Effect of Variable-Temperature Baking Technology on the Quality of ‘Beauty’ Oolong Tea

To explore the effect of variable-temperature baking technology on the quality of ‘Beauty’ tea, tea samples were prepared by indoor natural withering (IW) or outdoor solar withering (OW) alone or followed by one of three variable-temperature baking treatments, A: 65 ℃ for 4 h-75 ℃ for 2 h-85 ℃ for 1 h, B: 75 ℃ for 4 h-85 ℃ 2 h-95 ℃ for 1 h, and C: 85 ℃ for 4 h-95 ℃ for 2 h-105 ℃ for 1 h, and their taste quality was assessed by sensory evaluation, biochemical composition analysis and non-targeted metabolomics analysis. The results showed that for each withering method, the woodiness and mellowness of ‘Beauty’ tea decreased as the baking temperature increased, and the taste became mellower and richer. The floral and fruity aroma of the tea initially increased and subsequently decreased, and the sweet aroma continued to increase. The overall aroma changed from clean and woody to floral, fruity, nectar-like to sweet. The variations in the contents of caffeine, free amino acids, flavonoids, theaflavins and theabrownin were key to the formation of the difference in the flavor of ‘Beauty’ tea samples. In terms of specific metabolites, umami, sweet and bitter amino acids, as well as catechin components including epigallocatechin, catechin, epigallocatechin gallate and gallic acid, and theobromine were found to be the key differential metabolites, which played a key role in the mellow flavor of IW ‘Beauty’ tea and in the mellow, brisk, prominent honey-like flavor of OW ‘Beauty’ tea. Myrcene, geraniol, cis-linalool oxide, cis-linalool oxide (furans), β-ionone, phenylacetaldehyde and nerolidol were found in ‘Beauty’ tea regardless of the withering method used or the baking temperature, and their contents were the key factors affecting the difference in aroma. For IW, baking treatment B performed best among all baking treatments, being beneficial for the formation of the mellow, honey-like, elegant aroma of IW ‘Beauty’ tea. In contrast, baking treatment A imparted the best quality to OW ‘Beauty’ tea, contributing to the formation of the mellow and brisk taste and the honey-like, nectar-like and elegant aroma of OW ‘Beauty’ tea.