Black tea, like the other types of tea, is an infusion of processed leaves of Camellia sinensis, the tea plant, a shrub that belongs to the Theaceae family.
Black tea, a type of fully fermented tea, is the most consumed tea worldwide, accounting for about 78% of the consumed tea. It is preferred by Western populations, while the favorite tea in Asia, particularly in Japan and China, is green tea.
Processing of black tea
The processing of the leaves of Camellia sinensis, that leads to the production of black tea, proceeds through three main steps:
- withering or drying;
The last step, oxidation, gives it the specific organoleptic characteristics and composition in polyphenols, that are extremely different from those of green tea (green tea undergoes very slight oxidative processes during processing).
Withering or drying
The withering or drying step is the first, and most basic process during processing. In this step, water in the leaves is removed (about 75% of the leaf’s weight is made up of water), thus determining the concentration of the sap of the leaf itself. The withering also makes the next step easier.
Withering can be achieved in three different ways:
- exposing leaves to sunlight, that is, sun withering;
- heating in an appropriate manner the rooms where the leaves are placed;
- using machines that artificially ventilate the leaves.
The rolling step follows the withering of the leaves. It breaks the leaf tissue, facilitating the outflow of lymph; thus, it promotes the subsequent enzymatic oxidation of polyphenols. This step is essential for the creation of the aroma, color and flavor of black tea.
The oxidation, also improperly called fermentation, is the last stage of its processing, and is crucial in determining the quality of the tea. In this step, the enzymatic oxidation of about 90–95 % of the polyphenols occurs, accompanied by other changes that make the green tea leaves into red color.
Temperature (typically 25°C), pH, relative humidity (95% or more), ventilation, and duration are crucial factors too.
Black tea polyphenols
During the oxidation step, the main compounds that undergo oxidation processes, both enzymatic, by polyphenol oxidase, and chemical, by the action of atmospheric oxygen, are:
- monomeric catechins and gallate catechins;
- to a lesser extent, the glycosides of catechins, especially myricetin;
- but also not flavonoids compounds, such as teogallin (ester of gallic acid).
Therefore, throughout the process, a reduction in the concentration of monomeric catechins, characteristics of fresh leaves of Camellia sinensis and green tea, occurs, with the formation of complex polyphenols, such as:
- thearubigins, red-brown or dark-brown in color;
- theaflavins and theaflavic acids, red-orange in color.
Thearubigins, polymers of catechins not yet well characterized, are the major polyphenols in black tea, accounting for about 20% of extracted solids. In addition to the reddish color, thearubigins contribute the richness in taste, the so-called “body” to black tea.
Theaflavins, dimers of catechins much better characterized than thearubigins, account for about 3-5% of the solids in black tea infusion. Theaflavins contribute the astringent and brisk taste, as well as the red-orange color of the beverage.
The main theaflavins are:
- theaflavin digallate;
Black tea benefits and oxidized polyphenols
Although this type of tea is still able to improve health, oxidative processes suffered from the leaves during the processing attenuate its health benefits , which are instead reported after intake of green tea (particularly, the benefits of green tea are ascribed to its content of catechins, such as EGCG, epicatechin and epicatechin gallate).
Its caffeine content does not vary significantly.
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