Biotechnology Letters, Vol.37, No.12, 2411-2418, 2015
Removing chiral contamination of lactate solutions by selective metabolism of the D-enantiomer
Objective A bio-based process is appealing for purification of l-lactic acid, the major enantiomer of polylactic acid syrup, generated by thermochemical processes at the end of life of PLA-based plastics, from its chiral impurity, d-lactic acid, before reuse. Results Polylactic acid (PLA), a renewable alternative to petroleum-derived plastics, contains a mixture of l- and d-lactic acid (LA) isomers with the l-isomer dominating (up to 95 %). A novel bio-based process was developed to produce chirally pure l-LA from syrup produced during recycling of PLA-plastics. This process utilizes an engineered Escherichia coli (strain DC1001) containing novel gene deletions (lld, ykg) that eliminated the oxidative metabolism of l-lactate, leaving the membrane-bound d-lactate dehydrogenases to selectively metabolize the d-isomer. Strain DC1001 removed 8.7 g d-lactate l(-1) from a PLA-syrup containing 135 g total lactic acid l(-1) in 24 h. Average rates of removal of d-lactic acid were 0.25 g d-lactate h(-1) (g cell dry weight)(-1) and 0.36 g d-lactate l(-1) h(-1). Conclusion Bio-based purification of PLA-syrup utilizing E. coli strain DC1001 is an attractive process step during recycling of PLA-plastics. This selective oxidation process can also be used to remove chiral contamination of l-lactate in medical applications.