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kernels and in less than 0.1% of all oats and wheat kernels examined from a large number
of samples in the USA (Sauer et al., 1984).
The major food and feed commodities where A. flavus is found are peanuts
(McDonald, 1970; Pitt et al., 1993, 1998), maize (Diener et al., 1983; Pitt et al., 1993,
1998) and cottonseed (Simpson & Batra, 1984). Spices of many kinds frequently contain
A. flavus (ICMSF, 1998). From time to time, A. flavus occurs in most types of tree nuts,
including pistachios, pecans, hazelnuts and walnuts, copra and kola nuts (Pitt & Hocking,
1997). Aflatoxins are sometimes produced in these commodities (Pohland & Wood,
1987). Low levels of A. flavus in small grain cereals and pulses, and many other kinds of
foods, e.g. soybean, have been reported, but the possibility of significant aflatoxin accumulation
is much lower (Pitt et al., 1994; Pitt & Hocking, 1997; Pitt et al., 1998).
2.3.2 Aspergillus parasiticus
IARC MONOGRAPHS VOLUME 82
A. parasiticus seems to be less widely distributed than A. flavus. During a major
study, more than 30 000 A. flavus cultures from south-east Asian foods were isolated and
identified, but not more than 20 isolates of A. parasiticus were found. Although A. parasiticus
is certainly widely distributed in soils and foodstuffs in the USA, Latin America,
South Africa, India and Australia, it is essentially unknown in south-east Asia. Like
A. flavus, it is a tropical and subtropical species, less prevalent in warm temperate zones,
and rare in the cool temperate regions of the world. The most important food source is
peanuts, in which A. parasiticus is endemic. Other types of nuts may be infected, including
hazelnuts and walnuts, pistachios and pecans. A. parasiticus is much less common
than A. flavus on grains, and perhaps does not invade maize at all. A variety of other
minor sources have been reported (Pitt & Hocking, 1997).
2.4 Formation of aflatoxins in foods
A fundamental distinction must be made between aflatoxins formed in crops before
or immediately after harvest, and those occurring in stored commodities or food
products. In subtropical and tropical areas, certain crop plants, notably peanuts, maize
and cottonseed are associated with A. flavus, or in the case of peanuts, also for A. parasiticus,
so that invasion of plants and developing seeds or nuts may occur before harvest.
This is the cause of the frequent occurrence of high levels of aflatoxins in these crops,
and is the reason for the difficulties still being experienced in eliminating aflatoxins from
these commodities. In contrast, A. flavus is less common in other plants, seeds or nuts
before harvest. In consequence, aflatoxins are not normally a problem with other crops
at harvest and their elimination relies on preventing post-harvest contamination, by rapid
drying and good storage practice (Pitt, 1989; Chatterjee et al., 1990; Miller, 1995).
Therefore, if infection of peanuts and maize by A. flavus could be controlled before
harvest, excessive aflatoxin production would not normally occur in storage, even under
somewhat unsatisfactory conditions. In temperate maize production, A. flavus conta-