How Is Tea Made? One Plant, Six Colours and a Great Deal of Oxidation

Two main varieties are grown: the small-leaved Chinese variety, hardy and suited to cooler hill country, and the large-leaved Assam variety, found growing wild in northeast India and better suited to hot, wet lowlands, which is what most commodity black tea is made from. Left alone the plant becomes a tree; in cultivation it is kept as a waist-high bush so the new growth can be reached. What is picked matters as much as what is done afterwards, and the standard for fine tea is the bud and the two leaves below it, taken by hand, because those contain the highest concentration of the compounds that matter and the least fibre. Mechanical harvesting takes everything, which is cheaper and coarser. The same bush yields several flushes a year, and the first growth after winter dormancy is generally the most prized, which is why first flush Darjeeling commands what it does.

Inside a tea leaf, enzymes and polyphenols are kept in separate compartments. Bruise or roll the leaf and the cell walls break, the two meet in the presence of air, and the enzymes convert the pale, bitter catechins into larger coloured compounds called theaflavins and thearubigins, which are what make black tea dark, brisk and malty. The process is enzymatic browning, exactly what happens to an apple or an avocado, and it is often loosely called fermentation in the tea trade even though no microbes are involved. It can be stopped at any point by heat, which denatures the enzymes permanently, and this single decision, applied early, late or not at all, is what produces the categories. Everything else, the withering, the rolling, the shaping and the firing, is in service of controlling it.

The classical Chinese scheme divides tea into six, and the difference between them is a sequence of steps rather than a different crop:

• •Green: heated almost immediately after picking, by pan-firing in China or by steaming in Japan, so oxidation never begins. Steaming gives the marine, grassy character of sencha and gyokuro; pan-firing gives the nuttier, toastier Chinese style
• •White: the least processed, simply withered and dried with no rolling and no deliberate heating, so a little oxidation occurs at the edges. Made from buds and young leaves, and pale and delicate as a result
• •Yellow: a rare Chinese category, made like green tea with an extra slow smothering step under cloth that mellows the grassiness
• •Oolong: partially oxidised anywhere from about 10 to 80 percent, with the leaves repeatedly bruised at the edges, rested and shaken, which is the most labour-intensive and most variable category of all
• •Black, called red tea in China: fully withered, thoroughly rolled and left to oxidise completely before firing, which is roughly 85 percent of world production
• •Dark, including pu-erh: genuinely fermented by microbes after the enzymes are killed, aged for months or decades, and the only category where microbial action is the point

Most tea drunk in Britain, Ireland and much of the world is not made by the orthodox method at all. The crush, tear, curl process, developed in Assam in the 1930s, passes withered leaf through rollers with hundreds of small teeth that shred it into granules, which vastly increases the surface exposed to air, speeds oxidation to a couple of hours, and produces a tea that brews very fast, very strong and very dark, which is what a teabag with milk requires. It is efficient and it is not subtle, since the shredding destroys the volatile aromatic compounds that orthodox processing preserves. Grading terms on a packet describe leaf size and appearance rather than quality: orange pekoe, contrary to universal belief, refers to a leaf grade and has nothing to do with oranges or with flavour, and the abbreviations stacked in front of it describe how much bud is present and whether the leaf is broken.

The chemistry continues in the cup. Caffeine and the astringent catechins extract at different rates and at different temperatures, so water near boiling pulls out more of both, which suits a robust black tea and ruins a delicate green one by making it bitter; green tea is generally brewed between 70 and 80 degrees and gyokuro cooler still. Longer steeping extracts more tannins and more astringency, not more flavour, which is why a strong cup and an over-brewed one are different things. The amount of caffeine in the finished drink depends far more on the leaf quantity, the water temperature and the time than on whether the tea is green or black, a common belief that does not survive measurement. And the practice of adding milk, long assumed to be a matter of taste, has a chemical basis, since milk proteins bind the tannins that produce astringency, which is why milk suits a brisk Assam and does nothing good for a first flush Darjeeling.

All tea comes from Camellia sinensis, and the differences are made after picking by controlling enzymatic oxidation, the same browning reaction as in a cut apple, which is halted whenever heat is applied. Green tea is heated at once, white is only withered and dried, oolong is partially oxidised through repeated bruising and resting, black is fully oxidised, and dark teas such as pu-erh are the only ones genuinely fermented by microbes. Most commodity tea is shredded by the crush, tear, curl process for speed and strength at the cost of aroma.