EFFECT OF COAGULANTS ON ANTIOXIDANT CAPACITY OF MILK PROTEIN CURDS AND THEIR TRYPTIC HYDROLYSATES

This study investigated the influences of Ca lactate precipitation, lactic acid coagulation and renneting of proteins in skimmed milk on the physicochemical properties of milk protein powders. Scanning electron micrographs indicated that milk curd from Ca lactate precipitation had the densest network with the smallest protein particles compared with others. It also had the lowest antioxidant capacity: i.e., 0.27 and 0.05 µmol Trolox equivalent (TE)/mg protein, as measured by oxygen radical absorbance capacity–fluorescein (ORAC FL ) and Trolox equivalent antioxidant capacity (TEAC) assays, respectively ( P  < 0.05). Nevertheless, tryptic hydrolysis for 30 min increased the antioxidant capacity of all milk protein products to 0.85–1.11 and 1.17–1.19 µmol TE/mg protein in ORAC FL and TEAC assays, respectively, regardless of the coagulants used ( P  ≥ 0.05). Although milk protein hydrolysate obtained from Ca lactate milk protein had the lowest ζ‐potential, its antioxidant capacity remained unaffected by self‐association of peptides into large‐sized aggregates ( P  ≥ 0.05). PRACTICAL APPLICATIONS The results showed that Ca lactate milk protein powder could be used as a protein diet containing high Ca that would provide antioxidative peptides upon subsequent tryptic digestion. The study also suggested that milk protein hydrolysates with reasonable antioxidant capacity could be obtained from milk powder prepared from mixed proteins coagulated by Ca lactate, lactic acid or renneting and hydrolyzed by trypsin. This is important for developing countries with majority of the population suffering from lactose intolerance, i.e., having a milk surplus yet lacking the capability of fractionation of mixed milk proteins into individual proteins such as β‐casein, κ‐casein, β‐lactoglobulin and α‐lactalbumin.