Basic nutritional investigationEffects of heat treatment on structural modification and in vivo antioxidant capacity of soy protein

ObjectiveThe present study identified the effects of heat oxidation on protein carbonyl content and α,α-diphenyl-β-picrylhydrazyl (DPPH) free radical-scavenging activity in soy protein. The changes on antioxidant status in male mice fed a heat-oxidized diet were also investigated.MethodsSoy protein heated at 100°C for 30, 60, and 90 min was used to determine the protein carbonyl content and DPPH free radical-scavenging activity in vitro. Male KM mice (3 wk old) were fed a normal diet, an oxidized diet (HD) containing 12% heat-oxidized soy protein, or an HD supplemented with 0.1% lipoic acid. After 4 wk of feeding, apparent digestibility, reactive oxygen species, malondialdehyde, and total antioxidant capacity levels were measured. The antioxidant enzyme activities in serum and tissues were also assayed.ResultsHeat-oxidized soy protein showed a significant increase in protein carbonyl formation and a decrease in DPPH free radical-scavenging activity. The HD induced a significant decrease in food intake and apparent digestibility of dry matter and crude protein in mice. Increased levels of reactive oxygen species and malondialdehyde in serum and tissues accompanied by decreased total antioxidant capacity and antioxidant enzyme activities were also observed in HD-fed mice. These changes were partly restored in the lipoic acid-treated group.ConclusionHeat-oxidized soy protein showed a relatively higher protein carbonyl content and a loss of its free radical-scavenging activity in vitro. The heat oxidation also led the soy protein to generate reactive oxygen species, decrease the antioxidant status, and induce redox imbalance in vivo. The heat oxidation of food protein could be a potential health risk in humans.