Novel Ethanol Fermentations from Sugar Cane and Straw

Most agree that it is ultimately desirable to produce bulk chemicals such as ethanol from renewable resources; the questions focus on `where', `when' and `how'. For developing counties with abundant biomass, growing needs for bulk chemicals and energy, and balance of exchange problems, the time is now. Sugar cane is an obvious feedstock but the economics depend on whole-crop utilization and capital and energy input. The latter is currently met by burning the cane straw (bagasse), but hydrolysis of this and fermentation and the resulting sugars would increase ethanol yields. A high-temperature fermentation would reduce energy input and capital costs. A mutant strain of Bacillus stearothermophilus (LLD-15) can make ethanol from sucrose at 70 degrees C in yields and at rates equivalent to yeasts at 30 degrees C. Aerobically, the wild type grows rapidly on many sugars, and equally rapidly anaerobically producing mainly L-lactate and traces of acetate, formate and ethanol via pyruvate-formate lyase. In the mutant, the latter anaerobic pathway normally predominates. But under certain stresses pyruvate flux can be diverted exclusively via pyruvate dehydrogenase, resulting in quantitative conversion of sucrose to ethanol and CO₂ without growth. Hence the mutant offers potential for novel fermentation processes at high temperature, with high yield, high productivity and broad substrate range.