The Mystery Of Heavy Bitterness And Astringency Of Green Tea Produced In Sichuan Province & The Method To Modify The Bitterness And Astringency Of Green Tea

Green tea samples which were produced in Sichuan province in spring and summer tea season were studied to discover the mystery of heavy bitterness and astringency in some green teas. High-performance liquid chromatography (HPLC) was employed to analyze the compositions of main tastants of those green teas. Quantitative methods using sensory analysis were used to establish a forecasting model for the assessment of tea bitterness and astringency. Based on comprehensive analysis of chemical compositions and sensory evaluation, the factors causing heavy bitterness and astringency to green tea were investigated. In order to mask the bitterness and astringency of Sichuan green tea, several bitterness inhibitors reported recently were screened, and further a modified spreading and rolling process in Sichuan green tea manufacturing was employed. In the screening bitterness inhibitors, an ion-pair chromatography method was built up to test AMP (adenosine-5'-phosphoric acid) in tea samples.The results from chemical compositions of Sichuan green tea samples have showed that flavonoids which have the characteristic absorption wavelength at 370 nm existed in flavonoid glycosides including rutin as the main form. In Sichuan green teas, the amounts of total catechins ranged between 120.26 and 227.43 mg·g-1, while the amounts of caffeine in different samples ranged from 34.10 to 53.10 mg·g-1. Among catechins, epigallocatechin gallate (EGCG), epicatechin gallate (ECG) and epigallocatechin (EGC) accounted for about 60%,13% and 12%, respectively. The total amounts of free amino acids (AAs) ranged between 14.53 and 41.02 mg·g-1, in which theanine accounted for 31% to 54% is the most abundant free amino acid in tea samples. It is observed that there is significant difference of chemical compositions between summer Sichuan green tea and spring Sichuan green tea. In general, summer teas have higher amounts of catechins and caffeine, and lower amounts of total free AAs compared to those in spring teas. The comparative analysises on spring and summer tea samples from five local tea varieties have showed that spring tea have higher amounts of GCG, His, Met and Theanine, while summer teas have higher amounts of caffeine, EC, EGCG,Ile, Leu, Lys, Phe, and Val, all of which were reported as bitter compounds except Lys. Tea infusion which was prepared according to the standard sensory assessment method was tested, and the results from chemical analysis and sensory assessment showed that the flavor of Sichuan green teas was mainly affected by caffeine (4.0-6.9 times of threshold), EGCG (6.0-15.5 times of threshold), ECG (0.7～2.4 times of threshold), EGC (0.8～4.1 times of threshold) and flavonoid glycosides (rutin, etc.). The concentrations of other tastants discussed in this paper were lower than their thresholds, and they had limited contribution to the flavor of Sichuan green teas.The results from the sensory assessments have showed that caffeine is a bitter substance, while EGCG shows both bitter and astringent flavor. Rutin could enhance the bitterness of caffeine, but had no effect on bitterness and astringency of EGCG, while caffeine could increase astringency of EGCG. The mixed solution of caffeine and EGCG showed higher bitterness than any single compound of them, but the bitterness of the mixture could not reach the sum of the two compounds showed. Forecasting models which was based on the sensory assessments were established and the results have showed that the models could interpret 78% to 137% bitterness and 60% to 101% astringency of tea samples.On the whole, bitterness and astringency of tea infusion were directly determined by the concentrations of EGCG, ECG, EGC, caffeine and flavonoid glycosides, and flavor were indirectly affected by these factors which could affect the amounts in made teas and extracting rate of the bitter and astringent compounds when tea brewing. Although some tea samples had high amount of bitter and astringent compounds, the tea infusion showed no obviously bitterness and astringency due to slight rolling in tea processing which led to the low water extracting rate at tea brewing. The results showed that the sensory attributes could be modified by rolling extent in tea processing.The masking effects of some compounds reported recently on bitterness were studied. The results showed that AMP was an ideal bitterness inhibitor, but the endogenous AMP in tea samples had no obvious effects on bitterness of tea due to its limited amounts (3.71-6.40 mg·g-1). For tea AMP assay, an ion-pair chromatography method with simple pre-treatment was established, which had a wide linear range with high accuracy and good reproducibility. It suggests that increasing the endogenous amounts of AMP by the particular treatment in tea plants cultivation could decrease the bitterness and astringency of green tea.The extent of spreading and rolling in green tea processing had significant effects on the bitterness and astringency of tea infusion. The spreading procedure could decrease both bitter and astringent compounds especially EGCG and ECG in final made teas. The concentrations of bitter and astringent compounds in tea infusion increased when the rolling extent was increased. To sum up, long-time spreading and slight rolling in green tea processing were recommended to decrease the bitterness and astringency of final made teas.