Effects of electromagnetic roller-hot-air-steam triple-coupled fixation on reducing the bitterness and astringency and improving the flavor quality of green tea.

Despite the importance of fixation in determining green tea quality, its role in reducing the bitter and astringent taste of this beverage remains largely unknown. Herein, an electromagnetic roller-hot-air-steam triple-coupled fixation (ERHSF) device was developed, and its operating parameters were optimized (steam volume: 20 kg/h; hot-air temperature: 90 °C; hot-air blower speed: 1200 r/min). Compared with conventional fixation treated samples, the ratio of tea polyphenols to free amino acids and ester-catechins to simple-catechins in ERHSF-treated samples was reduced by 11.0% and 3.2%, reducing bitterness and astringency of green tea; amino acids, soluble sugars, and chlorophyll contents were significantly increased, enhancing the freshness, sweetness, and greenness; the color indexes, such as L/L* value of brightness and -a/-a* value of greenness, were also improved, and ERHSF-treated samples had the highest sensory scores. These results provided theoretical support and technical guidance for precise quality improvement of summer-autumn green tea.

New insights into the effect of fermentation temperature and duration on catechins conversion and formation of tea pigments and theasinensins in black tea.

BACKGROUND: The phenol oxidative pathway during fermentation remains unclear. To elucidate the effect of fermentation on phenol conversion, we investigated the effects of fermentation temperature and duration on the conversion of catechins and the formation of theasinensins (TSs), theaflavins (TFs), thearubigins (TRs), and theabrownins (TBs). RESULTS: During fermentation, TSs formation increased initially and then decreased. Long fermentation durations were unfavorable for liquor brightness (LB) and resulted in the production of large amounts of TRs and TBs. Low fermentation temperatures (20 °C and 25 °C) favored the maintenance of polyphenol oxidase activity and the continuous formation of TFs, TSs, and TRSI (a TRs fraction), resulting in better LB and liquor color. Higher temperatures (30 °C, 35 °C, and 40 °C) resulted in higher peroxidase activity, higher oxidative depletion rates of catechins, and excessive production of TRSII (a TRs fraction) and TBs. Analysis of the conversion pathway of polyphenolic compounds during fermentation showed that, during early fermentation, large amounts of catechins were oxidized and converted to TFs and theasinensin B. As fermentation progressed, considerable amounts of theaflavin-3'-gallate, theasinensin A, theaflavin-3-gallate, theaflavin-3,3'-digallate, and theasinensin C were produced and then converted to TRSI; in the final stage, TRSII and TBs were converted continuously. CONCLUSION: Different fermentation temperature and duration combinations directly affected the type and composition of phenolic compounds. The key conditions for controlling phenolic compound conversion and fermentation direction were 60 or 90 min and 25 or 30 °C. Our study provides insights into the regulation of phenolic compound conversion during black tea fermentation. © 2021 Society of Chemical Industry.

Aroma dynamic characteristics during the process of variable-temperature final firing of Congou black tea by electronic nose and comprehensive two-dimensional gas chromatography coupled to time-of-flight mass spectrometry.

The drying technology is crucial to the quality of Congou black tea. In this study, the aroma dynamic characteristics during the variable-temperature final firing of Congou black tea was investigated by electronic nose (e-nose) and comprehensive two-dimensional gas chromatography coupled to time-of-flight mass spectrometry (GC × GC-TOFMS). Varying drying temperatures and time obtained distinctly different types of aroma characteristics such as faint scent, floral aroma, and sweet fragrance. GC × GC-TOFMS identified a total of 243 volatile compounds. Clear discrimination among different variable-temperature final firing samples was achieved by using partial least squares discriminant analysis (R2Y = 0.95, Q2 = 0.727). Based on a dual criterion of variable importance in the projection value (VIP > 1.0) and one-way ANOVA (p < 0.05), ninety-one specific volatile biomarkers were identified, including 2,6-dimethyl-2,6-octadiene and 2,5-diethylpyrazine with VIP > 1.5. In addition, for the overall odor perception, e-nose was able to distinguish the subtle difference during the variable-temperature final firing process.

Far-Infrared Optimization of the Fragrance-Improving Process with Temperature and Humidity Control for Green Tea.

In this study, a Box-Behnken design was used to explore the effect of a new technology on green tea fragrance improvement and to optimize fragrance-improving with a bilayer far-infrared fragrance-improving machine with temperature and humidity control. Based on the results of previous single-factor experiments, the main biochemical composition and sensory evaluation scores of the fragrance-improved samples were used as investigation indices. The new fragrance-improving technology was compared with the traditional far-infrared fragrance-improving process, roller pot fragrance improvement, and hot air rotary fragrance improvement. The results show that the optimal parameter combination of the new technology consists of a temperature of 128.00 °C, relative humidity of 70.00 g/h, and transmission speed of 435.00 r/min. With these process parameters, the amino acids, tea polyphenols, flavonoids, soluble sugar, catechins, and caffeine in the fragrance-improved samples reached 3.86%, 32.29%, 5.59%, 4.45%, 8.97%, and 2.75%, respectively. The quality material weight value was 11.72%. The shape, color, taste, and aroma of the fragrance-improved samples made using these parameters were found to be best, with a sensory quality score of 87.40, which is significantly higher than that of other fragrance-improving methods. The energy consumption was 0.19 RMB/kg, which was reduced by more than 50% compared with the other methods, and the production efficiency was more than 30% higher than the traditional methods. This new far-infrared fragrance-improving technology overcomes the yellowish and grayish color of fragrance-improved tea samples that is caused by the traditional fragrance-improving approach, and will provide technical guidance for actual green tea production. PRACTICAL APPLICATION: Our proposed approach innovatively integrates humidity and rotational speed as factors for fragrance improvement in Chinese tea process. The findings of this work provide new technical for fragrance improvement processes.