Abstract: Turning-over is the core processing stage for the formation of aroma quality of oolong tea. In order to understand the effect of mechanical force during the turning-over process on the formation of aliphatic aroma compounds in oolong tea leaves, fresh tea leaves, turned-over tea leaves and spread tea leaves (collected at the same time point) of the oolong tea cultivar Camellia sinensis cv. Maoxie were used as test materials. We detected the contents of linoleic acid and linolenic acid by ultra-performance liquid chromatography tandem mass spectrometry, which are considered the precursors of fatty acid metabolism. The results showed that mechanical force during turning-over could significantly reduce the contents of linolenic acid and linoleic acid in tea leaves (P < 0.05), while spreading treatment could significantly only reduce the content of linoleic acid in fresh leaves (P < 0.05), although the content of traumatic acid in the spread tea leaves was lower than that of fresh tea leaves, it was still significantly higher than that of the turn-over tea leaves (P < 0.05). Also, we detected the contents of aliphatic aroma compounds in each kind of tea leaves by headspace-gas chromatography-mass spectrometry, and the results displayed that under the mechanical force, the relative content of C6-aldehydes in turned-over tea leaves descended significantly, whereas the contents of C6-alcohols and leaf alcohol ester derivatives increased obviously. The contents of C6-alcohols and leaf alcohol ester derivatives in spread tea leaves were far lower than in turned-over tea leaves. The results of quantitative polymerase chain reaction (qPCR) demonstrated that the LOX1 and ADH genes had a higher relative expression level in turned-over tea leaves, while the LOX2, LOX3 and HPL genes had a higher relative expression level in spread tea leaves. The correlation analysis illustrated that a highly significant negative correlation between the relative expression level of LOX1 and C6-aldehydes content for each kind of tea leaves (P < 0.01), and a significant positive correlation between the relative expression level of LOX1 and C6-alcohols content and leaf alcohol ester derivatives content (P < 0.05). In addition, a highly significant positive correlation between the relative expression level of ADH and leaf alcohol ester derivatives content. On the other hand, there was no significant correlation between the relative expression level of LOX2 and HPL and each aliphatic aroma compound (P > 0.05). To sum up, the LOX1 and ADH genes were up-regulated during turning-over, suggesting a close relationship with the formation of natural floral or fruity aroma of oolong tea.

Key words: oolong tea, mechanical force during turning-over, aliphatic aroma, lipoxygenase pathway, gene expression