Exploration of the substances and key processing steps related to the sweetness of Niangniang tea.

Niangniang tea (NNT) is processed using a unique twice bunch-drying method, which tastes very sweet. In order to explore the key substances related to the sweetness of NNT, we processed green tea (GT), yellow tea (YT) and NNT and conducted a sensory evaluation and metabolomic analysis. NNT was the sweetest, with the lowest contents of total polyphenol and caffeine and highest content of soluble sugar. A total of 365 differential substances were identified in all the samples, most of which were flavonoids, alkaloids, and terpenoids. Thirty differential substances, especially caffeine, turanose, kojibiose, -phenylalanine, L-leucine, and flavone, were explored as important factors contributing to sweetness. The second bunch-drying step was found to be critical to enhance the sweetness and reduce the bitterness of NNT. This study clarified the chemical basis for the sweetness of NNT and provides insights for the potential reduction of bitterness and astringency in summer and autumn tea.