What Is in Tea, Part Three

At the heart of the umami of tea is theanine.

Theanine is an amino acid structurally similar to glutamate. It is characteristic of the tea plant, Camellia sinensis, and it is the core of the savour of tea.

Where glutamate is the principal excitatory neurotransmitter in the brain, theanine acts to suppress excitation, and a calming effect may be expected of it. Specifically, it shows partial antagonism at the NMDA receptor, one type of glutamate receptor in the brain, which softens excessive stimulation by glutamate.

In fact, when glutamate over-stimulates the NMDA receptor, Ca²⁺ floods in and causes neurotoxicity — excitotoxicity. Theanine has a neuroprotective action that guards against this.

So in which teas might the calming effect of theanine be expected? In sencha, for instance, the proportions of the amino acids run theanine, then glutamate, then aspartate, then the rest. The theanine in green tea takes the L-theanine form, and it is this that is particularly associated with a calming effect.

The proportion of amino acids overall tends to be higher in spring first-flush teas and in teas containing many of the tips of young shoots. It tends to be lower in teas from leaf picked after summer, and in teas made from hardened leaf.

Theanine is converted during the fermentation stage of black tea manufacture, by enzymes such as polyphenol oxidase, into compounds that make up the colour and aroma of black tea. So the theanine content of fully fermented black tea leaf is a great deal lower than that of green or white tea.

Taken together: a theanine effect may be expected from high-grade green tea of the first-flush season, or from white tea, which is made with only light oxidative fermentation.

The bitterness that spreads slowly across the tongue when you drink tea is most likely caffeine. You will know the same sensation from coffee.

Caffeine is a bitter compound present in many plants — tea, coffee, cacao, the kola nut. In plant physiology it is a functional compound by which the plant defends itself against predators.

For animals, humans included, it is a physiologically active compound producing alertness and stimulation, and it is on the strength of caffeine’s effects that tea, coffee and cacao have taken hold as things we choose to consume.

In tea, caffeine is thought to contribute a great deal to what Lu Yu and Eisai called the supplementing of the heart. The stimulant action of caffeine does in fact raise the heart rate and improve circulation.

That said, the complex action of tea cannot be explained by caffeine alone. It is also thought possible that polyphenols such as catechins influence the absorption and metabolism of caffeine in some way, producing a gentler alertness overall. It is worth keeping in mind that compounds other than caffeine may be involved in the alertness tea brings.

There is a particular sensation to drinking tea — quiet alertness, a clear head with a settled body. This is generally attributed to the balance between calming theanine and stimulating caffeine.

Individual variation is large, but there are reports that a moderate amount of tea before bed does not reduce sleep quality, and that some people feel it relaxing. This is thought to be because the calming action of theanine works against the stimulant action of caffeine, producing a mild effect overall.

Even so, experience differs from person to person. Recent research has begun to show at the genetic level that some people metabolise caffeine poorly and others well.

If caffeine feels like too much, choosing a low-caffeine tea, or stopping tea for a while, is a reasonable option.

So what counts as a low-caffeine tea? To begin with, every type of tea contains caffeine — green, white, oolong, pu-erh, black, and hōjicha and microbially fermented teas too.

The caffeine originally present in the leaf is remarkably stable through almost any processing or ageing; the content in the leaf does not change greatly with the type of tea or the years of maturation.

On the other hand, teas made mainly from mature leaf tend to be lower in caffeine. These are teas made not from young shoots rich in umami and sweetness, but from hard leaf with a distinctive bitterness and savour quite different from caffeine — the teas called bancha.

You can also reduce the caffeine in a cup by how you brew. Caffeine extracts readily from the leaf at 80°C and above, and much less readily below that, so the proportion of caffeine in the resulting liquor comes out lower.

Theanine and caffeine are the best-known constituents of tea. What is worth remembering is that, as we have seen, how much of them ends up in your cup depends on the type of tea and on how you brew it.