Effect of Recovery Methods on the Characteristics of Antioxidative Milk Peptides

Abstract

This study investigated the influences of peptide recovery methods after tryptic (E.C.3.4.21.4) hydrolysis, together with the pH before freeze-drying on the antioxidant capacity of milk peptides. Isoelectric precipitation of milk hydrolysate yielded hydrophobic milk peptides while 50% ethanol
(EtOH) precipitated the hydrophilic ones. The hydrophobic peptides had significantly higher antioxidant capacity than the hydrophilic peptides (P < 0.05). The antioxidant capacity of hydrophobic peptides was 1.24 and 1.15 μmol Trolox equivalent (TE)/mg protein measured by the oxygen radical absorbance capacity-fluorescein (ORACFL) and Trolox equivalent antioxidant capacity (TEAC) assays, respectively. However, the hydrophilic peptides had ORACFL antioxidant capacity of 0.62 and TEAC antioxidant capacity of 0.37 μmol TE/mg protein. Nevertheless, the hydrophobic peptides were prone to flocculation and aggregation, particularly when freeze-dried at an acidic pH of 2.0 in lactose excipient. However, freeze-drying in lactose excipient at pH 6.5 resulted in reconstituted hydrophobic peptides with higher ORACFL antioxidant capacity than the freshly prepared peptides. This study suggested that the high antioxidant capacity of hydrophobic peptides could play significant roles as natural antioxidants after tryptic digestion in the gastrointestinal tract.