Optimiranje bioprocesnih parametrov proizvodnje etanola in kefirana z mikrobioto kefirnih zrn : doktorska disertacija
Optimization of bioprocess parameters for ethanol and kefiran production using microbiota of kefir grains : doctoral dissertation

slovenian

doktorska dela

2012

Univerza v Mariboru, Fakulteta za kemijo in kemijsko tehnologijo

This doctoral dissertation consists of two parts that dealing with the production of traditional kefir with the Caucasus kefir grains. The first partdeals with the optimization of the bioprocess parameters of ethanol production during kefir fermentation and the second part deals with the optimization of kefiran production from kefir grains. As a start the effect of kefir grain activation time on the change in kefir pH value and the effect on the production of ethanol during kefir fermentation was studied. The drop in the pH value of kefir was successfully described during a 24 h fermentation process with the suggested mathematical, exponental pH model. Additionally, the effect of kefir grain activation on the relation between theproduced ethanol and the number of yeasts in the grains and in the kefir was examined. The results have shown that the more active grains produce more ethanol and contain a larger amount of yeasts in comparison with the grains that have been activated for only a few days. The number of yeasts in kefir was increasing in relation with the grain activation time. The kinetic models, increasingly developed because of the industrial use of fermentation, allow for a description of microbe growth, product creation and substrate use in natural ecosystems and under certain process conditions. Using a non-structured mathematical model the kinetic fundamentals of ethanol production during a batch fermentation of kefir were described. To that end the kinetics of kefir grain growth and ethanol production were examined. The experimental data, acquired through a batch fermentation series that was carried out at 21 °C, the start mass concentration of kefir grains 42 g/L and set fermentation times in the time bracket (0-89) h, was used to assess the kinetic parameter values and to confirm the validity of the suggested sigmoid models. The kefir grain growth was described using a logistic model. The ethanol production was described using a modified version of the Gompertz model. It was established that both models offer a good description of the kefir fermentation kinetics in relation to the kefir grain growth and ethanol production and that they can be used to develop and optimize the biologic processes of ethanol production. A mathematical model for ethanol production kinetics during kefir fermentation as a function of temperature and time was also developed. The influence of temperature and fermentation time on the value of the biokinetic parameters of ethanol production was described using the modified Gompertz model. The value of biological kinetic parameters of the suggested model were assessed using the method of least squares on the basis of the experimental data that was acquired through a batch fermentation series. The influence of temperature on the growth of kefir grain biomass and the microbiological composition of kefir and grains was assessed. The temperature dependence of the maximum speed of ethanol production was analysed using the Arrhenius equation that was used to assess the value of activation energy for the ethanol production during the fermentation of milk with kefir grains (64,3 kJ/mol.) This mathematical model allows for a very good description of the ethanol production dynamics during kefir fermentation.The first dissertation part was concluded by examining the effect of time lapses during the mixer inactivity or the stirring frequency on the ethanol production and the microbiological properties of kefir and kefir grains. Several mathematical equations were developed that describe the relation of the stirring frequency and (1) the increase of kefir grain growth in 24 h, (2) the concentration changes of ethanol during the 24 h fermentation and (3) pH value of kefir after the 24 h fermentation. The values of the function parameters for all developed mathematical models were estimated using the method of least squares.

07.08.2023