Acetic acid production by Acetobacter pasteurianus TISTR 102 during successive saccharification and co-fermentation with Aspergillus spp. and Saccharomyces cerevisiae for the production of halal vinegar from purple sweet potato

Fruit vinegar and starch-based vinegar are essential condiments widely used in many countries. It is also known for its anti-bacterial, anti-inflammatory, anti-diabetic and antioxidative activities. The potential process of successive saccharification and cofermentation (SSCF) of mixed amylolytic fungi, baker’s yeast, and acetic acid bacteria for acetic acid production using purple sweet potato flour as the primary carbon source was investigated. The media containing an initial carbon (C) to nitrogen (N) ratio of 4.0:1 using carbohydrates from flour and EtOH produced by Saccharomyces cerevisiae TISTR 5050 supported the growth and acid production of Acetobacter pasteurianus TISTR 102 the most, compared to the C:N ratio of 14.8:1 or 2.82:1, with or without EtOH. The SSCF process using amylolytic fungi, Aspergillus usamii TISTR 3258 or Aspergillus shirousamii TISTR 3140, grown together with S. cerevisiae and A. pasteurianus, was investigated. During SSCF using mixed culture for 20 days, A. usamii saccharified purple sweet potato flour to glucose that was subsequently fermented to EtOH by S. cerevisiae and converted to titratable acidity of 163 g acetic acid equivalent (AAE)/L by A. pasteurianus in a static condition. In contrast, the carbohydrate saccharified by A. shirousamii resulted in titratable acidity of 153 g AAE/L (P<0.05). Nonetheless, the gas chromatography-flame ionisation detector (GC-FID) revealed that amylolytic A. usamii, or A. shirousamii, co-fermented with S. cerevisiae and A. pasteurianus produced acetic acid during the SSCF process with the acid conversion yield of 137% of the theoretical maximum after 20 days under static conditions (P≥0.05). Overall, this study demonstrated a suitable SSCF process to support the growth of A. pasteurianus and its acid production for a potential halal vinegar fermentation from purple sweet potato while maintaining EtOH concentration to below 75 g/L throughout the fermentation process of 20 days.