Oxidation

Oxidation is what turns a green leaf into everything that is not green tea. Once the leaf is bruised or torn, its polyphenols meet the air and the leaf's own enzymes convert them into darker, richer compounds. How long a maker lets that reaction run, and how evenly, is the single biggest lever in tea processing after the pluck itself.

What it is

A set of enzymatic reactions rather than one. Breaking the leaf's cells puts two enzymes, polyphenol oxidase and peroxidase, in contact with the catechins they were kept apart from and with oxygen present they start converting them. A maker has three choices: prevent the reaction outright, let it run to completion or stop it partway. Those three choices are most of what separates green tea from black tea from oolong and they are made on one plant's leaves.

What it does to the leaf

Catechins become theaflavins and thearubigins. Theaflavins carry briskness, a bright taste and a yellow colour; thearubigins carry depth, body and the red in the cup. At the same time chlorophyll converts to pheophytins and pheophorbides, which is the brown-black of an oxidised dry leaf, while lipids, amino acids and carotenoids break down into aroma. The carotenoid route is where much of the perfume comes from: beta-carotene becomes beta-ionone, which smells of violets, and neoxanthin becomes beta-damascenone, which smells of roses and apples. Taken to completion, theaflavins and thearubigins outnumber the catechins that made them.

How it's done

Control, not equipment. The lever is warm, humid, oxygen-rich air held over time and the temperatures are modest: the reaction runs best around 27-29°C (80-85°F). Chinese black tea is oxidised for eight to twelve hours at about 22°C with the leaf spread on the floor under damp cloths. Green oolong takes ten to eighteen hours at 20-25°C, stirred every one to four hours so the leaf works evenly. The end point is called by nose rather than by clock, which is the part of tea making that has stayed stubbornly manual.

What it tastes like

Lightly oxidised leaf keeps its vegetal, floral character, browning only at the edges and gives a yellower liquor. Fully oxidised leaf is unrecognisable next to the same leaf unoxidised: brisk, coppery brown when dry and red in the cup. Between those poles sits the whole oolong range, which is why a 30 percent leaf and a 70 percent leaf off the same bush are two different drinks. Published percentages are estimates a maker arrived at by smell rather than measurements and the same tea is routinely quoted at different figures, so a band is a more honest way to read them than a figure.

What people get wrong

Calling this fermentation. It is the most common error in tea writing and it survives because one real fermentation does exist in tea: the microbial pile that makes ripe puerh and heicha. What happens here is different in kind. It is driven by the leaf's own enzymes plus oxygen, no microorganism is involved and it needs no warm damp room. The usage is not settled even among specialists: Zhang, writing on puerh, uses fermentation as the umbrella term and names oxidation as one of its two kinds, which inverts the distinction chemists and most tea writing use. The distinction is worth holding because it predicts behaviour: an oxidised tea is finished when the maker stops it, while a fermented one keeps changing.

Where the boundary blurs

Fixing is a brake, not a wall. Heating the leaf to about 60-66°C (140-150°F) slows oxidation nearly to a stop and that is the halt every kill-green is really performing, but above that temperature the reaction still creeps along and drying slows it further without ever ending it. The same point runs the other way: a black tea with no kill-green step has not skipped the halt. Its drying is doing that job under a different name.

Styles where this step is distinctive

and 118 more

teabert, the tealytics teapot, keeper of the kettle

Oolong is a whole spectrum wearing one word. A jade oolong and a 70 percent one have less in common than the shelf label suggests.

Sources

oxidation is an enzymatic reaction, distinct from the microbial fermentation that produces puerh and heicha Gebely, Tea: A User's Guide (2016), ch. 4
oxidation runs best around 27-29°C (80-85°F) and is slowed nearly to a halt at 60-66°C (140-150°F), which is why heating the leaf is how a maker stops it Gebely, Tea: A User's Guide (2016), ch. 4
oxidation converts catechins into theaflavins and thearubigins; theaflavins give briskness, a bright taste and a yellow colour, thearubigins give depth, body and a reddish colour, and chlorophyll converts to pheophytins and pheophorbides Gebely, Tea: A User's Guide (2016), ch. 4
carotenoids are oxidised into aroma compounds: beta-carotene into beta-ionone, which smells of violets, and neoxanthin into beta-damascenone, which smells of roses and apples Francl, Steeped: The Chemistry of Tea (2024), ch. 2
Chinese black tea is oxidised for eight to twelve hours at around 22°C under damp cloths; green wulong for ten to eighteen hours at 20-25°C, stirred at one to four hour intervals Gascoyne et al., Tea: History, Terroirs, Varieties (2014), 'The Processing of Chinese Teas' and 'Processing Green Wulong Teas'
Zhang treats fermentation as the umbrella term for puerh's changes and names oxidation as one of its two methods, the inverse of the enzymatic/microbial split used here Zhang, Puer Tea: Ancient Caravans and Urban Chic (2014), appendix 1