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 the use of thermophilic microorganisms for ethanol production. Energy savings through reduced cooling costs, higher saccharification and fermentation rates, 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 adaptation, 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 temperatures. Thermotolerant yeasts have been investigated for the following potential applications: simultaneous saccharification and fermentation of cellulose, where the high fermentation temperature allows more rapid and efficient enzymatic cellulose hydrolysis; whey fermentation, where high salt and low fermentable substrate concentrations make conditions difficult; and fermentation of D-xylose and cellobiose, which is essential for efficient conversion of woody biomass to ethanol. Ethanol and temperature tolerance are important characteristics for commercial yeast strains. Both characteristics are interactive and generally decrease with increasing temperature and ethanol concentration. Considerable research has been directed towards investigation of fatty acid composition changes in response to these stresses and the role of heat shock proteins in tolerance mechanisms. If thermotolerant yeasts are to be used in commercial processes, bioreactor configuration will 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 have continuous flow systems, however, some of the newly isolated thermotolerant yeasts such as Kluyveromyces marxianus do not show the high growth rate under anaerobic conditions that is characteristic of Saccharomyces cerevisiae. Various immobilization techniques appear to offer a means of presenting and maintaining high biomass in anaerobic continuous flow reactors.