The Influence of Heat Treatment on Interaction of Green Tea Flavonoids with Milk Proteins

Several heat treatment norms are applied in the dairy industry during the manufacture of various products. The knowledge on the influence of temperature on the proteins-flavonoid binding is critical for optimization of process conditions. The aim of this study was to establish the effect of heat treatment on the interaction between milk proteins and flavonoids and also determine when to apply heat treatment. Reverse-phase high-performance liquid chromatography (RP-HPLC) analysis revealed that free flavonoids decreased in the presence of milk proteins and a binding ratio (%) was calculated based on this decrease. Three different heat treatment norms were selected (80, 85 and 90 °C  10 min). Analyses were carried out in both sodium caseinate and skimmed milk samples to observe the possible effect of milk serum proteins on the interaction. The binding ratio between flavonoids and proteins was found to be higher in the samples where green tea extract (GTE) was added before heat treatment than in the samples where GTE was added after heat treatment in both the skimmed milk and sodium caseinate systems. Results of protein surface hydrophobicity (PSH) index and protein partition also demonstrated that heat treatment must be applied after the addition of GTE to improve the amount of GTE binding to milk proteins.