Preparation of High-Optical-Purity _D-Lactic Acid from _DL-Lactic Acid Using Optical Isomer-Preferential Lactic Acid Assimilating Microorganisms

Abstract

　We screened optical isomer-preferential lactic acid assimilating microorganisms (LAAMs) for their ability to generate lactic acid with high optical purity from dl-lactic acid by isomer-specific assimilation. Fifty-seven strains of LAAMs were isolated from soils samples and plants. Five strains of LAAMs demonstrated preferential assimilation of l-lactic acid, and 10 strains of LAAMs demonstrated preferential assimilation of d-lactic acid. One such strain, LAAM001 produced a high ratio of d-lactic acid (94.0%) and had a high d-lactic acid yield (67.9%) ; this isolate may be suitable for preparation of lactic acid with high optical purity. LAAM001 was identified as Pichia manshurica by morphological observation, physiological analysis, and sequence analysis of the 26S rDNA-D1/D2 region. Other strains of P. manshurica also demonstrated preferential assimilation of l-lactic acid, although the optical purity of the resulting d-lactic acid was the highest with LAAM001. The optimal culture conditions for LAAM001 were investigated. Optimal temperature was 30℃ or 35℃, and optimal initial pH of the medium was 3.8. The use of LAAM001 for preparation of d-lactic acid with high optical purity by isomer-preferential assimilation from biosynthesized d-lactic acid with low optical purity was investigated. After 48 hours, l-lactic acid was not detected in the culture broth by HPLC, and 9.72 g/ℓ of d-lactic acid with high optical purity was obtained.