Functional properties and structural profiles of water-insoluble proteins from three types of tea residues

Green tea water-insoluble protein (GP), oolong tea water-insoluble protein (OP) and black tea water-insoluble protein (BP) were extracted from residues of green tea, oolong tea and black tea by an alkaline method, respectively. The foaming stability and emulsifying capacity of GP were better than that of OP and BP, but the oil absorption capacity (3.00 +/- 0.28 mL/g) and foaming capacity (41.00 +/- 1.41%) of GP was significantly lower than that of OP and BP. Furthermore, the analysis of protein surface hydrophobicity, amino acid composition, secondary structure, zeta potential and contact angle of different tea water-insoluble proteins (TWIPs) revealed that TWIPs were negatively charged with high hydrophobicity. Moreover, the random coil content (26.70 +/- 0.10%) of GP was higher than those of OP and BP in favor of emulsifying capacity. At the oil-water ratio of 5:5 and TWIP concentration of 2.0 g/100 mL, the creaming index of GP, OP and BP emulsions was 33.55 +/- 0.93%, 40.79 +/- 5.58% and 40.13 +/- 0.93% after 6 d. The emulsifying stability of GP better than that of OP and BP was due to the small size (112.67 +/- 2.52 mu m) and high zeta potential (-36.77 +/- 2.47 mV) of GP emulsion droplets. These results suggest that TWIPs, especially GP, are the potentially food-grade emulsifiers for preparing emulsions.