Tea instructions specify different temperatures and times for different types, and the specifications reflect the chemistry of what is being extracted rather than convention.
What is being extracted
Caffeine, which contributes bitterness and stimulation.
Polyphenols, particularly tannins, which contribute astringency and body.
Amino acids, which contribute sweetness and savoury character.
And volatile aromatic compounds.
Each extracts at different rates depending on temperature, which is why temperature changes the balance rather than only the speed.
The temperature effect
Higher temperatures extract tannins faster than amino acids.
Which means water that is too hot produces bitterness and astringency while under-extracting sweetness.
This is why green teas, which have more amino acids and less robust structure, are brewed cooler.
Black teas, fully oxidised and more robust, tolerate and require higher temperatures.
Oxidation
The variable that distinguishes tea types, all of which come from the same plant.
Green tea is heated quickly after picking to halt enzymatic oxidation.
Black tea is fully oxidised, which converts the polyphenols into different compounds.
Oolong is partially oxidised across a wide range, which is why the category covers such varied teas.
White tea is minimally processed.
Which means the same leaf produces entirely different results depending on processing, and the brewing must follow.
Fermented teas
A distinct category where microbial action is involved, unlike the enzymatic oxidation of other types.
Which produces earthy characteristics and teas that improve with age, unlike most tea which deteriorates.
These generally tolerate high temperatures and repeated brewing.
Multiple infusions
The approach used in several traditions, where the same leaves are brewed repeatedly with short steeps.
Which extracts different compounds in each infusion, producing a sequence of different flavours from one quantity of leaf.
It uses more leaf and less time per infusion than the single long steep common in western practice.
The results are genuinely different rather than merely more economical.
Leaf quality and grade
Whole leaves extract more slowly than broken ones, since surface area is smaller.
Which is why tea bags, containing fine particles, brew fast and can over-extract quickly.
Grade designations describe leaf size and appearance rather than quality directly, and the relationship between them is loose.
Water
Affects tea as much as coffee.
Hard water produces a dull flat brew and the surface film sometimes seen.
Very soft water can produce a thin result.
Freshly drawn water, boiled once, is the traditional advice, and the reasoning about dissolved oxygen is more folklore than chemistry, while reboiled water does concentrate minerals slightly.
Milk and additions
Milk proteins bind tannins, reducing astringency, which is why milk suits strong black teas and not delicate ones.
Whether milk goes in first is a long-standing argument with a practical origin — it prevented fine china cracking from hot tea.
Adding milk first does result in gentler heating of the proteins, which some argue affects flavour, and blind testing has produced mixed results.
Lemon lightens the colour by changing the pH, which affects the compounds responsible for it.
Storage
Tea absorbs odours readily and degrades with light, heat, air and moisture.
Which means an airtight opaque container away from spices and coffee, at room temperature.
Green teas deteriorate faster than black and benefit from refrigeration in sealed containers for long storage.
Quantity
Weighing produces consistency, since leaf density varies enormously between types.
A spoonful of a large-leaf tea and a spoonful of a fine one contain quite different amounts.
Cold brewing
Steeping in cold water for hours extracts less tannin and caffeine, producing a smoother sweeter result.
Which works well for green and delicate teas, and takes overnight.
Caffeine
Varies by type, by leaf and by brewing more than the common claims suggest.
The frequently repeated claim that a brief rinse removes most caffeine is not supported — caffeine extracts steadily rather than immediately.
Decaffeinated tea retains a small amount, which matters for anyone genuinely avoiding it.
Vessels
Material affects heat retention, which affects the brew.
Unglazed clay absorbs and retains flavour over time, which some traditions treat as desirable and which means the vessel is dedicated to one type.
Glass and porcelain are neutral and lose heat faster.
Herbal infusions
Not tea, since they come from other plants entirely.
Which means the brewing guidance for tea does not apply — most tolerate boiling water and longer steeping, since they lack the tannins that cause bitterness.
Some contain compounds that interact with medication, which is worth checking for anything consumed regularly.
Blends and flavourings
Blended teas are formulated for consistency across seasons and origins, which is a genuine skill.
Flavoured teas vary enormously in whether the flavouring is natural and in how much it masks the base.
Which is why a good flavoured tea uses a base worth drinking and a poor one uses flavouring to disguise it.
Which means the leaf is worth buying loose rather than in bags, where the quality is generally lower and the fine particles over-extract.