An unstructured model has been developed to predict microbial growth based on carbon or nitrogen substrate consumption, ammonia or carbon dioxide production and proton transfer. The model has been validated for batch cultures of Geotrichum candidum and Penicillium camembertii growing on peptones and peptones + lactate based media. The contributions of the considered kinetics to biosynthesis and cellular maintenance can be deduced from this model. The nitrogen source (peptones) was mainly utilized in biosynthesis: for P camembertii growing on peptones, 86% of the metabolized peptones. G candidum metabolized peptones preferentially to lactate as a carbon source, resulting in lactate utilization by a maintenance mechanism during the stationary state. In contrast, P camembertii, which metabolized fewer amino acids as a carbon source, utilized lactate mainly for biosynthesis (83% of the consumed lactate). Most (up to 71%) of the ammonia released was produced by deamination of amino acids utilized as both carbon and nitrogen sources by growth-associated metabolism. With peptones, proton transfer resulted from ammonia release, most likely as a result of the growth-associated mechanism, as supported experimentally (55-58% of the released ammonia for both microorganisms). The contribution of lactate to proton transfer resulted in 76% of protons exchanged by a growth-associated mechanism during P camembertii growth. For total carbon dioxide production, the contributions of the energy supplies for biosynthesis and cell maintenance were similar; except during P camembertii growth in the presence of lactate (65% of growth-associated CO2 production).