Among six major types of tea (white, green, oolong, yellow, black, and dark teas) from Camellia sinensis, oolong tea, a semi-fermented tea, with its own unique aroma and taste, has become a popular consumption as indicated by the increasing production. Representing the characteristic flavonoids of oolong tea, theasinensins are dimeric flavan-3-ols. Many recent studies have indicated that oolong tea and theasinensins possess several health benefit properties. We consider it significant and necessary to have a comprehensive review in the recent advances of oolong tea. Therefore, the aim of the present review is to provide a new perspective on oolong tea and its characteristic phytochemicals, theasinensins associated with health benefits, molecular action pathway, and chemical mechanism of theasinensin formation from scientific evidences available on the literature. Furthermore, the chemical characterization of the oxidation products and the model oxidation system to the chemical changes of theasinensins are also discussed.

Besides water, tea is one of the most popular beverages around the world. The chemical ingredients and biological activities of tea have been summarized recently. The current review summarizes tea aroma compounds and their formation in green, black, and oolong tea. The flavor of tea can be divided into two categories: taste (non-volatile compounds) and aroma (volatile compounds). All of these aroma molecules are generated from carotenoids, lipids, glycosides, etc. precursors, and also from Maillard reaction. In the current review, we focus on the formation mechanism of main aromas during the tea manufacturing process.