Canadian Grain Commission
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Spoilage and heating of stored agricultural products



Chapter 1 - Changes that occur during storage

Principles

Stored agricultural products are influenced by many factors that determine their keeping quality. These factors include product condition, storage container or structure, length of storage, and type of handling (Sinha 1973). Unlike inert materials such as sand, agricultural products in storage change physically and chemically and need to be managed carefully.

The original condition of a product is probably the most important factor affecting its storage. The product’s moisture content (M.C.) and temperature will influence and even direct events that occur during storage and may sometimes lead to spoilage and self-heating.

Moisture

During storage, moisture within the product reaches an equilibrium with the air within and between the product particles and produces a relative humidity level that may be suitable for the growth and development of deteriorative organisms. In stored seed, the lower limit of moisture content for mold growth is near the upper limit of moisture content in dry, that is, straight grade seed.

Table 1 shows the maximum moisture content levels at which cereal, pulse, and oilseed can be sold as straight grade, as permitted under the Canada Grain Act. The levels are subject to periodic revision. If seed is sold as straight grade and the moisture content levels exceed the values shown in Table 1, a penalty is charged. The amount of the penalty is determined by the amount of moisture content above the acceptable level. Because seed with the moisture content levels shown in Table 1 can be sold without penalty, such values are often assumed to represent safe levels (Moysey and Norum 1975). In practice, though, the safe moisture content levels are one or two percentage points below those given in Table 1. This is because some seed lots may have a higher moisture content or a higher level of damage than others, some may include green weed seed or other debris, and some may have suffered the effects of temperature variation or high temperature drying (see Part II).

Table 1 - Maximum moisture content levels for straight grade seeds*
Grain Maximum moisture content
* Percentage wet weight basis (Canadian Grain Commission 2005)
Barley 14.8
Canola/rapeseed 10.0
Corn/maize 15.5
Domestic buckwheat 16.0
Domestic mustard seed 9.5
Fababeans 16.0
Flaxseed and solin 10.0
Lentils 14.0
Oats 13.5
Peas 16.0
Rye 14.0
Safflower 9.5
Soybean 14.0
Sunflower 9.5
Triticale 14.0
Wheat 14.5

Relative humidity

Biological organisms that cause stored products to deteriorate require different levels of relative humidity for normal development. Generally, the level for bacteria is above 90%, for spoilage molds it is above 70%, and for storage mites it is above 60%. The levels required for insect development range from 30% to 50%. However, specifying only the relative humidity levels is oversimplifying the physical limits of deterioration. Both relative humidity and moisture content are dependent upon temperature. For example, if the temperature of an air sample having a relative humidity level of 50% is increased five degrees from 25°C to 30°C, its relative humidity level will decrease to 38%. If the temperature of the air sample is decreased five degrees from 25°C to 20°C, then the relative humidity level will increase to 69%. The effects and interactions of temperature, relative humidity, and moisture content on stored products and their associated organisms are complex. A concise explanation of the theory of moisture in stored produce is given by Mackay (1967).

Temperature

Important facts concerning temperature are as follows:

  • The high temperatures of grain harvested and binned on a hot day are retained within unaerated grain bulks for many months due to the insulation properties of grain.
  • Temperature and moisture influence enzymatic and biological activities and thus the rate of spoilage.
  • Temperature differences within bulk commodities favor mold development through moisture migration resulting from sinking colder, denser air, followed by rising warmer air and subsequent moisture adsorption near the top surface.

Safe storage guidelines

Moisture content and temperature determine the safe storage period for any grain or oilseed. The canola/rapeseed storage time chart (Fig. 1) predicts the keeping quality of canola/rapeseed over 5 months, under varying temperatures and moistures. If the temperature or moisture content of canola/rapeseed falls within the spoilage area of the chart, take steps to reduce one factor or both. To reduce the moisture content, either delay combining to allow further drying in the swath or artificially dry the seed. To reduce the seed temperature, aerate the bin contents. Safe storage guidelines have been developed to predict the long-term keeping quality of other commodities (Wallace et al. 1983).

Canola/rapeseed storage time chart based on seed moisture and temperature at binning

Figure 1 - Canola/rapeseed storage time chart based on seed moisture and temperature at binning.

Respiration and heat production

Respiration occurs in all living cells. Aerobic respiration, occurring in the presence of oxygen, is essentially responsible for the breakdown of carbohydrates, fats, and proteins to carbon dioxide, water, and energy. The energy liberated during aerobic respiration is used by the cells to fuel metabolic processes and is then released as heat.

Dry mature seeds in storage are largely dormant and have a very low respiration. However, freshly harvested, immature seeds or seeds with a high moisture content have a much higher respiration. This is because the seeds are still metabolically active and molds that are present on the surface and within the seed coats are actively respiring. Heat, which is produced by both seed and mold respiration, is manifested as an increase in grain temperature.