Polyphenols at a glance
- Definition: Polyphenols are a group of secondary plant compounds with multiple hydroxyl groups on aromatic rings.
- Found in: Tea, coffee, berries, red wine, and many other plants.
- Main groups: Phenolic acids and flavonoids, with catechins (including EGCG) as the most important flavonoid group in tea.
- Taste: They create bitterness and astringency – the slightly tightening mouthfeel typical of tea.
- Tea type difference: Green tea contains the most polyphenols, black tea the least, because oxidation converts part of them into theaflavins and thearubigins.
- Diversity: Over 8,000 different polyphenols are known, more than 2,000 of them in the tea plant alone.
Description
Polyphenols belong to a group known as secondary plant compounds – substances a plant doesn't need to survive, but uses to defend itself. In the tea plant, they protect against UV radiation, ward off pests, and shape the color and flavor of the leaves.
Chemically, polyphenols can be roughly divided into two main groups: phenolic acids and flavonoids. In tea, the flavonoid group of catechins dominates, led by the extensively researched epigallocatechin gallate, or EGCG for short. Catechins often make up the largest share of polyphenols in green tea and are water-soluble – they transfer directly into the cup during steeping. Together with caffeine and L-theanine, they're among the most defining compounds in the tea plant and also shape the characteristic umami taste of high-quality green teas.
How many polyphenols end up in your cup depends heavily on preparation: softer water below 80°C (176°F) dissolves more catechins than boiling, hard water, which breaks down part of them. The longer a tea steeps, the more polyphenols the water draws out of the leaf – though the ideal steeping time varies by type. Worth knowing: steeping longer mainly means more bitterness, not automatically "healthier."
Polyphenols across tea types
The degree of oxidation determines how many and which polyphenols end up in the cup. Green and white tea remain unoxidized or are only minimally processed, retaining most of their original catechins, especially EGCG – making them particularly catechin-rich. Oolong is only partially oxidized: some catechins have already converted, some remain intact, giving it a taste that sits noticeably between green and black tea. Black tea is fully oxidized, converting catechins into new polyphenols like theaflavins and thearubigins, which create its dark color, bold aroma, and milder bitterness. Pu-erh undergoes an additional microbial fermentation that transforms the polyphenol profile even further.
Historical background
Long before anyone spoke of "polyphenols," tea drinkers knew them by a different name: tannins. The term originates from leather tanning, where plant extracts were used to treat animal hides – polyphenols bind proteins, which is exactly what gives them their astringent, tightening effect on the tongue.
Today we know that tannins are just one subgroup within the much larger class of polyphenols, specifically polymerized – meaning larger, linked – flavonoids. It wasn't until modern analytical methods emerged in the mid-20th century that the full diversity of polyphenols in tea could be properly identified – and from the 1990s onward, their antioxidant properties became a major research focus.
In growing regions like China and Japan, the link between oxidation and flavor was put to practical use centuries earlier, long before the underlying chemistry was understood: depending on how much the leaves were oxidized, not just the color changed, but also the character of the astringency – from the fresh bite of green tea to the fuller, deeper intensity of black tea.
Fun facts
- Younger leaves, more polyphenols: The youngest leaf tips of a harvest tend to have the highest polyphenol concentration – one reason delicately picked first flush harvests are especially prized.
- Milk binds catechins: Proteins in milk can bind to catechins – a possible reason some tea connoisseurs skip milk with particularly polyphenol-rich teas.
- Not just in tea: Coffee, berries, and red wine are also major dietary sources of polyphenols.
- Cloudiness as a side effect: As tea cools, polyphenols can bind with other compounds and cause slight cloudiness – a purely visual effect, not a sign of poor quality.
- Studies point to antioxidant effects: Polyphenols are considered one of the compound groups with particularly pronounced antioxidant properties in research – their concrete effects on the human body continue to be studied.
