Canadian Grain Commission
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Spoilage and heating of stored agricultural products



Chapter 3 – Effects of molds

Growth and development of spoilage molds affect products in storage by causing adverse quality changes, aggregation of product, heat-damage, and production of toxins and allergens. Table 3 summarizes the effects and consequences of mold activities on stored products.

Table 3 - Effects and consequences of mold activities on stored products
Effects Consequences
1. Adverse quality changes
  • dull appearance
  • musty odors
  • visible molds
  • reduced germination
  • germ damage, discoloration
  • increased free fatty acids
  • possible degrading
  • rejection for seed purposes
  • degrading
  • rejection for processing
2. Aggregation of product
  • clogging of pipes, augers
  • sticking to bin walls
  • bridging of bin contents
  • aggregation and/or fusion of bin contents
  • interruption of operations
  • uneven pressure effects, partial bin collapse
  • dangerous air space
  • cleaning out costs, unusable facilities
3. Heating of product
  • bin-burning
  • damage to product and premises
  • possible degrading, rejection, extra costs
  • could lead to fire-burning, explosions
4. Contamination of product by harmful substances
  • mycotoxins





  • respiratory/allergenic effects
  • livestock poisoning, feed refusal
  • rejection of shipments
  • loss of markets
  • chronic human health problems
  • breathing problems in animals and humans
  • employment of other grain handlers may be required

Adverse quality changes

Growth of spoilage molds on the surface of seeds often results in a dull rather than a bright appearance as in normal seeds. Dull appearance is sometimes considered a degrading factor. The presence of spoilage fungi on seeds is also often associated with musty odors, which are a degrading factor (Canadian Grain Commission 2002; United States Department of Agriculture 1972).

Other important effects of spoilage fungi on seeds include reduction in germinability and discoloration of whole seeds or portions of them, including the germ. Under the right moisture conditions, spoilage fungi invade the germs with no visible signs of molding, weaken the seeds, and eventually cause seed death. Some strains of the spoilage molds Aspergillus restrictus, A. candidus, and A. flavus can cause severe damage and kill the germs quickly. As fungal invasion of the germs of seeds continues, the tissues of the germ become brown and then black (Christensen and Sauer 1982). Discoloration caused by fungi results in lower grades both in the USA (United States Department of Agriculture 1972) and in Canada (Canadian Grain Commission 2002).

Aggregation of grains

Mold activity in binned seed products can result in clumping and aggregation of grains in localized areas, formation of bridges of material across the top or within the bin contents, or adherence of material to bin walls (hang-ups), as illustrated in Fig. 3.

Types of hang-up and bridging problems caused by aggregation of materials in silos.

Figure 3 - Types of hang-up and bridging problems caused by aggregation of materials in silos (after Northern Vibrator Manufacturing Co., Georgetown, ON).

Clumping

Clumps of white grain within a bulk result from the mycelia, or hairlike filaments of spoilage molds, on, in, and among the seeds, thus binding them together. Burrell et al. (1980), studying moist rapeseed, observed that each clump was formed by mycelia radiating from a nucleus consisting of a foreign body or a badly damaged seed. Clumps generally develop when some localized areas of the bin contents are of higher moisture content, permitting mold development. Large clumps may block the free passage of seeds through augers and pipes and can result in interruption of plant operations. Clumps are the basic cause of a nonfree-flowing condition that can lead to off-centre flow in the bin-emptying process. Off-centre flow places the bin structure at risk. Sometimes mold growth results in large columns of aggregated material that may be found, for instance, below pipe line openings (Meronuck 1984), or even in fusion of entire bin contents. Such problems result in temporarily unusable facilities or extra chipping-out costs.

Upper bridging

A crust, usually several centimetres thick, consisting of rotted kernels, mold tissue occupying the space between kernels, and sprouted grain, sometimes develops at the top of grain bulks when they are undisturbed for several months. This development is known as upper bridging and is caused by uneven bin temperatures and uneven moisture levels resulting from convection currents within the bin (University of Kentucky 1984).

Upper bridging at the top of grain bulks causes a severe hazard to persons handling stored grains, since air spaces are created beneath the crust in partially unloaded bins. Workers may accidentally break through the surface and become trapped during unloading operations. Even if the bin is not being unloaded, workers may fall into a large void left from previous unloading operations and either suffocate or be forced to breathe toxic gases and microbial spores until they are rescued (see Chapter 7 Safety).

Upper bridging may also occur in sealed silos when conditions are favorable for microbial growth (Nichols and Leaver 1966). The surface grain mats together and adheres to the silo sides, resulting in an empty cone over the extraction auger. Lumps of matted grain may break off, fall into the empty cone, and be extracted with the clean material. As matting increases, extraction may become jerky and eventually, if the grain bridges across the silo, the flow ceases and the silo has to be opened.

Middle bridging

Numerous examples of middle bridging within grain bulks, consisting of matted, germinated, and molded seeds and mold mycelia, were observed in 1979 when the flood waters of the Red River entered grain bins near Winnipeg, Man. After the waters had subsided the contents were examined. It was found that bridges had developed just above the uppermost water level. Most grains above the bridges were salvageable if the farmer could move them before the putrefactive odors from the wet grain beneath the bridges permeated into the grain above (Mills and Abramson 1981).

Hang-ups

Moist bin contents sometimes adhere to the walls and gradually a collar of material with a central hole (rat hole, or donut hang-up) is built up across the bin. When viewed from above, the hang-up appears to be ring-shaped with the stored material flowing downward through the central hole. Frequently, managers of grains and grain products are unaware of the existence of hang-ups within bins, which only become evident when self-heating or insect infestations occur, or when the bin is emptied.

Bridge formation, adherence of material to bin walls, and clogging of some entry points in augers can result in uneven pressure effects and sometimes severe damage to bins. Walls may buckle under the uneven pressures caused by the flow of stored material in bins. Most bins are designed for centre emptying. Off-centre emptying causes uneven and, it is believed, increased loads (at least close to the wall channel). Hang-ups can cause wall buckling or denting (Jenike 1967). The subject of bin collapse due to pressure effects is described by Ravenet (1978). Bridged or adherent material in place for many months can also provide a harborage and breeding ground for insects, which move out from the bridge to infest good grain or processed materials.

Heat damage

Spoilage fungi including Aspergillus species such as A. candidus (white or cream) and A. flavus (yellow green) can, through their respiration, raise the temperature of stored products up to 55°C. Development of these molds frequently occurs in pockets of increased moisture within bulks. The pockets result from moisture migration, high-moisture weed seeds, plant debris, heavy rains, or melted snow.

The elevated temperatures result in internal browning or blackening of seeds, reduced seed quality, and lower or no germination. The effects of heat damage become progressively worse if the initial mold heating is succeeded by chemical heating. The presence of heated brown or black seeds and/or a burnt odor in a sample of grain lowers the grade in the USA and in Canada. The presence of only 2% heated, internally brown seeds in a sample of canola/rapeseed in Canada lowers the grade from No.1 Canada to No.3 Canada, with accompanying monetary loss. If more than 2% heated seeds are present in the sample, the seed is further degraded to Canola or Rapeseed Sample Account Heated (Canadian Grain Commission 2002). Similar fixed levels of permissible heated seeds exist for other crops. Seed lots with elevated levels of heated seeds cause problems for the processor, as oil from heat-damaged oilseeds requires extra decolorizing procedures during processing and this leads to extra costs.

Heat-damaged externally blackened kernels are classified as either bin-burnt or fire-burnt, depending on the severity of the heating. Fire-burnt beans are often shiny black on the outside, with large internal cavities, whereas bin-burnt beans, although often black on the outside, are brown to dark brown in cross sections, with no large, internal cavities. Furthermore, the fire-burnt beans are often fused together (Christensen and Kaufmann 1977). The same phenomena have been observed in our Winnipeg laboratory with bin-burnt and fire-burnt canola/rapeseed, wheat, and malting barley. Ultrastructure and mineral distribution in sound and heat-damaged canola/rapeseed have been studied by Mills and Chong (1977).

Heat-damage may also result from improper artificial drying. Seeds damaged by excessive heat in drying have reduced viability, are darker, and may have blistered pericarps, or seed coats. If heat-damage is extreme the seeds may explode or partially pop (Freeman 1980).

Toxins

Under suitable conditions of moisture and temperature, spoilage molds produce poisonous substances, called mycotoxins, on stored grains and processed feeds. When mycotoxin-contaminated grains are eaten by susceptible animals, disease conditions called mycotoxicoses can result. The effects of mycotoxins on animals vary, depending on the species and age of the animal, and the type and amount of toxin present in the feed. Disease effects include lack of weight gain, formation of tumors, loss in productivity, fetal abnormalities, and sudden death. In western Canada, ochratoxin A, produced by the spoilage mold Penicillium verrucosum var. cyclopium, and sterigmatocystin, produced by Aspergillus versicolor, have been found in damp or accidentally wetted stored grains associated with livestock health problems (Abramson et al. 1983). In the USA, aflatoxins, produced by Aspergillus flavus, sometimes occur in poultry feeds (Hamilton 1985). Aflatoxins have also been reported in grain dust, posing health problems for workers handling aflatoxin-contaminated corn in Georgia, USA (Burg et al. 1982). Recently, aflatoxin was shown to occur in fragments of fungal mycelium and other mycotoxins in the fungal spores, in grain dust (Palmgren and Lee 1986). For an overview of the worldwide risks from mycotoxins see Mannon and Johnson (1985). For a summary of available information on mycotoxins as they affect human and farm animal populations in Canada see Scott et al. (1985).

It is possible that other toxic substances including carcinogens are produced when grain and grain products become heated and/or burnt. If they are produced, such toxic substances and their effects on animals when heat-damaged products are incorporated into animal feeds require investigation.

Allergens

Spoilage fungi present in and on stored grains cause allergic health problems in both humans and animals. Two types of fungus-related health problems have been described in humans: bronchial asthma and farmer’s lung. Such health problems are caused by allergic reactions in the respiratory tract stimulated by allergens, primarily from fungal spores. In 1968, over 70% of the grain in the province of Saskatchewan in western Canada was harvested or initially stored in a tough or damp condition because of unusual harvest conditions. Subsequently, 20 out of 3200 farmers and elevator managers who had worked with the damp, heated, or spoiled grain developed acute farmer’s lung syndrome (Dennis 1973). For a review on the nature of grain dust, work exposure to the dust, and related health disorders see Manfreda and Warren (1984).