Sunday, April 29, 2007

Enhanced L(+)-Lactic acid production from corncob hydrolysate by an adapted strain of Rhizopus oryzae

Dong-Mei Bai, Center for Microbial Biotechnology, Technical University of Denmark, DK-2800 Kgs, Lyngby, Denmark, Shi-Zhong Li, Institute of New Energy Technology, Tsinghua University, Beijing, 100084, China, Z. Lewis Liu, National Center for Agricultural Utilization Research, USDA-ARS, 1815 N. University, Peoria, IL 61604, and Zhan-Feng Cui, Department of Engineering, University of Oxford, Oxford OX1 3PJ, United Kingdom.

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 can be used as a precursor for poly-lactic acid production.  Although current industrial lactic acid is produced by lactic acid 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 by R. oryzae using xylose has been reported, however, its yield and conversion rate are poor compared with that of using glucose.  The objective of this study was to develop more efficient strains of R. oryzae for economic production of L(+)-lactic acid using corncob hydrolysate.  In this study, we report an adapted R. oryzae strain HZS6 that significantly improved substrate utilization and enhanced production of L(+)-lactic acid from corncob hydrolysate.  It  increased L(+)-lactic acid final concentration, yield, and volumetric productivity from 36.2 g l-1, 38.3% and 0.46 g l-1 h-1 to 77.2 g l-1, 79.6% and 0.99 g l-1 h-1 respectively, compared with that of a control.  The optimized growth conditions were defined.  This development improves the producing cost of L(+)-lactic acid for poly-lactic acid production using corncob hydrolysate.