The Chemistry & Makeup of Black Tea

The Chemistry & Makeup of Black Tea
For a better understanding of orthodox (and unorthodox) processing of black tea, some knowledge of the chemistry of the tea leaf and the changes involved in the tea processing is needed. Precise knowledge of all factors, which are responsible for the quality of tea and of the way these and other factors operate, is still being researched.
                          
The basic constituents which have a considerable influence on taste and color characteristics of tea include polyphenolic bodies, caffeine, non-caffeine nitrogenous compounds, pectic substances, minerals and other compounds jointly or separately, but the exact role played by each of these is not well understood.

The earliest scientific studies on the quality of tea were conducted in Java toward the end of the last century. Analytical methods since then have improved substantially. It may be expected that with modern chromatographical methods, the important field of biochemical changes will be explored more exhaustively and more information will be gained on the complex problem of quality and flavor of tea.

A tea shoot consisting of two leaves and a terminal shoot (the bud), which constitutes the normal and best material for tea manufacture, contains 74 to 77% moisture (surface dry shoot) and 23 to 26% solid matter ( about half the solid matter is insoluble and is made up of crude fiber, cellulose, proteins, fats, etc.).

The soluble part includes some 30 polyphenolic bodies, over 20 amino acids, caffeine and organic acids. Traces of a number of substances, which may be connected with what is known as the essential oil, responsible for the aroma of tea, have also been recorded.

For example, the following table is the make-up of fresh shoots of Assam Tea.

In total the polyphenol group makes up about 30% of the solid matter of the tea shoot. The polyphenols are popularly called tannins, although they have no tanning properties. The group consists mainly of bodies called flavanols or catechins, six of which are present in fresh tea leaf as major components. Oxidizable matter, another term used to indicate the catechins, makes up about 1/5 of the solid matter.

The Chemistry of Green Tea

The green tea polyphenols (aka catechins) are the most important leaf components because of their ability to scavenge free radicals. In biological systems, oxygen routinely accepts electrons to become active oxygen and hydroxyl free radicals. In certain test systems catechins are better scavengers to active oxygen than vitamin C or E and are at least as effective at quenching radicals as BHA.

Also the catechins are the most abundant green tea solids, representing 30-42% of the extractable solids, and are the source of the condensation products formed during black tea production. The gallocatechins, especially EGEG (epigallocatechin gallate) have the most capacity to quench free radicals.

The main components of green tea beverage include:

Flavonols:                     5-10%
Theogallin:                     2-3%
Quinic Acids:                  2%
Caffeine:                        3-5%
Theophylline:                  .02%
Theobromine:                 .1%
Theanine:                      4-6%
Carotenoids:                   .02%
Mineral content:             6-8%
Catechins:                    30-42%

Oolong Tea Chemistry:

Oolong tea is the product of a partial or semi-oxidation process. The catechin content is thus about halfway between green and black tea. Nevertheless, Oolong is not merely a combination of black and green tea, but it can be distinguished by the presence of some unique flavonoids. One group of these, which has been called oolonghomobisflavins, is characterized as dimeric catechins where the two pyran rings are linked by methylene bridges.

For more information on the chemistry of tea and further reading on these excerpts, please consult: Tea: Cultivation to Consumption by K.C. Willson and M.N. Clifford, 1992. This work cites numerous studies, research and information from investigating the crop, tea history, botany, cultivation, production and the clinical and physiological effects of tea consumption.