A cup of brewed tea is roughly 99% water, but the remaining fraction contains a surprisingly complex mix of compounds. Polyphenols, caffeine, amino acids, minerals, tannins, and hundreds of aromatic molecules all dissolve into your cup during steeping. The exact profile shifts depending on whether you’re drinking green, black, white, or oolong tea, but every variety comes from the same plant, and they share a common chemical foundation.
Polyphenols: The Largest Group of Compounds
Polyphenols make up the single biggest category of active compounds in tea leaves, accounting for 20 to 30% of the dry weight in green tea. These are the molecules responsible for tea’s bitter and astringent taste, and they double as potent antioxidants. The most studied subgroup is the catechins, a family of compounds that protect cells from oxidative damage.
Green tea is especially rich in catechins because the leaves are heated early in processing, which prevents the oxidation that breaks them down. Lab analysis of green teas from China, India, Japan, Sri Lanka, Nepal, and South Korea shows that a 100 mL serving typically contains between 92 and 139 mg of the catechin called EGCG, the one most frequently linked to health benefits. Other catechins appear in even higher concentrations. One type, EGC, can reach up to 249 mg per 100 mL in Japanese and Korean green teas.
Black tea tells a different story. During production, the leaves are allowed to oxidize fully, and enzymes convert catechins into larger molecules called theaflavins and thearubigins. Together, these make up 30 to 40% of black tea’s polyphenol content. Theaflavins give black tea its reddish-gold color and brisk taste. They account for about 2% of dried black tea leaves by weight, while thearubigins, which are less well characterized, make up a much larger share. These oxidized polyphenols carry their own antioxidant properties, so black tea isn’t nutritionally inferior to green tea; it simply has a different polyphenol profile.
Caffeine
All true teas contain caffeine, though the amount varies by type and preparation. An 8-ounce cup of brewed black tea delivers about 48 mg of caffeine, while the same serving of green tea contains around 29 mg. For comparison, an 8-ounce cup of coffee typically has 95 mg or more. Factors like water temperature, steeping time, and the ratio of leaves to water all shift these numbers, so your cup at home may land above or below these averages.
L-Theanine: Tea’s Signature Amino Acid
Tea leaves contain between 1 and 4% amino acids by dry weight, and the most distinctive one is L-theanine. This amino acid is rare in the plant world and is one of the main reasons tea produces a different kind of alertness than coffee. L-theanine promotes a calm, focused state that tempers the jitteriness caffeine can cause on its own.
Green and white teas contain the most L-theanine, averaging around 6.3 to 6.6 mg per gram of dry leaf. Oolong comes in close at about 6.1 mg/g, while black tea drops to roughly 5.1 mg/g. The pattern makes sense: oxidation during processing reduces L-theanine levels. Pu-erh tea, which undergoes extended microbial fermentation, contains no detectable theanine at all.
Tannins and Iron Absorption
Tannins are a subset of polyphenols that bind to proteins and minerals, creating that dry, puckering sensation in your mouth. A single cup of tea contains roughly 25 to 80 mg of tannins per 150 mL serving. Three cups a day puts you in the range of 75 to 240 mg.
The practical concern with tannins is their effect on iron. They bind to non-heme iron (the type found in plant foods, eggs, and supplements) in the gut, forming insoluble complexes that your body can’t absorb. Single-meal studies consistently show this effect. However, the longer-term picture is more reassuring. Epidemiological studies and multi-meal research generally do not show that regular tea drinking worsens iron status in people who have adequate iron intake. If you’re at risk for iron deficiency, drinking tea between meals rather than with food is a simple way to minimize any interference.
Minerals and Fluoride
Tea picks up minerals from the soil, and several end up in your cup. The most notable is fluoride. The tea plant is unusually efficient at absorbing fluoride from the ground and concentrating it in its leaves. Black tea releases the most fluoride during brewing, averaging 2.65 mg per liter, with some samples reaching over 6 mg/L. Tea bags release about twice as much fluoride as loose leaves, likely because the finely broken leaf particles have more surface area.
Green tea releases comparable amounts of fluoride to black tea. White tea and pu-erh release less, while herbal teas (which aren’t from the tea plant at all) contain negligible amounts. In moderate quantities, fluoride supports dental health, but very heavy tea consumption, especially of lower-grade teas made from older leaves, can contribute to excessive fluoride intake. Older leaves on the tea plant accumulate dramatically more aluminum and fluoride than young leaves. Aluminum concentrations in young leaves range from 250 to 660 micrograms per gram, while old leaves can reach 4,300 to 10,400 micrograms per gram.
What Creates Tea’s Flavor and Aroma
Researchers have identified hundreds of volatile organic compounds in tea, and the mix varies by type. Green tea’s fresh, vegetal aroma comes largely from compounds like linalool (which smells floral), limonene (citrusy), and various aldehydes. Black tea has a more complex bouquet built from geraniol (rose-like), methyl salicylate (minty), benzyl alcohol, and jasmonone, which contributes a jasmine-like sweetness.
Oolong tea, which is partially oxidized, sits between the two. Its aroma profile features linalool alongside phenylacetaldehyde (a honey-like note) and phenylethanol (rose-like). White tea has its own character, with hexanal providing a grassy note and geraniol adding floral sweetness. Dark teas like aged pu-erh develop earthy, woody aromas from an entirely different set of compounds, including naphthalene derivatives and furans that emerge during prolonged microbial fermentation and aging.
The sheer number of these volatile compounds explains why tea tasting can be as nuanced as wine tasting. The same plant produces wildly different aromatic experiences depending on how the leaves are processed, where they were grown, and how you brew them.
How Processing Changes the Chemistry
Every type of tea starts as a leaf from the same species. The chemical differences in your cup come almost entirely from what happens after harvest. Green tea is heated quickly (by steaming or pan-firing) to deactivate the enzymes that would break down catechins, preserving the leaf’s original polyphenol profile. White tea undergoes minimal processing, just withering and drying, which keeps L-theanine and catechins relatively intact.
Oolong is partially oxidized, typically somewhere between 10 and 80%, creating a hybrid profile with both catechins and some theaflavins. Black tea is fully oxidized, converting most catechins into theaflavins and thearubigins. Pu-erh goes further, using microbial fermentation over months or years to produce compounds found in no other tea type, while eliminating L-theanine entirely.
This means that choosing a tea type is, in effect, choosing a different chemical cocktail. Green tea maximizes catechins and L-theanine. Black tea trades catechins for theaflavins and delivers more caffeine. White tea offers the gentlest chemical profile with less fluoride extraction. All of them deliver polyphenols, just different ones.