What is in the leaf
Tea leaf contains a substantial pool of free amino acids, and theanine — an amino acid characteristic of the tea plant — is the most abundant of them in most cultivars, typically accounting for a large share of the total. Glutamic acid, aspartic acid, arginine and serine are present alongside it. These are highly water-soluble and extract readily, including at low temperatures. In sensory terms they are associated with umami, the savoury brothy quality that also characterises stock and seaweed, and with a soft sweetness that accompanies it. A leaf with a high amino acid level relative to its polyphenols produces a sweet, thick, savoury liquor with modest astringency; one with the balance the other way produces a brisk, grippy, more bitter one. Almost everything interesting about Japanese green tea follows from that ratio. The ratio between this fraction and the polyphenols is used within the tea industry as an evaluative measure for green tea, which is a useful indication of how central it is understood to be. This catalogue does not reproduce any figure for it, for the reasons set out below, and the underlying idea is worth carrying: what determines the character of a green tea is not the level of any one component but the balance between two families that respond differently to light, to season and to brewing temperature.
Where theanine comes from and what light does to it
The standard agronomic account is that theanine is synthesised in the roots and transported up into the growing shoots, and that in the presence of light it is progressively converted along the pathway that leads to catechins. Shading the bush before picking limits that conversion, so shaded leaf retains more theanine and forms fewer catechins — which is the reason gyokuro, kabusecha and tencha taste sweet and brothy where an unshaded sencha from the same cultivar tastes brisker and greener. It is also why shading is described in this catalogue as an agronomic decision with a chemical purpose rather than as a tradition. The direction of the relationship is well established; the magnitudes vary with cultivar, shading method, duration and season, and no figures are given here. The root-to-shoot part of the account also explains a piece of agronomy that otherwise looks superstitious. Because the supply comes from the roots, the vigour and age of the root system and the plant’s recovery between harvests both bear on what the next flush will contain — which is why plucking intensity, rest periods and the treatment of the bush after a heavy harvest are managed as carefully as they are in the Japanese shaded-tea districts.
The other things that raise the ratio
Season is the largest after shading. Through winter dormancy the plant accumulates reserves, and the first flush of spring is generally richer in amino acids relative to catechins than the second or third pickings taken under strong summer light and rapid growth — which is the chemical account behind the premium on shincha, on ichibancha and on Chinese greens picked before the Qingming festival. Plucking standard matters, since the bud and youngest leaves differ from mature leaf. Cultivar matters substantially, and several Japanese cultivars are selected specifically for this character. Nitrogen availability in the soil is associated with amino acid levels in the leaf, which is why fertiliser practice in Japanese tea growing is unusually intensive and why it is treated as an agronomic and environmental question in its own right. Stems are the fourth factor and produce a whole product category. Stem and vein material carries a different balance from leaf blade, which is why kukicha and karigane are distinctly sweeter and lighter than the sencha they are separated from, and why they are made deliberately rather than sold as a by-product. A drinker who likes the sweetness of a shaded tea and does not want its price will often find the stem teas a better route than a lesser grade of leaf.
Why amino acids drive brewing practice
Because they extract well at temperatures where the polyphenols extract poorly. That asymmetry is the whole reason for the very low brewing temperatures used for gyokuro and for the highest grades of sencha: cool water collects the sweet savoury fraction and leaves much of the bitter and astringent fraction in the leaf. The same principle underlies cold brewing, where a long extraction at refrigerator temperature produces a strikingly sweet, low-astringency liquor from teas that would be harsh brewed hot, and it underlies the practice of a very short first infusion in some gongfu styles. It also explains a common disappointment: a fine shaded tea brewed at the temperature suitable for a black tea will taste bitter and thin, having collected the wrong fraction, and no amount of shortening the steep fully repairs it. The extreme version of the technique is worth knowing because it demonstrates the principle. Kooridashi, in which ice is placed on the leaf and allowed to melt slowly, produces a very small quantity of intensely sweet and savoury liquor with almost no astringency at all, from the same leaf that would be brisk and green brewed hot. It is not a better method, and it is a clear demonstration that temperature is selecting rather than merely pacing.
What happens to them in other categories
Amino acids do not survive processing unchanged. In oxidation they participate in reactions with other leaf components, and in firing and roasting they react with sugars in the Maillard reactions that generate roasted, nutty and caramel aromas — so a heavily roasted tea has traded part of its amino acid pool for aroma compounds, which is a genuine exchange rather than a loss. This is one reason black and roasted teas do not show umami the way a shaded green does, and why their sweetness reads differently: as malt and caramel rather than as broth. Matcha is the extreme case in the other direction, since the whole leaf is consumed rather than infused, so the amino acids present are all delivered rather than partially extracted, which is a large part of why koicha is as thick and savoury as it is. White tea sits in an interesting middle position. It is minimally processed and never fired hard, so a good deal of the fraction survives into the finished tea and a fresh silver needle can show a distinct sweetness alongside its hay and melon character. As the same material ages the balance shifts again, which is part of why aged white tea tastes so unlike the fresh product and why the category is best understood as two products rather than one.
The simplification to avoid
TeaHQ's own reading of well-attested general knowledge, with no single source behind it.
Theanine is routinely presented as the compound that explains umami in tea, and the truth is more combinatorial. Several amino acids contribute, glutamic acid among them and directly associated with umami in foods generally. The perception of umami and sweetness also depends on what else is present, since bitterness and astringency suppress both — so raising the amino acid level and lowering the polyphenol level produce the same perceptual result by different routes, and shading does both at once. And a substantial part of the sweetness attributed to this fraction is actually aromatic, arriving through the nose. TeaHQ therefore records umami and sweetness as separate sensory axes assigned by tasting rather than derived from any compositional claim, because the cup is the only place the combination can be observed. There is also a practical reason not to reason from composition to preference. Two teas with a similar balance can taste quite unalike because their aromatic fractions differ entirely, and aroma supplies most of what a drinker calls flavour. Composition explains why a category behaves as it does under heat and time; it does not predict whether anybody will enjoy a particular tea, which remains a question that only a cup answers.
What this page does not say
No quantity, ratio or percentage is given for theanine or for any other amino acid, in any tea. Values vary with cultivar, shading regime, season, plucking standard, growing conditions and analytical method, and a number would imply a precision this catalogue does not have. Nothing here describes any effect on a person: the account above concerns the composition of the leaf and its association with flavour and texture in the cup, and stops there. Where popular writing continues past that point, it is making a different kind of claim, governed by a different standard of evidence, and its presence in tea marketing is not a reason for a catalogue to repeat it. Nor does this page assert any comparison between categories. Statements that one type of tea contains more of this fraction than another hold as broad tendencies driven by shading and season, and fail routinely for particular pairs of teas. Where the catalogue records a relationship it records the mechanism and the direction — shading raises the balance in this direction, summer growth in that one — and leaves the magnitudes to whoever has measured the specific leaf in question.