| Retaining the original color and aroma of green tea beverage, and preventing it from precipitating are the key issues of green tea beverage process to be resolved. The precipitation phenomenon of green tea beverage is one of the important factors restricting the development of the market. There were many researches about the quality of tea beverage. However, seeking technological breakthroughs to produce characteristic and natural tea beverage, was a multidisciplinary and multi-field topic. The purposes of this paper were to study the main components participated in tea cream, set models and study the mechanism of interaction between TP and protein. The main contents and results are as follows:1. Beverage made from raw fragrant green teas was studied before and after sterilization with high temperature, and stored at the temperature of 37 ℃ and 4 ℃ for 7d, respectively, by measuring changes of color, transmittance, particle size, amounts and morphology of the cream, as well as polyphenols, amino acids, protein, flavonoids, total sugar, caffeine and catechin components. The results showed that (1)4 ℃ storage to a certain extent could effectively alleviate the color deterioration and turbidity of tea drink, and 37 ℃ storage would accelerate etiolation and turbidity of tea drink. (2) Different storage methods had significant impact on the other biochemical components. Tea drink could be maintained higher content of biochemical components at 4 ℃ and the flavor could be deteriorated at 37 ℃. (3) There were less amounts of precipitation at 4 ℃ than 37 ℃. The cream was gathered by many small molecules at 4 ℃ storage and large molecules at 37 ℃ storage.2. Model systems combined with tea polyphenols and caffeine, polyphenols and protein, caffeine and protein, polyphenols and caffeine and protein were set for studying the mechanism of green tea cream. The results showed that:Polyphenols-caffeine system:the transmittance of the solution decreased from 95.5% to 24.7%, and the amounts of the solution increased from 8 mg/L to 244 mg/L with the increase of polyphenols and caffeine after storing for 30 days. These changing trends strengthen with the concentrations of polyphenols and caffeine compound rising. Based on the test results, the stability of the green tea infusion was relatively higher when polyphenols concentration was less than 1200 mg/L and caffeine concentration was less than 200 mg/L.Polyphenols-protein system:the amounts of the solution increased with the increase of polyphenols and protein after storing for 7 days. This changing trend strengthen at the beginning but then weaken with the concentrations of polyphenols and protein compound rising.Caffeine-protein system:There was no tea cream.Polyphenols-caffeine-protein system:The solution was clear and transparent when the polyphenols concentration was 1200 mg/L, caffeine concentration was50mg/L, protein concentration was 15 mg/L after storing for 7 days.Based on test results, the interaction between TP and protein was the biggest effect of tea drinks clarity. So, the interaction between TP and protein might be the main cause of tea cream. By comparing the polyphenols-caffeine-protein and polyphenols-protein models, we had concluded that the amounts of tea cream had no increase when the polyphenols concentration was 400 mg/L. The amounts of tea cream had significant increase when the polyphenols concentrations were 800 mg/L and 1200 mg/L, and the amount of tea cream was biggest when caffeine concentration was 200 mg/L. What’s more, the increase ratio of tea cream was bigger when the polyphenols concentrations were 1200 mg/L than 800 mg/L. Hence, we should choose green tea that contains low content of caffeine and protein as raw materials to keep tea drink clear.3. Based on previous results (tea cream was more likely to produce in polyphenols-protein system), it was important for us to understand the mechanism of interaction between TP and protein. The fluorescence and ultraviolet spectroscopy studies were designed to examine the mechanism of interaction between BSA and (-)-epigallocatechin-3-gallate (EGCG), (-)-epicatechin gallate (ECG), the major components of TP. The results showed that:increasing concentrations of EGCG/ECG resulted in increased quenching of BSA fluorescence, and the fluorescence quenching was the static quenching. The fluorescence quenching results showed that the binding constants of polyphenols bind protein were KECG-BSA=2.4 (±0.6) ×107 M-1 and KEGCG-BSA=10.7(±0.8) ×107 M-1. The binding constants of EGCG-BSA complexes were bigger than ECG-BSA, which could be due to the presence of more OH groups associated with the EGCG. The binding sites n were 1.1 (ECG) and 1.3 (EGCG), respectively. The synchronous fluorescence spectra showed that EGCG/ECG should be located or near the Trp residues when bound to BSA.4. Model systems combined with tea polyphenols and caffeine, polyphenols and protein, caffeine and protein, polyphenols and caffeine and protein were set for studying the mechanism of green tea cream. The results showed that:(1) Catechins with more phenolic hydroxyl groups were more likely to participate in formation of a precipitate; (2) The interaction between TP and protein might be the main cause of tea cream. To reduce the content of protein could reduce the amount of precipitation; (3)With a large excess of BSA relative to TP, each TP molecule should be able to bridge between two BSA molecules, but it would be unlikely that BSA would be further bridged to others. This would result mainly in BSA dimers, smaller aggregates, and less average size. To increase the content of protein could reduce the amount of precipitation... |
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