Ethanol Production by Co-Cultivation of Yeast and Lactic Acid Bacteria on Starch

Abstract

The co-cultivation of GRAS amylolytic bacteria together with ethanol-producing yeast Saccharomyces cerevisiae in starch-containing media might be one of the ways solving the problem of starch-containing waste disposal with simultaneous formation of ethanol as a potential biofuel for increasing octane number of gasoline. The aim of the study was to test the combination of microorganisms (amylolytic lactic acid bacteria and yeast) suitable for co-cultivation on starch and to optimize the conditions for starch cofermentation. Methods. Conventional microbiological, biochemical and statistical methods, including serial dilution technique with counting colony forming units (CFU) for growth assessment of mixed cultures, Gas Chromatograph/Mass Spectrometer (GC/MS) for measuring ethanol concentration and Box-Behnken experimental design (Statistica 10) for bioethanol production optimization, were used in this work. Results. The combination of microorganisms for mixed cultures co-cultivation in single-stage starch fermentation was established: the strain of ethanol-producing yeast S. cerevisiae UCM Y-527 and the amylolytic strain of lactic acid bacteria Streptococcus bovis IMV B-7151. Mathematical simulation using a Box-Behnken (3k-p) design determined the optimal parameters for the fermentation of starch in the process of co-cultivation of yeast and bacteria: 10 g/L of starch in the medium at simultaneous inoculations of both cultures and co-cultivation for 72 hours. The theoretically obtained parameters data were experimentally verified: the maximum ethanol yield 1.95 g/L in the experiment corresponded to the theoretically calculated values. Conclusions. It was suggested and optimized a method of starch cofermentation using strains of amylolytic lactic acid bacteria S. bovis IMV B-7151 and yeast S. cerevisiae UCM Y-527, which can be used for one-stage process of hydrolysis and fermentation of starch and starchcontaining wastes with the production of bioethanol and microbial biomass.