Simultaneous utilization of mixed carbohydrates by lactobacilli: Physiology aspects and application on bio-based chemical production.

摘要

Lactic acid markets in worldwide are growing fast due to emerging applications of lactic acid as a food grade organic solvent and as a chemical intermediate for production of biodegradable plastics. Despite the merits as an alternative substrate for the biological production of lactic acid, utilization of lignocellulosic biomass is still problematic due to the heterogeneity of its composition. In this study, lactobacilli which lack the carbon catabolite repression apparently (CCR-) have been studied in order to develop a novel process of lactic acid production from acid-pretreated rice straw. Lb. brevis is able to consume any fermentable sugars simultaneously with glucose. The apparent CCR- phenotype is observed in amino acid utilization by Lb. brevis as well. This unusual phenotype is discussed in the context of CCR-related genetic elements observed in the Lb. brevis genome sequence. Various process parameters required for growth of Lb. brevis on rice straw hydrolyzate are examined. Simultaneous utilization occurs regardless of sugar type and composition obtained from rice straw. Potential inhibitors in rice straw hydrolyzate and high initial sugar concentrations up to 60g/L of glucose do not affect the lactic acid production. A batch fermentation using 100g-dry mass/L of rice straw hydrolyzate reproduces fermentation profiles carried out in rich media. The effect of lactic acid and hydrogen ion concentrations on lactic acid production by Lb. brevis are also presented. Kinetic equations describing the relationship between specific cell growth rate and lactic acid or hydrogen ion concentration are deduced. The change of substrate utilization and product formation in accordance with the lactic acid and hydrogen ion concentration in the media are described as well. Finally, a new fermentation scheme which combines enzymatic digestion of cellulose fiber and concurrent fermentation of resultant sugars is shown to increase lactic acid productivity and overall yield while decreasing the enzyme required. In an effort to optimize the new process, termed simultaneous saccharification and mixed sugar fermentation (SSMSF), the impact of temperature on lactic acid production and cellulose hydrolysis is examined. In addition, the effect of media composition on enzyme reaction, and the stability of enzyme mixture during fermentation is examined. In order to increase lactic acid yield from hexose sugars, the CCR - strain of Lactobacillus pentosus JH5XP5, a facultative heterofermentative lactobacilli, is isolated. This new strain exhibits a CCR phenotype which is stable during repeated fermentations. SSMSF of acid pretreated rice straw performed in batch and fed-batch mode results in the reduction of fermentation time, enzyme requirement and a increase in final lactic acid concentrations and yields..