INFLUENCE OF YEAST EXTRACT CONCENTRATION ON BATCH CULTURES OF LACTOBACILLUS-HELVETICUS - GROWTH AND PRODUCTION COUPLING

Lactobacillus helveticus was cultivated batchwise on whey permeate sup plemented with 2-30 g yeast extract l(-1). For low supplementation lev els, growth was slow, and a long maintenance phase in the absence of a ny cell autolysis was observed. When the yeast extract concentration w as increased, growth time increased slightly; under these circumstance s, a large increase of maximum biomass concentration was recorded, ind icating a clear nitrogen source starvation at low supplement concentra tion. At the same time the length of the maintenance phase decreased; for a 30 g l(-1) supplementation, it vanished and a death phase was ob served instead: this behaviour highlighted a shift from nitrogen to ca rbon source starvation. For more and more supplemented media, the fina l lactic acid concentration was constant, while fermentation time decr eased significantly; a growth-associated production phase was followed by a maintenance production phase. The part played by the former mech anism was under tight control of yeast extract concentration: for 30 g l(-1) supplementation, 83% of total lactic acid was provided by a gro wth-linked production process.