Lactobacillus helveticus growth and lactic acid production during pH-controlled batch cultures in whey permeate/yeast extract medium. Part II: kinetic modeling and model validation

A kinetic model was developed that describes growth and lactic acid production rates of Lactobacillus helveticus during pH-controlled batch cultures in whey permeate-yeast extract medium as a function of four variables: sugar, nitrogen substrate and lactic acid concentrations, and pH. Since available nitrogen substrate could not be measured directly, it was related to medium composition through a response surface model of maximum biomass concentration in nitrogen-limited growth cultures. The effect of pH on growth was modeled with a Gaussian equation instead of the traditional second order polynomial term. Product inhibition was modeled with a logistic term for lactate and an exponential term for undissociated lactic acid. The kinetic model accurately described the experimental data with few exceptions.