The Chemistry Behind Wuyi Rock Teas: Mineral Terroir and Masterful Roasting

12 mai

Rock teas, from the Wuyi Mountains in China, are a separate category of oolong. This tea is renowned for its rich flavor and complex aromatic signature between mineral, pyrogenic and floral, which is magnified by a long aftertaste. The most recent publications have focused on the impact of manufacture (the steps involved in making tea) on the modification of aromas. More specifically, a number of studies have focused on the formation dynamics of non-volatile compounds (polyphenols, etc.), volatile odorants that we detect during tasting, amino acids and sugars.

The manufacture of rock tea involves fine plucking of the leaves, withering (dehydration), partial fermentation (alternating rotating and cooling stages), fixing (inactivation of the enzymes in the leaf that modify the molecules), rolling and roasting.

Due to its gentle nature, the leaf wilting phase releases volatile molecules only to a limited extent, but modifies the overall morphology of the leaf. Although still poorly understood, this phase could modulate the flow of water and aromatic molecules. Partial fermentation enables partial oxidation of catechins, explaining the presence of other molecules such as flavonoids. The roasting phase “breaks down” certain molecules, such as sugars and fatty acids, to form new aromatic molecules.

It is the partial fermentation and roasting phases that have the greatest impact on the taste of rock tea. In a study of several fermentation and roasting stages (one to three) on Rou Gui rock tea, there were differences. The more often fermentation is repeated, the more molecules are released to produce a fruity/vegetal/flowery jasmine/astringent taste. The more the roasting is repeated, the more molecules are released to give a sweet caramel/woody/opulent rose-like flower/nutty taste.

The duration of roasting is said to have an impact on the molecular aromatic profile of Rou Gui tea. Low roasting forms a unique fruity, grassy/fresh leaf taste with more astringency. Moderate roasting reduces catechin content, which reduces bitterness and astringency. It also brings toasted notes. More advanced roasting further reduces catechin and L-theanine content, which diminishes the fresh, lively taste but enhances woody, burnt/smoky, cinnamon and opulent flowery scents.

These recent studies (2021 and 2022) provide unprecedented insights into the evolution of the aroma profiles of rock teas in different manufacturing processes, and are proving to be of real interest in identifying and modulating the stages in the aroma formation process. Certain stages still require further investigation, notably wilting and the impact of leaf morphology on the release of aromatic molecules.