Unstructured generalized models for the analysis of the inhibitory and the nutritional limitation effects on Lactobacillus helveticus growth—Models validation

During lactic acid fermentation, an inhibitory effect was recorded during seed cultures, usually carried out without pH control; while nutritional limitations (nitrogen) were observed during cultures, carried out at pH controlled at 5.9, which ceased when carbon became limiting. To avoid the use of two different expressions, depending on culture conditions, a generalized model for production was developed, involving a unique expression taking into account both a nutritional limitation and an inhibitory effect. In some conditions, namely at acidic pH control, the model failed to describe accurately growth- and non-growth-associated production. Therefore, the inhibitory term related to the undissociated lactic acid inhibition was added in the growth relation (modified Verlhust model), instead of the production model. The modified Verlhust model allowed a dissociation of the inhibitory and the nutritional effects. A modified Luedeking-Piret expression, involving a carbon substrate limitation to account for cessation of production was considered for the production model. This model proved to satisfactory describes growth and production data in the wide range of culture conditions examined; it also allowed a fine identification and characterization of growth and production parameters.