Development, Characterization, and Stability of Flavored Water Kefir: Impact of Fermentation and Storage

The increasing demand for functional beverages sparked greater interest in health-promoting craft drinks, highlighting the need to optimize production parameters and assess their stability. This study aimed to develop, optimize, and characterize a grape juice-flavored naturally carbonated water kefir, evaluating its sensory qualities, physicochemical and microbiological stability. Fermentation conditions (F1) were optimized using Central Composite Rotational Design, leading to the selection of 24 h at 30 °C with (6.5% w/v) brown sugar, ensuring efficient pH reduction to safe levels. Sensory analysis selected grape juice as the flavoring agent, and a mixture design coupled with the desirability function determined the optimal formulation as 50% kefired water, 46.4% grape juice, and 3.6% water, resulting in high overall sensory desirability. During 42 days of refrigerated storage (4 °C), the beverage exhibited progressive sugar consumption from residual metabolic activity, a dynamic antioxidant profile characterized by increases in total phenolic compounds and FRAP activity, stability in ABTS activity, and decline in DPPH activity. Lactic acid bacteria counts remained stable during storage, while acetic acid bacteria and yeast populations decreased. Furthermore, pH (~3.30) and alcohol content (~1.86 °GL) remained stable, although the latter requires clear labeling in compliance with regulations for similar fermented beverages.