Ethanol production at elevated temperatures and alcohol concentrations: Part I - Yeasts in general

There are a number of process advantages which could be exploited through t he use of thermophilic microorganisms for ethanol production. Energy saving s through reduced cooling costs, higher saccharification and fermentation r ates, continuous ethanol removal and reduced contamination have stimulated a search for routes to thermophilic or thermotolerant yeasts. These routes have included screening existing culture collections, temperature adaptatio n, mutagenesis and molecular techniques and finally isolating new strains. Varying success has been achieved, however, the most thermotolerant yeasts have come from fresh isolations from environments which experience high tem peratures. Thermotolerant yeasts have been investigated for the following p otential applications: simultaneous saccharification and fermentation of ce llulose, where the high fermentation temperature allows more rapid and effi cient enzymatic cellulose hydrolysis; whey fermentation, where high salt an d low fermentable substrate concentrations make conditions difficult; and f ermentation of D-xylose and cellobiose, which is essential for efficient co nversion of woody biomass to ethanol. Ethanol and temperature tolerance are important characteristics for commercial yeast strains. Both characteristi cs are interactive and generally decrease with increasing temperature and e thanol concentration. Considerable research has been directed towards inves tigation of fatty acid composition changes in response to these stresses an d the role of heat shock proteins in tolerance mechanisms. If thermotoleran t yeasts are to be used in commercial processes, bioreactor configuration w ill play an important part in the design of production processes. Batch and fed-batch systems have been shown to be useful in some circumstances as ha ve continuous flow systems, however, some of the newly isolated thermotoler ant yeasts such as Kluyveromyces marxianus do not show the high growth rate under anaerobic conditions that is characteristic of Saccharomyces cerevis iae. Various immobilization techniques appear to offer a means of presentin g and maintaining high biomass in anaerobic continuous flow reactors.