The colors of tea: a fascinating scientific enigma

Tea is known for its molecular composition beneficial to health: anti-inflammatory, anti-oxidant, antibacterial, etc. Other molecules will give the taste so particular to each color of tea. What are all these molecules? Are there molecules specific to each tea family?

Tea: a molecular cocktail

Everything is composed of molecules, tea leaves are no exception. We must distinguish two categories of molecules in the leaves: structural molecules and organic compounds trapped in the leaf.

The structural molecules are mainly sugars: pectin (gives the "soft" aspect of the leaves), cellulose (forms the wall of the plant cells) and hemicellulose (links the cellulose fibers together). They will form the skeleton of the leaf, allowing to structure the compartments of the leaf and to enclose the organic compounds which will interest us.

Inside the cells and between the cells, there are many molecules that will allow the functioning of the leaves as the vascularization of the leaves or photosynthesis. Some molecules will be mainly useful to the plant while others can be useful for our body. 

If we look at the molecules of therapeutic interest, more specifically the polyphenols, they have the highest proportion in the leaf (more than a third) while the amino acids and mineral salts do not even represent 10% of the leaf.

The polyphenols ?

Polyphenols are "Swiss Army knife" molecules for the plant. They help the plant to protect itself from external attacks (insects, pathogenic micro-organisms, etc.) or to develop (attraction for pollination, activation of germination, composition in certain plant hormones, etc.). Polyphenols are classified into families according to their chemical structure. In tea, several kinds of polyphenols are particularly studied in the research on tea and health:

• Flavonoids. In this family of molecules, we find the catechins, molecules whose antioxidant power is the most studied in research. Anthocyanins, molecules that give a purple pigment found in the leaves of tea plants used to make purple tea. These molecules attract attention for their antioxidant power.

• Theaflavins and thearubigins. This family derives from the oxidation of catechins. They have antioxidant and binding capacities to certain molecules. They give a strong taste to tea.

• Phenolic acids. This family of molecules is characterized by a cyclic chemical structure and a strong antioxidant potential. Gallic acid is a member of this family.

• Tannins are not a family per se. They are polyphenols whose chemical structure respects certain criteria. Anthocyanin, catechins and gallic acid are tannins. They give tea a bitter taste. Contrary to popular belief, theaflavins and thearubigins are not tannins in the strict sense of the word because they are derived from the oxidation of other polyphenols.

• Caffeine is an alkaloid, it has different effects and mechanism of action on the body than tea.

What we know from tea colors

• Green tea and yellow tea: Green tea and yellow tea are relatively close because the oxidation process of the leaves has been stopped. Yellow tea undergoes a post-fermentation stage compared to green teas. The preservation of catechins from oxidation prevents their transformation into theaflavin and thearubigin, which could explain why these are the two tea families with the most catechins. The preservation of these molecules with strong antioxidant potential explains the craze for green tea in health research. Gustatively, these molecules will bring bitterness, astringency and the characteristic freshness to many green teas. Differences in catechin, caffeine and gallic acid compositions may appear according to several criteria: leaf age (Green tea 2 weeks vs 3 months: +EGCG/-EC , -EGCG/+EC), growing region and manufacture (almost twice as much caffeine and +20% EGCG in Mao Jian from Guizhou than Lu'An Guapian from Anhui) and roasting (it seems to slightly decrease the amount of total polyphenols in green tea).

• Black tea: Unlike other teas, black tea is rich in theaflavins and thearubigins. Black tea is a tea whose leaves are completely oxidized, theaflavins and thearubigins are formed from the oxidation of catechins. This is what gives black tea its stronger aromatic profile compared to other families. The amount of caffeine in black tea is variable and traces of catechins may remain in the leaves. Like green tea, the composition of black tea also depends on the region or external treatments such as roasting. Theaflavins are thought to have an antioxidant action similar to that of catechins. However, the quantity of theaflavins remains relatively low, only a few therapeutic perspectives are currently considered such as metabolic disorders (bad balance of LDL and HDL cholesterol levels, type 2 diabetes, etc.), dental infections, anti-viral effects, osteoporosis.

• Oolong tea: Oolong tea is a tea whose leaves have undergone partial oxidation, which can range from 10% to over 50%. Unfortunately, in the scientific literature, there is little data available to establish a typical profile for oolongs with or without partial oxidation. It seems that the quantities of certain catechins vary according to the degree of oxidation (slightly oxidized: +EGCG and ECG, more oxidized: +GC and EGC). The duration of semi-fermentation could slightly increase the amount of theaflavin and decrease the amount of catechins in Tie Guan Yin. It is still difficult to evaluate the differences in composition of teas with particular processes such as Wuyi Mountain teas or Dancong.

• White tea: White tea is a tea that has been dried, it has only undergone a slight oxidation and fermentation. Like oolong tea, white tea has no fixed profile. It will be very dependent on the harvest period. Indeed, an autumn Shou Mei and compressed Shou Mei would be richer in theaflavins but poorer in caffeine and catechins than Hao Bao Yin Zhen and spring White Pony. White tea does not always taste fresh and light, autumn Shou Mei and old white teas are teas with a slightly tannic character.

• Cooked and raw Pu'Erh: Pu'Erh tea is a tea whose leaves have undergone fermentation by fungi and bacteria (accelerated: cooked, natural: raw). Of all teas, it is the one with the lowest quantity of molecules of therapeutic interest. On the other hand, the bacterial and fungal populations of Pu'Erh are studied for different properties, notably antibacterial. There are very few studies that have looked at the composition of raw and cooked Pu'Erh but it seems that the raw contains a little more flavonoids and tannins than the cooked (which would explain the more vegetal and astringent side of the raw) but that the latter has a more diverse bacterial population.

• The case of purple tea: Purple tea is not a tea family. It is about tea leaves from mutant tea plants, that is to say that they produce an additional polyphenol: anthocyanin (purple pigment). It is a molecule with a strong antioxidant potential. We can have for example green tea / purple as in Kenya, or Pu'Erh / purple as on some endemic species in China. It seems that purple/green or purple/black tea has no more chemical compounds than a counterpart of the same color. On the other hand, anthocyanin is one of the most abundant polyphenols in the leaves which would suggest a stronger antioxidant potential and a very marked difference in taste (personally I find that purple teas become astringent more quickly).

Amino acids and mineral salts

To understand if amino acids such as leucine/isoleucine/valine (well known by bodybuilders to limit the destruction of muscle fibers and to help protein fixation) and mineral salts such as calcium Ca2+ are in sufficient quantity in tea for our needs, I based myself on the recommendations formulated by the WHO and the ANSES. Unfortunately, it seems that this is not the case for a large number of compounds.

Leucine is an amino acid that is not synthesized by the body and that is found most in the tissues of our body. Daily intakes of leucine should be 37mg/kg/d (for a 70kg adult, 2590 mg/d). In this 2019 study, the authors extracted between 0.81 and 4.2mg of leucine / gram of leaves for all possible tea families. For one cup of tea (20c, 4g), that would be between 3.24 and 16.8 mg/ cup. For 4 cups of tea, the leucine intake does not exceed 2.5%. The same is true for other amino acids (less than 5% of total amino acid requirements for 4 cups).

Calcium is a key mineral for the functioning of our cells and for certain tissues such as bone. ANSES reports estimate that an adult should consume 1000 mg of calcium per day. In Reto's 2007 study on the composition of minerals in green tea infusion, it was shown that the level of calcium was only 3.5mg/L. For 4 cups of tea, there would be less than 2mg. This is not even 1% of the required calcium intake. Only the source of manganese in the tea could be an interesting supplement because it would represent 10 to 20% of the 2.5 mg/d recommended by the ANSES (for a single 20cl cup and 4g of Indian black tea).

The composition in polyphenols and alkaloids is very varied according to the family of teas (more catechins for green tea and yellow tea, more theaflavins for black tea) but also according to the period of harvest, the region of culture and the manufacture of the leaves. All these parameters are important to know to know the best sources of molecules, the taste that it will bring to the tea. On the other hand, tea is not a sufficient complementary source of amino acids and minerals (except perhaps manganese).

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