What is about tea that we like so much? First of all, we enjoy the comfort of a warm drink. There is also the fact that most tea is very inexpensive. But, more specifically, black tea leaf gives off a pleasant aroma, its infusion has an attractive red colour, and the drink has an agreeable, slightly astringent taste and delivers a stimulating lift afterwards.
These characteristics all arise from the various chemicals in the leaf, which include amino acids, carbohydrates, mineral ions caffeine and, especially, various polyphenolic components which are sometimes incorrectly called tannins.

PRODUCING BLACK TEA

To understand the chemistry of tea, we need to start with the tea plant, Camellia sinensis. It is now grown in large, actively managed plantations. Every few days, depending on the weather conditions, large numbers of workers pluck the terminal bud and the top two or three leaves of each plant. This tea 'flush’ contains 75-80% water, and the first stage in the manufacturing process-called withering-reduces the moisture content to 60-70% by a flow of cool air.

The second stage is the crucial one. Here the leaf is broken up physically by a mechanical process, such as rolling (the orthodox method), or crushing, tearing and curling with contra-rotating rollers (the CTC method). The CTC method produces smaller and more physically disrupted leaves, which are used in teabags.The tea is then allowed to 'ferment’ in air for 1-3 hours, originally in a thin layer on the

floor, but in more recent times in thicker layers with a forced air supply. During fermentation, the polyphenolic flavanols (or catechins) in the green tea are oxidized by the oxygen in the air to form aroma constituents and higher molecular weight polyphenolics-especially the theaflavins which give the tea brew its red coloration, and the dark thereubigins responsible for much of the 'mouthfeel’ of tea.

This oxidation comes about through the catalytic action of an enzyme called polyphenol oxidase. In the green leaf this enzyme is physically separated from the flavanols, but the maceration of the leaf brings them into contact and allows the oxidation to proceed.

Once the oxidation has run its course, the enzyme is deactivated by passing hot air over the tea leaves. This third stage of the manufacturing process, called firing, also develops some of the flavour characteristics of the black tea and reduces the water content to ca3%. The dried tea is then sorted into different grades by passing it over a series of vibrating screens of different grades by passing it over a series of vibrating screens of different mesh sizes. Electrostatically charged rollers preferentially attract and remove stalk and fibre. The colourful names of grades, like Broken Orange Pekoe, are simply indications of the size of leaf and the mechanical method used in the second stage.

The processed black tea has traditionally been packed and shipped in foil-lined plywood tea chests. However, slow oxidation continues during the months of transport and storage, and introduces undesirable flavour characteristics. Recent research has shown that such oxidative deterioration can be greatly reduced by using nitrogen flushing or vacuum packs. Tea protected in this way is now on the market and has a fresher taste.

TEA 'FERMENTATION'

Some 25% of the dry weight of green tea is made up of polyphenolic flavanols which, in the tea world, are often called catechins. They are colourless, water-soluble compounds with an astringent taste. Flavanols all contain two aromatic rings linked by an aliphatic C3 chain, and some have a galloyl group (3,4,5-trihydroxybenzoyl) attached. Epigallocatechin gallate is the predominant flavanol in green tea.

In the so-called 'fermentation’ process, the enzyme polyphenol oxidase catalyses the oxidation of these flavanols to quinines, which then react with each other. One set of products is theaflavin in its mono – and digallated forms – these structures all include an unusual seven-membered ring. Although black tea contains less than 2% theaflavins, their bright orange-red colour lends a distinctive hue to infusions of tea. Theaflavins can be determined spectrophotometrically by extracting them from the tea brew with isobutyl methyl ketone and then reacting them with flavognost (2-aminoethyl diphenylborate) to form a bright green complex.

Some 75% of the original catechins in the green tea are converted to a complex and still incompletely characterized group of substances known as thereubigins because of their red-brown colour. They constitute the largest group of compounds in black tea – upto 20% of its dry weight and 30-60% of the soluble material in tea brew.

Even more complex is the aroma of black tea. Although the volatile components account for less than 0.2% by weight of the leaf, over 550 individual chemicals have so far been identified by gas chromatography coupled with mass spectrometry. Over 40 hydrocarbons, 70 alcohols, 70 aldehydes, 80 carbonyl compounds, 70 acids and 80 esters have been characterized in black tea aroma, as well as more than 60 nitrogen-containing compounds, 20 sulphur-containing compounds and 50 miscellaneous oxygen-containing chemicals. Some of these compounds originate directly from the green leaf, but most are formed during tea processing. The aroma components strongly affect the flavour of a cup of tea through our sense of smell. However, the taste or mouthfeel of the tea brew derives mainly from the various tea polyphenolics modified by caffeine.

CAFFEINE

This is one of the most important chemicals in tea and acts as a mild stimulant. Tea leaf contains about 4% Caffeine and a typical cup of tea, 40±10mg. On average, a cup of instant coffee contains ca 60 mg caffeine and one of ground roast coffee, 85 mg. A cup of coffee drunk in the evening is therefore more likely to keep you awake than a cup of tea.

Caffeine is also a participant in a curious phenomenon. When a cup of black tea (without milk!) is cooled to below ca 400C, a colloidal precipitate appears which is called 'tea cream’. This is mainly the result of weak complexation between caffeine and theaflavins and thearubigins. It has been suggested that a similar association between the milk protein casein and the various polyphenolic tea compounds could explain why milky tea is less astringent on the tongue than tea without milk.

THE EFFECT OF WATER

The look of a cup of tea as well as its taste are significantly affected by the water used to make it. Tea brewed in distilled water, most soft water or even permanently hard water (which contains CaSO4) appears brighter than if it is brewed in temporary hard water. The latter has passed through beds of limestone and so contains calcium bicarbonate via the reaction : CaCO3(s) + CO2 + H2O = Ca2+ + 2HCO3

The higher pH of the bicarbonate-containing water causes greater ionization of the tea polyphenols, thus changing their spectra and making the infusion look darker brown. (Repressing polyphenol ionization by adding acid, as in lemon tea, turns the tea infusion yellow. Indeed, tea has been suggested as a teaching model for acid-base indicators). As regards taste, some teas are more suited to soft water, such as the orthodox manufactured Assam leaf, while higher-grown Ceylon and CTC manufactured teas are better with temporary hard water, as in London.

Many people living in areas of temporary hard water notice that their tea brew develops a dark skin or scum on the top. This phenomenon has recently been investigated in laboratories, where they deliberately grew large quantities of scum in big beakers kept hot for several hours. It turns out that, under the influence of calcium and bicarbonate ions, tea solubles are further oxidized at the liquid-air interface to form higher molecular weight products. This is what most of the scum consists of, together with some reformed CaCO3. One can avoid scum formation by removing either the calcium ions (for instance, by running the tap water through a filter containing a cation exchange resin), or the bicarbonate ions (by adding acid to convert HCO3 to CO2). Thus there is no scum on lemon tea, since the citric acid in lemon juice reacts with the bicarbonate ions as well as complexing the calcium ions.

Suprisingly, very little scum forms on a cup of very strong tea because the acidic tea polyphenols themselves partly neutralize the bicarbonate ions. It should be emphasized that in ordinary tea brewing, less than one milligram of scum is formed.

THE INFUSION PROCESS

The concentration (c) of caffeine (or indeed, of any tea soluble) increases with the time (t) when tea leaf is infused in water at constant temperature. The rate of increase of concentration progressively falls off with time until the concentration reaches an equilibrium value (c ) after a few minutes.At this stage, the caffeine is partitioned between the tea brew and the tea leaves which have absorbed a large amount of water, almost three times their dry weight.

The partition constant, K, is given by :

K = equilibrium concentration in the tea brew
equilibrium concentration in swollen tea leaf

The values of K lie between 0.1 and 0.7, depending on the constituent. The fact that the swollen tea leaves still contain a large quantity of tea solubles is the reason why we can squeeze a second cup of tea out of the teapot by pouring more hot water into it. However, the composition of the tea infusion in the second cup is not quite the same as in the first cup because the partition constants of the various tea solubles differ.

Experiments done under carefully controlled conditions then showed that the infusion rate constant :

• increases with decreasing size of tea leaf;
• is about twice as large for leaf treated by the more severe
  CTC manufacturing method than for leaf processed by the orthodox method;
• rises by ca 3.7% for every 10 C rise in brewing temperature;
• does not depend on the degree of stirring of the water.
• Only when teabags are used, faster stirring increases the brewing rate.

These experiments led to the conclusion that the rate-determining step in tea brewing is the slow diffusion of caffeine (and other tea solubles) through the swollen tea leaf.

TEA AND HEALTH

Of the several hundred different compounds in black tea, the most important are the polyphenols: flavanols, theaflavins, thearubigins. These compounds are able to bind iron in non-haem forms. On a more positive note, these polyphenols can play a beneficial role as antioxidants. Like vitamin C, they can act against oxygen radicals which have been implicated as

of certain degenerative diseases of the body. Evidence now accumulating appears to show that the polyphenols in tea, especially (-) epigallocatechin gallate (1), can reduce the risk of cancer and heart disease, and an international research programme has been set up to study the subject further.

Tea is therefore not only a pleasant drink, it may also be good for your health.