The effect of milk alpha-casein on the antioxidant activity of tea polyphenols

• Milk alpha-casein showed minor effect on the antioxidant activity of tea polyphenols.
• Hydrophobic contacts play a major role in casein–polyphenol complexation.
• Larger and bulkier polyphenols showed more antioxidant capacity than smaller ones.
• Different methods revealed differences in antioxidant activity of tea polyphenols.

In this study, we report how the antioxidant capacities of major tea polyphenols are affected by their interactions with milk alpha-casein (milk protein) using three complimentary oxidation methods: ABTS+ radical cation scavenging, cyclic voltammetry and lipid peroxidation inhibition. We found that using the ABTS+ assays, the antioxidant activity of all polyphenols was lowered by 11–27% in the presence of caseins. Using cyclic voltammetry, the overall current measured at the electrode was decreased by the presence of the protein, from 21% to 61%. The peak potentials were also shifted to higher values varying from 13 to 41 mV. However, using lipid peroxidation method, we noticed of the antioxidant activity of all the polyphenols changed (from 6% up to 75%) after the addition of alpha-casein. The results show using this method the larger gallate esters containing polyphenols epicatechingallate (ECG) and (epigallocatechingallate (EGCG) were less affected by the presence of casein than smaller polyphenols catechins (C), epicatechin (EC) and epicgallocatechine (EGC). Alpha-casein caused a small effect on the chain breaking antioxidant capacity of theaflavins as well. Therefore, casein has different effects on the overall antioxidant capacities of tea compounds depending on the methods used. We aim to understand those results with the types of protein–polyphenol interactions that take place in various settings and their effects on the antioxidant capacities of those compounds.

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