Conversion of coconut water to vinegar by acetic acid bacteria isolated from fruit waste: a two-stage fermentation and quality analysis

• Developed coconut water vinegar using acetic acid bacteria isolated from agro-waste. • Proximate composition and microbial counts confirmed product safety, stability, and enhanced nutritional value. • S4 sample (14°Brix, 10% inoculum) showed highest acetic acid yield, total phenolic content, and titratable acidity. • The study highlighted sustainable waste utilization and offers a scalable model for vinegar production in tropical regions. The mature coconut ( Cocos nucifera L.) water in Bangladesh is significantly underutilized for the production of edible items. This research investigates the viability of coconut water as a significant dietary component by transforming its liquid into vinegar using a two-step fermentation method. Coconut water with varying total soluble solids (TSS) concentrations of 10-14 % were subjected to sequential alcoholic and acetic acid fermentation, yielding S1, S2, S3, and S4 vinegar samples. The findings showed that when the concentration of TSS increased during anaerobic fermentation with Saccharomyces cerevisiae , the alcohol content increased and the pH decreased (p < 0.05). Among the samples, the 14% TSS vinegar (S4) exhibited the greatest acetic acid content (5.8 ± 0.2 g/100 ml), ash content (0.45 ± 0.01 g/100 ml), and phenolic concentration (68.3 ± 1.5 mg GAE/100 ml), confirming its enhanced nutritional profile. In addition, although the number of viable cells was elevated at the beginning of each stage of vinegar production, it began to drop after a few days. Acid fermentation and ageing were the most noticeable changes in the vinegar's colour, flavour, and overall acceptability, according to sensory analyses conducted after each fermentation and month of ageing. This research highlights the possibility of using coconut water waste to make value-added vinegar, offering a novel method to utilize this local resource sustainably.