Impact of different kefir inocula on fermentation of soy and milk substrates and their antimicrobial potential against H. pylori

Kefir is a fermented beverage known for its potential health benefits and nutritional profile. This study evaluated the effects of three kefir inocula - two traditional (K1 and K2) and one commercial freeze-dried (K3) - on the quality of fermented beverages produced from whole milk (K1L, K2L, K3L) and water-soluble soybean extract (WSSE) (K1S, K2S, K3S). Fermented beverages were assessed for physicochemical properties, in vitro gastrointestinal resistance, sensory attributes, and anti-Helicobacter pylori activity. WSSE kefirs reached the fermentation endpoint (pH 4.5) faster, while milk kefirs exhibited higher titratable acidity and a reduction in fat content. Lactic acid bacteria and yeasts from beverages fermented with traditional inocula (K1L, K2L, K1S, and K2S) demonstrated enhanced resistance to simulated gastrointestinal conditions. Milk kefirs were the most accepted by consumers, due to their creamy and whitish appearance, firm texture and pleasant flavor, while WSSE kefirs were less favored due to their yellowish appearance, watery texture and soy flavor. Traditional inocula metabolites inhibited H. pylori (>84%) in both substrates, whereas freeze-dried inoculum showed lower efficacy. This study reveals how inoculum and substrates affect kefir's nutritional, sensory and health properties, highlighting consumer preference for traditional milk kefir and the importance of creating attractive, functional plant-based alternatives.