Page 1053 - Veterinary Toxicology, Basic and Clinical Principles, 3rd Edition

Aflatoxins Chapter | 69 985

VetBooks.ir during harvest and damaged during handling in the com- colonization should be placed in moisture-proof bags to
prevent desiccation. As soon as possible after sampling,
modity systems are more susceptible to invasion by spe-
fungi.
other
Growth
the individual moisture content of at least 100 seeds or
Aspergillus
and
of
of
cies
aflatoxigenic fungi and AF production can rapidly occur pellets should be determined to identify moisture distribu-
especially in damaged high-moisture seeds. Experimental tion in the kernels or pellets. This test result provides
study on maize with 57% damaged kernels had 25 ppm valuable insights into the storability of the commodity.
AFB 1 at 6.5 days (Seitz et al., 1982). High levels of AF Culture colonization can subsequently be done on each
production can occur in maize before a 0.5% loss in dry surface sterilized seed to determine infection with myco-
matter occurs. High levels of AFs in corn are not linked toxigenic fungi.
to the production of kojic acid (black light fluorescence). Aflatoxigenic fungi prefer minimum temperatures of
.25 C, with the abiotic and biotic microclimate of a

geographic region having major effects on the genera and
Moisture Management of Grain
species of aflatoxigenic Aspergillus present and on pro-
The most common methods of preventing the growth of duction of AFs. Contamination of commodities in the
aflatoxigenic fungi in dry-stored commodities are by con- field with AFs can be divided into two distinct phases.
trolling the moisture level and temperature. Water avail- Growth of aflatoxigenic fungi and mycotoxin production
able for aflatoxigenic fungi in stored grain is best occurring preharvest in the developing crop and posthar-
indicated by equilibrium relative humidity or water activ- vest mycotoxin production after the crop has matured and
ity (a w , percent relative humidity/100) in the air around has been harvested. In the preharvest phase, wounds in
the developing crop including those caused by insects,
the grain. Temperature of the grain is important as the a w
increases with temperature. In cereal grains, aflatoxigenic birds, mammals, and hail provide entry points for afla-
fungi generally cannot grow when the a w is below 0.80. toxigenic fungi. Insect damage to ears, bolls, pods, or nuts
A lower a w is required for protection of oilseeds. Oilseeds has been shown to dramatically increase the risk of AF
have more available water on the seed surface because production in maize, cotton, peanuts, almonds, and pista-
the seed absorbs less moisture. For most cereal grains, chios. Controlling insect damage reduces the risk of fun-
storage moisture of 12% is recommended for a wide gal infection and mycotoxin production. Excessive heat
range of grain temperatures, and for oilseeds the storage and drought generally decrease the native resistance of
moisture from ,8% to 9% is recommended for a wide plants to aflatoxigenic fungi by physiological and anatom-
range of temperatures inside the storage unit since other ical mechanisms. Delayed irrigation and resulting drought
species of Aspergillus can grow in commodities at lower stress at silking can increase the risk for AF production in
a w than A. flavus. The growth of other fungi produce both maize. The postharvest fungal invasion and AF produc-
heat and metabolic water creating a favorable moisture tion can occur from maturation in the field to the point of
and temperature for A. flavus to grow rapidly. human and animal consumption. Postharvest AF contami-
Measuring moisture with a grain tester provides the nation can occur when the commodity is suitable for
average moisture of the kernels being tested, and the aver- growth of aflatoxigenic fungi at harvest, and during trans-
age value can misinterpret the storability of grain and oil- port, storage, and manufacturing. AF contamination in
seeds. If the moisture is heterogeneously distributed wheat is principally a postharvest phenomenon (Jacobsen,
among the seeds or finished feed pellets, hot spots can 2010), whereas maize contamination can be either prehar-
occur. This phenomenon occurs because a microbial vest or postharvest. Delayed harvest due to wet conditions
microcosm exists around each high-moisture seed and with sufficient heat to support growth of aflatoxigenic
that microcosm can meet the requirements for fungal fungi can result in high levels of AFs being produced. In
growth and mycotoxin production. This is the reason why some tree nuts, such as walnut, almond, and pistachio and
high- and low-moisture grain and finished feeds should in cotton, a natural opening in the nut hull or boll com-
not be blended to give a lower average moisture test. In bined with delayed harvest and warm, humid conditions
the microcosm with a favorable a w level, growth of afla- can result in significant fungal invasion and AF contami-
toxigenic fungi can occur around each seed and the nation. Damage to kernels or nuts during harvest, clean-
microcosm created by other organisms. The metabolism ing, and general grain handling weakens the mature seeds
of microorganisms in the microcosms produce moisture to fungal invasion. The suitability of grain for storage at
and some microorganisms can also produce heat. Insects harvest is adversely affected by moisture content, physical
and other pests in grain also produce moisture, and hot damage to the kernels, and the extent to which fungi have
spots can form because of insect and other pests in stored invaded the seed before storage. Moisture level is a risk
grain. Monitoring multiple areas of a storage unit and factor for mycotoxins in stored commodities (Jacobsen
controlling insects are essential for preventing the produc- et al., 2007). Blending grain of high moisture levels with
tion of mycotoxins. Samples for AF analyses and culture grain of lower moisture levels can produce microcosms