The Impact of Lactobacillus casei in Soy Yogurt for Healthy Aging

Aging is a physiological process influenced by various factors, including oxidative stress, which causes cell and tissue damage in the body. Oxidative stress can be inhibited by antioxidants. Soy yogurt has high isoflavone content, which has antibacterial and antioxidant potential. The fermentation process with lactic acid bacteria could increase the bioavailability of soy isoflavones. This study aimed to evaluate the effect of the addition of Lactobacillus casei on the antibacterial and antioxidant activities of soy yogurt fermented with Bifidobacterium longum and Streptococcus thermophilus. This study used four varying concentrations (Y0, Y1, Y2, and Y3) of L. casei to study antibacterial activity using the Kirby–Bauer method and antioxidant activity using DPPH and HPTLC methods. Research results statistically showed that the addition of L. casei increased the antibacterial activity of soy yogurt against Staphylococcus aureus and Escherichia coli. Sample Y3 showed the largest inhibition zones, 29 mm against S. aureus and 34.67 mm against E. coli. The antioxidant activity of soy yogurt, as measured by the DPPH method, also increased with the addition of L. casei, with the highest free radical inhibition percentage reaching 75.75% in sample Y3. The HPTLC method showed the presence of genistein, especially in samples Y2 and Y3, with retention factor (Rf) values of 0.57 and 0.59, respectively, which are close to the standard Rf value of genistein (0.59). This study concluded that the addition of L. casei can increase the antibacterial and antioxidant activities of soy yogurt fermented by Bifidobacterium longum and Streptococcus thermophilus.