Surface Film Formation in Static-Fermented Rice Vinegar: A Case Study

In the present study, we aimed to determine the cause of surface film formation in three rice vinegars fermented using the traditional static fermentation method. The pH and total acidity of vinegar were 3.0-3.3 and 3.0-8.7%, respectively, and acetic acid was the predominant organic acid present. Colonies showing a clear halo on GYC medium were isolated from the surface film of all vinegars. Via 16S rDNA sequencing, all of the isolates were identified as Acetobacter pasteurianus. Furthermore, field-emission scanning electron microscopy analysis showed that the bacterial cells had a rough surface, were rod-shaped, and were ${\sim}1{\times}2{\mu}m$ in size. Interestingly, cells of the isolate from one of the vinegars were surrounded with an extremely fine threadlike structure. Thus, our results suggest that formation of the surface film in rice vinegar was attributable not to external contamination, to the production of bacterial cellulose by A. pasteurianus to withstand the high concentrations of acetic acid generated during fermentation. However, because of the formation of a surface film in vinegar is undesirable from an industrial perspective, further studies should focus on devising a modified fermentation process to prevent surface film formation and consequent quality degradation.