Chemical, Microbiological, and Functional Characterization of Kefir Produced from Cow’s Milk and Soy Milk

Kefir is a functional beverage that contains lactic and acetic acid bacteria (LAB, AAB) and yeasts. This work's aim was to study the chemical, microbial, and functional characteristics of kefir produced from cow's milk and soy milk. After fermentation, free amino acids were 20.92 mg 100 mL^-1 and 36.20 mg 100 mL^-1 for cow's milk and soy milk kefir, respectively. Glutamic acid was majority in both, suggesting that microbial proteolysis leads to an increase in free amino acids including glutamic acid. 10⁸-10⁹ CFU mL^-1 LAB, 10⁶-10⁷ CFU mL^-1 AAB, and 10⁶-10⁷ CFU mL^-1 yeasts were counted in cow's milk kefir, whereas soy milk kefir contained greatly lower yeasts and AAB. <i>Lactococcus lactis</i>, <i>Kazachstania unispora</i>, and <i>Saccharomyces cerevisiae</i> were isolated as major microorganisms in both kefirs. <i>Acetobacter orientalis</i> only existed in cow's milk kefir. Cow's milk and soy milk showed ACE inhibitory activity, which significantly increased after fermentation. Both kefirs also exhibited antioxidant activity and bactericidal activity against <i>Escherichia coli</i>, <i>Salmonella</i> Typhimurium, and <i>Staphylococcus aureus</i>.