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You can’t beat a nice cup of tea. Few people would disagree with that,
and Theodore Roosevelt for one preferred it to brandy. But what distinguishes
a delicate, high-quality blend from the mundane cuppa? The chemical mix
that gives tea – black or green – its flavour is complex and is only now
beginning to be fully understood.

Tea producers, like producers of coffee, wine and some spirits, rely
on the discrimination of professional tasters to determine quality – and
therefore price. Trained tea tasters can assess over 50 teas an hour, but
producers on the trail of more objective ways to assess flavour have turned
to analytical chemistry for help in tracking down what, precisely, makes
a nice cup of tea. What we perceive as flavour often comes as much from
smell as from taste. Aroma molecules are found in teas, especially ‘flavoury’
teas such as Darjeeling and high-quality, high-grown Sri Lankan teas. So-called
‘plain’ teas are not considered flavoury, while the medium teas which constitute
most of the tea drunk in Britain have some aroma. Where does the taste of
medium teas come from?

We owe our first insight into the chemistry of black tea to the pioneering
work of E. A. H. Roberts, who worked for the Indian Tea Association in Butler’s
Wharf in London during the 1940s and 1950s. He discovered that black tea
is unique in containing two groups of phenolic substances which he called
theaflavins and thearubigins. They are produced during the fermentation
stage of tea manufacture when enzymes oxidise phenolic compounds called
catechins that occur naturally in the green leaf .

It is now known that there are four theaflavins. These natural chemicals
are unusual because their molecules each contain a ring of seven carbon
atoms: rings of five or six carbons are more familiar. The way electrons
are distributed around the theaflavin molecules gives them an intense, bright-red
colour which in turn gives the tea a quality called ‘brightness’. The same
molecules also impart what tasters call ‘briskness’, ‘freshness’ and ‘aliveness’
– words used to describe how the tea feels in the mouth. These characteristics
are essential components of all black teas, and are much sought after in
‘breakfast’ teas, particularly those from Kenya and the Assam region of
India.

Chemists know the structure of these molecules and understand fairly
well how they affect quality. But not all theaflavins contribute to briskness
to the same extent. In 1965, Gary Sanderson of the Thomas J. Lipton company
in the US discovered that one of the theaflavins, the digallate, is six
times as brisk as its parent molecule, theaflavin. But the ideal mix of
theaflavins is still uncertain, as is the ideal combination of their precursors,
the catechins, in the growing leaf.

The chemical structure of the thearubigins remains a mystery. Roberts
considered these molecules to be as important to the flavour and quality
of black tea as the theaflavins. Like the theaflavins they are coloured
phenolic molecules produced during fermentation, and according to Roberts
they make up as much as a fifth of the dry weight of black tea. It is these
molecules that are responsible for what in tasters’ parlance is known as
the ‘body’, ‘depth of colour’, ‘richness’ and ‘fullness’ of the tea liquor.
Too much of them, and the tea becomes what is known as ‘soft’.

Such a diverse mixture of compounds provides a challenge for analytical
chemists. At the Natural Resources Institute in Chatham, we have been working
with researchers from the University of Reading to try to pinpoint which
molecules are most important to flavour, and to devise better methods of
separating and identifying them. We now think that among the thearubigins
are compounds very like theaflavins. This fits in with what we already know
from practical experience, that increasing fermentation times increases
the amounts of thearubigins and cuts down the theaflavins. Tea tasters are
looking for teas that combine the highest levels of thearubigins with briskness.
This is what they call good liquor quality.

The traditional British cuppa is a medium black tea blended to produce
these high levels of theaflavins and the-arubigins. A typical example is
Co op 99, a tea that is well thought of in the trade because of its good
liquor quality and consistency. This type of tea is probably made up of
a blend containing 75 per cent African tea, half of which is likely to be
a high-quality grade from Kenya, with the rest made up of secondary grades.

But theaflavins and thearubigins are not the only molecules that give
tea its taste. Substantial quantities of the catechins survive fermentation.
They are known to be the basis of the astringency and bitterness characteristic
of Chinese and Japanese green teas, but no one has looked at what impact
they might have on the flavour of black tea. Phenolic compounds not involved
in fermentation, such as the flavonol glycosides and chlorogenic acids,
are also present in black tea leaves. Their contribution to flavour is not
certain, but they may have a similar taste to catechins. Chlorogenic acids
are already known to be important to the flavour of coffee but no one knows
about their role in tea.

All these molecules stay firmly in the tea: they are nonvolatile. Other,
more volatile ones are responsible for the aroma that is characteristic
of many of the most expensive teas. These flavoury teas are produced by
certain varieties grown at altitudes above 1200 metres in Sri Lanka and
in Darjeeling in India. Sri Lanka produces flavoury teas in the Dimbula
district during January and February and from the Uva district in August
and September.

People with a taste for these expensive teas probably do not realise
that their exquisite aromas are dependent on the vagaries of the weather.
Both Sri Lankan teas are produced during cool, clear, dry, windy weather
when the temperature is about 20 °C by day, falling to between 6 and
10 °C at night. These dry winds blow just before the onset of the monsoon
winds and rain and these conditions – which encourage a sort of withering
on the bush – must last unbroken for at least two weeks. The slightest rain
sets back the process to the beginning. The sought-after first and second
flush Darjeeling is produced in a similar environment when growth is slow
and the plants are under stress because of the climate. Some of the aroma
molecules produced under these conditions are already present in the green
leaf. Others are produced during processing by oxidative deamination of
amino acids, hydrolysis of terpene glycosides and oxidation of carotenoids
and unsaturated fatty acids. Chemists have identified more than 500 of these
aroma molecules using a combination of gas chromatography and mass spectrometry.

But there are problems in dealing with such large numbers of different
substances, sometimes at extraordinarily low concentrations. The total amount
of aroma substances accounts for only 0.2 per cent of the dry weight of
tea at most, and can be as low as 0.02 per cent. So a cup of tea contains
between 5 and 50 parts per million of these odorous molecules.

Methyl jasmonate, for example, which as its name implies is an important
constituent of jasmine flowers, is particularly important in the aroma of
some high-quality semifermented teas such as Oolong from China and Pouchong
from Taiwan. In 1988, Tei Yamanishi and her colleagues at Ochanomizu University
in Tokyo discovered that these teas contain an isomer of methyl jasmonate,
called methyl epijasmonate, in concentrations as low as 0.5 parts per billion.
Even this tiny amount was enough to impart a strong flavour to the tea because
this substance has an odour that is 400 times stronger than methyl jasmonate.
Seemingly, it is this molecule, not methyl jasmonate, that is responsible
for the detectable floral odour. This highlights the problems that tea tasters
share with those who work with coffee, whisky, wine, essential oils and
perfume: that substances present in quantities almost too small to detect
can have an overwhelming impact on how we perceive them.

Not all the aromas are pleasant – some of them smell ‘green’, grassy
or even rancid. Such unpleasant aromas tend to be straight-chain alcohols
which combine to give a fresh, leafy odour, or aldehydes such as trans-2-hexanal,
which has coarse, vegetable odour. But low concentra-tions of these molecules
can be essential components of the overall aroma. In Japan, for example,
the most popular tea, Sencha, is made from the spring flush and is valued
particularly for its briskness. This quality comes not from theaflavins
– green tea does not contain any – but from aroma molecules which in isolation
most people would consider unpleasant. Yamanishi identified these as cis-3-hexanol
and its hexanoate esters, and dimethyl sulphide, which has a strong odour
reminiscent of seaweed.

Pleasant aromas come from slightly larger molecules – esters and mono-terpene
alcohols such as geraniol and linalool. The perfume industry makes extensive
use of linalool, which is an important constituent of rose oil, neroli oil
and lavender oil. In 1985, Pardip Mahanta at the Tea Research Institute
in Tocklai in Assam discovered that high-grown Darjeeling teas contain more
than five times as many floral aroma molecules – such as linalool and its
oxides – as low-grown Assam teas. Yamanishi and others have developed the
useful technique of taking ratios of pleasant to unpleasant aromas as an
indication of tea quality. It seems that the quality of expensive teas is
determined mainly by their aroma molecules – smell rather than taste.

The astringency of tea comes from gallo groups on the catechin and theaflavin
molecules. When a cup of tea without milk is left to stand, a fine precipitate,
called ‘tea cream’, forms. This is thought to be a weak association between
theaflavins, thearubigins and caffeine. We think a similar association between
the milk protein casein and the catechins and theaflavins is what makes
milky tea less bitter than tea without milk.

Confusingly, flavour has nothing to do with how teas are graded. For
example, a traditional Ceylon tea can be graded as orange pekoe, flowery
pekoe, broken orange pekoe, broken pekoe, broken orange pekoe fannings,
and dust. These grades simply reflect the size of leaves. The word pekoe
is a corruption of the Chinese word peh-ho, which describes the white down
which covers certain leaves and buds. Today, the word pekoe refers to a
particular size of tea leaf.

To ensure the best quality from a particular tea, the pluckers have
to be skilled in harvesting the leaf from the bush. The acknowledged standard
is two leaves and a bud: three leaves or more is considered an inferior
plucking. Orange pekoe is a grade used for speciality teas. Flowery pekoe
is a grade with lots of buds or tips and its flavour can be delicate at
the height of the quality season. But this grading is only a description
of size and appearance, and in itself is no guarantee of quality of flavour.
The smaller leaf sizes, graded as broken orange pekoe and broken orange
pekoe fannings, predominate in the black tea which is put into tea bags.

The most flavoury China teas are the oolongs, but there are Chinese
black and green teas with interesting flavours. Keemun, a black tea from
Northern China, has a good aroma. Yunnan in Western China has been producing
tea for 1700 years, and people started producing black teas there in 1939.
The quality of the most famous grade, flowery broken orange pekoe broken
black, matches that of Indian Assam teas. The unusual smoky flavour of Lapsang
Souchong derives from the practice of withering it over open fires of pine
or cypress wood.

The three highest grades of China green teas, gunpowder, imperial and
hyson, refer only to the style of manufacture of the leaf, usually with
the district name added. A young English clerk named the grade gunpowder
because of the appearance of the tightly hand-rolled green balls which preserve
their flavour much longer than other grades. The smaller the pellets, the
more expensive the tea: the highest grade is called extra first pinhead
gunpowder.

Teas are often scented with plant essential oils such as bergamot, lemon,
rose and fragrant olive to enhance the natural flavour of the tea with their
sweet, floral characteristics. The most popular scented tea in Britain is
Earl Grey, which is flavoured with oil from the peel of bergamot, the canton
orange Citrus bergamia, by spraying the oil onto the tea just before packaging.
The Chinese used blossoms of this fruit to scent a tea called Tai Tai and
gave the recipe to the second Earl Grey, then British Prime Minister, when
he visited China in 1830.

Other teas are blended with flower petals such as chry-santhemum, spices
such as cinnamon and cloves, or dried leaves such as rosemary, camomile
and peppermint. You still can’t beat a nice cup of tea, but there is more
in it than you might expect.

Ian McDowell is a flavour chemist at the Natural Resources Institute
in Chatham, an executive agency of the British Overseas Development Administration.
Philip Owuor is head of the chemistry department at the Tea Research Foundation
in Kericho, Kenya.

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ONE BEFORE THE POT

There are three basic types of tea, all deriving from the Chinese shrub
Camellia sinensis: black, fermented tea is based mainly on the assamica
variety and makes up 75 per cent of world trade. Green, unfermented tea
from China, Japan and Taiwan, and semifermented oolong tea from China and
Taiwan account for the rest. Tea is big business: in 1989, as much as 2.4
million tonnes of it – 0.5 million tonnes of green tea and 1.9 million tonnes
of black tea – were produced worldwide. India produces the most black tea,
and China the most green. Black tea is made in five stages; green in four.

Withering. Once plucked, the leaves are spread out in a current of air
for between 4 and 18 hours. This reduces their moisture content to about
60 per cent of total weight so that they are more easily macerated. Green
tea is not dried at this stage but is steamed to kill the fermentation enzymes;
then it is macerated and dried.

Maceration. Mechanical breakdown of cell walls, to allow catechins,
enzymes and air to mix. The method used determines the type of tea. Mechanical
rollers mimic the traditional hand-rolling technique that produces the distinctive
twist of teas such as Darjeeling and Earl Grey. More modern methods, such
as CTC (crush, tear and curl) reduce the size of the leaves as much as possible
so that they will ferment rapidly, and because the smaller leaf will give
a quick brew in the pot.

Fermentation. This is a different process to the fermentation brought
about by yeasts. An enzyme called polyphenol oxidase catalyses the oxidation
of catechins, allowing them to react with each other to form theaflavins
and thearubigins, which give black tea its colour and taste. For CTC teas
this lasts as little as 90 minutes but rolled teas can take up to 6 hours
to ferment. Semifermented teas such as oolong are withered for 16 hours,
roasted at 160 °C for 20 minutes then fried. Finally, all teas are dried
and graded.