Canadian Grain Commission
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Spoilage and heating of stored agricultural products



Chapter 2 - Self-heating

When a stored material increases in temperature by generating heat without drawing heat from its surroundings the action is called self-heating. The increase in temperature occurs in two phases. Phase one is known as biological heating, which normally occurs up to 55°C and exceptionally up to 75°C. Phase two is known as chemical heating, which occurs from above 75°C to at least 150°C. Biological heating is caused by the activity of plant cells, molds, bacteria, insects, and mites. Chemical heating is caused by oxidation. This chemical reaction may increase the temperature to the ignition point, depending on the commodity and storage conditions (Fig. 2). For information on the theory of thermal ignition see Beever and Thorne (1982). For information on evaluating and controlling the hazards of self-heated material see Bowes (1984).

Figure 2 - Schema showing progression of self-heating in stored products.

Figure 2 - Schema showing progression of self-heating in stored products.

Stored materials vary widely in their tendency to self-heat (Table 2). Cornmeal feeds and fish meal have a high tendency to self-heat, whereas shelled peanuts and various grains have a relatively low tendency to do so (National Fire Protection Association 1949). Generally, processed products have a higher tendency to self-heat than do whole grains. Spoilage and heating problems are discussed in Part II of the manual.

Table 2 - Stored materials and their tendency to self-heat*
Tendency to self-heat
High Moderate Low Very slight Possible
* National Fire Protection Association, 1949. See table 14 for comprehensive list of commodities.
Alfalfa meal Brewers' grains Cottonseed Grain (various) Burlap bags
Cornmeal feeds Cocoa bean shells   Hides Sawdust
Fish meal Feeds (various)   Jute  
Fish scrap Hay   Linseed  
Tung nut meals Manure   Peanuts (shelled)  
  Wool wastes   Powdered eggs  
      Powdered milk  
      Sisal  

Chemical heating

When the biological heating exceeds 75°C, a purely chemical process may occur and raise the temperature of the material to ignition. This chemical process, known as oxidation, releases heat. The oxygen required for oxidation can be available either as free oxygen in the air or as liberated oxygen in chemical reactions. Chemical oxidation proceeds at a more rapid rate if preceded by biological heating.

Self-ignition

The self-heating of a stored commodity to its ignition temperature is called self-ignition. The terms spontaneous ignition and spontaneous combustion are also used but the term self-ignition is preferred. Self-ignition may be affected by a variety of chemical and physical factors (American Insurance Association 1983; Bowen 1982).

The following general conditions affect self-ignition:

  • The rate at which heat is generated by the fuel material.
  • The oxygen supply available.
  • The rate of heat loss to surroundings.

A number of vegetable and animal oils and fats undergo sufficient oxidation at normal temperatures in air to self-ignite. Examples are linseed, soybean, and fish oils. The reaction is promoted by exposure of a relatively large surface area of the material to the oxygen in the air as occurs when a fibrous material such as a cloth or bag is impregnated with the oil or fat. Enough air must be available to permit oxidation but not to dissipate the heat.

Some vegetable products are susceptible to self-ignition due to their inherent oxidizable oil content (examples are corn meal and rice bran), whereas others such as hemp, jute, and sisal appear to self-ignite only if impregnated with an oxidizable oil, even though they may heat when wet with water.

Moisture content is a factor in self-ignition. Although small amounts of moisture may increase the rates of oxidation and heat generation in many materials, moisture may also reduce the likelihood of ignition by promoting dissipation of the generated heat. However, high moisture content may contribute to the biological heating (American Insurance Association 1983).

Biological spoilage and heating

Enzymes

Enzymes are specialized proteins of living matter that catalyze, or speed up, chemical reactions. During the processes of growth and maturation, plant material in the field goes through a number of chemical reactions that are catalyzed by enzymes. Freshly harvested seeds entering the bin are often immature and may have increased enzymatic activity, resulting in high respiration rates and in heat production. High seed moistures and green weed seeds and debris also favor increased enzymatic activity. During this early storage period, carefully monitor the stored commodity.

Molds

Products in storage provide food and an environment for many organisms and microorganisms, including molds (fungi). Of these, the spoilage molds, certain storage or post-harvest molds (Christensen and Kaufmann 1969), are the most important cause of deterioration of grain and its products.

Spoilage molds exist as spores in soil, on decaying debris, in harvesting equipment, and within storage structures and are gathered by the combine harvester and distributed among the grains. The various types of spoilage fungi each require a different relative humidity level and temperature for their growth and development. Some species, like Aspergillus amstelodami (a yellow-green mold sometimes found growing on the top of homemade jams), grow at low humidities, affect seed germination, and produce water during their growth, which enable more damaging molds to grow. Such molds include Aspergillus candidus (colonies are white) and Penicillium species (green or blue green), both of which impair seed germination and are frequently associated with hot spots in bins of grain. Hot spots are areas within a bulk commodity that have a higher temperature than the surrounding material.

The development of an artificially induced hot spot was studied in wheat bulks by Sinha and Wallace (1965). Heating was initiated by Penicillium species growing in a grain pocket at -5°C to +8°C and with 18.5 to 21.8% moisture. The hot spot reached a maximum of 64°C and cooled in 2 weeks.

Preharvest molds, originating from the growing plant, also occur on grains in storage. Some preharvest molds may produce harmful toxins on developing grains in the field. For information on pre-harvest and post-harvest molds see Christensen and Sauer (1982).

Bacteria

Although bacteria outnumber molds on grain surfaces and in flour, they are not usually important during storage of these commodities in Canada. This is because during most years crops are harvested and binned dry, and species of bacteria require a high relative humidity (90-95%) for their growth. Numbers of bacteria decrease during storage when the moisture content is too low for growth. Their numbers are also decreased during artificial drying of grain. When the moisture content is adequate, their growth contributes to self-heating and to production of sour and putrid odors (Semeniuk 1954). A general account of the bacteria associated with stored grain is given by Wallace (1973).

Insects

More than 60 species of insects can occur in stored grain and grain products in Canada (Sinha and Watters 1985). Insect metabolic activity within dry grain bulks containing 15% M.C. or less can result in heating up to 42°C (Cotton and Wilbur 1982). Insect-induced hot spots occur most frequently in southern Alberta, where grain is often binned at an ambient temperature of 30°C. Ambient temperature is the temperature of the surrounding medium, in this case, the atmosphere. A further consequence of this localized insect metabolic activity is an increase in product moisture content above15% in the vicinity of the hot spot, permitting spoilage molds to grow and sometimes producing temperatures up to 62°C.

Several of the insects infesting farm-stored grain are destructive. These include rusty grain beetles, red flour beetles, squarenosed fungus beetles, sawtoothed grain beetles, granary weevils, hairy spider beetles, and meal moths (Loschiavo 1984).

Some of the insects that occur in stored products in the Prairie Provinces survive freezing temperatures (the rusty grain beetle survives -5 to -10°C for long periods), but they cannot reproduce below 17°C. Where grain temperatures remain above 17°C for long periods, as occurs in the centre of unaerated grain bulks, insects can do extensive damage. The effect of insect damage is worsened in high moisture grains.

Mites

Mites are fragile creatures and are difficult to see. Their presence gives a strong minty odor to grain, which, when heavily infested, becomes unpalatable as animal feed. About eight kinds of mites are common in stored grain in Canada, all of which can withstand low temperatures. Mites feed on broken grain, weed seeds, and molds present on the grain and thrive in moist grain. They spread mold spores on and in their bodies, and through their metabolic activity can, like insects, encourage development of spoilage molds (Sinha and Wallace 1973).

Advanced biological heating

Mold-induced heating of stored grains, pellets, feeds, and hay attains temperatures of 55°C and remains at this level for weeks. The heating then either gradually subsides or passes into the next stage where thermophilic molds take over. These sometimes carry the temperature to 60°C and may be succeeded by thermophilic bacteria and actinomycetes that carry it up to 75°C, the maximum temperature attained by microbiological activity (Christensen and Sauer 1982).