Characterization of volatile composition, aroma-active compounds and phenolic profile of Qingxin oolong tea with different roasting degrees

Due to the premium flavor, Qingxin oolong tea (QOT), which is grown and produced in Taiwan, is widely appreciated. Roasting is a crucial process for shaping its flavor. However, the effect of roasting on the aroma and taste of QOT was not fully understood. In this study, the volatile composition, aroma-active compounds and phenolic profile of QOT with different roasting degrees, including the unroasted (UR), the medium roasted (MR) and the heavy roasted (HR) were analyzed. As revealed by headspace solid phase microextraction gas chromatography-mass spectrometry (HS-SPME-GC-MS) and multivariate statistical analyses, UR and MR demonstrated similar volatile profiles, while heavy roasting changed the volatile composition greatly. Multiple nitrogen-containing compounds, such as 3,5-diethyl-2-methylpyrazine and 2-ethyl-3,5-dimethylpyrazine, were uniquely found in heavy roasted samples, which can be ascribed to the extensive Maillard reaction. Moreover, eight compounds were identified and verified as key aroma-active compounds by using gas chromatography-olfactometry-mass spectrometry (GC-O-MS), aroma recombination and omission experiments. The aldehydes and alcohols, such as linalool, benzeneacetaldehyde and α-farnesene, with a floral scent were the dominant aroma-active compounds in UR and MR, while 2-ethyl-3,5-dimethylpyrazine and 3,5-diethyl-2-methylpyrazine with a roasted scent were the main aroma-active compounds in HR. Furthermore, as revealed by ultra-high performance liquid chromatography and Orbitrap high resolution mass spectrometry (UHPLC-Q-Orbitrap-MS), flavanols and phenolic acids were significantly lower in abundance in UR and MR as compared to those in HR, whereas flavonols were less affected by roasting. These findings provide useful information for understanding the flavor characteristics of this popular oolong tea for the first time.