Composition & Flavour · the Ratio

Whether a cup leans fresh-mellow or bold is no mystery — it depends on the balance of three substances: the polyphenols that make tea astringent and bitter, the caffeine that adds bitterness and lift, and the theanine that brings freshness and sweetness. Dial them yourself below:

Raise theanine and lower polyphenols and the tea leans fresh-sweet (like shaded gyokuro); reverse it and it turns bold and astringent (like assamica for black tea). Their levels and ratio are the “mixing desk” of a tea’s style.

The three leads and their tastes

Absolute amounts set strength; the ratio between them sets the style. But “polyphenols bitter-astringent, amino acids fresh-sweet” is only the beginning — inside each substance there are distinct characters worth separating.

Catechins: the skeleton of astringency and bitterness

Roughly 70–80% of tea polyphenols are catechins, and catechins fall into two families[1]:

• Galloylated (ester-type) catechins: EGCG, ECG, GCG and others, carrying a galloyl group;
• Non-galloylated (simple) catechins: EGC, EC, GC and others, without that group.

That one group makes a world of difference in taste: galloylated catechins bind salivary proteins far more strongly, making them the dominant source of astringency and the more bitter ones; simple catechins are far less astringent and carry a faint returning sweetness[1][3][4]. Note that astringency is strictly not a taste but a tactile sensation in the mouth — polyphenols cross-link and precipitate proline-rich salivary proteins, stripping the lubricating film from the oral mucosa and producing that “sandpaper” contraction[1].

German sensory chemists Scharbert and Hofmann gave black tea taste a “molecular dissection” using quantitative analysis, taste reconstitution and omission experiments: they first quantified a dozen-plus tastants in black tea, then dissolved them back into pure water at their measured concentrations — a panel judged the reconstitute almost indistinguishable from real infusion; omitting compounds one by one showed the largest contributions came from caffeine and a group of astringent compounds (galloylated catechins, flavonol glycosides, etc.)[3]. Threshold measurements on individual catechins further confirmed that galloylated catechins (EGCG, ECG) have markedly lower bitter/astringent thresholds than simple ones[4] — which is why summer teas and large-leaf cultivars, rich in galloylated catechins, taste so much “biting”.

Theanine: where umami comes from, and how it is tuned

L-theanine is a non-protein amino acid nearly unique to the tea plant, often accounting for about half of the total free amino acids in tea[1]. Its metabolic pathway is itself the key to understanding the polyphenol/amino-acid ratio:

1. Synthesised in the roots: theanine is formed from glutamic acid and ethylamine in the roots;
2. Transported to the shoots: it travels up through the stem and accumulates in young buds and leaves — one reason buds taste the freshest;
3. Converted in the leaf: under light, theanine is gradually broken down and its carbon skeleton feeds into catechin synthesis — “strong light → more polyphenols, fewer amino acids” is essentially a rerouting of the same metabolic flow[1][6].

Unno et al. (2018) showed that under stresses such as shading, theanine-synthetase gene expression is up-regulated while the conversion of theanine into catechins is suppressed, so free theanine accumulates[6] — shading and generous nitrogen fertilisation (which supplies raw material for amino-acid synthesis) both work by “damming” this pathway (see leaf chemistry for the shading mechanism).

The taste side holds a counter-intuitive twist: Kaneko et al. (2006), in a molecular sensory study of sencha, found that theanine’s threshold on its own is actually quite high — far above its concentration in the infusion; its umami contribution relies largely on synergy with other umami substances such as glutamic acid[5]. That is why total amino-acid content predicts a green tea’s freshness better than theanine alone.

Caffeine: beyond bitterness — the “complexation” plot

Caffeine brings bitterness and stimulation — common knowledge[1]. Less known is its mechanistic subplot: caffeine molecules form complexes with catechins and theaflavins through hydrophobic association and hydrogen bonding.

Jöbstl et al. (2005) studied black-tea “creaming” (tea cream) — the milky haze that appears as a strong infusion cools — using small-angle X-ray scattering and NMR: theaflavins first self-associate into nanoclusters about 3 nm across, and caffeine then fills the gaps within the clusters, together forming the bulk of the haze[7]. This has two flavour implications:

• A proxy for quality: a black tea that “creams down” conspicuously was traditionally prized as bold — it signals an abundance of both polyphenol oxidation products and caffeine[1][7];
• Softening bitterness and astringency: complexation temporarily “locks away” some caffeine and polyphenols inside colloidal structures, lowering their free (tasteable) concentrations. This is considered one reason why strong infusions often taste less bitter-astringent than the sum of their parts would predict — though this is a mechanism-level explanation, and actual perception also depends on temperature and matrix[1][7].

In the same cup: orders of magnitude of taste thresholds

Whether a compound “contributes” depends on whether its concentration in the infusion exceeds its detection threshold. The table below gives order-of-magnitude thresholds in pure water alongside typical infusion concentrations, to build intuition[3][4][5]:

Thresholds vary several-fold — even by an order of magnitude — with method, matrix (pure water vs real infusion) and individual sensitivity; the figures here are orders of magnitude only. What actually matters in sensory science is the “taste activity value” (concentration ÷ threshold): only above 1 is a compound likely to be perceived[3].

Absolute amounts set strength, while the ratio between them sets the style — and the most-used yardstick is the polyphenol/amino-acid ratio.

The key index: the polyphenol/amino-acid ratio

The ratio of polyphenol to amino-acid content is an important index of quality and style (especially for green tea)[2]:

• Low ratio → relatively more amino acids → fresh and mellow, ideal for fine green tea;
• High ratio → relatively more polyphenols → bold and astringent, better suited to black tea.

The concept is abstract; three typical scenarios make it concrete:

• Early-spring, bud-only fine green tea: slow growth in cool weather and tender plucking push amino acids up to 3–4.5% of dry weight with moderate polyphenols, so the ratio often sits low, around 4–6 — the material basis of freshness. Wang and Ruan (2009), analysing Longjing and other green teas, found amino-acid content positively correlated with sensory quality scores, while the polyphenol/amino-acid ratio correlated significantly and negatively[8].
• Summer tea: strong light and heat raise polyphenols (especially galloylated catechins) and lower amino acids; the ratio can climb to 8–12 or beyond, giving a harsh, bitter-astringent cup mostly used for commodity tea or as base material for further processing[1][2].
• Yunnan large-leaf (assamica): polyphenols often exceed 30% of dry weight, and the ratio is markedly higher than in small- and medium-leaf varieties — precisely the biochemical basis of its suitability for bold black tea[1].

Note: absolute ratios depend on the analytical method (total amino acids by ninhydrin vs individual amino acids by HPLC use different measures) and on sampling, so numbers from different sources should not be compared directly. The examples above are typical magnitudes common in textbooks and studies — meant for building a sense of direction, not for judging any specific tea[1][8].

What lowers the ratio

Anything that relatively raises amino acids and curbs polyphenols lowers the ratio and makes tea fresher:

• Tender plucking: buds and young leaves are higher in amino acids, lower in polyphenols (early-spring buds taste fresher)[2].
• Shading: suppresses the conversion of theanine into polyphenols (gyokuro, matcha) — see leaf chemistry[6].
• High altitude / low temperature / spring: slower growth lets amino acids accumulate[2].
• Sound nitrogen management: nitrogen is the raw material for amino-acid synthesis — the heavy nitrogen regime of gyokuro gardens applies exactly this principle[1].

Conversely, strong sun, heat, summer and large-leaf cultivars tend to give a higher ratio, better for bold black tea.

Where does aroma come from? A teaser

The three leads above decide taste on the tongue; tea’s aroma comes from another cast of trace volatiles — several hundred have been identified across tea types so far[9][10]. Interestingly, much of the aroma in fresh leaves is not free but “locked” as glycosidically bound precursors, released by enzymatic hydrolysis or heat during processing (wounding, rolling, drying)[11]. So the same leaf can smell utterly different depending on the process it goes through — the full “from compounds to aroma” story is told on the flavour wheel page.

Composition across the six tea types

The chart below gathers the typical dry-weight ranges of the main components across the six categories (synthesised from tea-biochemistry literature). Pick a component to compare the six teas:

Full data:

Values are typical ranges of dry-tea weight percentage and vary widely with cultivar, origin, plucking standard and process; theaflavins / thearubigins / theabrownins are products of oxidation and post-fermentation. Synthesised from Wan Xiaochun, Tea Biochemistry (3rd ed.), Chen et al. (2009), Wang et al. (2010), Xu et al. (2018), etc. [12][13][14]

See also

• The Flavour Chemistry of the Fresh Leaf — where the three substances come from
• The Flavour Wheel — how composition becomes specific aroma and taste
• Brewing Basics — how brewing also shifts the bitter/sweet balance

References

1. Wan Xiaochun. Tea Biochemistry (3rd ed.). China Agriculture Press, 2003.
2. Wang Yuefei, et al. Tea Culture and Tea Health. Zhejiang University Press, 2021. / Wan Xiaochun. Tea Biochemistry (3rd ed.). China Agriculture Press, 2003.
3. Scharbert, S., & Hofmann, T. (2005). Molecular definition of black tea taste by means of quantitative studies, taste reconstitution, and omission experiments. Journal of Agricultural and Food Chemistry, 53(13), 5377–5384. https://doi.org/10.1021/jf050294d
4. Xu, Y.-Q., et al. (2018). Quantitative analyses of the bitterness and astringency of catechins from green tea. Food Chemistry, 258, 16–24. https://doi.org/10.1016/j.foodchem.2018.03.042
5. Kaneko, S., Kumazawa, K., Masuda, H., Henze, A., & Hofmann, T. (2006). Molecular and sensory studies on the umami taste of Japanese green tea. Journal of Agricultural and Food Chemistry, 54(7), 2688–2694. https://doi.org/10.1021/jf0525232
6. Unno, T., et al. (2018). Stress-induced synthesis of theanine in tea plants Camellia sinensis. Journal of Plant Physiology, 231, 261–267. https://doi.org/10.1016/j.jplph.2018.10.006
7. Jöbstl, E., et al. (2005). Creaming in black tea. Journal of Agricultural and Food Chemistry, 53(20), 7997–8002. https://doi.org/10.1021/jf0506479
8. Wang, K., & Ruan, J. (2009). Analysis of chemical components in green tea in relation to perceived quality. International Journal of Food Science & Technology, 44(12), 2476–2484. https://doi.org/10.1111/j.1365-2621.2009.02103.x
9. Ho, C.-T., Zheng, X., & Li, S. (2015). Tea aroma formation. Food Science and Human Wellness, 4(1), 9–27. https://doi.org/10.1016/j.fshw.2015.04.001
10. Recent Advances in Volatiles of Teas. Molecules / PMC, 2018. https://pmc.ncbi.nlm.nih.gov/articles/PMC6273888/
11. Li, P., Zhu, Y., Lu, M., et al. (2019). Variation patterns in the content of glycosides during green tea manufacturing by a modification-specific metabolomics approach. Food Chemistry, 274, 975–982. https://doi.org/10.1016/j.foodchem.2018.11.148
12. Chen, G.-H., et al. (2009). Comparison of major chemical components in different kinds of tea. Journal of Agricultural and Food Chemistry, 57(15), 7325–7330.
13. Wang, K., et al. (2010). Comparison of catechins and amino acids in white, green, oolong, black, and pu-erh teas. Food Chemistry, 121(2), 338–344.
14. Xu, Y.-Q., et al. (2018). Chemical compositions and biological activities of different types of tea. Comprehensive Reviews in Food Science and Food Safety, 17(2), 432–452.

The interactive is an illustrative model, not a precise measurement; the real ratio uses measured polyphenol and amino-acid contents.