An unstructured model has been developed by assuming a sigmoidal variation with time of specific growth rate and introducing an additional term in Luedeking and Piret production kinetics. The model fitted growth and production kinetic data. The model was helpful for examining the influence of initial yeast extract concentration on growth and production coupling. In richly supplemented media, the growth associated production mechanism prevailed, while in the case of poor nitrogen supplementation, most lactic acid production was non-growth-associated. Both contributions might be calculated from model parameters; this was especially interesting when the forms of growth and production kinetics did not allow a clear conclusion. The additional term in the production rate expression was a convenient way to characterize ＇usable nitrogen＇ concentration in the medium. From the model a clear criterion has been derived for optimizing the nitrogen supplementation from an economic point of view.