Berry Kefirs as Functional Non-dairy Beverages: Linking Microbial Activity, Phenolic Profiles, and Immunomodulatory Properties

The present study investigated the physicochemical, sensory, and biological properties of non-dairy kefirs produced by fermenting strawberry, blueberry, and blackberry juices with kefir grains. Fermentation was performed at 21 ± 1 °C for 48 h, followed by refrigerated storage at 4 ± 1 °C for 7 days. All fruit kefirs exhibited a decrease in total soluble solids and pH, and an increase in lactic and acetic acids, confirming the active metabolism of the kefir consortia. Viable mesophilic and lactic acid bacteria counts increased from 2.4–4.2 to 7.0–9.0 log CFU/mL after fermentation and remained stable during refrigeration, whereas microbial viability declined in the water kefir control. Polyphenolic profiling by HPLC-QTOF revealed extensive transformation of anthocyanins, phenolic acids, and flavonols; the blackberry kefir retained the highest content of cyanidin derivatives and showed greater phenolic stability. Total phenolic compounds and antioxidant activity values decreased after fermentation (up to 40%) but remained stable during refrigeration in the blackberry and blueberry kefirs, whilst being negligible in the water kefir control. Sensory evaluation indicated high consumer acceptance (flavour ≈4/5), particularly for the blueberry kefir, with flavour emerging as the main driver of acceptability, and polyphenol composition (notably total anthocyanins and TPC) shaping sensory quality. In vitro assays using HCT116-Dual reporter cells demonstrated the anti-inflammatory potential of preferentially strawberry juice, and after fermentation and storage under basal and inflammatory conditions. Blackberry kefir partly inhibited TNF-α-induced IL-8 secretion, consistent with its anthocyanin profile. These findings demonstrate that fruit-based kefirs are promising non-dairy fermented beverages in which the fruit matrix enhances microbial performance and phytochemical stability, representing a new generation of naturally fermented beverages with health-promoting potential. • Berry juices were successfully fermented into non-dairy kefirs using kefir grains. • Fermentation led to polyphenols losses, with 35–60% reductions in anthocyanin. • Blueberry kefir showed the highest flavour and overall sensory acceptability. • Fermentation reduced anti-inflammatory potential, albeit partially in blackberry kefir. • Fruit matrix and fermentation dynamics determine kefir bioactivity and stability.