All right. So let's start by making clear that there's tea tea, and various herbal teas (infusions) where you can use a shitload of different plants (examples: peppermint, yerba-mate, coca...).
Tea itself is prepared out of mostly leaves, but also twigs or buds of the plant Camellia sinensis
(and sometimes also closely related species such as taliensis
), sometimes with addition of other herbs for flavour. The plant is a shrub that can be easily maintained at height below 2m for easy harvesting, and is grown in rainy subtropical regions – the plant requires a lot of water and deep soil. Tropical highlands are also suitable (Kenya, Ceylon). While majority of teas derive from Camellia sinensis
, there are two main varieties being grown – var. sinensis
in East Asia, and var. assamica
, with larger leaves, mainly grown in India and other places where the British started plantations, but also deriving from southern China. Tea can then also be divided by regions where it is grown, but let's not go into too much details here.
What's more important is that there are different ways of processing the leaves. Green tea is dried in a pan or steamed quickly soon after picking, which prevents the leaves from fermenting (rotting). Some Japanese green teas are shaded – the leaves are covered while growing, which changes their chemical composition (kabusecha, gyokuro) White tea is processed in a similar manner, but drying is done usually by sundrying, so it is done slower (but presents less thermal shock to the tea) – white tea is usually prepared only from immature leaves and buds. Oolong is processed by withering under sun more strongly after partially fermenting the leaves. Black tea (and red tea, there's no real difference aside from region) is produced by again letting the leaves ferment for longer in warm humid rooms, exposing the leaves to pine smoke during this process produces lapsang souchong. Pu'er is also fermented, but after
drying by pressing the leaves together, and exposing them to microbial fermentation in controlled humid atmosphere for a long period of time. A special type of tea is also kukicha, which is green tea produced from twigs. Tea can also be powdered (matcha, in Japan from green tea) and used either in recipes or dissolved in water or milk and consumed this way, or eaten as leaves (lahpet, in Burma).
What are benefits of tea? General health benefits are unclear, but it seems to have positive effect on cardiovascular diseases and slowing down progression of Alzheimer's.
Clearer are tea's psychoactive properties. The main substances found in tea are caffeine (around 3% of dry weight) and ʟ-theanine (around 1% of dry weight, higher in shaded teas). ʟ-theanine is more sensitive to oxidation, which means that in order to extract it one needs to use lower steeping temperature (around 70-75°C), and that black tea and other more fermented varieties contain lower contents. A cup of tea (250mL) will contain about 60-80g of caffeine and 20-25g of ʟ-theanine, of course depending on steeping time, amount of tea used, kind of tea...
Caffeine is a stimulant and globally the most widely used psychoactive substance, chemically it's a xanthine; I will not discuss this much as caffeine is well-known.
ʟ-theanine is an aminoacid derived from ʟ-glutamine. It positively reinforces serotonin, dopamine, glycine, and GABA, along with stimulating some specific receptors. As a result, ʟ-theanine causes relaxation (both physical and psychological), countering the most unwanted effects of caffeine, while increasing mindfulness, concentration, and cognition. Personally I'd say that ʟ-theanine "high" is the zen state of mind – sharp, focused, yet calm. For that reason, ʟ-theanine is commonly used as a nootropic – for increasing productivity, especially when learning.
A bit less wanted side effect is that tea is often grown in soils naturally rich with fluoride (South China and India have geological history of volcanism while East Africa and Japan still tectonically active – which brings fluoride to the surface) so consuming a lot of tea is basically the same as drinking fluorinated water. South China and Japan have generally lower fluoride concentrations so those teas are safer in this regard.
Besides that, the whole custom of brewing (and sharing) tea is a good method to relax your mind.