The nutrient profiling of cow and goat milk kefir and their cytotoxicity evaluation of crude proteins evolved with biogenic silver nanoparticles

Fermented beverages and foods are essential components of the human diet, providing crucial nutrients and contributing to disease prevention. Lactic acid bacteria and yeasts play critical roles in fermentation, with specific strains, termed probiotics, offering distinct health benefits. This study focuses on the production of kefir from goat and cow milk, offering a comprehensive analysis of their nutritional profiles. Our objective is to provide valuable insights into the unique characteristics of goat milk kefir (GK) and cow milk kefir (CK) by examining their pH values, titratable acidity, total free amino acid content, and total dry matter over an eight-day period. Additionally, this research focused on protein profiling using SDS-PAGE electrophoresis to elucidate the detailed water-soluble protein extracts (WSPE) composition of cow and goat milk kefir. Beyond these analytical endeavours, our emphasis on protein profiling lays the groundwork for exploring the cytotoxicity of proteins treated with biogenic silver nanoparticles (B-AgNPs). During the eight days of storage at 8 °C, the pH values and titratable acidity of both cow and goat milk kefir decreased. There was no significant difference between cow and goat milk kefir in terms of these parameters. However, the amino acid content increased, ranging from 1.35 to 2.45 for cow milk kefir and from 2.15 to 3.98 for goat milk kefir. The titratable acidity decreased from 0.92 to 0.74 and 0.95 to 0.81 for cow milk kefir and goat milk kefir, respectively. In terms of protein content, the Bradford method revealed a total protein range of 1.35 to 2.45 for cow milk kefir and 0.70 to 1.30 for goat milk kefir. When analysing protein profiles using SDS-PAGE, no significant difference was observed between cow and goat milk kefir, despite the initial variation indicated by the Bradford method. When assessing the antimicrobial activity of proteins treated with B-AgNPs, only Gram-positive bacteria showed an effect, whereas Gram-negative bacteria did not exhibit any response. CK-WSPE and GK-WSPE showed no toxic effects on normal foreskin fibroblast (BJ, CRL-2522) cells, with proliferative concentrations determined as 62.5 and 500 µg/mL, respectively. Remarkably, while B-AgNPs exhibited high toxicity individually, this toxicity was mitigated when combined with CK-WSPE and GK-WSPE in the 1:2 mixtures. Only CK-WSPE demonstrated proliferation at a concentration of 15.625 µg/mL after being mixed with B-AgNPs, which is lower than the concentration required when CK-WSPE is applied independently to treat the cells.