Influence of yeast extract concentrationon batch cultures of Lactobacillus helveticus: growth and production coupling

Lactobacillus helveticus was cultivated batchwise on whey permeate supplemented with 2–30g yeast extract l^-1. For low supplementation levels, growth was slow, and a long maintenance phase in the absence of any cell autolysis was observed. When the yeast extract concentration was increased, growth time increased slightly; under these circumstances, a large increase of maximum biomass concentration was recorded, indicating a clear nitrogen source starvation at low supplement concentration. At the same time the length of the maintenance phase decreased; for a 30gl^-1 supplementation, it vanished and a death phase was observed instead: this behaviour highlighted a shift from nitrogen to carbon source starvation. For more and more supplemented media, the final lactic acid concentration was constant, while fermentation time decreased significantly; a growth-associated production phase was followed by a maintenance production phase. The part played by the former mecha nism was under tight control of yeast extract concentration: for 30gl^-1 supplementation, 83% of total lactic acid was provided by a growth-linked production process.