The Chemistry of Bitterness, Astringency, and Sweetness

With pu'er, many people remember bitterness first, then an astringency that tightens the tongue, and a little later a sweetness rising in the throat. These three sensations seem like one continuous experience, but they come from different constituents and work in different ways: bitterness and sweetness are tastes, while astringency is a sense of touch.

Bitterness: Caffeine and Ester Catechins

There are two main sources of bitterness in the liquor:

• Caffeine: an alkaloid with a clean, direct bitterness. It dissolves quickly in hot water, and the hotter the water and the longer the steep, the more is extracted. Its content and how raw and ripe differ are covered in Caffeine.
• Ester catechins: catechins with a "gallate ester," such as EGCG and ECG, which are more bitter and astringent than non-ester catechins (EGC, EC).

The two also amplify each other: when caffeine binds with catechins, the perception of bitterness changes. Young raw tea is noticeably bitter precisely because both groups are still abundant; after wodui in ripe pu'er, or years of storage in aged raw tea, ester catechins decrease greatly, and bitterness recedes with them. How catechins change is covered in Tea Polyphenols and Catechins.

Astringency: A "Tightening" Sensation of Touch

Astringency isn't a flavor the tongue tastes but a tactile sensation on the surfaces of the mouth. Tea polyphenols bind with proline-rich proteins in saliva to form complexes or even precipitates, thinning the slippery protective layer in the mouth, so it feels dry, tight, and rough.

The main constituents behind astringency are:

• Ester catechins: the strongest, sharpest astringency.
• Flavonol glycosides: such as glycosides of quercetin and kaempferol. Though present in small amounts, they bring a fine, velvety astringency.
• Phenolic acids such as gallic acid: astringency with a hint of sourness.

So bitterness and astringency often appear together but are two separate things. For how to tell them apart and how to deal with them through brewing, read Bitterness and Astringency: Two Different Things.

Sweetness: Sugars, Amino Acids, and Polysaccharides

Pu'er's sweetness isn't as direct as sugar water; it's gentle and emerges later. Its sources include:

• Soluble sugars: sucrose, glucose, fructose, and so on. Present in small amounts, but they form the base of sweetness.
• Amino acids: theanine has a savory, slightly sweet taste, and glycine and alanine lean sweet. Sun-dried maocha retains amino acids well, so young raw tea often tastes fresh and sweet. See What's in Pu'er.
• Polysaccharides and their conjugates: during wodui in ripe pu'er, some large molecules are broken down and reassembled. Recent research has isolated from ripe pu'er liquor a polysaccharide conjugate made up of rhamnose, galactose, arabinose, and other sugars, with a molecular weight of about 22.93 kDa, and suggests that together with theabrownin it forms ripe pu'er's "mellow, thick, smooth" mouthfeel.

The sweet smoothness and thickness of ripe pu'er on the palate come mainly from these sugars and large molecules; the sweetness of aged raw tea is mostly a matter of sweetness becoming easier to perceive once bitterness and astringency recede.

Huigan: A Phenomenon Without a Settled Explanation

Huigan is one of pu'er's most captivating qualities: after swallowing, sweetness rises in the mouth and throat. There's no single agreed explanation yet; common ones include:

• The contrast theory: after bitter and astringent substances linger in the mouth and are diluted by saliva, the brain perceives the shift back to neutral as "sweet."
• The constituent theory: some research suggests certain phenolics (such as gallic acid and some flavonoids) are related to the strength of huigan; the more of these in the liquor, the more noticeable the huigan.
• The salivation theory: astringency prompts the mouth to produce more saliva, the mouth becomes moist, and the taste turns sweet. That's also why "shengjin" (salivation) and huigan often appear together when tasting tea.

These explanations aren't mutually exclusive and may work together. How to describe them when tasting is covered in Huigan and Shengjin.

How Raw, Ripe, Young, and Aged Taste Different

Adjusting Taste Through Brewing

Since bitterness and astringency come mainly from caffeine and ester catechins, which are extracted most at high temperatures over long times, brewing offers a few levers:

1. Shorten steeping time: pour the first few infusions of young raw tea quickly, so sweetness comes out before bitterness and astringency catch up.
2. Use less leaf: a looser leaf-to-water ratio dilutes bitterness and astringency. See Leaf-to-Water Ratio.
3. Slightly cooler water: for young raw tea with plenty of tender buds, water around 90°C is gentler. See Water Temperature.
4. Don't let it stew: lift the lid after pouring so the spent leaves don't keep steeping in the heat.

The full set of fixes for an overly bitter or astringent brew is in What to Do When Your Brew Is Too Bitter.

FAQ

Where does pu'er's bitterness come from? Mainly caffeine and ester catechins (such as EGCG and ECG); the hotter the water and the longer the steep, the more is extracted.

What causes astringency? Tea polyphenols bind and precipitate with proteins in saliva, reducing the mouth's lubrication, so it feels dry and tight. It's a sense of touch, not taste.

What causes huigan? There's no settled answer. Common explanations include a contrast effect after bitterness and astringency recede, the action of certain phenolics, and the taste turning sweet as the mouth salivates. They may all work together.

Where does the sweetness come from? From soluble sugars and amino acids such as theanine; ripe pu'er also has polysaccharide conjugates that bring a sweet mellowness and thick smoothness.