What Makes Tea Different: Catechins, Tannins, and the Chemistry of Steeping

Every cup of green tea and every cup of black tea started from the same leaf. Camellia sinensis, the tea plant, is the source of all true tea. The dramatic differences in flavor, from the grassy, vegetal taste of a good sencha to the malty richness of an Assam, come entirely from what happens to the leaf after it’s picked.

Understanding that transformation is really understanding oxidation chemistry. And understanding why tea gets bitter when brewed too long is a simple story about tannins.

The Polyphenol Foundation

Fresh tea leaves contain high concentrations of polyphenols, with catechins as the dominant class. The most abundant catechin in fresh tea is epigallocatechin gallate (EGCG), which you’ve probably seen on green tea product labels. These catechins have a slightly astringent taste on their own, but at moderate concentrations they contribute complexity and a pleasant clean finish.

The leaf also contains caffeine (which is bitter), amino acids like L-theanine (which has a mellow, savory quality and partially tempers the bitterness of caffeine), aromatic compounds, and chlorophyll. The interplay of all these molecules is what makes tea taste like tea.

Oxidation Makes Black Tea

After picking, tea leaves contain an enzyme called polyphenol oxidase. When the cell walls are damaged by wilting, rolling, or cutting, this enzyme contacts the catechins and begins oxidizing them. This is the same class of reaction that turns a sliced apple brown or darkens a bruised avocado.

In tea processing, oxidation converts catechins into two groups of new molecules: theaflavins and thearubigins. Theaflavins are bright orange-red and contribute to black tea’s briskness and color. Thearubigins are darker, brownish-red polymers that make up the largest fraction of black tea’s polyphenol content and give it depth and body.

This is the key difference: green tea is quickly heated after picking (steamed in Japanese style, pan-fired in Chinese style) to deactivate polyphenol oxidase before oxidation begins. The catechins stay intact. Black tea goes through full oxidation and ends up with an almost completely different polyphenol profile.

Oolong sits in between. Partial oxidation gives it characteristics of both. A lightly oxidized oolong can taste floral and green. A heavily oxidized oolong can approach black tea in flavor profile.

Why Tannins Cause Astringency

The term “tannins” refers to a group of polyphenols with a specific property: they bind tightly to proteins. Historically, tannins from tree bark were used to tan (preserve) animal hides, which is where the word comes from. In your mouth, tannins bind to proline-rich proteins in your saliva.

Saliva’s proteins normally lubricate the mouth and tongue. When tannins bind to them, they precipitate, and lubrication decreases. That dry, puckering sensation you feel when you drink over-steeped black tea is your mouth’s lubrication system being temporarily disrupted. It’s not a taste receptor response. It’s a physical protein-binding reaction.

The sensation is called astringency. It’s distinct from bitterness (a taste receptor signal from caffeine and some catechhin breakdown products). Over-steeped tea can be both bitter and astringent at the same time, which compounds the unpleasantness.

Temperature and the Extraction Problem

Water temperature matters more in tea than most people expect, and the effect is different for different tea types.

Hot water is more aggressive at extracting both the desirable flavor compounds and the tannins. Green tea is particularly vulnerable because its catechins, while not identical to tannins, contribute bitterness and astringency at high concentrations, and they extract very quickly at high temperatures. The mellow, sweet compounds in green tea (including L-theanine) also extract faster at higher temperatures, but they hit their peak earlier. So water that’s too hot extracts a huge rush of tannins after those sweet compounds have already been extracted.

Using cooler water (160-175°F for green tea, 175-190°F for oolong) extracts selectively. You get more of the sweet, floral, and savory compounds before the tannin extraction gets out of hand.

Black tea, having been oxidized, has a different chemical structure. Its theaflavins and thearubigins don’t behave exactly like green tea catechins. Black tea can handle higher temperatures (200-212°F) and in fact benefits from them for full flavor development. The malty, rich quality of black tea comes from theaflavin-thearubigin interactions that are better expressed at higher temperatures.

The Steep Time Problem

Time and temperature compound each other. Think of it like a variable-speed pump. High temperature pumps compounds out of the leaf faster. More time means more total extraction regardless of temperature.

For most black teas, 3-5 minutes at proper temperature extracts a balanced cup. Beyond 5 minutes, you’ve entered the zone where tannin concentration keeps climbing while the desirable flavor compounds are fully extracted and can’t increase further. The ratio tips toward bitterness.

For delicate green teas like gyokuro, the window can be as short as 60-90 seconds at 140-150°F. Japanese matcha sidesteps the steep-time problem entirely by grinding the leaves into powder and whisking them directly into water. You’re consuming the whole leaf, so there’s no extraction step to manage.

The fix for bitter tea is almost always temperature, not time. Dropping the water temperature from boiling to 175°F for green tea makes a larger difference than cutting steep time in half at the same temperature. Temperature is the primary variable.