Metabolomics Analysis of Dynamic Changes in Metabolites in Tieguanyin Oolong Tea during Wrapping-Twisting

In order to investigate the effect of wrapping-rolling on the quality of Tieguanyin oolong tea, non-volatile and volatile metabolites in rolled tea leaves before and after repeated wrapping-twisting for up to three times were analyzed by widely targeted metabolomics based on headspace solid-phase microextraction-gas chromatography-mass spectrometry (HSSPME-GC-MS). The results showed that a total of 973 non-volatile metabolites belonging to 12 categories were identified, including 468 differential metabolites. The total amount of non-volatile metabolites showed a decreasing trend during the wrapping-twisting process. Particularly, the decrease in the content of differential metabolites such as flavonol glycosides and phenolic acids contributed to the reduction in bitterness and astringency during the wrapping-twisting process. The content of differential lipids increased, which provided favorable conditions for the conversion of volatile fatty acids during the subsequent drying process. A total of 47 volatile components belonging to six categories were identified. During the wrapping-twisting process, the contents of β-red myrcene, heptanal, (E)-2-hexenyl hexanoate and cis-3-hexenyl isobutyrate, responsible for fatty or grassy aroma, decreased, while the contents of methyl jasmonate, benzyl benzoate, geranyl acetone and 3,7-dimethyl-1,5,7-octatrien-3-ol, contributing to floral or fruity aroma, increased. It was noteworthy that the odor activity value (OAV) of the major aroma components in Tieguanyin oolong tea, such as linalool, jasmonolactone, cis-jasmonone, α-farnesene and basilene, were still greater than 1 at the end of the wrapping-twisting process although their contents decreased during this process. It could be seen that the wrapping-twisting process was beneficial to purify the quality of Tieguanyin tea, and provided it with a more mellow taste and more obvious floral and fruit aromas. This study provides new insights into the metabolite changes and the flavor formation of oolong tea during the wrapping-twisting process.