Hot processing of tea leaves

The types and quantities of aromatic compounds in tea leaves vary according to the tea’s place of origin, variety, fertiliser application and processing method. Of these, the variety and processing method have a decisive influence. For example, there is a very marked difference in aroma profile between Assam tea and Chinese medium- and small-leaf tea varieties; the former has a high content of linalool compounds, whilst the latter has a high content of geraniol.

Different processing methods yield different aromatic compounds. For instance, as Sencha is produced using a steamed green tea process, it contains higher levels of sulphur-based aromatic compounds (such as dimethyl sulphide), whilst Longjing tea is characterised by higher levels of floral compounds—such as linalool, geraniol and 2-phenylethanol—as well as pyrazine and pyrrole compounds, which impart a roasted aroma. Oolong tea, on the other hand, is characterised by high levels of floral aroma compounds such as nerolidol, jasmine lactone, methyl jasmonate and indole, a characteristic determined by its unique processing method. Furthermore, there are numerous other factors that influence the aroma composition of tea.

Aromatic substances in tea are also known as ‘volatile fragrance components (VFC)’. This is a general term for the substances in tea that are perceived as aroma by the sense of smell.

Based on their chemical composition, these can be classified as: low-boiling-point non-terpenoid alcohols (VFCI), including 1-penten-3-ol, cis-2-penten-1-ol, cis-3-hexen-1-ol, trans-2-hexenol and trans-2,4-heptadienal;

terpenol high-boiling-point components (VFCII), including linalool, linalool oxides, geraniol, α-p-cymene, β-p-cymene, nerol, cis-jasmone, cedrol and indole, amongst others.

Classified by fragrance type:

The fresh, crisp fragrance of young leaves: cis-3-hexen-1-ol and its esters; the delicate floral scent of lily of the valley: linalool and its oxides; the gentle floral scent of roses and wild roses: geraniol and 2-phenylethanol; β-Irisone and other irisone-derived compounds, such as cis-jasmone and methyl jasmonate, which characterise the sweet and rich floral notes of jasmine and gardenia; jasmine lactone and other lactones, as well as thearhelone, which characterise the fruit and dried fruit fragrance profiles; Woody notes: cedrol, 4-vinylphenol and guaiacol-based compounds; sesquiterpenes; heated notes: pyrazines, pyrroles, furans, etc.

Maillard reaction flavourings refer to a series of aromatic substances synthesised through the Maillard reaction, which is a reaction occurring between amino compounds and reducing agents or other carbonyl compounds. As the reaction products are dark brown in colour, this process is also known as the non-enzymatic browning reaction.

The ProC-TOF online mass spectrometer can monitor the Maillard reaction process in real time and has gained prominence and further development in research fields such as food chemistry, food technology, nutrition, tobacco chemistry and flavour chemistry.