Enhanced D-Lactic Acid Production from Dairy Residues Using Engineered Lactiplantibacillus plantarum

The dairy industry produces large quantities of lactose-rich residues which could be fermented into a suite of valuable products. In this study, we sought to ferment concentrated cheese whey (CCW), with an initial lactose concentration of about 150 g/L, into D-lactic acid (D-LA) using an engineered strain of Lactiplantibacillus plantarum ATCC BAA-793 that lacks the L-lactate dehydrogenase gene (ldhL1), referred to as LdhL1. In experiments using lactose-supplemented laboratory MRS media, greater than 99% of the lactic acid generated by LdhL1 was the D-stereoisomer, compared to equimolar production of D- and L-lactic acid (L-LA) with the wild-type (WT) strain. When CCW was used as the major source of lactose and essential nutrients, most lactose was fermented in 7 days. However, in mixtures of CCW and MRS, fermentations with high-density LdhL1 cultures took less than 1 day, yielding D-LA titers of ~75 g D-LA/L, with 94% of the LA being the D-LA isomer, and with a production rate of ~3.7 g D-LA/(L h). This fermentation process achieves some of the highest titers and rates of D-LA production from dairy residues, an important step toward valorizing lactose-rich dairy residues.