Abstract:

Casein consists of about 80% of the proteins of milk or milk powder, which is susceptible to oxidation during
food processing and storage. Fenton reaction-induced casein oxidation and the associated chemical and structural changes
were investigated. The effects of the main components in the Fenton reaction system as well as reaction temperature and
time on casein oxidation were investigated by using sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE),
and the concentrations of Fe2+, ascorbic acid, and H2O2 were optimized. Fenton reaction-induced casein oxidation and its
chemical and structural changes were assessed by changes in solubility, degree of protein denaturation, and total sulfhydryl
and carbonyl contents of casein. Results showed that casein was oxidized to a great degree in the Fenton system containing
0.8 mmol/L FeSO4, 1 mmol/L EDTANa2, 10 mmol/L H2O2, 0.8 mmol/L ascorbic acid, 5 mg/mL casein after incubation at
37 ℃ for 4 h. The electrophoretic patterns indicated decreases in band intensity of casein and increases in density of the high
molecular weight (HMW) protein bands. These changes were greater with increasing concentrations of Fe2+, ascorbic acid,
and H2O2 in the Fenton reaction system as well as temperature and time. Smaller changes in the casein bands were seen with
increasing its concentration. Solubility and total sulfhydryl content were decreased, and degree of protein denaturation and
carbonyl content were increased after oxidization of casein. In conclusion, casein oxidation leads to cross-linking into high
molecular weight (HMW) substances and modification of functional groups of amino acid residues, which consequently
results in decreased protein solubility and increased protein denaturation degree.

Key words: casein, Fenton reaction system, oxidation, optimization, chemical and structural properties