Modelling of batch kefir fermentation kinetics for ethanol production by mixed natural microflora

Non-structured kinetic models are proposed for the mathematical description of principal kinetics involved in ethanol production during kefir fermentation in a batch reactor system. Experimental data collected from a series of batch fermentations, which were performed at constant temperature (0-21 C) and initial kefir grains mass concentration (gamma(KG,0)=42 g L-1) over different fermentation periods in the interval from (0-89) h, were used to estimate parameters and also to validate the proposed models. The optimal values of the kinetic parameters were approximated by minimizing the discrepancy between the model predictions and corresponding experimental data. The growth of kefir grains could be expressed by a logistic function model, which describes the growth as a function of initial kefir grains concentration, time, specific growth rate, and final kefir grains concentration. The modified Gompertz model, which gives the lag time, the specific production rate and the maximum ethanol mass concentration, was able to describe the formation of ethanol as the fermentation proceeded. In both cases, the model simulation matches well with the experimental observations, which made it possible to elucidate the fermentation characteristics of kefir grains during efficient ethanol production. These models thus can be employed for the development and optimization of bio-based ethanol production processes. (C) 2009 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.