A temporal view of the water kefir microbiota and flavour attributes

Water kefir is a fermented beverage often produced in a household setting through the inoculation of a microbial community (water kefir grains) into a sugar rich solution. To date, a detailed understanding of how this community evolves during the fermentation, and the resulting impact on the final beverage flavour has not been elucidated. To address this metagenomics, flavour analysis and sensory science were used to resolve the temporal evolution of a model water kefir. We observed that the diversity of the water kefir studied is relatively unique, being dominated throughout the fermentation by the gluconic acid producer Zymomonas mobilis. We describe the metagenome-assembled genome of two likely new Curvibacter species, that is mostly found in the liquid fraction. Finally, we show that, in addition to Z. mobilis, members of the Bifidobacterium, Acetobacter and Saccharomyces species as well as former members of the Lactobacillus genus, were the main contributors to flavour. Water kefir has become an increasingly attractive alternative natural beverage for consumers, while also serving as an interesting model to enhance our understanding of microbial communities consisting of eukaryotes and prokaryotes. By integrating metagenomics, chemical profiling and sensory evaluation, we illustrate and clarify the dynamic nature of water kefir fermentations. In addition to elucidating its detailed temporal diversity, we also highlight the potential flavour and sensory contributions of the different microbial members of the community. These combined findings represent a significant milestone in the understanding of water kefir microbiology while also paving the way for the development of defined starter cultures that can enable the manufacture of a beverage which reproduces the main attributes of a traditionally brewed water kefir.