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ThusF. culmorumandF. graminearumcan cause both stem rot and head
blight of wheat and barley in the field and these field infections may lead
to more extensive post harvest spoilage of these commodities if they are
stored at too high a water activity. By contrast the so-called storage fungi
seem to be well adapted to the more constant conditions of cereals in
storage, and generally grow at lower water activities (Table 5.10).
The most important genera of the storage fungi are Penicilliumand
Aspergillus, although species of Fusarium may also be involved in
spoilage when grain is stored under moist conditions.
Water activity and temperature are the most important environmental
factors influencing the mould spoilage of cereals, and the possible
production of mycotoxins, and Table 5.11 shows the relationship be-
tween water content and water activity for barley, oats and sorghum
while Figure 3.10 shows the same information as an isotherm for wheat.
Although xerophilic moulds such as Eurotium spp. and Aspergillus

Table 5.10 Minimum water activity requirements

of some common field and storage fungi

Species Minimumaw

Field Fungi

Fusarium culmorum 0.89

Fusarium graminearum 0.89

Alternaria alternata 0.88

Cladosporium herbarum 0.85

Storage fungi

Penicillium aurantiogriseum 0.82

Penicillium brevicompactum 0.80

Aspergillus flavus 0.78

Aspergillus candidus 0.75

Eurotium amstelodami 0.71

Wallemia sebi 0.69

Table 5.11 Equilibrium relative humidity, water activity and moisture content
(as % wet weight) of cereals at 25 1 C

Equilibrium relative
humidity (%)

Water

activityaw

Water potential

(MPa)

Water content (% wet weight)

Barley Oats Sorghum

15 0.15
261 6.0 5.7 6.4
30 0.30
166 8.4 8.0 8.6
45 0.45
110 10.0 9.6 10.5
60 0.60
70 12.1 11.8 12.0
75 0.75
39 14.4 13.8 15.2
90 0.90
14.5 19.5 18.5 18.8
100 1.00 0.0 26.8 24.1 21.9

148 Microbiology of Primary Food Commodities