The Chemical Composition of Tea-Part 1:Tea polyphenols,Protein and Theanine

In the fresh tea leaves, moisture accounts for 75% -78%. Dry matter may be as high at 22%-25%.The dry matter of tea leaves is made up mostly by 93.0% -96.5% organics with 3.5% -7% inorganic components.Up to today more than 700 organic compounds are known to be isolated and identified in tea, including primary metabolites such as proteins, sugars, fats and secondary metabolites of the tea tree - polyphenols, pigments, theanine, alkaloids, aromatic substances, soaps, etc. The inorganic compounds in tea leaves are mainly mineral elements and their oxides.

Chemical Components Related to the Quality of Tea Leaves Part 1

Tea polyphenols

Tea polyphenols is the general name of all polyphenols in tea leaves, also known as "tea tannins". Tea polyphenols are one of the main chemical components in tea leaves, with a general content of 18% -36% (dry weight) in fresh tea leaves. Tea polyphenols are widely distributed in the tea tree, and are found in all organs of the plant; however, tea polyphenols are mainly concentrated in the new growing parts of the tea tree, and the content decreases from buds, leaves, old leaves, stems and roots in order. The tea polyphenol content changes very obviously under different quality, different seasons and different environmental conditions, and has the most significant effect on tea quality.

The tea polyphenols found in the new leaves of tea tree mainly consist of catechins (flavanols), flavonoids and flavonols, anthocyanins and anthocyanins, phenolic acids and phenolic acids, of which catechins have the highest content, accounting for about 70% to 80% of the total tea polyphenols. Except for phenolic acid and phenolic acid, all of them have 2-phenylchroman as the main structure, which are collectively called flavonoid substances.

1.Catechins

Catechin in tea belongs to flavanols, which is the main component of tea polyphenols, with 12% -24% content in tea leaves. Catechin is a white solid, highly hydrophilic and easily soluble in hot water. Catechin is one of the main taste substances of tea soup, with bitterness and astringency.

Tea catechin class mainly includes epigallocatechin (EC), epigallocatechin (EGC), epigallocatechin gallate (ECG) and epigallocatechin gallate (EGCG), etc.. During the processing of tea leaves, these cis-catechins are also converted to form catechin (C), gallocatechin (GC), catechin gallate (CG), gallocatechin gallate (GCG). C, GC, EC, EGC are called non-ester catechins or simple catechins, CG, GCG, ECG, EGCG are called ester catechins or complex catechins.

2.Flavonoids and flavonoid glycosides

Flavonoids in tea tree are also known as anthocyanins, mainly flavonols and their glycosides. The glycosides are mainly formed by the combination of flavonols and sugars at the C3 position. Tea flavonols mainly include kaempferol, quercetin and myricetin; the glycosides that are more abundant in tea are mainly rutinoside, quercetin and kaempferol. Flavonoids and flavonols are generally insoluble in water, while the solubility of flavonoid glycosides in water is greater than that of glycosides, and their aqueous solution is greenish-yellow.

The content of flavonols and glycosides in tea leaves accounts for 3%-4% of the dry mass, generally higher in spring tea than in summer tea. Flavonoids are the main substances of the water-soluble yellow pigment of tea leaves, which is an important component of green tea soup color.

3.Anthocyanins and leucoanthocyanidin

Anthocyanins, also known as anthocyanins, are an important class of water-soluble pigments. In general, anthocyanins in tea leaves account for about 0.01% of the dry mass, while in purple bud tea, they can reach 0.5% ~1.0%. The taste of anthocyanin is bitter and its content has a great impact on the quality of tea.

Leucoanthocyanidin, also known as "4-hydroxyflavanols", are a class of so-called reduced flavonoids, with a content of about 2% to 3% of the dry mass in the new tips of tea trees. In the fermentation process of black tea,Leucoanthocyanidin can be completely oxidized into colored oxidation products.

4.Phenolic acids and depsides

Phenolic acids are a class of aromatic compounds with carboxyl groups and hydroxyl groups in the molecule; depside is formed by the interaction of the hydroxyl group on the depside with the hydroxyl group on another molecule. Phenolic acids and depsides in tea mainly include gallic acid, caffeic acid, chlorogenic acid, condensed derivatives of quivic acid, etc. Phenolic acids and depsides are easily soluble in water, accounting for about 5% of the dry weight of fresh tea leaves.

Protein and theanine in tea

Tea leaves are rich in protein, accounting for 20% -30% of the dry weight of tea leaves, but the protein that can be soluble in water is very little, accounting for 1% - 2%. Tea soluble protein not only contributes to the clarity of tea broth and the stability of tea broth colloidal solution, but also enhances the taste and nutritional value of tea broth. The proteins of the fresh tea leaves are hydrolyzed during processing to produce various amino acids which are closely related to the quality of tea leaves and have different effects on the taste, aroma and nutritional value of tea leaves. With the growth of new tea leaves, the protein content gradually decreases, therefore, the protein content can be used as one of the indicators of tea tenderness to a certain extent.

The amino acid content in tea is very rich, there are 26 kinds of amino acids found and identified, in addition to 20 kinds of protein amino acids, there are 6 kinds of non-protein amino acids, which are secondary plant substances, the highest content of which is theanine. Amino acids are an important factor in the quality of tea, and the taste characteristics of various amino acids are different. Some amino acids have freshness and sweetness, which are the main fresh taste components of tea, and play an important role in the formation of aroma and soup color of tea. In general, the content of amino acids in young tea leaves is higher than that of old leaves, and spring tea is higher than summer and autumn tea.

The amino acid content of tea leaves accounts for 1% -4% of the dry weight of tea leaves, and the amino acid content of Anji white tea can be as high as about 7%. Theanine is a unique amino acid in tea, and it is also the main part of free amino acids in tea, accounting for more than 50% of the total free amino acids in tea. Theanine is mainly distributed in the buds, young stems and young roots, and about 70% of the amino acids in the new tips of tea trees are theanine. Theanine is highly soluble in water, and the aqueous solution has the fragrance of caramel and the fresh taste similar to MSG, with a low taste threshold of 0.06%. Theanine can suppress the bitterness and astringency of tea soup. The addition of theanine to low-grade green tea can improve its taste quality.

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