Effects of Brewed Tea on Flavor Quality, Protein Structure and Glycemic Index of Food Product

Objective Effects of using brewed tea to replace plain water in preparing custard and in cooking rice to improve the sensory and healthful qualities of the products were investigated.

Method Brewed green and black teas were used as an ingredient in making custards and in cooking rice. A sensory panel was employed to evaluate the eating quality of the custard and rice. Scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) were applied to examine the changes on the microstructure and egg protein of the custard and the molecular structure of the rice protein. An in vitro digestion test was conducted to determine the glycemic index (GI) of the prepared rice that might be affected by the altered protein molecular structure.

Result Both tea-containing custards and cooked rice tasted fresh and mellow with a pleasant aroma. Black tea made the mouthfeel of the products more tender and smoother than control, which used plain water as an ingredient or for the cooking. Green tea accentuated the sweetness taste with a hint of the typical green tea aroma and chestnut flavor note and increased the elastic texture of the custard and rice. The SEM and FTIR images on the products showed that (1) the particles of the black tea-containing custard were smaller than the green tea-containing counterpart or control, and the surface appeared smooth and uniform; (2) the absorption peaks of protein amide I band of the custards containing either green or black tea were significantly higher than that of control, while the relative content of random coils in protein secondary structure increased and that of α-helix decreased over control; (3) the increase on random coils and decrease on α-helix in the egg protein of the black tea custard were greater than those of the green tea counterpart; (4) the cross-sectional microstructure of green tea custard showed fewer holes with a surface smoother and more compact than that of control; (5) the peak position of 1022-1000 cm⁻¹ of the rice cooked with the brewed green tea shifted toward the high band, while that with the black tea to the low band, indicating an increased orderliness of starch by green tea addition could result in an impeded rice digestion; (6) at 1080 cm⁻¹, the bandwidth on the rice cooked with either the brewed green or black tea was wider than that of control, indicating that more hydrogen bonds might be formed in the starch due to the presence of certain chemical compounds in the teas; and (7) the addition of green or black tea could significantly reduce GI of rice.

Conclusion By using the brewed black tea in making custard or in cooking rice, significant improvements on sensory quality of the finished products could be achieved. The addition altered the protein structure that may otherwise cause allergies in humans from rice consumption. Green tea added pleasant aroma and flavor to custard and rice which would enhance the commercial value of the products. Both green and black tea could help lower the GI of rice, which is a healthful benefit for people who are diabetic or concerned with elevated serum glucose.