Thermophilic fungi were discovered as chance contaminants of bread or potato that had been inoculated with garden soil (see Cooney and Emerson, 1964; Maheshwari et al., 2000). Their habitats and growth conditions were discovered when Hugo Miehe (1907) of Germany was drawn to investigate the cause of self-heating and the spontaneous combustion of damp stacks of hay. He studied the role of microbial flora in thermogenesis. From the self-heating haystacks, Miehe isolated several microorganisms, including four species of thermophilic fungi: Mucor pusillus (renamed Rhizomucor pusillus, http://helios.bto.ed.ac.uk/bto/microbes/thermo.htm), Thermomyces lanuginosus (syn. Humicola lanuginosa, http://helios.bto.ed.ac.uk/bto/microbes/ thermo.htm), Thermoidium sulfureum (renamed Malbranchea cinnamomea) and Thermoascus aurantiacus (http://helios.bto.ed.ac.uk/bto/microbes/thermo.htm). To assess their role in the self-heating of agricultural residues, Miehe inoculated moist hay and other plant materials kept inside thermal flasks with pure cultures of individual fungi. Whereas sterilized hay did not generate heat, that inoculated with the fungus did and the final temperature attained by the material showed a correlation with the maximum temperature of growth of the fungus used. Further, by controlled experiments, Miehe demonstrated that the naturally occurring microorganisms in moist haystacks or other plant materials caused its heating. Cooney and Emerson (1964) explained spontaneous combustion as follows: Initially, the heat produced from the exothermic metabolic reactions in the mesophilic fungi raises the temperature of the compacted mass of vegetable matter to approximately 40°C, with this warm environment favoring the development of thermophilic fungi and actinomycetes present therein. The latter raise the temperature to about 60°C or higher, corresponding with their upper temperature limit of growth. Above 60°C, the mycelial growth of thermophilic fungi declines and they survive as heat-resistant spores, whereas the actinomycetes raise the temperature of the mass up to their maximum around 75°C. Beyond this temperature, autocatalytic chemical reactions are triggered which ignite the haystack. Figure 10.2 is a photograph of the spontaneous combustion of a large pile of sugar cane bagasse near a sugar manufacturing factory. The discovery of thermo-philic fungi provided a link in the puzzle of spontaneous combustion of stored agricultural products.
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