Catalytic resolution of racemic ethyl lactate to produce D‐lactic acid by immobilized cells of Rhizopus sp. WS0128

Abstract

AbstractD‐Lactic acid can be produced by either chemical or biochemical methods and is an important raw material or intermediate used in many fields including pesticides and herbicides production, and as polymers with promising applications in medical areas. Esterase produced by Rhizopus sp. WS0128 was effective in the catalytic resolution of racemic ethyl lactate to produce optically pure D‐lactic acid. The cells of Rhizopus sp. WS0128 were immobilized by entrapping them into calcium alginate gels, and the fermentation of the immobilized cells was carried out in an external‐loop airlift bioreactor (ELAB) (11.5 l). Optimal growth conditions for the microorganism were determined such that the maximum esterase activity of the immobilized cells reached 57.9 U/g cells after 48 h of fermentation. The immobilized cells of Rhizopus sp. WS0128 containing abundant esterase could be used directly as a biocatalyst in the enantioselective hydrolysis of ethyl lactate. The optimal hydrolysis reaction conditions were initial substrate concentration 160 g/l, temperature 40 °C, pH value 7.5 and reaction time 3.5 h. Under these conditions, the hydrolysis yield of ethyl lactate was 43.4% and the optical purity of D‐lactic acid was 96.3%. The immobilized cells were quite stable and could be utilized repeatedly in the biocatalytic process for at least six batches, where the average hydrolysis yield of ethyl lactate and the average optical purity of D‐lactic acid were 43.1% and 95.9%, respectively. This research work showed a promising process of D‐lactic acid preparation by biotransformation. Copyright © 2010 Curtin University of Technology and John Wiley & Sons, Ltd.