Characterisation of the key aroma compounds in a Longjing green tea infusion (Camellia sinensis) by the sensomics approach and their quantitative changes during processing of the tea leaves

Longjing tea is one of China’s most renowned green teas and is enjoyed by tea enthusiasts all over the world. Although its unique aroma is the most important quality attribute, the key aroma compounds were not yet systematically analysed. A sensory-guided screening of the aroma distillate isolated from a high-grade Longjing green tea infusion by application of an aroma extract dilution analysis revealed 60 aroma-active domains in the gas chromatogram with flavour dilution (FD) factors in the range from 32 to the highest FD factor of 4098, as found for 3-methylnonane-2,4-dione (hay-like, aniseed-like) and vanillin (vanilla-like). A quantitation of the 42 aroma compounds eliciting the highest FD factors by means of stable isotope dilution assays (SIDA) followed by a calculation of odour activity values (OAV; ratio of concentration to odour threshold) showed that 30 aroma compounds exceeded their odour threshold in the tea beverage. By mixing 30 purified reference compounds of these odorants in water in exactly the same concentrations as determined in the tea beverage, the genuine aroma of the Longjing tea could successfully be simulated. In ensuing studies, quantitative changes during the traditional manufacturing process of the tea leaves as well as differences induced by the hot water treatment were systematically evaluated based on SIDAs. The results indicated the great importance of the initial pan-frying for the formation of key aroma compounds in the tea leaves. A comparison of the concentrations of selected key aroma compounds in the finished tea leaves and the tea infusion prepared thereof indicated further that large quantities of, for example, 2-methylbutanal, dimethyl sulphide, and 3-methylnonane-2,4-dione were obviously formed from yet unknown precursors by the hot water treatment.