Novel thermotolerant Acetobacter pasteurianus strains isolated from Moroccan local products destined to industrial vinegar production using substandard fruits

Six strains of acetic acid bacteria were isolated from Moroccan local products and their potential as industrial strains was evaluated in lab-bioreactor. Two of them, namely AF01 and CV01, isolated from apple and cactus fruit, respectively were selected and their responses to high temperature were assessed. Biochemical and molecular study based on 16s rDNA sequence analysis confirmed that the two isolated strains belong to Acetobacter pasteurianus with 99% homology. Their growth and acetic acid production were compared with (1) the thermoresistant reference strain, Acetobacter senegalensis and (2) other mesophilic strains of Acetobacter pasteurianus (LMG 1632, LMG 1607, LMG 1701, DSM 2324 and DSM 4491). The two strains showed abundant growth and noticeable acetic acid production ability at high temperatures (38 to 41°C). A thermophilic character was observed for AF01 strain. Indeed, this bacterium grew better at 38 than 30°C. Subsequently, a semi-continuous fermentation was performed in a 20L bioreactor for the both selected strains. A total amount of 7.64% and 10.08% (w/v) acetic acid was achieved at the end of fermentation for AF01 and CV01 strains respectively. Interestingly, CV01 was able to produce acetic acid and gluconic acid simultaneously during acetic acid fermentation. This is an important feature for AAB used in vinegar industry. Finally, the enzymatic study showed that the two strains exhibited high ADH and ALDH enzyme activity at either 30 or 38°C as compared to the mesophilic reference strain LMG 1632 which exhibited remarkable thermal inactivation rate. Since the selected strains exhibited appropriate features, it is concluded that they can be considered as industrial starins for production of vinegar from substandard fruits.