A generalised unstructured model for batch cultures of Lactobacillus helveticus

During lactic acid fermentation, seed cultures are usually carried out without pH control to obtain active cells, while cultures are usually carried out at pH maintained at the optimal value, 5.9, to overcome inhibitory effects. The Luedeking–Piret expression was therefore previously modified by introducing an additional term (i) involving the residual lactose concentration to account for the carbon substrate limitation, responsible for cessation of production during cultures at controlled pH, on one hand; (ii) or involving the undissociated form of the lactic acid, the main inhibitory species, in case of an absence of pH control, namely the usual seed culture conditions, on the other hand. To avoid the use of two different expressions, depending on the culture conditions, a generalised model for production was developed, involving a unique expression taking into account both a nutritional limitation and an inhibitory effect. The nutritional limitation term was modified by introducing the carbon limitation constant, in place of the residual lactose concentration, which varied in absence of pH control, namely in absence of carbon limitation. The model proved to be satisfactory in a large range of culture conditions; and also allow to deduce accurately the growth- and non-growth-associated parts of the production.