High Gravity Ethanol Fermentations and Yeast Tolerance
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High gravity (HG) fermentations save energy consumption for both ethanol distillation and subsequent discharge treatment. However, yeast cells suffer from various stresses under HG conditions, which often result in stuck or sluggish fermentations with more sugars remained unfermented, and thus reduce efficiency of ethanol fermentation. This chapter focuses on stresses affecting ethanol fermentations under HG conditions and their impact on yeast growth and ethanol production. The HG condition associated with osmotic pressure may repress yeast cells for ethanol fermentation from sugar-based feedstocks such as molasses but less likely for starch-based feedstocks that are fermented by simultaneous saccharification and fermentation process in industry. However, ethanol inhibition is a major stress for ethanol fermentation from both sugar- and starch-based feedstocks in which ethanol concentration as high as 12–14% (v/v) can be commonly achieved, and higher ethanol concentration of more than 15% (v/v) is expected. On the other hand, it is a less concern for ethanol fermentation from hydrolysate of lignocellulosic biomass since ethanol concentration achieved is usually lower than 12% (v/v), a tolerable level for ethanologenic yeast. Instead, overcoming the inhibition of toxic by-products generated during biomass pretreatment is a major issue for ethanol production from lignocellulosic biomass. Strategies for developing stress-tolerant strains and bioprocess engineering aspects to alleviate the impact of stresses on yeast cells are discussed.
KeywordsYeast Cell Ethanol Production Lignocellulosic Biomass Ethanol Fermentation Fermentation System
The assistance of Dr. Z. Lewis Liu is gratefully acknowledged. Without his great effort and valuable comments, it would not have been possible for us to make the chapter accommodated to the theme of this book.
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