is increased beyond this then the aseptate fungi
(Zygomycota) and Oomycota are the first to stop
growing (their lowest limit is about −4 MPa). But
many septate fungi will grow at between −4 MPa and
−14 MPa, and are not considered to be particularly stress-
tolerant. In fact, the most stress-tolerant fungi will grow
at near-maximum rates at −20 MPa and will make at
least some growth at −50 MPa. These fungi with high
stress tolerances include the yeast Zygosaccharomyces
rouxii, used in the traditional production of soy sauce;
its lower limit for growth is −69 MPa in sugar solutions.
From these comments it is clear that fungi as a
whole can grow in environments where few other
organisms can grow. This ability to tolerate water
stress is one of the special features of fungi. But, there
is an important qualification because the response of
a fungus to water stress depends on how this stress is
generated. Most fungi tolerate sugar-imposed osmotic
stress better than salt-imposed stress – they are inhib-
ited by salt toxicity long before they are inhibited by
osmotic potential as such.
Ecological and commercial aspects of
water-stress tolerance
Water-stress-tolerant fungi have major economic
significance in the spoilage of stored food products,
including cereal grains. No fungus can grow on stored
grain that has been dried to 14% (w/v) moisture con-
tent, but even a slight rise to 15–16% water content
will enable the stress-tolerant Aspergillusspp. (sexual
stage, Eurotium) to grow. For example, Aspergillus
amstelodamiwill initiate spoilage at −30 MPa in a
slightly moist pocket of a grain store. This can set off
a chain reaction, when the spoilage fungi degrade
starch to glucose, and thence to CO 2 and water. The
metabolic heat generated in this process can cause
the water to evaporate and condense elsewhere in
the grain mass, so that moulding spreads progressively
and eventually paves the way for the growth of less-
stress-tolerant fungi.
Figure 8.10 shows how predictive models of post-
harvest grain spoilage can be developed in laboratory
studies, by combining different environmental factors- in this case, temperature and water potential. The lines
on these graph are growth rate isopleths(the com-
binations of temperature and water potential at which
different growth rates are seen). The broken white
lines represent “minimal” growth rates of 0.1 mm per
day on agar plates, the broken black lines represent
2.0 mm per day, and the solid black areas represent
4.0 mm or more per day. Areas that lie outside all of
these zones represent potentially safe storage conditions.
The data accord with the known biology of the grain-
storage fungi. Aspergillus amstelodamiis very tolerant of
water stress and, together with another stress-tolerant
fungus, A. restrictus, often initiates moulding in grain
stores. Aspergillus fumigatusis less water-stress-tolerant
but the isopleths are skewed, indicating its preference
for higher temperatures (maximum 52°C). Fusarium
culmorum, like many Fusariumspecies, is regarded as a
“field fungus”. It causes rotting of grains in field con-
ditions if there is a wet harvest season and the grain
cannot be dried quickly enough to a safe level.
Similar models can predict the conditions for myco-
toxin production (Fig. 8.11). For example, Aspergillus
flavusproduces aflatoxin over most of the range of
temperature/water potential that supports growth of this
fungus. Penicillium verrucosumproduces ochratoxin A
ENVIRONMENTAL CONDITIONS 153
Fig. 8.10Growth rate isopleths for three fungi that cause spoilage of cereal grains. (Data from Ayerst 1969, for Aspergillus
spp., and from Magan & Lacey 1984, for F. culmorum.)