The grain ecosystemGrain stored in bulk forms a man-made ecosystem with a limited, non-regenerating energy supply. However, frequent interference results in an immature, unstable ecosystem with a relatively simple structure and composed of species with limited specialisation, and high growth and reproductive rates. Microbial colonisation commences soon after ear emergence, by what have been termed "field" fungi. These usually have a minimum water potential for germination of -22.4 MPa (> 0.85 aw), have to contend with large diurnal changes in environmental conditions and seldom develop further in store. Sometimes, especially in humid tropical climates, species more typical of stored grain ("storage fungi") also develop before harvest, and even produce mycotoxins. This differentiation between fungi characteristic of field or store is historical and their classification on ecological requirements would be preferable.When grain is harvested, it becomes contaminated with storage fungi and the ecosystem is drastically disturbed. Subsequent hot air drying may increase the risk of moulding in store. Conditions in stored grain become more stable than in the field, with smaller diurnal temperature changes and with water availability controlled by grain water content. Microclimatic conditions, especially temperature, water potential and gaseous regime, determine the microorganisms that can grow, the rate of their growth and the extent of spontaneous heating. Aspergillus and Penicillium spp. are the most characteristic storage fungi but, in heated grain at high water potential, thermotolerant fungi and actinomycetes occur. The predominant species can be good indicators of the previous storage conditions. Field fungi persist at low water potential but a small increase can greatly increase the activity of storage fungi so that field fungi decline rapidly. The most xerophilic storage fungi grow to about -59.3 MPa (0.65 aw).Storage fungi are usually considered obligate aerobes but often the concentration of O 2 has to be decreased to <0.14% to halve linear growth rates. Their growth is sometimes stimulated by low CO 2 concentrations and tolerance of high CO 2 concentrations may be diminished by decreasing O 2 concentrations. Many interactions between abiotic and biotic variables occur in stored grain. Water potential, temperature and period of storage all affect moulding and insect infestation, while competitive interactions are found between field storage fungi, within each group and between fungi and arthropods. Insects and mites grow and reproduce on some fungi and are inhibited or infected by others. Their faeces provide substrates for microbial colonisation and they assist fungal invasion of grain through feeding damage, increased water availability and increased temperature.
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