Influences of first-drying methods on the quality of Congou black tea using partial least squares-discrimination analysis

Processing of Congou black tea usually includes five procedures: withering, rolling, fermentation, first-drying and second-drying. Water content of a tea leaf is an important indicator for the black tea processing, particularly water loss most in first-drying, from 58%-64% of the fermented leaves to 20%-25% of the first-drying leaves. With a large amount of evaporation from leaf water and a rapid rise of leaf temperature, the enzymatic reaction in the leaves can be inhibited, while a large number of violent thermochemical reactions occur, in order to promote the transformation and upgrade of quality substances, and thereby to obtain the formation of excellent aroma, taste, color and other qualities of black tea. In this paper, two leaves and a bud of Fuding Dabai species were selected as raw material to make trial production of Congou black tea, in order to explore the influence of rotary pot-hot air coupling first-drying with electromagnetic heat on first-drying characteristic and quality formation of Congou black tea. Four methods were used, including chain plate hot air first-drying (CHFD), box hot air first-drying (BHFD), rotary pot first-drying (RFD), and rotary pot - hot air coupling first-drying with electromagnetic heat (RHFD). Multiple features and indicators were compared, such as 29 kinds of non-volatile tea indexes (polyphenols, catechins, tea pigments, soluble sugar, caffeine, amino acids, et al), 114 kinds of volatile aroma indexes that tested by GC-MS/MS, 10 kinds of evaluation indexes of leaf shape and liquor color, quality evaluation of artificial sensory and performance indexes of first-drying methods (production efficiency, energy cost, thermal efficiency, etc.). Partial least squares-discrimination analysis (PLS-DA) was then used to analyze the influence of first-drying methods on the taste, liquor color, and aroma quality formation of high-quality Congou black tea, finally to obtain the typical features of difference compounds. The results show that: 1) in RHFD treatment, the contents of tea polyphenols and total amount of catechins were the lowest, and the contents of total simple catechins, thearubigins, soluble sugar, TDE and 10TFRB were the highest, whereas the effect of first-drying methods on the total amount of theaflavins was not significant (P<0.05); 2) the total amount of volatile compounds was observed in the order of RHFD, RFD, BHFD, and CHFD; 3) the RHFD treatment can achieve the highest content of aromatic hydrocarbons, and terpenes. The rotary pot-hot air coupling first-drying (RHFD) method indicated some advantages, including fast heating (only 14 minutes), uniform temperature distribution and good stability, high thermal efficiency (50.0%), high production efficiency (220 kg/h), and low production cost (only 0.32 Yuan/kg). Moreover, the black tea after RHFD treatment has been significantly improved, indicating soup color brightness (La value), aroma sweetness, taste sweetness and refreshing, and the total score of sensory quality reaches the highest (88.1). PLS-DA also shows that Congou black tea can be distinguished by four kinds of first-drying methods from non-volatile (43 types obtained) and volatile (18 types obtained) compounds. The typical compounds are obtained, such as 2,4,6-tris(1,1-dimethylethyl)-4-methylcyclohexa-2,5-dien-1-one, geraniol, 3-octanone, methyl salicylate, theaflavin (TF), theabrownins (TBs), total soluble sugar (SSs), epicatechin (EC). These compounds can be used as indicators to identify the first-drying methods of Congou black tea, and then to produce “sweet fragrance aroma”, “sweet alcohol taste”, “high brightness liquor color” and other high-quality Congou black tea. This study can provide a technical reference and theoretical guidance to process much high quality of black tea.