvantages include short penetration distance (the probe probably does not reach the actual problem heating area), and excessive time and labor. A more desirable method is to attach thermometers to metal wire and to insert them into steel pipes installed in the grain, thus permitting increased depth of penetration and multiple-indicating points along the tube (Medders 1975). Note that mercury-in-glass thermometers and the steel pipes can take 10 to 30 min to reach the grain temperature because of the low thermal diffusivity of grain.

Thermocouples consist of a pair of metal wires, usually copper and constantan, joined at one end electrically. Thermocouples change in impedence when exposed to temperature differences and, when connected to a temperature monitor, detect changes in temperature. They are available for monitoring temperatures from -70°C to 400°C and for higher temperatures if ceramic cable is used. Grain bin monitoring devices based on thermocouples vary from simple thermocouple wires or probes periodically attached to portable monitors, to commercially available systems employing multistrand cables permanently attached to continuously recording monitors with alarm systems. Thermocouples are inserted into grain either before or after filling the bins (Lyster 1983).

Thermistors, small devices similar in appearance to resistors, are used to measure temperature changes with the advantage of using ordinary speaker wire rather than more expensive thermocouple wire (Anonymous 1985). They are particularly valuable in smaller bins. Because thermocouples and thermistors pick up temperature changes only at points relatively close to them, it is advisable to locate sensors where heating is most likely to occur. In western Canada, the preferred locations for sensors are at the top centre of bins at depths of 30 cm, 45 cm, 1 m, and 2 m.

Temperature-sensing cables are usually recommended for storages of 544 t capacity and above. For 544-t bins, suspend four cables from the roof (Fig. 7). Mount the centre cable to one side of the bin centre to reduce drag on the cable when unloading grain. Space sensing points at intervals of 1.2-1.5 m along each cable (McKenzie et al. 1980). Use more cables in bins of larger volume (Boumans 1985; Foster and Tuite 1982). Attach support brackets to roof and bin walls for cables longer than 6-9 m, as most bin roofs are not able to take the weight and the cables or the roof might be pulled down (G. Henry, pers. com. 1986).

Bin temperature monitoring system of four sensing cables A-D suspended from the roof

Figure 7 – Bin temperature monitoring system of four sensing cables A-D suspended from the roof. Cables A, B, and D are located halfway from wall to bin centre and C is located close to the centre. Note: Cables longer than 8-12 m require support brackets (McKenzie et al. 1980).

Thermography is sometimes used in large silo complexes to detect heating of inaccessible stocks and electrical and mechanical equipment. Thermography is particularly useful for detecting and determining the extent of grain fires in large silos, which can smolder unnoticed for many months (Boumans 1985; Rispin 1978; Wishna 1979). Thermal-imaging cameras were used to determine the location of fires and heat levels in a stubborn shipboard fire involving wet animal feed, oil, and other substances generating intense smoke (Fire Protection Association 1986).

Temperature-sensitive paint or labels are commercially available to provide quick visual monitoring. Temperature-sensitive paint is applied in small strips to the roof and walls of a bin, and thermally sensitive labels are attached to equipment. The disadvantage is that these monitoring aids are not reusable, and temperature rises have to be excessive before the roof or wall temperature rise is greater than that due to other causes, such as solar radiation.

Steel rods, 1 cm in diameter, that are vertically inserted 3-4 m into the grain mass for 15 min, then withdrawn and held to the back of the hand, provide a quick check for rising temperatures.

Limitations of temperature recording systems

Temperature recording systems are an important component of stored grain management, as significant temperature rises can be detected early if frequent measurements are made. However, they have their limitations and for maximum effectiveness need to be used in conjunction with other detection methods. Sometimes deterioration occurs before there is any detectable rise in temperature and it is not recognized by temperature sensors. Hot spots can remain undetected, because temperature sensors are only sensitive for distances of 30-60 cm and the heat from a pocket of heating grain moves very little (Lyster 1983). A slight rise in grain temperature at the thermocouples above and lateral to a hot spot may be the only indication of severe spoilage in a large silo. If this is disregarded or missed, a considerable volume of grain may be in the final stages of spoilage before trouble is recognized (Christensen and Kaufmann 1972). To detect any hot spots that may be occurring between sensor locations it is necessary to use a grain thermometer or portable temperature probe attached to a monitor and/or take samples with a grain probe.

Temperature sensors may also miss insect infestations, which develop in warm grain. Much grain was combined at 30°C and above in western Canada in the fall of 1981, with the grain at the centre of bins remaining at these high temperatures for several weeks. A farmer checking the bins would have found no change in temperature and assumed that the grain was in good condition. However, many of these bins contained rapidly multiplying populations of the rusty grain beetle. The presence of this insect is not detectable by temperature measurements alone but is easily detected with the use of a simple grain probe (Lyster 1983) or insect detection traps inserted into the grain (Loschiavo and Atkinson 1973).


Changes in grain moisture are usually monitored by removing samples from bulks and conducting moisture determinations, using laboratory equipment such as electrical moisture meters or drying ovens. Remote sensors are now available for monitoring moisture changes that occur within grain bulks (Gough 1974, 1980; Waterer et al. 1985) (Fig. 8).

Diagrammatic view of sensor for monitoring moisture in stored grains

Figure 8 – Diagrammatic view of sensor for monitoring moisture in stored grains (after Gough 1980).

Reethorpe moisture sensors have been described (Gough 1974), and modified forms are used in New South Wales, Australia to monitor moisture in 4-m deep horizontal bins of paddy rice. They were also used by storage engineers of the Tropical Development and Research Institute, London, England, to detect moisture content changes that occurred in bulk brown rice in 100 t steel silos in South Korea, which has a tropical summer and a continental winter. The unaerated rice was stored at 13.5% M.C. (wet basis) for 8 months and the top surface became moldy at the end of the storage period. Sensors inserted into the bin before and during bin filling indicated that moisture had migrated to the top of the bin and to the north bin wall, giving rise to local moisture content increases of 7 and 3%, respectively (Gough et al. 1987). Reethorpe moisture sensors placed mainly at surface locations can detect climatically induced moisture changes and they may be able to detect incipient spoilage. In the tropics, detection devices are often put in the wrong place in bins. This is because their correct placement is dependent upon solar orientation rather than on positions taken from moisture sensing in temperate climate installations, which are commonly in the middle of the grain bulk (J.A. Hallam, pers. com. 1986). Moisture sensing has the disadvantage of being expensive compared to temperature sensing, but the advantage of direct indication of moisture content justifies the additional cost.

Carbon dioxide (CO2)

Low-level grain spoilage caused by molds, mites, and insects can be detected in bins by measuring CO2 concentration in the intergranular air. These organisms produce CO2 as they respire; therefore, by measuring the level of CO2 concentration their presence can be detected before serious grain damage occurs. The following levels of CO2 concentration apply: in atmospheric air 0.03%, in low-level spoilage 0.08-0.1%, in serious spoilage 2.0% or higher, in hot spots 5.0 to 7.0%.

To measure levels of CO2 concentration in stored grains and other commodities, a simple device (Fig. 9) consisting of a 50-mL plastic syringe, a commercially available CO2 analyzer tube, and polyethylene and rubber tubing has been developed at the Cereal Research Centre. To determine CO2 levels, the polyethylene tubing is inserted into the grain, the syringe and CO2 analyzer tube are attached to one end, air is drawn through into the syringe, and the level of CO2 present read from the color-coded scale. Analyzer tubes are commercially available for CO2 measurement with scales of 0.01-0.3, 0.1-1.2, 0.5-6.0, 0.5-10.0, 1.0-20.0, and 5-60% CO2. The device and its operation are described in detail in Wilkins (1985a).

Device for detecting grain spoilage by CO2 measurement

Figure 9 – Device for detecting grain spoilage by CO2 measurement.

The technique permits accurate detection of spoilage and insect infestations in grain bins much earlier than is possible with temperature and/or moisture measurement devices, and is particularly useful in large bins, as described in the following example. In the spring of 1985, a 544 t previously aerated bulk of wheat, binned at 10.1-14.7% M.C. (mean 14.2%) and 0.03% CO2 (normal) levels the preceding fall, developed a sudden rise in CO2 levels. Three gas sampling tubes were located near the wall, at the centre and midway between these locations, 30 cm above the floor. The ends of the tubes were covered with mesh to prevent them from being plugged with debris. During the winter and early spring, moisture increased near the floor and around the periphery of the bin walls. In early March, CO2 values suddenly increased from 0.1 to 1.1% over 5 weeks in this region. The grain was aerated to remove moisture. Although less than a tonne was affected, without early detection the potential for damage was considerable in such a large bin. Furthermore, the grain was intended as feed for pigs and serious health problems could have resulted if mold had developed and been integrated in their rations.


Regular sampling of grain stocks enables existing or potential spoilage and/or heating problems to be detected before considerable damage has occurred. Sampling procedures used to locate trouble areas include sampling on a systematic and spot basis. Sampling of stocks is needed at weekly or more frequent intervals at the outset to ensure that moisture and temperatures are acceptable. Sampling intervals may be lengthened to monthly or longer periods, provided the moisture and temperature levels have stabilized and the temperature inside the bin is below 0°C. Sampling should also be initiated when obvious signs of deterioration are apparent, for example musty or off-odors or water vapor coming from the grain mass.

Representative grain samples are obtained with specialized sampling equipment, using standardized methods. The partitioned grain trier (Fig. 10a) is the most widely used sampler. This device is used to obtain samples to determine insect infestation, grain damage, and moisture, and consists of a 1.5-m-Iong, brass double tube divided into compartments for sampling at specific depths. The trier is filled by inserting it full length into the grain at a 10° angle from vertical with compartments closed and facing up, twisting the handle to open the compartment doors, and moving it up and down quickly three times in the grain. The trier is then removed after the doors are closed and emptied by laying it across a piece of cloth to catch the grain as the doors are opened. To obtain surface samples, the trier is pushed horizontally about 7.5 cm below the grain surface. The deep-cup, or bin probe (Fig. 10b) allows samples to be taken from greater depths than are possible with the grain trier. The brass sample cup is inserted into the grain and 90-cm extensions added to reach the desired depth. A short pull of the handle opens the top, allowing grain to flow into the cup. Pneumatic grain samplers (Fig. 10c) can obtain grain samples from deep silo bins. Preferred locations are the centre core and near walls warmed by the sun or other heat sources. Sections of sampling tube are attached to a cyclone air pump, which provides the suction force for pulling up the sample grain and for pushing the probe further into the mass. Using this equipment, two people can make six or seven 24-m probes during a working day, but this may vary with the type of grain and its moisture content.

Equipment for deep grain sampling

Figure 10 – Equipment for deep grain sampling: A, deep bin fin trier; B, torpedo probe; C, pneumatic grain sampler (Seedburo Equipment Co., Chicago).

Sampling plans may be utilized to locate hot spots or pest populations within storages. Details of such plans are given for full upright circular bins, flat storages, and overfilled bins by the University of Kentucky (1984). Detailed plans are given in Laewer et al. (1981) for representative sampling of 4.5-18-m diameter circular bins for moisture and temperature determinations, and in Kramer (1968) for representative sampling of covered hopper cars.

Sample examination

At the laboratory each sample is coded, and details of its origin, history, date obtained, crop, variety, and other details are recorded. Each sample is then thoroughly mixed and portions set aside for specific tests, many of which can be rapidly performed to give an assessment of sample condition.

Moisture content

It is vital to know the range of moisture content of grains within the bin or silo, as this largely determines the storage risk. If the moisture content of some grains in a bin is sufficient for mold development and spoilage, early detection of such material prevents spoilage and heating problems from occurring.

Many methods measure grain moisture, including the hot-air oven and the electronic (electrical capacitance, electrical resistance) methods. The hot-air oven is widely used as a check method and procedures have been established for many crops. For wheat of less than 25% M.C., for example, the American Society of Agricultural Engineers (ASAE) Standard is to heat 15 g in a hot-air oven at 130°C for 19 hours. After oven drying, samples must be cooled before weighing, otherwise the convective air currents caused by the hot sample dishes would affect the weighings. Electronic methods are in practical use in many grain storage facilities. They are relatively accurate and fast, but they also have faults. Most electronic meters for measuring moisture content are not suitable for high moisture content grain and their sensitivities decrease with increasing moisture. Further, the calibration of such devices needs to be checked periodically against results obtained with the hot-air oven method. For recent evaluations of grain moisture meters see Prairie Agricultural Machinery Institute (1981).

Color and odor

The external and internal color and odor of seeds in a sample provide much useful information on their storage condition. Dull seeds indicate the likelihood of spoilage molds and other spoilage problems. Brown or black seeds accompanied by a tobacco-like odor indicate bin-burn. This can be seen when the seeds are viewed in cross section. Black vacuolated seeds, usually fused together and accompanied by a fire odor, indicate fire-burn. A few bin-burnt or fire-burnt seeds in a sample results in significant crop degrading and monetary losses.

With canola/rapeseed the quickest way to detect prior heating is to crush the seeds. The crush test (Canola Council of Canada 1974) consists of attaching 100 seeds to masking tape, crushing once with a hard roller, then counting the number of dark brown seeds. It also quickly permits an assessment of the number of immature green or mature yellow seeds present. The number of green seeds in a sample is indicative of crop immaturity, the amount of seed moisture content, and potential heating problems in storage.

Molds, seed germination

Seeds plated on filter paper moistened with water for 7 days and exposed to light usually produce green shoots. Variously colored molds may be present on the seed surface. If shoots are absent, the sample is likely old and the germs possibly damaged by spoilage molds. Surface-sterilized seeds plated on moistened filter paper or agar (a jelly-like substance) containing salt (NaCI) may have white, yellow, orange, blue, or green post-harvest molds on the surface. The abundance of such molds indicates spoilage and heating problems. By plating seed samples from selected locations in a bin it is possible to detect the onset of spoilage before widespread damage occurs, and to learn the storage history and the keeping quality of the stocks. The presence of Monascus sp. and Paecilomyces varioti fungi on acid-treated grain indicates that certain chemical treatments are beginning to fail (Tuite and Foster 1979). For information on the detection of molds in foods see King et al. (1986).

Insects, mites

The presence of insects and mites in stored grain samples can be detected within 16 hours, using a Berlese funnel. This equipment, which consists of a metal funnel screened at the bottom, is filled with 150 g of grain. The heat from a 30-W bulb, placed just above the grain surface, drives any insects and mites from the grain into a jar, which contains a 70%-alcohol preservative. The method is not reliable for detecting stages of insects that live inside the grains.

A more rapid method for detecting mites and some insects is sieving. Grain samples are placed on a mesh sieve, then shaken. Insects can be detected visually. Mites, barely visible to the naked eye, fall through the mesh onto the collecting tray beneath and can then be examined microscopically.

Physiological changes

Deteriorating seeds change in physiology, some of which are readily detectable and indicative of changes occurring in storage. Fat acidity value (FAV), is a measure of the chemical changes occurring within deteriorating seeds. FAV measurement involves grinding a known weight of a sample of seeds, extraction in a solvent (petroleum ether) for 16 hours, followed by titration against a standardized potassium hydroxide solution. The higher the FAV the higher the level of deterioration. Electrical conductivity is a measure of the condition of the cell membranes of seeds. Its measurement involves soaking seeds in deionized water for 80 min, then reading the conductivity of the leachate water with a conductivity meter. Elevated conductivity levels, indicative of leaking cell membranes within seeds, are usually associated with seeds that are deteriorated (Mills and Chong 1977).