High gravity lignocellulose bioprocess development for ethanol and lactic acid production by multi-feed simultaneous saccharification and fermentation

Second generation bioethanol production is becoming established in production plants across the world. The process can also be viewed as a model biorefinery concept for biotechnological conversion of recalcitrant lignocellulosic raw materials to a range of chemicals and other products. We have developed a Multi-Feed SSCF process, which is a systematic, model-driven design of feeding schemes of solid substrate, active and robust cell factories that are adapted to the present substrate, and enzymes, to fed-batch simultaneous saccharification and co-fermentation of steam-pretreated lignocellulosic materials in standard stirred tank reactors. The concept has been applied not only to ethanol production with S. cerevisiae, but also to lactic acid production from wheat straw by a thermophilic, cellulolytic strain of Bacillus coagulans(MA-13), isolated from bean processing waste. High Gravity operation, i.e. fermentation at high concentrations of water insoluble solids (WIS), pushes the process towards higher product concentrations and productivities, and improved energy and water economy. By using the multi-feed SSCF approach, the ethanol process was pushed towards final product concentrations above 60 g/L, at about 90% of the theoretical yields on consumed substrate, using 22% w/w accumulated WIS additions of acid- and steam explosion-pretreated wheat straw. Bacillus coagulansMA-13 was found to secrete cellulolytic enzymes and ferment lignocellulose-derived sugars to lactic acid; thus, it may be a potential platform for consolidated bioprocessing of lactic acid. We investigated its performance in multi-feed SSF and found that a pre-adaptation to the liquid fraction of steam-pretreated lignocellulosic materials improves lactate productivity and reduces the SSF time from 33 to 12 hours.