During the years of the Second World War, the need for finding alternate sources of rubber led to studies of the latex-bearing guayule plant Parthenium argentatum as a potential source. The extractability and quality of rubber improved if, before milling, this shrub was chopped and stored in a mass ("rets") that self-heated to temperatures between 65 and 70°C. From the guayule rets, Paul J. Allen and Ralph Emerson (1949) isolated some ten species of thermophilic fungi with temperature limits up to 60°C and all species capable of decomposing resin. The improvement from retting resulted primarily from the reduction in the amount of contaminating resin in the latex by a thermophilic microflora. Allen and Emerson found that for the optimal development of thermophilic fungi in the guayule rets, its moisture content, porosity and size were crucial factors for the build up of microbial protoplasm, for their aerobic respiration and for insulation against the loss of heat produced by their metabolism.
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