Influence of thermo- and ethanol-tolerant acetic acid bacteria on the production process of prickly pear (opuntia ficus indica (l.) mill.) vinegar

Resumen

Vinegar produced from different raw materials is used mainly by the world population as a food flavoring and consumed as a healthy beverage. Its production on an industrial scale is subject to constraints, including the low tolerance of acetic acid bacteria (AAB) to the high temperatures and the high concentrations of ethanol. In this study, we used Opuntia ficus-indica L from two Moroccan regions as a basic source to isolate thermo- and ethanol-tolerant AAB strains. The isolation and identification of AABs from this fruit allowed us to select a total number of 140 AAB strains. Ten showed significant production of acetic acid at temperatures up to 40°C and at high concentrations of ethanol up to 12%. The identification of these strains by 16S rDNA gene and internal transcribed spacer regions of the gene 16S-23S rDNA sequencing and by PCR-RFLP allowed us to differentiate these AAB as Acetobacter malorum, Acetobacter cerevisiae, and Gluconobacter oxydans. The influence of these different bacterial inoculums used at fermentation temperatures of 30 and 37°C on the chemical composition and on the antioxidant activity of prickly pear vinegar produced by surface culture, shows a decrease in the content of volatile compounds in the vinegar when the fermentation was carried out at a higher temperature. On the contrary, the highest content of polyphenolic compounds was detected in vinegars produced at 37°C. On the other hand, Prickly pear wine showed greater antioxidant activity than juices or vinegars. The study of these same parameters in the case of submerged culture, also allowed us to show that the concentration of volatile compounds was significantly affected by the fermentation temperature. The lower concentrations were detected as the temperature increased, whereas a minor significant difference was observed when comparing vinegar produced by A. malorum and G. oxydans. Regarding phenolic compounds, significant increases were shown when the temperature increased from 30°C to 40°C. Additionally, vinegar produced by A. malorum had a higher concentration of phenolic compounds than those produced by G. oxydans. The study of the effect of the production method on the quality of vinegar showed that the acetification conditions affect the chemical characteristics of the vinegar by positively affecting the quality and the nutritional value of the vinegar. The volatile compounds are dominated by esters and acids when the submerged culture was used and dominated by alcohols when the surface culture is used. Statistical analysis showed that the amounts of phenolic compounds in vinegar from the surface culture acetification were higher than that from the submerged process. From an efficiency point of view, the submerged process was found to be faster and more efficient in generating a higher concentration of acetic acid in vinegar compared to surface culture. Regarding the study carried out on the sensory properties of the vinegar generated, we have found that some vinegars produced by submerged cultivation are more aromatic with more fruity and floral notes and characteristic sharpness than those produced by surface cultivation. These best vinegars, made from submerged culture, have been obtained using the strains of A. malorum, while those obtained with G. oxydans are characterized by spicy and dried fruit notes.