Unstructured model for batch cultures without pH control of Lactobacillus helveticus—Inhibitory effect of the undissociated lactic acid

During lactic acid fermentation, the positive effect of precultivating without pH control is now clearly established. At acidic pH, growth was inhibited. To account for this inhibition, the well-known Luedeking–Piret model was modified by introducing an additional term involving the undissociated form of the lactic acid, the main inhibitory species. The Henderson–Hasselbach equation was also considered in the model to describe the relationship between pH, also involved in growth inhibition, and both the dissociated and the undissociated forms of lactic acid. The model was found to describe satisfactory experimental growth and production data in a wide range of culture conditions. Indeed, whey permeate supplemented by a large range of nitrogen supplementations was considered and analysed, since the growth- and non-growth-associated part of the production can be easily deduced from the model. High supplementation of whey permeate, namely 20 g l−1 yeast extract and 10 g l−1 peptones, the usual seed culture supplementation, led to the main part of the lactic acid produced by a growth-associated mechanism (74%).