This section is from the book "Practical Dietetics With Special Reference To Diet In Disease", by William Gilman Thompson. Also available from Amazon: Practical Dietetics with Special Reference to Diet in Disease.
Tea is a preparation made from the leaves of various species of a hardy evergreen shrub called Thea. The manufacture consists in plucking the young leaves of the plant and placing them in the sun; after they have become withered they are rolled and twisted. This process is sometimes conducted by hand, or even by the feet of the natives in China, and sometimes by machinery. The leaves are next pressed into small masses or rolled into balls and allowed to ferment while still moist, after which they are dried over a fire of a temperature sufficient to evaporate all the moisture. They are finally sifted and assorted into different qualities. The value of the tea depends upon its flavour, and this is mainly influenced by the process of fermentation, which must be supervised with great care.
The distinction between black and green teas is due to the variations in their mode of preparation, and not to separate species of the plant. Green tea is made by steaming the leaves before they are rolled and dried. The further difference between green and black tea consists in the relatively larger quantity of astringent material (tannin) which predominates in green tea. The following table, from an analysis by Mr. Y. Kozai, illustrates this point and exhibits the proportionate quantity of some of the more important ingredients of tea. It presents the difference in percentage composition between green and black tea prepared from the same plant:
Because green tea contains more than twice as much of the astringent tannin than black tea it is generally regarded as less wholesome than the latter. It is also believed to have a somewhat less stimulating effect upon the nervous system, though this can hardly be accounted for by the slight variation in the percentage of theine shown to exist by the above table, this latter substance being the alkaloid, which is chiefly responsible for the stimulating influence of tea upon the nerves.
There has been some discussion in regard to the identity of theine with caffeine, and by many writers they are believed to be the same alkaloid. Rice says that most of the commercial caffeine is derived from tea leaves, but May finds that in frogs, at least, "theine produces spontaneous spasms and convulsions, while caffeine does not Theine impairs the nasal reflex early in the poisoning process, while caffeine does not, if at all, until the very last stage".
The peculiar stimulating properties which tea possesses, as well as its colour and agreeable flavour, depend upon the season of the year at which the leaves are gathered, the variety of the plant, the age of the leaves, which naturally become tough as they grow older, and the care exercised in their preparation. The flavour is produced by the formation of volatile oils which develop during fermentation. It is these substances which cause the minor differences in effect of tea and coffee.
The aroma as well as the flavour of tea is often artificially increased by the addition of such substances as the leaves of orange flowers, jasmine, or roses.
Tea made of small leaves packs closely, and if measured by the spoonful gives a stronger beverage than the coarser-grained varieties.