Continuous Simultaneous Saccharification and Fermentation of Delignified Rice Straw by a Combination of Two Reversibly Soluble-Autoprecipitating Enzymes and Pentose-Fermenting Yeast Cells.

Abstract

Direct ethanol production from delignified rice straw was performed continuously using a combination of two reversibly soluble-autoprecipitating enzymes, [Meicelase and Cellulase Y-NG were individually immobilized on hydroxypropyl methylcellulose acetate succinate (AS-L)] and Pichia stipitis cells. Cellulase Y-NG immobilized on AS-L (X-AS) as well as Meicelase immobilized on AS-L (M-AS) proposed previously (Hoshino et al., 1994) exhibited a sharper response for the solubility depending on the pH changes; insoluble below 3.5 and soluble above 5.0. When delignified rice straw was hydrolysed using two immobilized enzymes at a ratio of X-AS/M-AS = l, the amount of soluble sugars obtained was higher than that using M-AS alone. Moreover, both immobilized enzymes in an insoluble state had good self-sedimentation properties and were coprecipitated with a large portion of P. stipitis cells.Ethanol production was performed continuously by a reactor system equipped with a mixing vessel and a separation vessel. Both immobilized enzymes and the yeast cells were separated continuously from a solution containing ethanol by co-precipitation in the separation vessel and they was returned to a main reactor for reuse. In continuous ethanol production from the rice straw, high ethanol productivity of up to 0.113 kg/(m3·h) was achieved at a dilution rate of 0.050 h-1. This continuous fermentation system suggests potential for effective ethanol production from delignified rice straw.