References:

On Food and Cooking, pages 214-225.

The origins of tea and coffee
The processing of tea and coffee
The action of caffeine

Tea is made from the leaves of a plant in the Camellia family, Camellia sinensis.

Tea has probably been used as a beverage since prehistoric times in southeast Asia. Tea was introduced to western Europe in the 16th century by Venetian spice merchants. Early records of tea cultivation date from 400 AD in China. Early methods of tea preparation included forming the leaves into "cakes" that were then boiled and grinding up the leaves and whipping them into a frothy hot drink, much like early chocolate drinks. Steeping individual eaves did not become popular until the 14th century in China. Tea became particularly popular in England during the 18th century. In 1700, England imported 20,000 pounds of tea. In 1800 England imported 20 million pounds of tea. Tea was and is consumed mainly as a breakfast beverage, where it replaced ale (in England), a transition that met with a lot of criticism at the time.

Tea can be grown in a wide range of habitats, but commercially is grown mainly in tropical and subtropical regions of southeast Asia. Japan and China are major producers but do not export that much. Taiwan, Indonesia, India and Sri Lanka are major producers and exporters.

Young leaves are the choice "pluckings" from the tea bush, with the terminal bud on a branch plus the two youngest leaves being those typically plucked. The bushes are regularly cut back to encourage new growth (more terminal buds and young leaves). In the tropics, bushes can be plucked every 10 days. The colder it gets, the less frequently the tea can be harvested, which is why commercial tea production is mainly in the tropics.

The fresh tea leaf alone does not brew a beverage with any flavor. The leaves must be crushed so that enzymes in the cells can act upon other molecules in the cells to produce the molecules that give tea its desirable properties. The last significant invention relative to tea production was the invention of the silk tea bag in 1904 by Thomas Sullivan, a New York merchant.

There are four stages to tea production: withering, rolling, fermenting and firing.

Leaves are withered by  drying them out, which takes a day in open air, or a few days in a drum dryer (sort of like a clothes dryer, but set at different temperatures). Leaves are then rolled (which was once done with the heel of the hand, making this a very laborious part of tea production). Rolling is what causes cells to be ruptured and cell contents to be mixed. Leaves will turn from green to a dark brown during rolling as the chemical components of the cells mix together, giving tea its characteristic color, flavor, aroma, and astringency. Rolling also gives the leaves a twist, which reduces the rate at which they give up their essence to hot water.

Fermentation begins as soon as the leaves are rolled. Fermentation is the time allowed for the molecules released from the cells to react with each other. Different kinds of tea are produced by fermenting tea for different amounts of time. Green tea is not fermented at all; black tea is fermented for about 3.5 hours. During fermentation, colorless, tasteless molecules called PHENOLICS are converted to brown colored molecules called TANNINS. Tannins bind to proteins on the surfaces of cells in the mouth, resulting in a "full bodied" sensation (astringency). The transformation of the phenolics to tannins is caused by the enzyme POLYPHENOLOXIDASE, which causes many fruits to turn brown when cells are cut and exposed to the air. The action of this enzyme is in steps, first producing yellow, then orangish reddish, and finally brownish complexes.

The grade of tea is determined partly by the size of the leaves, because different sized leaves ferment at different rates (small leaves ferment faster than larger leaves, in general). Point of origin is also important to the quality of teas. Some teas have other essential oils added, such as Earl Grey tea, which has Bergamot (a type of inedible orange) oil added.

Tea is brewed by mixing the leaves with hot water for a specific amount of time, depending on how strong you want the tea. Generally, the color of the tea develops before the caffeine and flavors are extracted. Tea should be brewed from cold water heated just to boiling, because hot water or reheated water contains less oxygen, and oxygen is required for the chemicals to be react properly and generate the proper flavor and astringency. Adding milk to tea destroys the astringency of tea as the tannins will bind to the milk proteins.

When making iced tea, some of the dissolved substances will come out of solution (they will precipitate), making the solution cloudy, when the hot tea is poured over ice, or cooled down in any manner. This happens because solubilities are higher at higher temperatures and lower at lower temperatures. The way to avoid this is to not make the tea too strong, although this could produce a weak tea.

Coffee

Coffee is made from the fruits (beans) of a small tree Coffea arabica, native to the mountains of Ethiopia. Coffee beans are actually berries with large seeds. The flesh of the berry has traditionally been made into a wine. The large seed actually contains large amounts of protein and carbohydrate, and is very nutritious, and has thus been used as a food source. However, because of the ability of the coffee bean to deliver large amounts of caffeine into a hot beverage, coffee is now very widely used to make a hot drink. The making of the hot drink coffee began in the middle east with the establishment of coffee houses. Coffee spread to western Europe in the 15th century when it was introduced by Venetian spice merchants. Coffee became quite popular in western Europe, despite opposition to the point of being banned, as it were by Charles II in England, who had to repeal this ban due to public outcry. Coffee houses are still quite frequent in western Europe and related cultures.

Two types of coffee trees are planted. The original Coffea arabica yields the most robust beans, however it is difficult to grow in many regions, leading tot he development of Coffea canephora into Coffea robusta, which is not, as its name suggests, particularly robust, but it is more frost tolerant. Interestingly, most coffee from Africa is robusta, while most coffee in the Americas is arabica. Better blends of coffee in the US generally contain more arabica while lower quality blends are mostly robusta.

Coffee is processed in two stages. First the berries  are removed from the trees and the fleshy coverings are cleaned from the seeds (producing the beans) where they are grown. Seeds are then dried and shipped to their market destination. Beans are roasted to release flavor molecules. The hotter and longer the beans are roasted, the more flavor they can release. Beans should be ground just prior to use. Beans should be stored in a freezer in an airtight container. Contact with the air causes the flavor producing molecules to be degraded. Oils in the seeds (beans) can become rancid.

Caffeine

Caffeine is  a term used to describe alkaloids found in coffee, tea and chocolate. The alkaloids are actually slightly different in these three drinks. We have seen theobromine. Tea contains theophylline and coffee contains caffeine. These alkaloids affect several parts of the body. They affect the cortex of the brain, and in small doses can improve attention, concentration and coordination. Caffeine acts on the kidneys to increase water elimination. Caffeine stimulates the heart, and causes blood vessels to widen everywhere but in the brain, where it causes them to constrict. Caffeine also interacts with calcium ions to increase the contraction power of skeletal muscles, and makes them less susceptible to fatigue. Overdoses can cause an irregular heartbeat and muscle trembling.

Tea leaves are 2% caffeine by weight, coffee beans 1%. A cup of coffee, however, contains twice as much caffeine (100mg) as a cup of tea (50mg). A cup of cocoa contains 15mg of caffeine.