Simplified modeling of fed-batch alcoholic fermentation of sugarcane blackstrap molasses

Abstract

Simplified modeling based on material balances for biomass, ethanol and substrate was used to describe the kinetics of fed-batch alcohol fermentation of sugarcane blackstrap molasses. Maintenance requirements were previously shown to be of particular significance in this system, owing to the use of massive inoculum to minimize inhibitions; therefore, they were taken into consideration for kinetic modeling. Average values of biomass and ethanol yields, productivities, and substrate consumption rates, calculated at the end of runs performed either at constant or exponentially varying flow rates, demonstrated that all of these parameters were influenced by the initial sugar-feeding rate, F(o)S(o). Under conditions of substrate shortage (F(o)S(o) </= 300 g(S) h(-1)), the amount of carbon dioxide produced was higher than that corresponding to the stoichiometry of sucrose fermentation to ethanol, indicating that an appreciable fraction of the carbon source was likely consumed by respiration. Besides, the biomass yields either on substrate, Y(X/S), or ethanol, Y(X/E), as well as the product yield on substrate, Y(E/S), notably decreased. These results are in agreement with the relatively high specific rate of anaerobic substrate consumption for maintenance estimated for this system (m(a) (s) = 0.789 g(S) g(X) (-1) h(-1)), which was responsible for the consumption of more than 70% of the fed carbon source. The proposed equations derived from the Monod model proved to be a useful tool to easily predict the performance of this process.