White, green, oolong and black teas are all made with leaves from the same evergreen shrub, camellia sinensis. Each tea’s particular look, taste and smell are the result of processing.
Besides white tea, green is the least processed. After picking, it’s steamed and sun-dried, then tumble-dried or pan-fried in a dry wok, which quickly halts oxidation (i.e. browning) and results in a mild grassy flavour.
In contrast, black tea is rolled or crushed, then subjected to heat and humidity to accelerate oxidation, concentrating the flavour. This makes for a bolder, more astringent cup. The partially oxidised oolong sits somewhere between the two.
Tea doesn’t contain useful amounts of essential nutrients, but it’s been suggested – inconclusively – that other compounds in it may protect against cancer, heart disease and Alzheimer’s.
Brewed tea does contain a significant amount of caffeine (which can vary sizeably depending on how the brewing method) and small amounts of theobromine and theophylline, two other stimulants. Despite this, tea is noticeably less likely to induce coffee-like jitters. It’s thought this is because another molecule in the leaves, theanine, has a calming effect on mood.