Tag Archives: processing of tea leaves

Black tea: definition, processing and polyphenols

What is black tea?

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.

“Tea is drunk to forget the din of the world.”
T’ien Yiheng

Processing of black tea

The processing of the leaves of Camellia sinensins, that leads to the production of black tea, proceeds through three main steps:

  • withering or drying;
  • rolling;
  • oxidation

The last step, oxidation, gives black tea 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

Black Tea: Withering or Drying  of Tea Leaves
Fig. 1 – Withering or Drying of Tea Leaves

The withering or drying step is the first, and most basic process during processing of black tea. 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.

Rolling

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.

Oxidation

The oxidation, also improperly called fermentation, is the last stage of black tea 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;
  • theaflavin-3-gallate;
  • theaflavin-3′-gallate.

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 health benefits of black tea, 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).

Black tea’s caffeine content does not vary significantly.

References

Asil M.H., Rabiei B., Ansari R.H. Optimal fermentation time and temperature to improve biochemical composition and sensory characteristics of black tea. Aust J Crop Sci 2012;6(3):550-8 [PDF]

Kuhnert N. Unraveling the structure of the black tea thearubigins. Arch Biochem Biophys 2010;501(1):37-51 [Abstract]

Li S., Lo C-Y., Pan M-H., Lai C-S. and Ho C-T. Black tea: chemical analysis and stability. Food Funct 2013;4:10-18 [Abstract]

Menet M-C., Sang S., Yang C.S., Ho C-T., and Rosen R.T. Analysis of theaflavins and thearubigins from black tea extract by MALDI-TOF mass spectrometry. J Agric Food Chem 2004;52:2455-61 [Abstract]

Sharma V.K., Bhattacharya A., Kumar A. and Sharma H.K. Health benefits of tea consumption. Trop J Pharm Res 2007;6(3):785-792 [Abstract]

Green tea: definition, processing, properties, polyphenols

What is green tea?

Green tea is an infusion of processed leaves of tea plant, Camellia sinensis, a member of the Theaceae family.
It is the most consumed beverages in Asia, particularly in China and Japan.
Western populations consume black tea more frequently than green tea. However, in recent years, thanks to its health benefits, it has been gaining their attention.
Currently, it accounts for 20% of the tea consumed worldwide.

“You can never get a cup of tea large enough or a book long enough to suit me.”C.S. Lewis

Processing and properties of green tea

Green Tea
Fig. 1 – Camellia sinensis

As all other types of tea, it is produced from fresh leaves of Camellia sinensis.
The peculiar properties of the beverage depend on the type of processing that the leaves undergo. In fact, they are processed in such a way as to minimize both enzymatic and chemical oxidation processes of the substances contained in them, in particular catechins.
Therefore, among the different types of tea, it undergoes the lowest degree of oxidation during processing.
At the end of the processing, tea leaves retain their green color, thanks to the little chemical modifications/oxidations they have undergone. The infusion shows off a yellow-gold color.
Finally, the processing of tea leaves ensures that green tea flavor is more delicate and lighter than black tea.

The three main steps in the processing of green tea

After harvesting, tea leaves are exposed to the sun for 2-3 hours and withered/dried; then, the real processing starts.
It consists of three main steps: heat treatment, rolling and drying.

Heat treatment

Heat treatment, short and gentle, is the crucial step for the quality and properties of the beverage.
It occurs with steam (the traditional Japanese method) or by dry cooking in hot pans (a large wok, the traditional Chinese method). Heat treatment has the purpose of:

  • inactivate the enzymes present in the tissues of the leaves, thus stopping enzymatic oxidation processes, particularly of polyphenols;
  • eliminate the grassy smell in order to stand out tea flavor;
  • evaporate part of the water present in the fresh leaf (water constitutes about 75% of the weight of the leaf), making it softer, so as to make the next step easier.

Rolling

The rolling step follows the heat treatment of the leaves; this step has the purpose of:

  • facilitate the next stage of drying;
  • destroy the tissues of the leaves in order to favor, later, the release of aromas, thus improving the quality of the product.

Drying

The drying is the last step, which also leads to the production of new compounds and improves the appearance of the product.

Green tea polyphenols

Gree Tea
Fig. 2 – EGCG

All types of tea are rich in polyphenols, compounds that are also present in fruits, vegetables, extra virgin olive oil, and red wine.
Fresh tea leaves are rich in water-soluble polyphenols, especially catechins (or flavanols) and glycosylated catechins (both belonging to the class of flavonoids), molecules which are believed to be the responsibles of the benefits of green tea.
The major catechins in green tea are epigallocatechin-3-gallate (EGCG), epigallocatechin, epicatechin-3-gallate, epicatechin, epicatechin, but also catechin, gallocatechin, catechin gallate, and gallocatechin gallate are present, even if in lower amount. These polyphenols account for 30%-42% of the dry leaf weight (but only 3%–10% of the solid content of black tea).
Green tea caffeine accounts for 1,5-4,5% of the dry leaf weight.

How to maximize the absorption of green tea catechins

In vitro studies have shown the high antioxidant power of catechins, greater than that of vitamin C and vitamin E. In vitro, EGCG is generally considered the most biologically active catechin.
In vivo studies and several epidemiologic studies have shown the possible preventive effects of green tea catechins, especially EGCG, in preventing the development of:

  • cardiovascular disease, such as hypertension and stroke;
  • some cancers, such as lung cancer (but not among smokers) and oral and digestive tract cancers.

For these reasons, it is essential to maximize the intestinal absorption of catechins.
Catechins are stable in acidic environment, but not in non-acidic environment, as in the small intestine; also for this reason, after digestion, less than 20% of the total remains.
Studies with models of the digestive tract of rat and man, that simulate digestion in stomach and small intestine, have shown that the addition of citrus juice or vitamin C to green tea significantly increases the absorption of catechins.
Among tested citrus juices, lemon juice is the best, followed by orange, lime and grapefruit juices. Citrus juices seem to have a stabilizing effect on catechins that goes beyond what would be predicted solely based on their ascorbic acid content.

References

Clifford M.N., van der Hooft J.J.J., and Crozier A. Human studies on the absorption, distribution, metabolism, and excretion of tea polyphenols. Am J Clin Nutr 2013;98:1619S-1630S [Abstract]

Dwyer J.T. and Peterson J. Tea and flavonoids: where we are, where to go next. Am J Clin Nutr 2013;98:1611S-1618S [Abstract]

Green R.J., Murphy A.S., Schulz B., Watkins B.A. and Ferruzzi M.G. Common tea formulations modulate in vitro digestive recovery of green tea catechins. Mol Nutr Food Res 2007;51(9):1152-1162 [Abstract]

Huang W-Y., Lin Y-R., Ho R-F., Liu H-Y., and Lin Y-S. Effects of water solutions on extracting green tea leaves. ScientificWorldJournal 2013;Article ID 368350 [Abstract]

Sharma V.K., Bhattacharya A., Kumar A. and Sharma H.K. Health benefits of tea consumption. Trop J Pharm Res 2007;6(3):785-792 [Abstract]

Tea polyphenols: bioactive compounds from leaves of tea plant

Tea polyphenols: from the leaf to the cup

Tea Polyphenols
Fig. 1 – Camellia sinensis

The leaves of the tea plant, Camellia sinensis, and tea are rich in bioactive compounds.
More than 4000 molecules have been found in the beverage.
Approximately one third of these compounds are polyphenols, the most important molecules in determining nutritional value and health benefits of the beverage.

Tea is a cup of life.” Anonymus author

Tea polyphenols are mostly flavonoids, such as catechins in green tea (e.g. EGCG), and thearubigins and theaflavins in black tea.
Other bioactive compounds present in tea are:

  • alkaloids, such as caffeine, theophylline and theobromine;
  • amino acids, and among them, theanine (r-glutamylethylamide), that is also a brain neurotransmitter and one of the most important amino acids in green tea;
  • proteins;
  • carbohydrates;
  • chlorophyll;
  • volatile organic molecules, that is, compounds that easily produce vapors and contribute to the odor of the beverage;
  • fluoride, aluminum and trace elements.

These molecules provide the nutritional value of the tea, affecting human health in many ways. Their composition is highly influenced by processing of tea leaves.

Biological activities of polyphenols

Polyphenols, both in vivo and in vitro, have a broad spectrum of biological activities such as:

  • antioxidant properties;
  • reduction of various types of tumors;
  • inhibition of inflammation;
  • protective effects against hyperlipidemia and diabetes.

Therefore, they have a protective role against the development of many diseases.
Thanks to the abundance of tea polyphenols, there has been a growing interest in recent years about the possible preventive effects of beverage against several diseases, particularly cardiovascular disease, for example in the development and progression of atherosclerosis.

Mechanisms of action of tea polyphenols

Currently, there is limited information on how tea polyphenols exert their effects at cellular level.
It seems, at least in vitro, that catechins in green tea, and theaflavins and thearubigins in black tea are the bioactive compounds responsible for the physiological effects and health benefits of tea.
And among the observed mechanisms by which tea polyphenols act at the cellular level, in addition to the antioxidant effect, it has been observed, as a consequence of polyphenol binding to specific receptors on the cell membrane, changes in the activity of various protein kinases, and growth and transcriptional factors.
Moreover, it seems that these molecules, or at least EGCG, may enter the cell and directly interact with their intracellular specific targets.

References

Dwyer J.T. and Peterson J. Tea and flavonoids: where we are, where to go next. Am J Clin Nutr 2013;98:1611S-1618S [Abstract]

Grassi D., Desideri G., Di Giosia P., De Feo M., Fellini E., Cheli P., Ferri L., and Ferri C. Tea, flavonoids, and cardiovascular health: endothelial protection. Am J Clin Nutr 2013;98:1660S-1666S [Abstract]

Lambert J.D. Does tea prevent cancer? Evidence from laboratory and human intervention studies. Am J Clin Nutr 2013;98:1667S-1675S [Abstract]

Lenore Arab L., Khan F., and Lam H. Tea consumption and cardiovascular disease risk. Am J Clin Nutr 2013;98:1651S-1659S [Abstract]

Lorenz M. Cellular targets for the beneficial actions of tea polyphenols. Am J Clin Nutr 2013;98:1642S-1650S [Abstract]

Sharma V.K., Bhattacharya A., Kumar A. and Sharma H.K. Health benefits of tea consumption. Trop J Pharm Res 2007;6(3):785-792 [Abstract]

Yuan J-M. Cancer prevention by green tea: evidence from epidemiologic studies. Am J Clin Nutr 2013;98:1676S-1681S [Abstract]

Tea: cultivation, processing and preparation

Tea: form bush to cup for your health

Tea is an aromatic infusion extracted from the dried leaves of Camellia sinensis, a member of the Theaceae family.
Tea is a beverage with very ancient origins, dating back to almost 4000 years ago, and it is one of the most consumed beverage worldwide, particularly in Asia, with an estimated per capita consumption of approximately 0.12 L/d.
Owing to its high consumption, even small effects on person’s health could have large effects on public health.

Cultivation of Camellia sinensis, the tea plant

Tea Fields
Fig. 1 – Tea Fields

Camellia sinensis is an evergreen plant, native to South, East, and Southeast Asia, which is currently cultivated in at least 30 countries, mainly in tropical or sub-tropical climates, even if some varieties grow in Cornwall, in Europe, and Washington State, in the USA.
In nature, if left undisturbed, Camellia sinensis can grow up to 15-20 meters (49-65 ft), while in plantations it is generally pruned to height less than a meter and a half, that is, like a small tree or bush, to facilitate cultivation and harvesting of the leaves.
It can also be cultivated in mountain, up to 1500-2000 meters (4900-6550 ft) above sea level. Many of the high-quality teas are produced from mountain crops, as the plant grows more slowly acquiring a better flavor.
Currently, the two most used varieties, of the four ones recognized, are:

  • Camellia sinensis var. sinensis, native to China;
  • Camellia sinensis var. assamica, native to India.

Tea is liquid wisdom.” Anonymus author

Processing of tea leaves

All the types of teas commercially available are produced from fresh leaves of the plant. During harvesting, young leaves are generally picked, as the older ones are considered to be inferior in quality.
The differences between the types of teas, e.g. green tea, oolong tea and black tea, depend on how Camellia sinensis leaves are processed after harvesting, since processing may cause a different degrees of oxidation of the substances present, in particular of catechins, a flavonoid subgroup, and the main responsibles of the benefits of green tea.
The organoleptic characteristics of the different types of tea are influenced, in addition to the processing of the leaves, even from the cultivar, the characteristics of the soil where the plant grown up, the methods of cultivation, the altitude, the climate, and the time of year in which the harvest of the leaves occurs.

How to prepare a perfect cup of tea

  • Due to the sensitiveness of dried leaves, it is good to store the packaging in cool dry place, free of fragrances that may alter its aroma.
  • Use fresh water and warm it to a temperature of 95-100°C for black tea, and about 90°C for green tea.
  • In order not to alter tea flavor, it is advisable to use a ceramic or porcelain teapot, avoiding those of steel. For teapot washing, avoid detergents, preferring water plus baking soda.
  • To prevent sudden changes in water temperature during the infusion, it is advisable to preheat the teapot pouring a bit of boiling water. Then, emptied the pot, add hot water (about 200-250 mL/filter).
  • How many filters/g of leaves to use? Typically, a filter (about 1.5-2 g) per person, or a teaspoon of loose tea leaves per person.
    If you prepare the beverage for some people, you add a filter/teaspoon more than the number of persons.
  • The infusion time should not exceed 10 minutes in order to avoid the development of bitter flavors; it should be 3-4 minutes for black tea, and 2-3 minutes for green tea.
    If you are using filters, you should remove them at the end of the infusion time.
    Approximately 30% of the material present in the leaves is extracted in the water.

Now, it’s time to enjoy your tea.

References