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Ask Paul: What Is the Difference Between Black and Green Tea?

If they're from the same plant, why are they so different?
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Published Dec. 22, 2021.

L. asked: What is the difference between green and black tea?

My last column of 2021 begins with an old joke.

Q: Why do anarchists only drink herbal tea?

A: Because proper tea is theft.

Herbal tea is any brew made from the leaves, roots, fruits, or any other part of any plant other than Camellia sinensis, the tea plant. In other words, it’s a free-for-all.

Proper tea can be made from the leaves of Camellia sinensis in quite a variety of ways, with much of the variation having to do with how the leaves are processed after harvesting.

When tea leaves are picked, they rapidly start to undergo chemical changes. These changes are commonly called “fermentation,” but that’s (almost always) a misnomer. Fermentation is caused by external microorganisms, while the changes in tea are wrought by the plant’s own enzymes and a process of oxidation facilitated by those enzymes.

Oxidation causes the green leaves to turn deep reddish-brown, and gradually develop an assortment of typical tea flavors, including astringency.

To make black tea, the leaves are often rolled, cut, or crushed, and/or held at warm temperatures, all of which encourage those enzymatic reactions.

Conversely, to make green tea, the objective is to prevent the reactions, so the leaves stay green and the flavor more delicate. As soon as possible after harvest, the leaves’ enzymes are deactivated with a blast of heat. That process, called fixation, preservesmany flavor compounds that would otherwise be transformed or destroyed, including amino acids such as theanine and glutamine, which give green tea an umami savor as well as some of the calmly alert mental effect it’s known for.

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Green and black varieties make up the great majority of the tea that is drunk in the world, but not all of it.

In between green tea and black tea, one finds oolong tea (sometimes referred to as “blue tea”), a type that is allowed to undergo a short period of enzymatic oxidation before fixation stops the process, resulting in a dark green leaf.

White tea is harvested younger than the other teas, and minimally oxidized, so it’s typically even more delicate than green tea.

Another kind of tea, pu’er, is the exception to what I said above, that tea is not truly fermented. Pu’er is made from leaves that are dried a little but not enough to prevent microbes—bacteria and yeasts—from slowly working a variety of transformations that produce floral, fruity, and fascinating flavors that are never encountered in other teas. Pu’er tea is fermented, and can be aged for a decade or more; some connoisseurs collect it like wine.

Note: All of these categories are broad, loose, and incomplete, as might be expected from a beverage that’s been cultivated and enjoyed for millennia.

Ask Paul Adams, senior science research editor, about culinary ambiguities, terms of art, and useful distinctions: paul@americastestkitchen.com

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