Production Of D-lactic Acid By Serratia Marcescens

Lactic acid is an important chemical for various applications in both the food and non-food industries, including cosmetic, pharmaceutical, and chemical industries. In particular, L-lactic acid has attracted considerable attention over the past two decades as a raw material for the synthesis of poly L-lactic acid (PLLA). PLLA is a promising biodegradable plastic material, but it exhibits a low melting point of175℃, which leads to difficulties in its application. Recently, there has been an increased focus on the stereocomplex of PLLA and poly D-lactic acid (PDLA) because they have approximately50℃higher melting point than that of their respective single polymers and have more biodegradability. Presently, the production of D-lactic acid has become considerably significant as well as that of L-lactic acid.In this paper, we tried to produce D-lactic acid by using Serratia marcescens, the detailed works were introduced as following:1. The comparing and choosing of Serratia marcescens strains. In the study we got three Serratia marcescens strains (H30, G1and R1) and made a careful comparison between them. It was found that the recombinant strain Serratia marcescens R1has several features favored the production of D-lactic acid compared to other two strains:the higher yield of D-lactic acid from glucose and the less formed foam which can prevent the microbial pollution. Considering these advantages, the strain Serratia marcescens R1was used to produce D-lactic acid in the following researches.2. Optimization of fermentative conditions and medium compositions. By using single factor experiment, Plackett-Burman design and Response Surface methodology, the flask fermentation conditions and the concentrations of medium components were optimized. The results showed that the best production process is to aerobic culture the strain at pH of7.0and28℃in order to obtain the largest biomass; to anaerobic culture the stain at pH of6.0and44℃in order to get the highest production of D-lactic acid. And the optimal medium (g/L) was obtained:glucose60; yeast extrace20; NaCl5; KH2PO45; MgSO40.5; MnSO4.3. The screening of sodium lactate-tolerance strain and experiments in3.7L bioreactor. The recombinant strain Serratia marcescens R1was cultured in flask or plate in the fresh medium contained50g/L sodium lactate in order to select the colony which grows best. The selected strain was cultured in3.7L fermentator. To obtain more biomass, large volume of seed culture were centrifuged and inoculated into3.7L bioreactor. Than after a long anaerobic culture process, the obtained D-lactic acid concentration can achieve83.5g/L.