| Rots and germ damage may develop in stored
small-grain seeds when storage fungi (commonly called storage molds)
are present and when the moisture content is above 13 percent. As
the moisture content increases above 13 percent, the likelihood of
invasion of the kernel germ or embryo by storage fungi increases with
increasing temperature and time. In wheat, oats, and barley having
a moisture content of 13.5 to 14.5 percent, slow invasion by storage
molds can cause germ and heat damage without any temperature increase
(Figure 1). In the absence of insects and mites, heating in stored
small grains generally indicates that the moisture content is above
15 percent. Seasonal or even diurnal temperature fluctuations in storage
bins cause moisture migration and condensation that permit the growth
of storage molds on and in grain that is otherwise suitably dry.
Nonseed debris, mostly dirt and chaff, is a reservoir for storage
fungi, stored grain insects, and moisture. Mechanically damaged
and broken kernels are more prone to invasion by these fungi than
are seeds which are intact. Samples of wheat and other small grains
free of storage molds have been dried to 16- to 18-percent moisture
and stored at temperatures of 60 to 80 F (16 to 27 C) for more than
8 months without any germ damage developing. This indicates that
normal processes within the seeds themselves do not cause germ damage
during storage.
In the temperature range of 40 to 50 F (5 to 10 C), storage molds
grow very slowly; while at 80 to 110 F (27 to 44 C), growth is very
rapid. Grain that is to be stored for only a few weeks before processing
may contain a higher moisture content, have more extensive invasion
by storage molds, and be kept at a higher temperature without serious
problems than can grain stored for longer periods. However, grain
stored for only a few weeks at any combination of moisture content
and temperature that permits even moderate invasion by storage fungi
(usually not detectable by grain inspectors, but easily seen with
the aid of a microscope) will be a high risk if kept in continued
storage.
Grain moderately invaded by storage fungi or molds develops damage
at lower combinations of moisture content and temperature and in
a shorter time than grain free or almost free of storage fungi.
Once storage molds become established, they continue to develop
at moisture and temperature levels below those required for the
initial invasion of sound grain.
The damage done by fungi growing in stored grain is the end product
of storage conditions. The responsibility for it lies with those
who store the grain. Storage rots and germ damage reduce the feeding
value of grain and lower the market grade. Occasionally, certain
storage molds may produce toxins that seriously affect poultry and
livestock. Wheat with discolored and damaged embryos is discounted
by millers because the germs are brittle and crumble easily. Such
embryos show up in the flour as unsightly, black specks. Flour milled
from wheat containing more than 20 percent "sick" kernels
may yield "off-flavor" bread that is also smaller in loaf
volume.
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Figure
1. Small grain storage mold - heat damage. (Courtesy C.M. Christensen)..
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Cause
More than 25 different species of fungi cause germ damage and spoilage
and are known to invade stored, small-grain seed. With the exception of
Fusarium (Gibberella)–fungi that cause head blight or scab of wheat,
barley, oats, and rye as well as ear and kernel rots of corn–storage
molds do not normally enter undamaged seed until the seed matures. If
the seed is sound at harvest, Fusarium will not attack it in storage,
although Fusarium may invade corn of high-moisture content stored on the
cob in cribs.
The fungi responsible for most of the damage in stored grain are species
of Aspergillus and Penicillium. Aspergillus glaucus and A. restrictus
colonize grain with over 13.5 percent moisture. At 15 percent moisture
and above, A. candidus, A. ochraceus and A. flavus begin to develop. Species
of Penicillium begin to grow once the grain moisture exceeds 16 percent.
These fungi are common to and destructive in stored grain when the moisture
content is 14 to 17 percent or more, and the temperature is above 50 F
(10 C). At least one species of Aspergillus can grow slowly on and in
wheat, oats, and barley that has a moisture content of only 13 percent.
Small-grain seed stored at a relative humidity (RH) of 70 percent will
have an equilibrium moisture content of 13.5 percent. At 85 percent RH,
the moisture increases to 18 percent. Within a moisture range of 13.5
to 18 percent, the majority of storage molds grow most rapidly at 85 to
91 F (30 to 33 C)–although Aspergillus flavus develops quickly on
and in moist grain at 113 F (45 C); and A. candidus, at 131 F (54 C).
Most of these fungi can grow between 41 and 104 F (5 and 40 C). A few
species of Penicillium can grow slowly at temperatures lower than 32 F
(0 C).
Storage molds work like a "bucket brigade" at a fire. Each
fungus is active within rather narrow limits. When those limits are reached,
another fungi or other fungi begin to colonize the grain quickly, resulting
in a succession of organisms colonizing the grain.
All storage molds give off heat and moisture–utilized, in turn,
by their successors. This accelerates the rotting process. Increased temperature
and moisture content (within limits) leads to more rapid rotting. Insects
and mites are often present in spoiled grain. They take advantage of and
contribute to the heat and moisture given off by the molds. Insects and
mites also bring in and promote the development of storage molds.
Determining the number and kinds of fungi in a given lot of grain often
indicates the moisture content and temperature at which the grain has
been stored; and sometimes, the approximate length of the storage time.
Symptoms
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Storage fungi produce a sharp decrease in germination; dark germs
(also called "sick" or germ-"damaged"); toxins
that may be a health hazard for man and animals; musty or sour odors;
heating; "caking"; and bin burning. These are the end
results, caused by storage molds invading the grain. However, a
complete invasion and killing of the embryo commonly occurs within
the kernel before growth or symptoms are visible from the outside.
Germ discoloration can be detected by removing the pericarp (germ
covering) and examining the embryo (Figures 2, 3, 4). Germs that
are lightly discolored throughout (or discolored only on the tip)
are likely to be moldy (Figures 5 and 6), and may later turn dark
brown to black. The mold growth may be tan, white, black, blue,
bluish green, or pinkish red.
Production of Mycotoxins
Under conditions of high relative humidity and high temperatures,
fungi commonly found colonizing grain may produce a variety of mycotoxins.
These fungal metabolites cause a number of diseases (mycotoxicoses)
in animals and man brought about by consuming food and feed that
have been invaded by toxin-producing fungi. Some of these mycotoxicoses
result in feed refusal; vomiting and diarrhea; stunting or "poor
growth" and performance; infertility; abortion; tremors and
convulsions; edema; lesions in the liver, brain, and kidney tissues;
teratogenic and hepatocarcinogenic effects; hemorrhaging of the
liver and lung; and even death–especially in young animals.
Mycotoxins consumed in the feed may lower the animal's resistance
to infection by parasites. Ten parts per billion (ppb) of aflatoxin
consumed regularly by sensitive animals (chickens, ducks, and turkeys)
can result in fatal liver cancer. (By way of analogy, 1 ppb is equivalent
to 1 inch in nearly 1,600 miles). For 1 ppb of alflatoxin to appear
in the milk or meat of dairy cows, edible organs and flesh of beef
and pigs, and eggs and flesh of poultry, the feed must contain several
hundred to several thousand ppb of aflatoxin.
Toxins produced chiefly by Fusarium (Gibberella) cause (1) the
estrogenic syndrome, externally characterized by a swollen edematous
vulva in females and enlarged mammary glands in young males and
(2) the "refusal factor" or vomitoxin (deoxynivalenol),
in the trichothecene mycotoxin group. If the feed ration contains
more than about 5 percent of grain that is visibly damaged and containing
one or more toxins, pigs may refuse to eat it.
Clinical signs and lesions in affected swine in Illinois in late
1981 and 1982 included feed refusal, a few instances of vomiting,
lack of weight gain, poor feed efficiency, failure of mature sows
to return to estrus, reduced fertility, high mortality of nursing
pigs, intestinal-tract inflammation, and acute diarrhea in young
pigs. Examinations of dead young pigs revealed hemorrhaging into
the abdominal cavities, and pale, friable livers.
The production of toxins by storage molds is highly variable–depending
on the strain and species of fungus, storage temperature and moisture
content, type of grain, length and type of storage, and probably
other still-unknown factors. Once a toxin is produced in grain,
it is extremely durable under most conditions of storage, handling,
and processing of grain or other plant parts, or in foods and feeds
made from them.
Toxins ingested with the feed by dairy cattle may be excreted in
the milk. Their effects on humans are largely unknown, although
aflatoxin has been implicated in primary liver cancer. Aspergillus
flavus produces aflatoxins at moisture contents greater than 18
percent in equilibrium with 85 percent RH and temperatures of 54
to 108 F (12 and 48 C) with an optimum of 81 to 86 F (27 and 30
C). Under optimum conditions for growth A flavus can produce some
aflatoxin within 24 hours and a biologically significant amount
in a few days. Other mycotoxin-producing storage molds generally
grow in the moisture range of 17 to 40 percent and at temperatures
of 32 to 131 F (0 to 55 C). The U.S. Food and Drug Administration
has set a maximum level of 20 ppb for aflatoxin in food or feed
shipped in interstate commerce. The aflatoxin metabolite M1 tolerance
for milk is 0.5 part per billion.
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Figure 2.
The seeds as observed before pericarps covering the germs were removed;
no damage is evident.
Figure 3.
Germs of wheat with the pericarps removed. Left, sound germ, white
or off-white in color. Right, dark and damaged or "sick"
(dead and decayed) germ.
Figure 4.The
same seeds with pericarps removed to expose the dark germs covered
with storage fungi. The warehouseman claimed that none of the grain
had a moisture content over 12.5 percent at any time during storage.
But the particular kinds of storage fungi that grew from this seed
proved conclusively that the grain had a moisture content of 15
to 16 percent for at least some months or these fungi could not
have invaded it as they did.
Figure 5.
Storage fungi (Aspergillus species) fruiting on the germ ends of
wheat kernels.
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Sources of Error in Determining the Moisture Content of Grain
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Many warehousemen take an "average" sample from each
lot of grain as it goes into storage and determine its moisture
content with an electric or electronic moisture meter. The overall
or weighted average of these samples is then considered to be the
average moisture content of the grain throughout its storage life.
This assumption has several sources of error.
1. An average sample from a given lot of grain does not indicate
the range in the moisture content of the entire bulk. A range of
± 1 to 2 percent may be expected in the moisture content
of any carload, truckload, or small bin. In large bins, the range
may be greater. For safe storage, it is essential to know the highest
moisture content of any portion of a given lot. Grain is only as
dry as the wettest grain in terms of the risk of damage from storage
molds.
2. The accuracy of the moisture meter should be checked frequently–determining
the moisture content of check samples by oven-drying, or by submitting
samples to federally licensed grain inspectors. The figure obtained
by some types of electric moisture meters may be 1 percent below
that obtained by oven-drying the same sample in the laboratory.
The standard Motomco meter is quite sensitive but even under ideal
conditions, it probably is not possible to determine the moisture
content of a given sample by machine more precisely than ±
0.4 percent.
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Figure 6.
A storage fungus (Aspergillus ochraceus) growing from the germ of
a split kernel of wheat.
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3. The moisture content will change and fluctuate with time and
from place to place within a bin. If the temperature is not uniform
throughout the bulk, a slow movement or circulation of air causes
moisture to accumulate in the colder portions of the grain. Large
and rapid shifts in moisture are likely when the moisture content
of the grain is between 13 and 15 percent, when the temperature
of the grain is 75 to 80 F (24 to 27 C), and when the grain going
into storage is heavily invaded by actively growing storage fungi.
The migration of moisture is usually greatest in the winter when
the cool, upper layers of grain in the center of a bin may accumulate
a moisture content 5 to 10 percent or more above that in the bulk
(shown on the elevator operator's record books) (Figure 7).
4. When different lots of grain with high and low moisture contents
are blended to achieve an average (presumed to be safe for storage,
or to meet a certain grade), the moisture content may never equalize.
Many such mixes are a very poor storage risk.
5. In a supposedly uniform lot of grain, the moisture content may
vary from seed to seed by as much as 1 percent.
6. Activities of grain-infesting insects and mites can rapidly
increase the moisture content of grain–up to 1 percent a week,
and 10 percent in a few months. Fumigation may rid the grain of
insects (some grain fumigants do not kill mites), but storage molds
continue to develop. The amount of grain damaged by the molds may
considerably exceed the amount infested by the insects or mites.
All of these factors combined explain why the moisture-content
figure in an elevator operator's books often bears little relation
to the actual moisture content of the grain being stored.
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Figure 7.
Small grain storage mold - heat damage. (Courtesy C.M. Christensen)..
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1. Be certain that harvesting equipment is operating properly and set
correctly. Cylinder speed and settings should be checked to insure the
lowest possible damage to harvested grain.
2. Wherever possible, store grain (free of dirt, debris, chaff, and broken
kernels) in a clean, tight bin at a moisture content below 13 percent
and a temperature below 50 F (10 C). If the grain is sound and dry when
stored, it can be kept for years under these conditions without damage.
3. Aerate high-moisture grain as soon as possible to provide a temperature
of 35 to 50 F (2 to 10 C) throughout the bulk. At such temperatures, insects
and mites are dormant; and most storage fungi grow very slowly if at all.
4. After the grain has been stored, take probe samples at weekly to monthly
intervals from different portions. Use calibrated moisture meters, "official"
drying ovens, or the distillation methods of determining moisture. Avoid
taking several samples from a bin and averaging them. The highest moisture
content, not the average, determines storability. Examine each sample
for moisture content, damage, and the number and kinds of fungi. No germ
or heat damage will develop in grain when the moisture content is below
13 percent.
5. Measure and keep a record of temperatures in different parts of the
grain. Even a slight rise in temperature means that some spoilage is occurring.
Only vigorously feeding insects or rapidly growing storage fungi can cause
stored grain to heat up. If a "hot spot" is found, determine
its size and location, as well as the condition of the grain in and near
it. Do this by sampling and examining the grain. Temperature detection
cables or other devices to measure the temperature at various places within
bulk grain are an aid to–but not a substitute for–the intelligent
handling of stored grain. Relatively dry grain is a good insulator. A
rise in temperature of even a few degrees (as indicated by a thermo-couple)
may mean that the grain in the region of highest temperature is already
spoiled.
6. When "hot spots" or a crust of moldy grain are found, take
the following corrective measures:
a. The rotted and moldy grain should be removed and dried, and either
fed or sold.
Moldy grain should be fed with extreme caution to all classes of poultry
and livestock, but if mixed with sound grain, it can be fed with less
risk to livestock being finished for market. Moldy grain is considered
unsafe for lactating cows and all breeding and potential breeding animals.
b. The moisture content of the remaining grain should be checked.
c. The remaining grain should be turned and thoroughly mixed to redistribute
moisture and allow heat to escape.
7. Aeration fans should be installed to move small quantities of air
through the grain. Doing this will maintain a uniform temperature and
help prevent "wet" spots. It is cheaper and more effective to
maintain the temperature and moisture throughout a bin with aeration than
to transfer the grain from bin to bin. Such transfers also increase the
amount of cracked and broken kernels.
REFERENCES
• Storage Rots of Corn. University of Illinois Report on Plant Diseases
No. 206.
• Scab of Cereals. University of Illinois Report on Plant Diseases
No. 103.
• Grain Storage: The Role of Fungi in Quality Loss. The University
of Minnesota Press, Minneapolis. 1969.
• Storage of Cereal Grains and Their Products. American Association
of Cereal Chemists, St. Paul, MN. 1974.
• Mycotoxic Fungi, Mycotoxins, Mycotoxicoses (3 volumes). Marcel
Dekker, Inc., NY. 1977-1978.
• Molds and Mycotoxins in Feeds. The University of Minnesota Extension
Service Folder AG-FO-3538. 1988.
• Maintenance of Quality in Stored Grains and Seeds. The University
of Minnesota Press, Minneapolis. 1986.
• Mycotoxicology. The Pennsylvania State University Press, University
Park. 1987.
• Trichothecenes and other Mycotoxins. John Wiley & Sons, NY.
1985.
• Molds in Grain Storage. The University of Minnesota Extension
Service Folder AG-FO-0564. 1987.
• Mycotoxins and Mycotoxicoses. University of Illinois Report on
Plant Diseases No. 1105.
• Illinois Pesticide Applicator Training Manual 39-8 - Grain Facility
Pest Control. University of Illinois Extension, Urbana, IL. 1997
• Stored Product Management. Cooperative Extension Service, Division
of Agricultural Sciences and Natural Resources, Oklahoma State University;
U.S. Department of Agriculture, Federal Grain Inspection Service; U.S.
Department of Agriculture, Extension Service; U.S. Department of Agriculture,
Animal and Plant Health Inspection Service Circular E-912.
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