Bovine milk antioxidant properties: effect of in vitro digestion and identification of antioxidant compounds

Milk proteins contained encrypted in their sequence biologically active components that can be released by enzymatic hydrolysis. Among the biological activities recognized in milk components, the antioxidant activity is of great interest. The objective of the present study was to analyse the antioxidant properties of whole, semi-skimmed and skimmed milk during simulated gastrointestinal digestion and to identify the compounds responsible for the antioxidant activity. Simulated digestion increased the 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid (ABTS)+ radical scavenging activity of milk. In digested whole milk, the main contribution to ABTS+ radical scavenging activity was due to high molecular weight fraction (>3 kg.mol−1). For semi-skimmed and skimmed milk, the main contribution was due to low molecular weight fraction (<3 kg.mol−1). Twelve major peaks were collected from low molecular weight fraction of digested skimmed milk by reversed-phase high-performance liquid chromatography and evaluated for their ABTS+ radical scavenging activity. Among the different fractions, three (F2, F3 and F5) showed high ABTS+ and hydroxyl radical scavenging activity and lipid peroxidation inhibitory capacity. The compounds (free amino acids and peptides) present in these fractions were identified with nano-LCQTOF MS/MS analysis. The amino acids tryptophan and tyrosine seemed fundamental in the ABTS+ and hydroxyl radical scavenging capacities whereas the amino acids phenylalanine and histidine played an important role in the lipid peroxidation inhibitory activity of the peptides. The results reported in this study suggested that milk proteins could act as a carrier for the delivery of antioxidant compounds in the gastrointestinal tract possibly protecting the gastrointestinal tract itself from the oxidative damage.