Abstract: In order to explore the effect of different heat transfer modes during secondary drying on the quality of Gongfu black tea, Gongfu black tea was made from the Cuifeng variety. Three heat transfer modes were set up: thermal radiation (far-infrared and microwave drying), thermal convection (box-type hot air drying), and thermal conduction (carding machine and rotary pot drying). The color parameters (L, a and b values) of tea and those (LL, La and Lb) of tea infusion as well as more than 60 of its quality components including tea polyphenols, catechin, theaflavin, thearubin, theaflavins, flavonoid glycosides and soluble sugars were measured and compared under the different secondary drying modes. The quality of teas prepared with these secondary drying modes was evaluated by sensory evaluation and electronic tongue technology. Partial least squares (PLS) analysis and significance analysis were used to investigate the effect of these heat transfer modes on the high brightness, and sweet and mellow taste of tea infusion as well as the black bloom of tea, and landmarks to distinguish the heat transfer modes were identified. The results showed that, thermal convection was the most beneficial to the retention and leaching of catechins except epigallocatechin gallate, theaflavins, thearubigins, theaflavins, flavonoids except vitexin-glucoside (Vit-Glc), amino acids, total soluble sugar and other components, rotary pot drying being not as effective as carding machine drying, followed by thermal radiation and thermal convection was the least effective. L and b values, as well as La and Lb values were all significantly optimized by heat conduction, a value by microwave radiation, and LL value by heat convection. The electronic tongue analysis showed that radar profiles were significantly different among heat transfer modes. Heat radiation, heat conduction and heat convection resulted in sour and umami taste, bitter and salty taste, and sweet taste and comprehensive attributes, respectively. Sensory evaluation exhibited that tea produced by heat conduction showed an appearance with gray color and its infusion had a bright red color and strong taste with astringency, while heat radiation produced a sweet aroma, strong and refreshing taste and orange-red color in tea infusion. The PLS model clearly distinguished tea infusion color and taste characteristics among the three heat transfer modes, and distinguished the appearance of tea produced by heat conduction from that obtained by heat radiation and heat convection. Through significance analysis, the key components that contributed to the difference in quality characteristics of tea and tea infusion among these three heat transfer modes were identified as follows: theaflavin (TF), theaflavin-3,3’-gallate (TF-D-G), theaflavin-3-gallate, soluble sugar, arginine (Arg), and phosphoethanolamine for tea infusion color; TF, theaflavin-3’-gallate, TFs, TBs, Vit-Glc, Glc, citrulline (Cit), and glycine (Gly) for tea color; and TF, TF-D-G, TBs, quercetin-3-O-β-D-gentiobiose , kaempferol-3-O-rutinoside, Arg, Cit, and Gly for tea infusion taste. This study provides a theoretical basis and technical guidance for the directed and standardized production of high-quality Gongfu black tea with high brightness, sweet?and?mellow?taste and black bloom.

Key words: Congou black tea, heat transfer modes of secondary drying, quality components, color difference, electronic tongue, partial least squares analysis