Untargeted Metabolomics Combined with Bioassay Reveals the Change in Critical Bioactive Compounds during the Processing of Qingzhuan Tea.

Qingzhuan tea (QZT) is a typical Chinese dark tea that has a long-time manufacturing process. In the present study, liquid chromatography coupled with tandem mass spectrometry was used to study the chemical changes of tea samples during QZT processing. Untargeted metabolomics analysis revealed that the pile-fermentation and turnover (post-fermentation, FT) was the crucial stage in transforming the main compounds of QZT, whose contents of flavan-3-ols and flavonoids glycosides were decreased significantly. The bioactivities, including the antioxidant capacities and inhibitory effects on α-amylase and α-glucosidase, were also reduced after the FT process. It was suggested that although the QZT sensory properties improved following pile-fermentation and aging, the bioactivities remained restrained. Correlation analysis indicated that the main galloylated catechins and flavonoid glycosides were highly related to their antioxidant capacity and inhibitory effects on α-amylase and α-glucosidase.

Novel insight into the role of withering process in characteristic flavor formation of teas using transcriptome analysis and metabolite profiling.

Withering is an indispensable process for improving flavors in green, black and white teas during their manufacturing. The effects of the withering process on the formation of tea flavors were investigated using transcriptome and metabolite profiling in withered tea leaves. A total of 3268, 23,282 and 25,185 differentially expressed genes (DEGs) were identified at 3 h (68%, water content), 12 h (61%) and 24 h (48%) of the withering process, respectively. The DEGs, involved in flavonoid biosynthesis were significantly downregulated, which could be correlated with the reduction of catechins. Enhancement of terpenoids and alpha-linolenic acid metabolism could trigger an increase in the total content and number of volatiles. The increase in free amino acid-content could be related to 261 DEGs. Our study suggests that dehydration stress during withering induced significant changes in the gene transcription and content of the tea flavor compounds, which promoted the special flavors in various teas.