Black tea, like the other types of tea, is an infusion of dried and processed leaves of Camellia sinensis, a shrub belonging to the Theaceae family.
Unlike what happens during green tea production, during black tea production the almost complete oxidation of the substances contained in the leaves occurs, particularly catechins, polyphenols of the flavonoid group.
The color of the processed leaves is dark, whereas the beverage is brown-red in color.
Black tea is prepared with one tea bag per person, or one teaspoon per person in case of loose tea leaves, with an infusion time of 3-4 minutes in water at 95-100 °C.
Tea is a beverage with ancient origins, dating back to almost 4,000 years ago. It is one of the most-consumed beverages, particularly in Asia, where the favorite tea is green tea, especially in Japan and China, whereas black tea is preferred by Western populations and at global level, accounting for about 78% of the tea consumed.
The oxidation of the compounds present in the leaves during processing reduces the potential beneficial effects ascribed to the polyphenols initially present.
How black tea is made
All the types of teas are produced from fresh leaves of Camellia sinensis. During harvesting, young leaves are preferred, as the older ones are considered inferior in quality.
The processing that leads to the production of loose dried tea leaves ready for brewing black tea proceeds through four steps: withering, rolling, oxidation, and drying. Such processing leads to the near complete oxidation of the substances present, particularly catechins.
Withering is the process by which the water present in the leaves, about 75% of the leaf’s weight, is removed, thus causing sap concentration. Withering, which makes the next step easier, can be achieved in three different ways:
- by exposing leaves to sunlight;
- by appropriately heating the rooms where the leaves are stored;
- by machineries that ventilate the leaves.
The rolling step follows the withering of the leaves, and, breaking the leaf tissues, facilitates the outflow of lymph thus promoting the subsequent oxidation of polyphenols.
The oxidation step is also improperly called fermentation. In this step, the oxidation by atmospheric oxygen and polyphenol oxidase (EC 188.8.131.52) of 90-95% of the polyphenols occurs, accompanied by other changes that color the leaves with a red color. Temperature, typically 25°C, pH, relative humidity, 95% or more, ventilation, and duration are crucial factors, too. This step is crucial in determining the quality of the tea, as it gives it its organoleptic characteristics and composition in polyphenols, quite different from those of green tea, which is produced in such a way as to minimize oxidation processes.
Note that caffeine content does not vary significantly.
Drying is the last step. It is carried out at a temperature of 80-90 °C for about 20-25 minutes. The high temperature inactivates polyphenol oxidase, and then stops enzymatic oxidation processes.
Thearubigins and theaflavins
The oxidative processes that occur during black tea production affect monomeric and gallate catechins, to a lesser extent catechins glycosides, especially myricetin, and non-flavonoid compounds, such as teagallin, and leads to the formation of complex polyphenols such as thearubigins, theaflavins and theaflavic acids.
Thearubigins, brownish in color, are polymers of catechins not yet well characterized and the major polyphenols in black tea, accounting for about 20% of the dry leaf weight. They contribute to the richness in taste and color.
Theaflavins, red-orange in color, are dimers of catechins and account for about 3-5% of the dry leaf weight. They contribute to the astringent and brisk taste, as well as the red-orange in color.
The main theaflavins are:
- theaflavin 3-gallate;
- theaflavin 3′-gallate;
- theaflavin 3,3’-digallate, the most abundant.
Health benefits of black tea
The health benefits of black tea are largely due to its complex polyphenols, thearubigins and theaflavins, being catechins largely oxidized during leaf processing.
Here are three examples.
- Theaflavins have been highlighted as having antiviral activity which, similarly to catechins, appears to be particularly effective against positive single-stranded RNA viruses. These viruses also include SARS-CoV-1 and SARS-CoV-2, viruses belonging to the Coronaviridae family.
Like catechins, the galloyl groups appear to be important for the antiviral activity of theaflavins.
- The phytochemicals present in black tea, like those in green tea, seem to be able to reduce the glycemic index of starchy foods. The effect appears to be due to the inhibition of the activity of pancreatic alpha-amylase and other digestive enzymes, and to the direct interaction with starch, that would reduce the surface available to enzyme activity. The inhibition is greater on gluten free foods; this seems to be due to the action of gluten on complex polyphenols that would not be able to interact with the polysaccharide. For more information, see the article on tea polyphenols.
- Thearubigins and theaflavins seem to have anticariogenic effects due to the inhibitory action on salivary and bacterial amylase, and seem to be more effective than green tea catechins.
Moreover, black tea seems to be able to inhibit acid production in the oral cavity.
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