Use of dynamic step response for control of fed-batch conversion of lignocellulosic hydrolyzates to ethanol

Optimization of fed-batch conversion of lignocellulosic hydrolyzates by the yeast Saccharomyces cerevisiae was studied. The feed rate was controlled u sing a step response strategy, in which the carbon dioxide evolution rate w as used as input variable. The performance of the control strategy was exam ined using both an untreated and a detoxified dilute acid hydrolyzate, and the performance was compared to that obtained with a synthetic medium. In b atch cultivation of the untreated hydrolyzate, only 23% of the hexose sugar s were assimilated. However, by using the feed-back controlled fed-batch te chnique, it was possible to obtain complete conversion of the hexose sugars . Furthermore, the maximal specific ethanol productivity (q(t.max)) increas ed more than 10-fold, from 0.06 to 0.70 g g(-1) h(-1). In addition, the via bility of the yeast cells decreased by more than 99% in batch cultivation, whereas a viability of more than 40% could be maintained during fed-batch c ultivation. In contrast to untreated hydrolyzate, it was possible to conver t the sugars in the detoxified hydrolyzate also in batch cultivation. Howev er, a 50% higher specific ethanol productivity was obtained using fed-batch cultivation. During batch cultivation of both untreated and detoxified hyd rolyzate a gradual decrease in specific ethanol productivity was observed. This decrease could largely be avoided in fed-batch cultivations. (C) 2001 Elsevier Science B.V. All rights reserved.