Different inhibition of Lactobacillus delbrueckii subsp. bulgaricus by D‐ and L‐lactic acid: effects on lag phase, growth rate and cell yield

Inhibition of Lactobacillus bulgaricus by lactic acids at constant pH is governed by two factors: the total concentration of lactic acids and the D/L ratio of lactic acid stereoisomers. L‐Lactic acid is more inhibitory than the D‐isomer, the latter being the only isomer produced by Lact. bulgaricus. The major inhibitory effect is an extremely prolonged lag phase (2–39 h for the range 0–25 g 1 ‐1 added lactic acids). Added lactic acids reduce the maximum cell concentration obtained in pH controlled fermentations, but have virtually no effect on maximum specific growth rate and on the yield of biomass on produced lactic acid. The added lactic acids have no effect on the growth‐related ATP consumption nor on the non‐growth related ATP consumption. The culture is heterogeneous since a part of the culture is unable to grow in the presence of L‐lactic acid. A different inhibitory effect of D‐ and L‐lactic acid has implications for yoghurt manufacture and potential for improved food preservation.