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The Chemical Composition of Tea-Part 3: Aromatic Substances and Sugar

The Chemical Composition of Tea-Part 3: Aromatic Substances and Sugar

  • Sunday, 01 January 2023
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The Chemical Composition of Tea-Part 3: Aromatic Substances and Sugar

Aromatic substances in tea

Aromatic substances in tea leaves are the general term of volatile substances in tea leaves. In the total chemical composition of tea leaves, the content of aromatic substances is not much, generally about 0. 02% of the dry weight in fresh leaves, 0. 005% ~ 0. 02% in green tea, 0. 01% -0. 03% in black tea. Although the content of aromatic substances in tea is not much, but its species is very complex, so far has been found and identified about 700 kinds of aroma components, there are alcohols, aldehydes, ketones, carboxylic acids, esters, lactones, phenols and their derivatives, miscellaneous oxygen compounds, sulfur compounds, nitrogen compounds ten categories. Generally speaking, the aromatic substances contained in the fresh leaves of tea are relatively small, about 80 kinds; in green tea, there are more than 260 kinds; in black tea, there are more than 400 kinds.

The aroma of tea leaves is one of the important factors determining the quality of tea leaves, and different tea types have different aroma characteristics due to different processing methods. The aromatic substances in fresh tea leaves are mainly alcohol compounds and mostly exist in the form of aroma glycoside. The aroma of green tea is mainly composed of pyrazine, pyrane  and pyrroles with roasted fragrance, forming "chestnut fragrance" and "caramel fragrance", etc. The aroma of black tea is mainly composed of aldehydes, ketones, acids, esters and other aroma compounds, forming the unique sweet flower fragrance of black tea.

Sugars in tea

Sugars are also known as carbohydrates. The sugars in tea leaves include monosaccharides, oligosaccharides and polysaccharides, which account for 20% -25% of the dry weight of tea leaves. Monosaccharides and disaccharides are easily soluble in water, with a content of 0.8% -4%, which is one of the substances that make up the taste of tea. The monosaccharides and disaccharides in tea leaves are mostly stored in the old leaves and less in the young leaves. In the tea processing, due to the presence of enzymes, heat or amino compounds, these sugars will undergo hydrolysis, caramelization and Maillard reaction to produce monosaccharides, poly-pigments and aroma substances.

The polysaccharides in tea mainly include cellulose, hemicellulose, starch and pectin, which account for more than 20% of the total dry matter of tea leaves. Generally speaking, the cellulose content of tea leaves is less, the fresh leaves are tender and good, and it is easier to make tea into strips and make shapes, which can produce high-quality famous tea. As the maturity of the leaves increases, the cellulose content increases, and thus the cellulose content is one of the sign components of the old and young tea leaves.

Starch is a storage substance in the tea tree, with the most content in tea seeds and less in leaves, and the content of old leaves is higher than that of young leaves. Starch is insoluble in water and usually cannot be used when brewing, so it has little nutritional value. However, in tea processing due to enzymes or hydrothermal action, the starch can be hydrolyzed into soluble sugars,which can improve the taste, aroma and soup color of tea.

Protopectin is a structure with branched chains formed by pectin and arabinoxylan, which binds together with cellulose and hemicellulose and becomes the constituent material of plant cell wall, insoluble in water. Pectin in tea leaves mostly exists in the form of protopectin, and the protopectin content in the fresh leaves of one bud and three leaves is generally about 8%. Under the action of protopectinase, protopectin is hydrolyzed to form water-soluble pectin. Water-soluble pectin can increase the sweetness, fragrance and thickness of tea broth; and water-soluble pectin has viscosity, which can help to knead and curl into strips and make tea leaves look oily.

The content of pectin is related to the maturity of tea varieties and new leaves, with the third and fourth leaves having higher pectin content, while the content of water-soluble pectin decreases with the maturity of new leaves. When processing green tea, the higher killing temperature is beneficial to the increase of water-soluble pectin content; during the withering process of black tea, the increase of pectinase activity is beneficial to the increase of water-soluble pectin content of fresh tea leaves.

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