Heating is the most common indicator of a problem in stored grains and oilseeds. High grain
temperatures normally indicate either microbial or insect activity. If left unchecked, this may lead to
heat-damaged or charred grains due to the phenomenon of stack burning. Heating in cereal grains
peaks at about 58Â°C (135Â°F) then declines to ambient temperature. At the peak temperature, insects
and molds are killed, thus making the process self-limiting. In soybeans, is also initiated by microbial
and insect activity. However, as heating progresses above 50Â°C (120Â°F), the oxidation of oil in soybeans
becomes a self-sustaining process. Temperatures above 150Â°C (300Â°F) may occur. At this extreme
temperature, charring will definitely occur and spontaneous combustion or fire becomes a distinct
possibility, if sufficient oxygen is present at the hot spot. Because of this danger, hot spots in stored
soybeans must be cooled or dissipated before they reach the critical level. If no action is taken when
heating in soybeans occurs, either the product will be lost by stack burning (charring) or at worst,
the entire facility will be lost through fire. Aerating soybeans when fire has already started makes the
situation worse. A temperature monitoring system in soybean storage silos is essential. Immediate
corrective measures for heating cannot be over-emphasized.
Change In Color And General Appearance
In general, sound soybeans are plump with bright uniform tan and not green color and free from
unusual spots and shrivelled appearance. Discolored soybeans usually indicate inferior quality and
lower market value. The change in color is usually associated with mold invasion accompanied by
microbial respiration and subsequent heating. This deterioration process can be detected by periodic
drawing of samples from the stored soybeans as part of an integrated approach to quality maintenance.
Once detected, appropriate measures can then be taken such as cooling the grain either by aeration
or use of a portable cooling unit. Another corrective measure is to transfer the grain to another silo
thus breaking any hot spots present and cooling the soybeans during the conveying process. However,
this should be done only as a last resort since it is costly and will increase the amount of broken or
Mustiness And Off-odor Condition
Musty odor usually indicates an advanced stage of insect or mold infestation and should be dealt
with immediately. If this is detected, the soybeans should be aerated to remove the bad odor and
cool the material. Beans should then be used at the earliest opportunity. The grain should be fumigated
immediately if insects are present. A sharp odor may indicate rancidity due to chemical changes in
the oil component.
Presence Of Storage Insects
The presence of large population of weevils and small moths usually indicate an advanced stage of
infestation. Sitophilus granarium (granary weevil) may infest whole soybeans but not soybean meal
while Tribolium castaneum (red flour beetle) and Trogoderma granarium (khapra beetle) will infest
soybean meal at relative humidities above 75% and temperatures above 30Â°C (86Â°F). Ephestia cautela
(almond moth) can develop even at 8.8% moisture (wet basis) and temperature of 25Â°C (77Â°F) .
Lumping And Caking
Lumping and caking indicate a very advanced stage of fungi invasion in soybeans and soybean meal.
In metal bins, caking usually occurs on the bin walls as a result of â€œsweatingâ€ or moisture condensing
on the inner surface of the cold bin wall. The condensing moisture is absorbed by the adjacent grains
resulting in either sprouting or mold growth. Lumping may also occur in spots where the grain moisture
increased due to a leaky roof or moisture migration or translocation by natural convection (see Figure
7). In a bag system of storage, caking of soybeans and soybean meal may also occur as a result of
increased moisture content adsorbed from the atmosphere, leaky roof, or due to capillary moisture
from the floor. Capillary moisture can be eliminated by putting the bags on pallets. Concrete floors
can be made water-proof during construction by installing plastic sheets as moisture barriers before
pouring in the floor slab.