A research group at Nara Institute of Science and Technology has revealed for the first time that "Nara-zuke," a type of pickled vegetable with a history of over a thousand years, is a fermented food produced by microorganisms.
Nara pickles have long been a popular type of pickle known for their long shelf life, achieved by using sake lees containing high concentrations of alcohol as a pickling bed to suppress the growth of unwanted bacteria. Traditionally, Nara pickles were considered a preserved food rather than a fermented food, as the pickling bed provided an extreme environment where many microorganisms struggled to thrive.
However, through joint research with a long-established Nara pickle manufacturer, it was discovered that Nara pickles, both during the pickling process and as finished products, contain the lactic acid bacterium Fructilactobacillus fructivorans (hereinafter referred to as F. fructivorans), which overwhelmingly dominates the environment and almost completely monopolizes other microorganisms.
When pre-made Nara pickles were added to fresh sake lees, it was confirmed that F. fructivorans autonomously proliferated, leading to increased lactic acid accumulation. Furthermore, an increase in umami and richness components was observed in the sake lees after fermentation, suggesting that F. fructivorans not only induces lactic acid fermentation but also contributes to improving the flavor of the Nara pickles.
Even more interestingly, while the standard strain of F. fructivorans has its growth inhibited in high-concentration ethanol environments such as sake lees, the strain isolated from Nara pickles (hereinafter referred to as the Nara pickle strain) showed "ethanolophilia," meaning its growth was actually promoted in the presence of ethanol. This means that the Nara pickle strain, having been exposed to the Nara pickle manufacturing environment for many years, has undergone unique adaptations to become a "lactic acid bacterium for Nara pickles."
Furthermore, fermentation did not occur when salted vegetables were pickled in sake lees instead of pre-made Nara pickles. In the production of Nara pickles, a traditional method of reusing sake lees after pickling is passed down. In other words, it became clear that Nara pickles have been passed down to the present day by the Nara pickle strain of F. fructivorans that grew in sake lees being passed down from generation to generation.
This study elucidated the advanced environmental adaptability of F. fructivorans, which has acquired the unique property of being ethanolophilic, demonstrating its scientific significance beyond its value as a traditional food in Nara pickles. Future developments are expected to include industrial applications such as the development of fermentation microorganisms and enzymes that function even in high concentrations of ethanol and organic solvents.
