Metagenomic and metabolomic analysis of Korean rice vinegar productions shows a large variability among different producers
Korean traditional fermented rice vinegar is part of the nation's food culture. Rice vinegar fermentation processes start with saccharification and alcoholic fermentation of rice by using nuruk, a traditional Korean fermentation starter, to make rice wine, followed by its conversion into vinegar through an acetic fermentation. At two production facilities, producers A and B, two fermentation processes were regularly sampled for the entire duration, namely 55 days for producer A and 90 days for producer B.  For producer A, the nuruk used contained species of Lactobacillus, Pichia, and Weissella. During vinegar production, Saccharomyces and Lactobacillus species prevailed during the alcoholic fermentation phase, whereas Acetobacter and Lactobacillus species prevailed during the acetic fermentation phase. The latter phase was initiated by the addition of vinegar with a very low occurrence of Acetobacter species, nevertheless able to lower the pH from 4.2 to 3.6. The duplicate fermentation processes showed little divergence, most likely because the fermenting liquids were stirred regularly, and due to the backslopping step to initiate the acetic fermentation phase. Acetic acid and esters were only produced after backslopping, simultaneously with the consumption of ethanol, until ethanol depletion.  For producer B, the fermenting liquid was less well stirred compared to that of producer A. In one of the two vinegar production processes, no Acetobacter species were found and consequently ethanol was not converted into acetic acid. In the other process, at day 0, Lactobacillus and Leuconostoc species were present, and around day 20, a mixture of different microorganisms was found, encompassing species of Acetobacter, Enterococcus, Lactobacillus, Leuconostoc, Pediococcus, and Saccharomyces. From day 20 onwards, Acetobacter species were found with an increasing relative abundance, together with Lactobacillus and Leuconostoc. Ethanol and lactic acid were converted into acetic acid and acetoin, respectively, and esters were produced from the beginning of the production. At producer B, a less clear separation between the alcoholic and vinegar fermentation phases occurred.  The comparison of two traditional rice vinegar production facilities showed that the fermentation practices applied can result in large differences of not only the end-products but also the reproducibility of the processes.