Abstract: To investigate the effect of different full firing methods on the aroma of black tea, four full firing methods, namely high temperature (80 ℃), low temperature (40 ℃), dehydrated hot air (heat pump) and vacuum freezing, were used to process Congou black tea samples using the fresh autumn tea leaves of ‘Jinmudan’. The samples were analyzed by gas chromatography-mass spectrometry (GC-MS), gas chromatography-olfactometry (GC-O), odor activity value (OAV), aroma character impact value (ACI) and aroma evaluation methods were used to compare the types and contents of key aroma compounds and their contribution to the aroma of black tea by different full firing methods. The results show that there were significant differences between the four full firing methods in terms of aroma type, classes and content of aroma compounds. The results of aroma evaluation show that: high temperature dried black tea showed an unpleasant sulky odor. Low temperature dried black tea showed no floral odor and a heavy grassy odor. Heat pump dried black tea showed a refreshing floral odor. Vacuum freezing dried black tea showed a harmonious odor with good retention of floral and fruit odor and no grassy odor. The contents of linalool and its oxides, which were determined as active aroma components by OAV and GC-O, were significantly lower in low temperature full-fired black tea. While the contents of active aromas such as (Z)-2-pentenol, which present a grassy aroma, were significantly higher than that of traditional high temperature full-fired black tea, and the contents of linalool and its oxides, benzaldehyde, phenylacetaldehyde, methyl salicylate, geraniol, etc., which show floral and fruit aromas in heat pump full-fired black tea, were significantly higher than that of traditional high-temperature full-fired black tea. The contents and contribution of linalool and its oxides, benzaldehyde, benzylaldehyde, methyl salicylate, geraniol and (Z)-2-pentenol, which show floral and fruity aromas, were higher in black tea full-fired by heat pump than in black tea full-fired by other full firing methods. The results of orthogonal partial least squares discriminant analysis (OPLS-DA) and variable importance in projection (VIP) show that the OPLS-DA model was reliable and 19 compounds with VIP value higher than 1 were screened. The results show that both heat pump and freeze-drying technologies can effectively reduce the sulky aroma of traditional high temperature full firing processes and can be used to improve the floral and fruit aroma of black tea. This study provided a technical reference and theoretical guidance for the improvement of aroma quality in black tea and process selection in processing practice.

Key words: black tea, aroma, drying method, GC-MS, GC-O