Oxidation is a series of chemical reactions that result in the browning of tea leaves and the production of flavor and aroma compounds in finished teas. Depending on the type of tea being made, oxidation is prevented altogether, or deliberately initiated, controlled then stopped.
Much of the oxidation process revolves around polyphenols and the enzymes polyphenol oxidase and peroxidase. When the cells inside tea leaves are damaged and the components inside are exposed to oxygen and mix, specifically when polyphenols in the cell’s vacuoles and the peroxidase in the cell’s peroxisomes mix with polyphenol oxidase in the cell’s cytoplasm1 a chemical reaction begins. This reaction converts the polyphenols known as catechins into flavanoids called theaflavins and thearubigins (which are also polyphenols). Theaflavins provide tea with its briskness and bright taste as well as its yellow color, and thearubigins provide tea with depth, body and its reddish color. Also, during oxidation chlorophylls are converted to pheophytins and pheophorbides (pigments that lend to the black/brown color of dry oxidized tea leaves); and lipids, amino acids and carotenoids degrade to produce some of tea’s flavor and aroma compounds. Tea producers use special methods to initiate, fix, or even prevent oxidation in order to produce different flavors in a finished tea and inherently, different types of tea.
Oxidation begins when the cell walls within tea leaves are damaged. To achieve cell damage, tea producers macerate, roll or tumble tea leaves to intentionally initiate oxidation. Maceration is the quickest path to full oxidation because the leaves are, well… macerated which exposes much more of the insides of the leaves to oxygen and results in a greater mixture of the chemicals within. Maceration is typically used in mass production methods to create CTC (cut tear curl) tea or other broken-leaf teas and is achieved using a rotorvane or a CTC machine. Rolling results in a much slower and gentle oxidation and is usually done using a rolling table or by hand. Tumbling is an even gentler way to initiate oxidation and is achieved using large cylinders wherein the leaves are tumbled or by hand by shaking the leaves on top of a shallow bamboo basket. Regardless of the method of initiation, great care must be taken up to this point as any damage to the leaves before processing will cause premature oxidation and result in an unevenly processed finished tea.
Control over oxidation is maintained by introducing warm, moist, oxygen-rich air over time. The extent to which oxidation is allowed to occur has an astounding effect on the finished tea. Oxidation occurs best between 80-85F and is slowed, nearly to a halt at 140-150F2. Thus, when the tea producer wishes to halt oxidation, they heat the leaves. This heating process, known as fixing, denatures the enzymes responsible for the reaction. Oxidation is further slowed by drying the leaves. But it never completely stops, it just slows way down.
Fixing is a process in tea manufacture used to stop oxidation once it has started or to prevent it altogether. Fixing works by denaturing polyphenol oxidase and peroxidase – the enzymes primarily responsible for oxidation. Fixing is also commonly referred to as de-enzyming, denaturing or kill-green. The term kill-green is derived from the Chinese term shaqing 杀青, which translates to killing the green. In tea, the leaves must be heated to approximately 150 degrees Fahrenheit to halt oxidation. The process requires precise control of the temperature and length of heating, each has to be adjusted depending on the size and thickness of the leaves and the amount being processed.
Most common fixing methods:
Less common fixing methods:
When oxidation is prevented altogether, the tea leaves will keep their green color and vegetal characteristics in the cup as the catechins will be left largely intact. Think of an apple, once it is sliced open, it quickly turns brown; but yet, the apples in apple pie are not brown because the heat used to bake the pie denatured the polyphenol oxidase and peroxidase in the apples and prevented enzymatic browning (same goes for potatoes, avocados, bananas, etc).
When a semi-oxidized tea is being produced, some catechins will have converted to theaflavins and thearubigins, resulting in a slight browning in the leaves and yellower liquor. Lipids, amino acids, and carotenoids will have also begun to break down into flavor and aroma compounds.
When oxidation is allowed to run its course, the leaves will have undergone a full transformation and exhibit an aroma and taste profile completely unrecognizable from a finished tea that was exempted from oxidation. Theaflavins and thearubigins will now outnumber catechins resulting in a brisk tasting tea with a reddish color in the cup, the cholorophylls will have been converted to pheophytins and pheophorbides, turning the leaves a coppery brown, and a myriad of new volatiles will have developed. In this case, the leaves are often just dried to halt any still-occuring reactions and to bring the tea to a shelf-stable moisture level. This is a bit of a grey area in tea processing because here, drying can be considered a form of fixing. Heat is being used and oxidation is being halted. This is a great example of why it is sometimes important to view tea processing as more of a continuum rather than as a distinct set of steps.