d -Lactic acid biosynthesis from biomass-derived sugars via Lactobacillus delbrueckii fermentation
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ORIGINAL PAPER
D-Lactic
acid biosynthesis from biomass-derived sugars via Lactobacillus delbrueckii fermentation
Yixing Zhang • Praveen V. Vadlani
Received: 18 December 2012 / Accepted: 1 May 2013 Ó Springer-Verlag Berlin Heidelberg 2013
Abstract Poly-lactic acid (PLA) derived from renewable resources is considered to be a good substitute for petroleum-based plastics. The number of poly L-lactic acid applications is increased by the introduction of a stereocomplex PLA, which consists of both poly-L and D-lactic acid and has a higher melting temperature. To date, several studies have explored the production of L-lactic acid, but information on biosynthesis of D-lactic acid is limited. Pulp and corn stover are abundant, renewable lignocellulosic materials that can be hydrolyzed to sugars and used in biosynthesis of D-lactic acid. In our study, saccharification of pulp and corn stover was done by cellulase CTec2 and sugars generated from hydrolysis were converted to D-lactic acid by a homofermentative strain, L. delbrueckii, through a sequential hydrolysis and fermentation process (SHF) and a simultaneous saccharification and fermentation process (SSF). 36.3 g L-1 of D-lactic acid with 99.8 % optical purity was obtained in the batch fermentation of pulp and attained highest yield and productivity of 0.83 g g-1 and 1.01 g L-1 h-1, respectively. Luedeking– Piret model described the mixed growth-associated production of D-lactic acid with a maximum specific growth rate 0.2 h-1 and product formation rate 0.026 h-1, obtained for this strain. The efficient synthesis of D-lactic acid having high optical purity and melting point will lead to unique stereocomplex PLA with innovative applications in polymer industry.
Y. Zhang P. V. Vadlani (&) Department of Grain Science and Industry, Bioprocessing and Renewable Energy Laboratory, Kansas State University, Manhattan, KS, USA e-mail: [email protected]
Keywords D-Lactic acid Fermentation Corn stover Pulp Biosynthesis List of symbols lmax Maximum specific growth rate (h-1) C0 Initial glucose concentration (g L-1) Cp Product concentration (g L-1) YPS Product yield (g lactic acid g-1 glucose) 0 Y PS Product overall yield (g lactic acid g-1 biomass) YXS Yield of cell dry mass from substrate (g cell dry mass g-1 glucose) YPX Yield of product from cell dry mass (g D-lactic acid g-1 cell dry mass) qPS Product formation rate (h-1) calculated based on the equation qPS = 1S dP dt Qp Productivity (g L-1 h-1)
Introduction Lignocellulosic biomass is gaining importance as a potential source of renewable energy and chemicals as the fossil fuel reserves are eventually getting depleted. Demand continues to increase for production of high-value chemicals and materials from renewable resources to attain domestic selfsufficiency and enhanced national security. Lactic acid is an important and multifunctional organic acid that has wide applications in the food, pharmaceutical, and chemical industries [1, 2]. It exists in two optical isomeric forms, L (?) and D (-) poly-lactic acid (PL
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