Inferring the role of microorganisms in water kefir fermentations

Water kefir is a slightly alcoholic, lactic and acetic beverage fermented by yeasts, lactic acid bacteria and acetic acid bacteria that are associated with the polysaccharide of the water kefir grains. In this study, the three main metabolic products of microorganisms were evaluated during a traditional 192-h water kefir fermentation and also after inoculating the microorganisms in fresh medium or sterilised broth from different fermentation stages. The first process to occur was alcoholic fermentation, carried out in particular by Saccharomyces cerevisiae. After 24 h, lactic and acetic acid accumulation was generated by Lactobacillus hilgardii and Acetobacter tropicalis. By the end of fermentation, ethanol had been almost entirely consumed and oxidised to acetic acid, possibly by a dissimilatory route of Acetobacter species. An original hypothetical diagram is proposed for the carbon flux from sucrose, and the metabolic role of the main yeasts and bacteria is assigned for the distinct stages of water kefir fermentation.