Britain is a nation of tea drinkers. Every year, we consume more than 60 billion cups of tea. That equates to 900 cups each for every man, woman and child annually, or 2-3 cups each a day. When a major sporting event or soap opera on TV is interrupted by a commercial break, the National Grid has to crank up the dial at power stations across the land to supply enough electricity to power the millions of kettles suddenly starting to boil water to make tea. It wakes us up in the morning and keeps the builders, plumbers, office workers and aristocrats of the country going throughout the day. And that’s not to mention the enormous popularity of tea in countries as populous and diverse as China, India and Japan, albeit each with their own local variations in style. This seemingly basic foodstuff is more than just a cultural phenomenon. It is in fact a cornucopia of chemistry. From the elaborate enzyme-catalyzed oxidation processes that give black tea its colour to the panoply of molecules responsible for its distinctive flavour, tea is a complicated and fascinating example of the chemistry at work in plants.
1. Tea is made from the leaves of the tea plant, whose Latin binomial name Camellia sinensis hints at its origins in southwest China several thousand years ago. Tea leaves may have been eaten as food in prehistoric times, and were later boiled up with other plants for use as medicines (M. E. Harbowy, D. A. Balentine, Crit. Rev. Plant Sci. 1997, 16, 415-480). Around 2000 years ago, tea finally started to be consumed as a drink for refreshment. While Europeans made water safe to drink by using it to brew beer from hops and barley, people in East Asia found an alternative method of purification: boiling water to make tea (M. L. Heiss, R. J. Heiss, The Story of Tea: A Cultural History and Drinking Guide, Ten Speed Press, New York, 2007).
Caffeine is a bitter-tasting substance you would normally associate with coffee, but it is present in quite large amounts in both green and black tea, accounting for up to 5% of the mass of a dried leaf. A typical cup of tea contains about half as much caffeine as a cup of coffee at 20 mg of caffeine per 100 g of liquid. Caffeine is produced by the tea plant as a natural pesticide to protect itself from insects that would otherwise try to eat it. When we drink tea, the caffeine can improve our mood and help us maintain concentration by stimulating the central nervous system, increasing the circulation of blood and the rate of respiration. Sometimes the stimulant effects of caffeine are undesirable, and they vary from person to person. An overdose of caffeine can cause discomfort in the form of restlessness, insomnia and an increase in heart rate. You might crave a cup of tea just before bedtime, but how can you avoid staying awake all night on a caffeine-fuelled high? At the start of the twentieth century, chemists devised a solution: decaffeination. 2. Decaffeination involves removing the caffeine from coffee or tea, while leaving all the other chemical components responsible for flavour and nutrition behind. Being an organic compound – that is to say, carbon-based – caffeine readily dissolves in organic solvents, so the simplest decaffeination technique is to wash the tea with such a solvent. Dichloromethane (CH2Cl2) was the solvent of choice until the 1970s, but has since fallen out of favour over concerns that it may cause cancer and damage the ozone layer. A more benign alternative is ethyl acetate, a substance that easily breaks down into ethanol and acetic acid (essentially alcohol and vinegar) in the body or the environment. Water can even be used, although special precautions are needed to avoid dissolving all the flavour alongside the caffeine.
Recently, the decaffeination solvent of choice has become supercritical carbon dioxide (CO2). We’re more used to hearing about CO2 in the context of global warming, dry ice or fire extinguishers, but it can also be a highly selective solvent for caffeine under the right circumstances. CO2 is a gas under normal conditions, but it has an amazing property that turns out to be incredibly useful. Heat it above 32 °C when the pressure is 73 times greater than atmospheric and CO2 becomes a supercritical fluid, one that is dense like a liquid but low in viscosity and effusive – freely expanding – like a gas. In this state, it is ideally suited to removing caffeine from tea without affecting any of the other molecules essential for taste, smell or colour. Better yet, releasing the pressure allows the CO2 to escape the tea without leaving any residue behind – not that it would matter too much, as CO2 is non-toxic. Supercritical CO2 decaffeination does require some sophisticated equipment and operators, but nothing that a large tea factory can’t accommodate. 3. The tea plant has more in its medicine cabinet than just caffeine. Chemists in China have recently investigated how tea is able to lower your cholesterol levels. The molecules responsible are known as catechins, and they make up the lion’s share of the chemistry of tea. They have a common structural core (see figure below), consisting of a benzene ring and a dihydropyran ring fused together. The inclusion of phenyl and hydroxy groups as substituents on the aliphatic ring qualifies these molecules as flavanols, as distinct from their cousins the flavonols which bear a carbonyl group in the ring. The benzene rings are decorated with several hydroxy groups, leading to the epithet polyphenol and making the rings so rich in electrons that they are easily oxidised.
While green tea is green because of chlorophyll from the leaves, black tea gets its reddish-yellow colour from oxidised catechins known as theaflavins (H.-D. Belitz, W. Grosch, Food Chemistry, 2nd ed., 1999, Springer). The enzyme polyphenol oxidase is present in tea leaves and it catalyses the reaction of catechins with oxygen.
To make green tea, you have to prevent this reaction by firing the leaves to inactivate the enzyme. If you want black tea, however, you have to avoid the firing step to preserve the enzymes. The leaves are rolled to crush their cells, and polyphenol oxidase gains access to the oxygen it needs to convert catechins to theaflavins.
The molecular structure of theaflavin is responsible for its bright red colour. The seven-membered benzotropolone ring at the heart of the molecule is the actual chromophore, or colour-bearing part. Next time you stare into your teapot as it brews, think of the many trillions of molecules of theaflavin, each one abuzz with electrons constantly being excited as they absorb blue light and give your tea its beautiful red hue.
The possible mechanisms of formation of theaflavins has been discussed and studied. The first step is certainly the oxidation of an ortho-benzenediol fragment in a catechin to its corresponding ortho-quinone. What follows depends on the concentration of quinone, with two distinct routes being plausible. At low quinone concentrations, a biphenyl-based intermediate forms by conjugate addition of a reduced catechin to an oxidised quinone. The biphenyl species is subsequently oxidised by polyphenol oxidase to a bridged intermediate. At high quinone concentrations, the bridged intermediate may be formed directly. However the bridged intermediate is generated, it is hydrolysed to a seven-membered ring and a sequence of decarboxylation, elimination and oxidation eventually leads to theaflavin.
Contributors: Ben Mills (research, words, photos, images and schemes), Jenny Slaughter (photos, ideas, editing, tea flower).