Tea Polyphenols and Catechins: How the Source of Bitterness Changes

The puckering on your tongue and tightness in your cheeks when you drink young raw tea, and the smoothness of aged tea: much of the difference comes from the same group of constituents, tea polyphenols. They're the "source of bitterness and astringency" in pu'er, and the main actors in how pu'er "changes."

What Are Tea Polyphenols?

What are tea polyphenols? They're not a single substance but the collective name for a large group of polyphenolic compounds in tea leaves, including catechins, flavonoids, anthocyanins, and phenolic acids. Catechins are the largest group, divided into two kinds:

• Ester catechins: such as EGCG (epigallocatechin gallate) and ECG, with the strongest bitterness and puckering.
• Non-ester catechins: such as EGC, EC, and C, less bitter and astringent, with a touch of huigan.

In testing, tea polyphenols are measured colorimetrically with Folin-Ciocalteu reagent according to national standard GB/T 8313; catechins are separated and quantified one by one using high-performance liquid chromatography.

One hallmark of Yunnan large-leaf tea is its rich content of tea polyphenols and catechins, which is why young raw pu'er tastes strong and punchy, and also why it has the "capital" to transform.

The Catechin Profile of Raw Tea

A 2026 study comparing five teas made from Yunnan large-leaf tea measured the catechin composition of raw pu'er (per gram of dry tea):

In raw tea, the two ester catechins EGCG and ECG alone account for more than 70%, which explains why young raw tea is so noticeably bitter and astringent.

How Much Do Raw and Ripe Tea Differ in Polyphenols?

How much do raw and ripe tea differ in polyphenols? By a lot.

• In Yunnan Agricultural University's study of wodui using special-grade sun-dried maocha, tea polyphenols decreased by about 59.74% after processing, while theabrownin increased greatly, the two showing a highly significant negative correlation (correlation coefficient −0.993).
• In the study in the table above, ripe tea had only about a tenth of raw tea's catechins, a reduction of more than 90%.

Where do the lost polyphenols go? The hot, humid environment of wodui and the enzymes secreted by microbes cause catechins to oxidize and polymerize into theaflavins and thearubigins, which polymerize further into larger theabrownin molecules. Some ester catechins are also hydrolyzed, releasing gallic acid, which is why ripe tea actually has several times more gallic acid than raw tea. The result: much less bitterness and astringency, a deep red liquor, and a mellow mouthfeel. The details of the pigments are in Theaflavins, Thearubigins, and Theabrownin.

The National Standard's Polyphenol Requirement

What does the national standard require for tea polyphenols? Among the physicochemical indicators in the national standard for pu'er, GB/T 22111-2008:

• Sun-dried tea and raw tea: tea polyphenols ≥ 28.0%, that is, at least 28.0%.
• Ripe tea (loose and compressed): no minimum for tea polyphenols.

Raw tea has a minimum to ensure the material is rich enough in constituents; ripe tea has none because wodui inherently transforms much of the tea polyphenols, often reducing them by about 60% in research. One requirement loose and one strict, they reflect the different directions of the two processes: raw tea keeps its polyphenols and lets them change slowly in storage, while ripe tea pushes them forward all at once during wodui.

What each physicochemical indicator means is covered in Lab Quality Indicators for Tea, and the full standard in GB/T 22111, the National Standard for Pu'er, Explained.

How Catechins Change with Aging

How do catechins change with aging? Raw tea changes much more slowly than in wodui, but in a similar direction:

• Catechins decrease year by year: a study tracking a 7542 raw tea showed tea polyphenols, catechins, and free amino acids decreasing with age, while flavonoids increased.
• There are turning points: another study analyzed raw tea from the same origin, factory, and storage, kept for 1–10 years, and found it could be divided into three stages by composition, with turning points around year 3 and year 8. The liquor shifted from greenish yellow to orange-yellow and reddish brown, astringency weakened, and sweet mellowness increased.
• Ripe tea changes too: a study comparing ripe tea stored 0, 5, 10, and 15 years found both catechins and caffeine decreasing with age.

Whether aging is driven by oxidation, microbes, or enzymes is debated in both the tea world and the research world; the views are summarized in The Chemistry of Raw Pu'er Aging.

What You Taste When You Drink

• Young raw tea: clearly bitter and astringent on entry, but it fades fast with a quick huigan, a sign of plenty of catechins.
• Raw tea at 5–10 years: astringency softens, the liquor turns orange-yellow, and sweetness increases.
• Ripe tea: almost no astringency, a deep red liquor, and a mellow, smooth mouthfeel.

Brewing also affects how much polyphenol is extracted: hotter water and longer steeping mean more bitterness and astringency. To make young raw tea gentler, shortening the steeping time is the most direct approach; details are in Where Bitterness and Astringency Come From.

FAQ

What are tea polyphenols? The collective name for a large group of polyphenolic compounds in tea leaves, mainly catechins, and the main source of pu'er's bitterness, astringency, and puckering.

How much do raw and ripe tea differ in polyphenols? In research, wodui reduced tea polyphenols by about 60% and catechins by more than 90%; ripe tea has only about a tenth of raw tea's catechins.

What does the pu'er national standard require for tea polyphenols? GB/T 22111-2008 requires at least 28.0% tea polyphenols for sun-dried tea and raw tea, with no minimum for ripe tea.

What happens to catechins when raw tea is stored a long time? They decrease year by year, turning into pigments and other constituents. Research found clear turning points around year 3 and year 8, with astringency softening and sweet mellowness increasing.