Canadian Grain Commission
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Protection of farm-stored grains, oilseeds and pulses from insects, mites and moulds



8. Pulse crops

Insects

Insects are rarely a problem in stored pulse crops. The exceptions are weevils of the family Bruchidae which can infest seed in the field and continue to multiply during storage (Bruchus brachialis Fahr., the vetch weevil; Bruchus pisorum (L.), the pea weevil; Bruchus rufimanus Boh., the broadbean weevil; Acanthoscelides obtectus (Say), the bean weevil).

Fababeans (Vicia faba L.)

Relative storage risk: Low

Moisture content standards:

  • Dry: up to 16.0%
  • Tough: 16.1 to 18.0%
  • Damp: over 18.0%

Safe storage guidelines

The maximum recommended moisture content for storing sound fababeans is 16% in Canada and 15% in Britain. Fababeans of 14.2% moisture content that had not undergone frost damage were safely stored for 2 years in Manitoba. Low-quality, frost-damaged beans that had been overwintered and had a moisture content above 15% often heated during the following summer.

Drying guidelines

Drying at a maximum of 32°C is recommended. Drying should be done in two stages if more than 5% moisture content is to be removed to attain a 16% storage moisture content. Allow a few days between each stage to permit internal moisture to move to the surface. Do not dry beans rapidly at high temperatures because this cracks the seed and reduces viability. The beans may also become overdried on the outside and underdried within. Underdried beans result in a pasty meal, which on prolonged storage becomes rancid and heated. At drying temperatures above 40°C, the skin wrinkles or splits, particularly with high moisture beans. Avoid cracking the seed coat as microorganisms can then gain entrance and cause spoilage.

Degrading factors

Fababeans are degraded when they contain heated and/or rotted beans, or have a distinctly heated or musty odour. Entire beans and pieces of beans are considered in the grading. Fababeans are graded Sample if they contain over 1% heated and/or spoiled beans, or have a distinctly heated or musty odour.

Appearance of heated and spoiled beans

Heated beans and/or spoiled fababeans are those which are materially discoloured as a result of heating or rotting. Seed coats are dark brown to black, and the cotyledon tissue on dissected beans is either tan or brown.

Storage problems

Beans are normally harvested when the pods are black and stems have shriveled. Because water loss is slow from the thick fleshy pods and large seeds, a prolonged period of ripening and drying may be required before combining, particularly in cool climates. If the crop is harvested too soon, the beans in the topmost pods will be immature. They will also be higher in moisture content than those in lower pods. Because of problems associated with prolonged ripening, late harvesting, frost damage, and prolonged drying, fababeans are frequently binned in a nonuniform state and consequently need to be carefully monitored during storage.

Field beans (Phaseolus vulgaris L.)

This heading includes white pea beans, also known as white beans or navy beans (most important), light and dark red kidney beans, black beans, pinto beans, pink beans, small red beans, Great Northern white beans, yellow eye beans and cranberry beans.

Relative storage risk: Low

Moisture content standards:

  • Dry: none
  • Tough: none
  • Damp: over 18.0%

Safe storage guidelines

A moisture content of 18% or less is recommended for safe storage of field beans. For long-term storage, a moisture content of 18% is too high, even at 5°C for beans required for seeding purposes (Table 6). The maximum moisture content for safe storage of pea beans for up to 1 year is 17.0%. Beans should be harvested when most of the pods are dry and the beans have hardened but before the seeds begin to shatter. The optimum moisture content for combining beans is 16 to 18%. At moisture content levels lower than this, damage can be severe and costly, as broken or cracked beans can only be used for livestock feed.

Table 6 – Estimated number of weeks for decreased germination to occur in brown beans
Moisture content (wet basis, %)
Storage temperature (°C) 11 12 13 14 16 18 20.5 23
Maximum safe storage (weeks)
25 31 22 16 11 7 4 2 0.5
20 55 40 28 19 13 7 3.5 1.5
15 100 75 50 30 20 12 6 3
10 200 140 95 60 38 20 11 4.5
5 370 270 170 110 70 39 20 9

Drying guidelines

Drying is necessary when beans are harvested damp because of poor weather or because of excessive harvesting losses due to shattering. Maximum drying temperatures for beans are 27 to 32°C. Dry beans slowly and, if necessary, remove excess moisture in two stages (see fababeans). Great care must be taken during drying otherwise splits develop, even at relatively low temperatures. Hairline cracks, a degrading factor, increase at elevated temperatures. During drying keep the relative humidity of the heated air above 40%.

Degrading factors

Beans are degraded when they contain heated or mouldy beans, or have a heated or distinctly musty odour. Beans are graded Sample if they contain over 1% heated beans or have a heated or distinctly musty odour, or if they contain over 1% mouldy beans. Mouldy beans are characterized by the presence of dark blue exterior moulds that have developed in crevices on machine-damaged beans, and yellow to black interior moulds that have developed in the concave centre area common to light and dark red kidney beans.

Appearance of heated kernels

Heated pea beans have a dull-coloured seed coat varying from cream to mahogany. The colour is more intense in the hilum area. Cotyledons vary in colour from tan to dark brown when viewed in cross section. Very light cotyledons are classed as damaged rather than as heated. Heated light and dark red kidney beans have a dull, dark red to black seed coat. Beans must be split to determine the degree and intensity of heat damage.

Storage problems

Mechanical handling damage is a problem which becomes more severe at low temperture and moisture levels. To reduce damage, use belt conveyors or front-end loaders rather than augers to handle beans wherever possible. Avoid dropping beans from excessive heights, particularly onto concrete floors.

Peas (Pisum sativum var. arvense (L.) Poir.)

Relative storage risk: Low

Moisture content standards:

  • Dry: up to 16.0%
  • Tough: 16.1 to 18.0%
  • Damp: over 18.0%

Safe storage guidelines

Peas are harvested when they are mature and hard in the pod. Yellow-seeded cultivars are harvested beginning at 16% moisture content. Green-seeded cultivars are harvested at 18% moisture content, or higher, to maintain good colour, then dried down to 16%, or lower, for safe storage.

Drying guidelines

The maximum drying temperatures are 45°C for seed required for seeding purposes, 70°C for commercial use, and 80 to 100°C for feed. Temperatures higher than 45°C will harm germination of seed peas, especially green peas.

Degrading factors

Peas are graded Sample if they contain over 0.2% heated seeds, or have a heated, fire-burnt or distinctly musty odour.

Appearance of heated seeds Heated peas have dull seed coats and discoloured cotyledons, ranging in colour from light tan to dark brown.

Storage problems

Peas of about 15% moisture content may develop a surface crust during the winter as a result of moisture migration and snow seepage, particularly when they are stored warm without aeration. The seeds tend to clump and if left undisturbed become blackened as a result of mould activity. To prevent clumping, periodically walk across the top of the bin or move the top 30 cm of stocks with a shovel.

Before moving the first load in the spring, examine the top surface of the stocks. If there is any black crust remove it with a shovel, otherwise the first load will be ruined by admixture. Crusting is a particular problem in overfilled steel bins, and it also occurs in stocks stored in Quonset huts. It can be prevented by using a front-end loader to divide the stocks and disturb the surface layers. Because of their size and shape, peas exert a greater lateral pressure than wheat; therefore, if grain bins are also used for storing peas they may require reinforcement.

Soybeans (Glycine max (L.) Merrill)

Relative storage risk: Moderate

Moisture content standards:

  • Dry: up to 14.0%
  • Tough: 14.1 to 16.0%
  • Damp: 16.1 to 18.0%
  • Moist: 18.1 to 20.0%
  • Wet: over 20%

The maximum permissible moisture content limits for soybean grades (U.S.) 1, 2, 3 and 4 are 13, 14, 16 and 18% respectively.

Safe storage guidelines

In dry fall weather, mature soybeans dry in the field from about 15% moisture content in the early morning to 10% at noon. They absorb moisture again during the following night to repeat the cycle the next day. Soybeans can be harvested at a low moisture but only at the expense of added field losses and excessive mechanical damage. These effects can be minimized if beans are harvested at a higher moisture content before pods are completely mature, then dried to a safe moisture content for storage.

The safe moisture content for commercial seed is 13% for up to 1 year and 10% for up to 5 years. These guidelines do not take into consideration such things as accumulation of fines under the spout lines. Soybeans are more difficult to store than shelled corn at the same moisture content and temperature. This is because the equilibrium moisture content of soybeans at a relative humidity of 65% and 25°C is 2% less than for shelled corn.

Table 7 – Moisture content scale for market and seed stocks.
Moisture content (%) Market stock Seed stock
10-11 4 years 1 year
10-12.5 1 to 3 years 6 months
13-14 6 to 9 months Questionable, check germination
14-15 6 months Questionable, check germination

Storage fungi can slowly invade soybeans stored at 12 to 12.5%, with the rate of invasion increasing above this moisture content level. Invasion of soybeans with 12.5 to 13.0% moisture content is unlikely to result in any loss of processing quality within a year even if the temperature is favourable for the growth of fungi; although it may cause some loss of germination. The invasion of soybeans at moisture content levels of up to 13.0% by storage fungi can, however, be dangerous because it may result in a sudden unexpected, and perhaps uncontrollable, increase in fungus growth and heating.

For continued silo storage, soybeans that are already lightly or moderately invaded by storage fungi are at higher risk than sound beans, and progress toward advanced spoilage more rapidly. Once the seeds have been moderately invaded by storage fungi, the fungi may continue to grow and cause damage at slightly lower moisture contents and temperatures than they would in sound beans.

Drying guidelines

The maximum safe drying temperatures are 43°C for soybeans intended for seeding purposes, and 49°C for commercial use.

Degrading factors

Soybeans are degraded when they contain heat-damaged, mouldy or rancid beans, or have a heated, distinctly musty or unpleasant odour. Heated beans are degraded numerically according to established grade specifications. Mouldy and rancid beans are considered in combination with heated beans for grading purposes. Soybeans are graded Sample if they contain over 5% heated beans or have a distinctly heated or musty odour.

Appearance of heated, mouldy and rancid soybeans

Heated soybeans have an olive to dark brown seed coat and, when bisected, have tan to dark brown cotyledons. Mouldy soybeans are wrinkled, misshapen, medium to dark brown and often have a superficial covering of grey mould. They may also have a spongy texture and an unpleasant odour. Rancid soybeans have a deep pink discolouration.

Storage problems

Most cases of serious loss of quality in stored soybeans occur because managers in charge of the beans do not know precisely the conditions prevailing in different portions of the bulk. The seed moisture contents and temperatures within the bulk must be known at all times and maintained at low levels to prevent mould development for safe storage. The condition of the stocks at the beginning of storage has an important bearing on their future keeping quality. Storage problems are aggravated by binning beans already lightly or moderately invaded by storage moulds, the presence of significant amounts of cracked and split beans, and the presence of fines in the bin spout lines. The cracked and split beans and fines (mainly weed seeds) form focuses for heating and subsequent deterioration. Spoilage commonly begins in soybeans in the spout line because the high-moisture weed seeds pack densely, preventing air penetration during aeration. Even if the beans at binning contain only 2 to 5% fines, the spout line may consist of 50 to 80% fines.

Sweating, which occurs when cold grain is removed from storage and exposed to air that has a high relative humidity and is more than 8 to 10°C warmer than the grain, is also of concern. Under these conditions, moisture from the air actually condenses on the beans, and when rebinned the cumulative effect of this sweat, or moisture, can cause heating problems in storage.

There is a genuine danger of self-ignition in soybeans because, unlike temperatures during heating of cereals which do not usually exceed 55°C, temperatures during heating of soybeans can exceed 200°C. The heat-damaged seeds lose at least 30% of their dry weight when temperatures reach 200°C.