Effects of second-drying process parameters on the hot-air drying characteristics and quality of congou black tea

Drying is an essential procedure in the processing of fresh tea leaves, in order to enhance the flavors of a tea and ensure its long shelf-life, for making herbal tea. This paper aims to study the effect of temperature and duration on the characteristics and quality of congou black tea during hot-air second drying. An experiment was performed on four second-drying temperatures of 130, 110, 90 and 70 ℃, to determine the contents of biochemical components (water content, catechins, tea pigment, tea polyphenols, caffeine, amino acids, soluble sugar), and thereby to evaluate sensory quality when the tea was dried to 10, 20, 30, 40, 50 and 60 min. The drying characteristics of congou black tea were analyzed under different second-drying temperatures, to obtain the influence of second-drying conditions on the biochemical components and sensory quality of congou black tea. Partial least squares discriminant analysis (PLS-DA) was used to explore the key differential components of second-drying temperature. The results showed that the higher the second-drying temperature, the lower the moisture content of tea, and the higher the drying rate would be, as well the shorter the drying time would be until the moisture content was less than 5%. In terms of drying rate, the drying process at 130 ℃ could be divided into two stages of acceleration and deceleration, while the drying process at 110, 90 and 70 ℃ could be divided into three stages of acceleration, deceleration and constant velocity, where the rapid acceleration and deceleration process were completed within 20min in all second-drying temperature conditions. The effective diffusion coefficient of moisture varied from 1.729×10-8 to 3.648×10-8 m2/s, while the average activation energy was 15.40 kJ/mol. The Page model can well describe the change of moisture with the drying time in the drying process of tea, where the coefficient of determination (R2) ranged from 0.924-0.995, the root-mean-square (RMSE) ranged from 0.008~0.023, the sum of squares (χ2) ranged from 1.043×10-4-7.866×10-4. With the increase of second-drying temperature, the content of some biochemical components would significantly enhance, including (−)-Epogallocatechin, (+)-Catechin, (−)-Gallocatechin, total simple catechins, (−)-Epigallocatechin Gallate, (−)-Gallocatechin Gallate, total ester catechins, total catechins, tea polyphenols, caffeine, whereas, the content of theaflavins, amino acids, soluble sugar would be reduced, where all changed dramatically before drying 20 min. Compared with that of 130 and 110 ℃, the case of the second-drying temperature at 90 and 70 ℃ can better improve the quality of congou black tea. The optimal liquor color effect, aroma effect and taste effect can be achieved in the case of 90 ℃ drying 20-30 min, 70 ℃ drying 40-50 min, 70 ℃ drying 20 min, respectively. The condition of 90 ℃ drying 40 min was the optimum drying parameter of black tea. PLS-DA results showed that four kinds of second-drying temperature conditions could be effectively distinguished, and the caffeine, soluble sugar, amino acid, catechin C and theaflavins were the distinctive characteristic components of congou black tea under different second-drying conditions, which can be served as the control indexes for the drying process of black tea. This finding can provide a theoretical basis to optimize the processing parameters of high-quality black tea.