Influence of peptide supply and cosubstrates on phenylalanine metabolism of Lactobacillus sanfranciscensis DSM20451(T) and Lactobacillus plantarum TMW1.468.

Bread spoilage is mainly due to the growth of filamentous fungi, and metabolites produced during sourdough fermentation by lactobacilli can inhibit fungal growth. One of these metabolites is phenyllactic acid (PLA), which is a catabolite from phenylalanine. In this work, the influence of peptide supply and cosubstrates was determined on PLA formation from phenylalanine by Lactobacillus plantarum TMW1.468 and Lactobacillus sanfranciscensis DSM20451(T). Transport of single amino acids is not efficient in lactobacilli, and only 1% of the offered phenylalanine was converted to PLA. PLA yields were increased 2-4-fold when peptides instead of single amino acids were used as a substrate. The accumulation of phenylalanine after peptide addition indicated that, after transport, transamination was the second limiting factor. In L. plantarum TMW1.468, PLA yields were increased from 5 to >30% upon the addition of alpha-ketoglutarate. In L. sanfranciscensis DSM20451, a combination of both citric acid and alpha-ketoglutarate increased PLA formation. The combined effect of citric acid and alpha-ketoglutarate can be attributed to changes in the NAD/NADH ratio.