Enzyme Hydrolysis and Ethanol Fermentation of Liquid Hot Water and AFEX Pretreated Distillers' Grains at High-Solids Loadings

Authors:Youngmi Kim, Rick Hendrickson, Nathan S. Mosier, Michael R. Ladisch, Bryan Bals, Venkatesh Balan, Bruce E. Dale
Journal: Bioresource Technology, 99, 5206-5215 (2008).

Abstract

The dry milling ethanol industry produces distillers' grains as major co-products, which are composed of unhydrolyzed and unfermented polymeric sugars. Utilization of the distillers' grains as an additional source of fermentable sugars has the potential to increase overall ethanol yields in current dry grind processes. In this study, controlled pH liquid hot water pretreatment (LHW) and ammonia fiber expansion (AFEX) treatment have been applied to enhance enzymatic digestibility of the distillers' grains. Both pretreatment methods significantly increased the hydrolysis rate of distillers' dried grains with solubles (DDGS) over unpretreated material, resulting in 90% cellulose conversion to glucose within 24 h of hydrolysis at an enzyme loading of 15 FPU cellulase and 40 IU ß-glucosidase per gram of glucan and a solids loading of 5% DDGS. Hydrolysis of the pretreated wet distillers' grains at 13-15% (wt of dry distillers' grains per wt of total mixture) solids loading at the same enzyme reduced cellulose conversion to 70% and increased conversion time to 72 h for both LHW and AFEX pretreatments. However, when the cellulase was supplemented with xylanase and feruloyl esterase, the pretreated wet distillers' grains at 15% or 20% solids (w/w) gave 80% glucose and 50% xylose yields. The rationale for supplementation of cellulases with non-cellulolytic enzymes is given by Dien et al., later in this journal volume. Fermentation of the hydrolyzed wet distillers' grains by glucose fermenting Saccharomyces cerevisiae ATCC 4124 strain resulted in 100% theoretical ethanol yields for both LHW and AFEX pretreated wet distillers' grains. The solids remaining after fermentation had significantly higher protein content and are representative of a protein-enhanced wet DG that would result in enhanced DDGS. Enhanced DDGS refers to the solid product of a modified dry grind process in which the distillers' grains are recycled and processed further to extract the unutilized polymeric sugars. Compositional changes of the laboratory generated enhanced DDGS are also presented and discussed.

 

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