In this study, a metabolic flux analysis (MFA) is firstly applied, at each time instant, for determining the range of admissible fluxes based on glucose and ammonium uptake rates and ethanol production rate estimated from measured concentrations in Saccharomyces cerevisiae fed-batch cultures. Given the similarity between the upper bound of the biomass production flux and its estimation through concentration measurements, a flux balance analysis (FBA), aiming at maximizing the biomass growth, is proposed and allows narrowing the admissible flux ranges within the metabolic network. Finally, after withdrawing the information corresponding to the ethanol production rate, original inequality constraints describing overflow metabolism of glucose are added to the FBA linear program in order to compensate for the lack of information regarding ethanol production. This final FBA model allows predicting biomass growth and ethanol production rates, which are consistent with the experimental measurements.