On-line control of fed-batch fermentation of dilute-acid hydrolyzates

Dilute-acid hydrolyzates from lignocellulose are, to a varying degree, inhi bitory to yeast. In the present work, dilute-acid hydrolyzates from spruce, birch, and forest residue, as well as synthetic model media, were fermente d by Saccharomyces cerevisiae in fed-batch cultures. A control strategy bas ed on on-line measurement of carbon dioxide evolution (CER) was used to con trol the substrate feed rate in a lab scale bioreactor. The control strateg y was based solely on the ratio between the relative increase in CER and th e relative increase in feed rate. Severely inhibiting hydrolyzates could be fermented without detoxification and the time required for fermentation of moderately inhibiting hydrolyzates was also reduced. The feed rate approac hed a limiting value for inhibiting media, with a corresponding pseudo stea dy-state value for CER. However, a slow decrease of CER with time was found for media containing high amounts of 5-hydroxymethyl furfural (HMF). The s uccess of the control strategy is explained by the conversion of furfural a nd HMF by the yeast during fed-batch operation. The hydrolyzates contained between 1.4 and 5 g/l of furfural and between 2.4 and 6.5 g/l of HMF. A hig h conversion of furfural was obtained (between 65-95%) at the end of the fe eding phase, but the conversion of HMF was considerably lower (between 12-4 0%). (C) 2000 John Wiley & Sons, Inc.