How Black Tea and Green Tea Are Made: Oxidation, Fixing and the Gardens That Make Both

Our blog explained it, with input from our procurement team: “During oxidation, chlorophyll is broken down to release tannins, similar to the browning of bananas, apples and other foods.” In more exact terms, an enzyme in the leaf acts on its catechins, turning them into the theaflavins and thearubigins that give black tea its color and body; the loss of green is a side effect.

Rolling is what sets it off. Frank Miller, who tasted tea professionally for decades and wrote for us, put it this way: “Rolling extracts polyphenols and enzymes from leaf cells, and this sap oxidizes. Oxidized tea polyphenols create the color and clarity, briskness, taste and aroma of brewed tea.”

Tea makers often call this step “fermentation,” though no yeast is involved; it is the leaf’s own enzymes at work. As our team has put it, “Oxidation is stopped at varying times, depending on the type of tea being produced. Only black teas are 100% oxidized.” Green tea is the opposite case: the enzymes are switched off with heat before oxidation can begin.

The order is the key. In green tea, rolling comes after fixing, so the leaf can be shaped without darkening. In black tea, rolling comes before oxidation and is what starts it. Our 2023 tasting deck describes green tea leaf going straight to the factory to be “fixed,” or “de-enzymed,” by steaming or pan-firing.

How dry each tea must be

Both teas end in the dryer, and our buying rules check the result. We set about 4.5% moisture as the norm for green tea and for Darjeeling first and second flush blacks, and 5% for Assam, with 6% as the most we accept for any of them. Green tea, the lighter and less processed of the two, is held at least as dry as black.

Black tea comes in two main styles. Orthodox tea keeps the leaf whole or in large pieces, for aroma and nuance; CTC (crush, tear, curl) tea is cut into small granules that brew dark and strong in minutes. See Orthodox vs CTC tea, and the full journey from bush to cup in How tea is made.

In September 2021, our master tasters and procurement managers tasted the Mangalam Gold Summer Black from Assam at four oxidation times, and their sheets recorded what each one did to the cup:

• 2 hours 30 minutes: “The flavors in this cup are muted.”
• 2 hours 45 minutes: “A light malty cup with some character towards the end.”
• 3 hours: “Slightly brisker with higher layer of malt and a hint of maple syrup.”
• 3 hours 30 minutes: “Let’s launch this! A perfect combination of strength, briskness with notes of maple syrup and hints of cocoa!”

A sample left longer was turned down because its flavors had deteriorated “due to excessive oxidation.” So “full” oxidation does not mean as long as possible. It means as long as the tea needs to reach its best, which the tasters judge cup by cup.

That is the gap with green tea in plain numbers: a black tea spends hours oxidizing, while green tea leaf is fixed before oxidation can start at all. You can taste the result in Assam Mangalam Gold Summer Black.

Is Darjeeling fully black?

Not quite, and that matters when you compare. As our founder has written, “Darjeeling is not really a black tea. It’s less oxidized.” Its teas “are not anywhere near as light as greens, but they share some of the characteristics of partially oxidized oolongs.” A spring Darjeeling sits closer to green tea than an Assam ever does, which is why it suits drinkers who find Assam too heavy.

Japan’s method has a date. In 1738, during the long period when Japan restricted its contact with the outside world, the tea producer Soen Nagatani invented a way of “fixing the green” by steaming rather than frying the leaves. Because steamed leaf holds more moisture, Japanese green tea “often undergoes three or more periods of drying compared to one or two for Chinese greens.”

The two methods give two different teas. Steaming yields leaf that is “vivid green in color with prominent marine and herbaceous notes,” while stir-frying gives “yellower leaves” with a taste that “tends toward the vegetal, nutty and floral side.” Read A short lesson on Asian green tea.

India mostly follows the Chinese way. In the Nilgiris, Pascoes Woodlands makes high-grown pan-fired greens, close to 0.7 million kg a year, and Burnside rolls its pan-fired green into semi-balls. More on India’s greens in Indian green tea.

• Glenburn, Darjeeling: sent us a Classic Spring Green, launched on 24 April 2018, and a Classic Spring Chinary Black, launched on 25 May 2018, from the same spring.
• Kangra, Himachal Pradesh: the Dharmsala Tea Company, which owns the Mann, Hoodle and Towa estates, makes “the base teas (black, green, oolong and white).” Kangra was once “the main source of supply of green tea to Central/West Asia and Russia.”
• Chota Tingrai, Assam: a 600-hectare estate with “a state-of-the-art green tea processing plant” beside its black tea, whose Assam black we sell as Assam Chota Tingrai Summer Black.

A green tea factory in Assam

When we visited Chota Tingrai in 2016, its new green tea factory was run by Simi, “all of 5 feet tall, 24 years old,” who had started as a management trainee and was quickly given charge of the whole unit. Read the full story of the Jalans at Chota Tingrai.

The estate’s young owner, Mrityunjay Jalan, had strong views on why. His father set up a green tea unit there in 1978, which later shut. Reviving it, he told us: “In India, what you get as green tea is not really green tea. It’s a farce. It’s not produced the right way.” He went to Japan to find partners and planned to launch “authentic green teas by April 2016.”

That is the practical difference between the two teas for a garden: black tea needs withering troughs, rollers and an oxidation room, while green tea needs fast heat, a pan or a steamer, close to where the leaf comes in.