Low-pH production of d-lactic acid using newly isolated acid tolerant yeast Pichia kudriavzevii NG7.

Lactic acid is a platform chemical for the sustainable production of various materials. To develop a robust yeast platform for low-pH production of d-lactic acid (LA), an acid-tolerant yeast strain was isolated from grape skins and named Pichia kudriavzevii NG7 by ribosomal RNA sequencing. This strain could grow at pH 2.0 and 50°C. For the commercial application of P. kudriavzevii NG7 as a lactic acid producer, the ethanol fermentation pathway was redirected to lactic acid by replacing the pyruvate decarboxylase 1 gene (PDC1) with the d-lactate dehydrogenase gene (d-LDH) derived from Lactobacillus plantarum. To enhance lactic acid tolerance, this engineered strain was adapted to high lactic acid concentrations, and a new transcriptional regulator, PAR1, responsible for acid tolerance, was identified by whole-genome resequencing. The final engineered strain produced 135 g/L and 154 g/L of d-LA with productivity over 3.66 g/L/hr at pH 3.6 and 4.16 g/L/hr at pH 4.7, respectively.