Bitterness and astringency come up often in our conversations with partners — not just as flavors, but as physical sensations. Drawing from fifteen years of tea research as well as scientific training, we explored what really shapes these textures at the molecular level. Here's what we found.
Catechins and tannins
Tannins and catechins are often spoken of together, but they behave differently.
Tannins — found in wine — are large, heavy polymers. They evolve slowly across years, building a grip that softens over time. Catechins — found in tea — are nimble. They shape the experience immediately, a structure that appears, holds, and fades with each sip.
They also have a different chemistry. Catechins belong to the flavan-3-ol family: flat, reactive rings designed for hydrogen bonding. Tannins, by contrast, are polymers — long, linked chains of flavan-3-ols and other flavonoid units — creating larger, more complex structures.
The word ‘tannin’ itself comes from an older, material tradition. It originally referred to the crushed oak bark used in tanning animal hides — an ancient process practiced by the Chinese, Greeks, and Romans. Plant tannins could transform skin into leather. Catechins from tea, by contrast, could not; their structure is too light and fleeting to produce such lasting physical change.
Today, tannins and catechins still shape sensory experience, but on a much finer scale — especially in tea. Within the catechin family, there are four forms: epigallocatechin gallate (EGCG), epigallocatechin (EGC), epicatechin gallate (ECG), and epicatechin (EC).
Small differences between them matter. Gallated catechins like EGCG and ECG bind more quickly and tightly to proteins, creating sharper, more immediate sensations. Non-gallated catechins like EGC and EC produce a softer, more rounded finish.
Having them both in one sip defines how clean, firm, or relaxed a tea feels as it opens across the tongue.
The effect of agriculture and steaming on bitterness
Growing conditions play a huge role in the formation of astringency and catechins in tea.
Under full sun, the tea plant invests heavily in defensive compounds — especially EGCG — as natural sunscreens against UV radiation and pests.
Shaded tea operates differently. It lowers this defense mechanism and redirects energy toward building amino acids, sugars, and more complex flavor structures. Spring harvest tea, known as Shincha, shows this transformation most clearly.
Intentional growing shifts the leaf’s chemistry toward lower catechins and higher amino acids. But full flavour formation passes through one more stage. To complete it, farmers either steam the leaves to fix the chemistry and lock in clarity, or pan-fry them to soften the structure and allow oxidation to reshape the flavor.
Processing decisions sharpen these differences. Light steaming, used for Sencha and Gyokuro, locks in the catechin structure and holds sharper tension. Pan-firing, common in the west of Japan, allows partial oxidation and softens the catechins, broadening the mouthfeel.
The depth of steaming further shapes the final texture. Light steaming (asamushi) holds onto fine clarity. Medium steaming (chuumushi) balances structure and softness. Deep steaming (fukamushi) breaks the leaf down, creating thicker, richer textures with less tension but greater body.
If the farmer allows oxidation to continue, catechins polymerize into theaflavins and thearubigins. This expands the mouthfeel from silken precision to velvety density. The exact moment the farmer halts enzymatic activity defines whether a tea stays vivid or softens into richness.
Closing reflections
In the finished brew, the balance announces itself immediately. The tea at Circadian shows its quiet tension within seconds, then resolves into clarity. It moves through the mouth like a well-made object moves through the hand — designed to be felt, not flaunted. A grand Cabernet Sauvignon, by contrast, builds its structure to hold for decades before easing.
