Oxidation results in the browning of the leaves and intensification of their flavor compounds. From the moment they are plucked, the cells within the tea leaves are exposed to oxygen and the volatile compounds within them begin to undergo chemical reactions. It is at this stage that polyphenolic oxidase, including theaflavin and thearubigin, begin to develop within the leaves. Theaflavins lend briskness and brightness to the tea while thearubigins offer depth and fullness to the liquor that's produced. In order to bring out specific intensities in flavors, tea makers control the amount of oxidation the leaves undergo. Controlled-oxidation is typically carried out in a large room where the temperature is maintained at 25–30 °C and humidity stands steady at 60–70%. Here, withered and rolled leaves are spread out on long shelves and left to ferment for a fixed period of time, depending on the type of tea being made. To halt or slow down oxidation, fermented leaves are moved to a panning trough where they are heated and then dried. Due to oxidation, the leaves undergo a complete transformation and exhibit an aroma and taste profile that's completely different from the profile of the leaves that do not undergo this process. Less oxidized teas tend to retain most of their green color and vegetal characteristics due to lower production of polyphenols. A semi-oxidized leaf has a brown appearance and produces yellow-amber liquor. In a fully oxidized tea, amino acids and lipids break down completely, turning the leaves blackish-brown. The flavors in such a tea are more brisk and imposing.
