Antioxidant activity and identification of food proteins by digestive enzyme supplementation and fermentation with Lactobacillus kefiri

Casein, gluten, and soy protein are widely used in food processing for structure, texture, and flavour improvement. These large proteins might be hydrolysed to shorter peptides or amino acids, which provide antioxidant activities through enzymatic and fermentative food processes. Casein, gluten, and soy protein were digested with an enzyme supplement product containing dipeptidyl peptidase IV (DPPIV) and protease in this study. Then, each protein was hydrolysed by Lactobacillus kefiri strain. 2,2, diphenyl 1-picryl hydrazyl (DPPH) radical scavenging activity and reducing power (RP) were measured for undigested and digested samples. According to our results, all proteins were hydrolysed. Soy protein demonstrated the highest IC50 value of DPPH for undigested (2.64 mg/ml) and digested samples (1.56 mg/ml) as well as the highest RP value (0.171 for undigested and 0.234 for digested at 700 nm). On the other hand, casein provided the weakest DPPH radical scavenging activity (1.58 ± 0.041% for undigested and 21.86 ± 0.012% for digested samples). A strong correlation was found between cell growth and antioxidant activity of casein during the microbial fermentation. In addition, the changes in protein expression levels by microbial fermentation were analysed by using sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Mass spectrometry-based protein identification studies revealed that EF-G, DNA-K, and DNA-J were mainly overexpressed indicating L. kefiri adapts itself by changing the overall proteome.