Isolation, identification and characterization of thermo-tolerant acetic acid bacteria for semi-continuous acetous fermentation at high temperature

Nowadays, vinegar is industrially produced by mesophilic acetic acid bacteria (AAB). However, temperature fluctuation during acetous fermentation is inevitable and may cause process disturbances. This can be mostly avoided using thermo-tolerant AAB. The main purpose of the present study was to isolate thermo-tolerant AAB and then evaluate the fermentation performances. Twenty-eight different isolates were isolated from traditional Iranian vinegar. One of the isolates was able to grow and produce acetic acid in minimal culture media containing 5% (w/v) ethanol at 30-42 ºC. The 16s rRNA gene analysis showed that the selected thermo-tolerant isolate was Acetobacter tropicalis (L31). Acetous fermentation in a Lab-bioreactor showed that Acetobacter tropicalis (L31) grew in minimal culture medium, and produced 5% (w/v) acetic acid at 37°C in batch and semi-continuous fermentation mode. Fermentation time was significantly dependent on the dissolved oxygen (DO) concentration and acclimation of cells to low pH and acid stress. On average, acclimated cells produced 2.3-2.7 g L-1h-1 acetic acid during the production phase. The final yield was 87% at 37ºC in low and high DO concentrations. Low DO concentration (15%) during acetous fermentation caused longer fermentation time, but a large part of the cells (91%) grown under such condition were still viable even if oxygen flow was interrupted. In contrast, cells grown under high DO concentration were not able to tolerate oxygen deficiency. In conclusion, since Acetobacter tropicalis (L31) grew well in minimal culture medium and showed tolerance to high temperature and low DO, it is a potential isolate for vinegar starter production.