Simultaneous Saccharification and Co‐Fermentation of Crystalline Cellulose and Sugar Cane Bagasse Hemicellulose Hydrolysate to Lactate by a Thermotolerant Acidophilic Bacillus sp.

Abstract

Polylactides produced from renewable feedstocks, such as corn starch, are being developed as alternatives to plastics derived from petroleum. In addition to corn, other less expensive biomass resources can be readily converted to component sugars (glucose, xylose, etc.) by enzyme and/or chemical treatment for fermentation to optically pure lactic acid to reduce the cost of lactic acid. Lactic acid bacteria used by the industry lack the ability to ferment pentoses (hemicellulose-derived xylose and arabinose), and their growth and fermentation optima also differ from the optimal conditions for the activity of fungal cellulases required for depolymerization of cellulose. To reduce the overall cost of simultaneous saccharification and fermentation (SSF) of cellulose, we have isolated bacterial biocatalysts that can grow and ferment all sugars in the biomass at conditions that are also optimal for fungal cellulases. SSF of Solka Floc cellulose by one such isolate, Bacillus sp. strain 36D1, yielded l(+)-lactic acid at an optical purity higher than 95% with cellulase (Spezyme CE; Genencor International) added at about 10 FPU/g cellulose, with a product yield of about 90% of the expected maximum. Volumetric productivity of SSF to lactic acid was optimal between culture pH values of 4.5 and 5.5 at 50 degrees C. At a constant pH of 5.0, volumetric productivity of lactic acid was maximal at 55 degrees C. Strain 36D1 also co-fermented cellulose-derived glucose and sugar cane bagasse hemicellulose-derived xylose simultaneously (SSCF). In a batch SSCF of 40% acid-treated hemicellulose hydrolysate (over-limed) and 20 g/L Solka Floc cellulose, strain 36D1 produced about 35 g/L lactic acid in about 144 h with 15 FPU of Spezyme CE/g cellulose. The maximum volumetric productivity of lactic acid in this SSCF was 6.7 mmol/L (h). Cellulose-derived lactic acid contributed to about 30% of this total lactic acid. These results show that Bacillus sp. strain 36D1 is well-suited for simultaneous saccharification and co-fermentation of all of the biomass-derived sugars to lactic acid.

Enhanced Cellulase Production by a Novel Thermophilic Bacillus licheniformis 2D55: Characterization and Application in Lignocellulosic Saccharification

“…It has been reported that acidic pH induces the production of cellulases in thermophilic cellulolytic Bacillus sp. (Patel et al 2005). The high tolerance of this strain to acidic pH suggests that it has the potential for use in the harsh environments encountered during industrial fermentation.…”

Metabolic engineering of a thermophilic bacterium to produce ethanol at high yield

“…Use of the hemicellulolytic organisms alone at high temperatures could potentially lower the quantity of added cellulase required. For example, Patel et al (11) report lactic acid production from cellulose at low cellulase loadings, by using a thermophilic Bacillus species. T. saccharolyticum JW/SL-YS485 is one such hemicellulolytic organism with the ability to hydrolyze xylan and ferment the majority of biomass-derived sugars at thermophilic temperatures.…”

Evolution of D-lactate dehydrogenase activity from glycerol dehydrogenase and its utility for D-lactate production from lignocellulose

“…With cellulose as a feedstock, however, commercial fungal cellulases represent a significant process cost. This cost could be reduced by the development of thermotolerant biocatalysts that effectively ferment under conditions that are optimal (pH 5.0, 50°C) for fungal cellulases (14,15).…”