Antimicrobial activity, antioxidant capacity and organic acid synthesis of fruit drink fermented by kefir grains

Vodeni kefir je blago kiseli, nisko alkoholni fermentirani napitak, čija fermentacija započinje dodatkom vodenih kefirnih zrnaca. S visokim koncentracijama šećera i malim udjelom aminokiselina, vodeni kefir predstavlja kompleksnu hranjivu podlogu. U ovom radu, proces fermentacije vodenog kefira proveden je tijekom 14 dana kako bi se otkrila dinamika združene kulture bakterija mliječne kiseline (rod Lactobacillus), octene kiseline (rod Acetobacter) i kvasaca (vrste Saccharomyces cerevisae i Dekkera bruxellensis), kao i kinetika potrošnje supstrata i proizvodnje metabolita. Glavni supstrat, saharoza, je u potpunosti metaboliziran nakon 6 dana fermentacije, što se podudara s proizvodnjom većeg dijela polisaharida vodenih kefirnih zrnaca. Glavni metaboliti fermentacije bili su mliječna i glukonska kiselina. Octena kiselina i etanol proizvedeni su u niskim koncentracijama. Najveći dio ovih metabolita nastao je tijekom prvih 96 sati fermentacije, pri čemu se pH smanjio s 3,80 na 3,52. Antimikrobna aktivnost je testirana na bakterijama vrste Escherichia coli, Bacillus subtilis, Salmonella typhimurium, Staphylococcus aureus, te kvascu vrste Candida albicans. Uočena je izvrsna antimikrobna aktivnost na bakterije i kvasac, no ne i na bakteriju vrste S. aureus. Za dokazivanje antioksidacijskog kapaciteta fermentiranog napitka provedena su tri određivanja: redukcijska snaga, sposobnost vezanja radikala i uspješnost keliranja iona željeza. Svi uzorci pokazali su izvrstan antioksidacijski kapacitet. Rezultatima ovog istraživanja je potvrđeno da je dobiven voćni napitak izvrsni izvor antioksidacijskih sastojaka.Water kefir is a mildly sour, low alcoholic fermented beverage, of which the fermentation is started with water kefir grains. With its high sugar content and low amino acid concentration, water kefir as a medium represents a demanding habitat. In this work, a water kefir fermentation process was followed during 14 days to unravel the symbiotic community dynamics of lactic acid bacteria (genus Lactobacillus), acetic acid bacteria (genus Acetobacter), and yeasts (species Saccharomyces cerevisiae and Dekkera bruxellensts), as well as the kinetics of substrate consumption and metabolite production. The major substrate, sucrose, was completely converted after 6 days of fermentation, which coincided with the production of the major part of the water kefir grain polysaccharide. The main metabolites of the fermentation were lactic and gluconic acid. Acetic acid and ethanol were produced at minor concentrations. The major part of these metabolites was produced during the 96 h of the fermentation, during which the pH decreased from 3.80 to 3.52. The antimicrobial activity was tested against bacteria Escherichia coli, Bacillus subtilis, Salmonella typhimurium, Staphylococcus aureus, as well as yeast Candida albicans. Antimicrobial activities were observed in the fermented samples against the investigated bacteria strains and C. albicans but S. aureus was not inhibited. To determine the antioxidant capacity of the fermented beverage, three determinations were made: reducing power, the radical scavenging ability, and the chelation efficiency of iron ions. All the samples showed excellent antioxidant capacity. The results of this study confirmed that the fruit drink obtained is an excellent source of antioxidant ingredients