Submitted to: Applied Biochemistry and Biotechnology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/9/2007
Publication Date: 11/7/2007
Citation: Bai, D., Li, S., Liu, Z., Cui, Z. 2007. Enhanced L-(+)-lactic acid production by an adapted strain of Rhizopus oryzae using corncob hydrolysate. Applied Biochemistry and Biotechnology. 144:79-85. Interpretive Summary: L-(+)-lactic acid (L-LA) is a natural organic acid widely applied in food, chemical, pharmaceutical, and personal care industries, as well as production of propylene oxide, propylene glycol, and acrylic fibers. It can be used as a precursor for production of poly-lactic acid (PLA), a biodegradable and thermoplastic biomaterial commonly used as high-volume, biodegradable plastics for packing and other applications. Current industrial LA fermentation is mostly carried out with LA bacteria from relatively expensive carbon sources such as glucose, lactose, and starch with rich nutrient. Rhizopus oryzae is, however, preferred for the production of L-LA owing to its exclusive formation of the L-isomer with a simple nutritional requirement; and the product can be recovered easily. Most L-LA producing R. oryzae strains are limited in corncob xylose utilization and L-LA yield. This study developed a more efficient strain that completely utilized xylose derived from corncob and increased over 100% of yield for L-LA production compared with a parental control. Results of this research impact research and development on utilization of low-cost feed stock corncob for higher valued bioproducts, specifically for L-LA production.
Technical Abstract: Corncob is an economic feedstock and over 20 million tons of corncobs are produced annually in China. Abundant xylose can be potentially converted from the large amount of hemicellulosic materials in corncobs, which makes the crop residue an attractive alternative substrate for a value added production of a variety of bioproducts. Lactic acid (LA) can be used as a precursor for poly-LA production. Although current industrial LA is produced by LA bacteria using enriched medium, production by Rhizopus oryzae is preferred due to its exclusive formation of the L-isomer and a simple nutrition requirement by the fungus. Production of L-(+)-lactic acid (L-LA) by R. oryzae using xylose has been reported; however, its yield and conversion rate are poor compared with that of using glucose. In this study, we report an adapted R. oryzae strain HZS6 that significantly improved efficiency of substrate utilization and enhanced production of L-LA from corncob hydrolysate. It increased L-LA final concentration, yield, and volumetric productivity over twofold compared with its parental strain. The optimized growth and fermentation conditions for Strain HZS6 were defined.