The tea plant is native to China and belongs to the family Theaceae, genus Camellia, and species Camellia sinensis.
Through long-term historical changes and natural as well as artificial selection, the tea plant has developed diverse botanical characteristics.
Tea plants can typically be divided into large-leaf (C. sinensis var. assamica) and small-leaf (C. sinensis var. sinensis) varieties based on leaf area. Leaves with an area greater than 60 cm2 are considered extra-large, those between 40 and 60 cm2 are large, those between 20 and 40 cm2 are medium-sized, and those less than 20 cm2 are small. There are also significant differences in the content of internal substances.
This article discusses the advantages of the internal substances in Yunnan large-leaf tea, focusing on the two major secondary metabolites that most directly influence tea quality: tea polyphenols and Caffeine. We will also consider the water-soluble extract content, which affects the concentration and body of the tea infusion.
Overview of Internal Substances in Tea
To date, over 700 compounds have been identified and isolated from tea.
These include primary metabolites such as carbohydrates, proteins, and lipids, which are essential for the survival of living organisms, as well as secondary metabolites like tea polyphenols, catechins, caffeine, amino acids (Theanine), flavonoids, pigments, terpenoids, and aromatic compounds, which are formed through enzymatic catalysis using primary metabolites as raw materials. These substances not only give tea its unique color, aroma, and taste but are also closely related to human health.
Advantages of Tea Polyphenol Content in Yunnan Large-Leaf Tea
Tea polyphenols (TP) are a complex of polyhydroxy phenolic compounds and are referred to as the “seventh nutrient,” comprising 18% to 36% of dry tea weight.
They are classified into five main categories: catechins (flavan-3-ols), flavones and flavonols, anthocyanins, phenolic acids and condensed phenolic acids, and hydrolysable tannins. Among these, catechins account for about 70% of total tea polyphenols.
The tea polyphenol content in fresh leaves of Yunnan large-leaf tea plants is higher than that in small-leaf varieties. Studies show that the tea polyphenol content in the one-bud-two-leaves of Yunnan large-leaf tea is generally greater than or equal to 36%, while that of Longjing tea (small-leaf variety) is around 30%.
The level of tea polyphenols can determine the suitability of a tea variety for processing, the sensory quality of the tea, and its functionality. For example, black tea processed from the fresh leaves of Yunnan large-leaf tea can better exhibit the characteristic qualities of black tea, such as being “red, concentrated, bright, and clear” and having a “concentrated, strong, and fresh” flavor. Pu'er tea, known for its “rich, sweet aftertaste, and saliva-inducing” taste experience and “getting better with age” quality, is favored by consumers due to being made from the fresh leaves of Yunnan large-leaf tea with high tea polyphenol content.
Studies have shown that the primary physiological activity of tea polyphenols is the elimination of harmful free radicals, thereby achieving antioxidant, anticancer, and anti-aging effects, acting as a preservative for the human body. In general, among people of the same age, those who drink tea tend to look younger than those who do not. Academician Liu Zhonghua summarized the antioxidant function of tea polyphenols with the phrase, “It's not that you age easily, it's just that you don't drink enough tea.” Therefore, Yunnan large-leaf tea with high tea polyphenol content would excel in eliminating free radicals, antioxidation, anticancer, and anti-aging compared to small- and medium-leaf teas.
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High Caffeine Content in Yunnan Large-Leaf Tea
Caffeine is the predominant alkaloid in tea, accounting for approximately 2 to 4% of dry tea weight. Apart from seeds, all other parts of the tea plant contain caffeine, with the highest concentration found in the leaves and less in the stems. The caffeine content decreases with the maturity of the new shoots, making it a useful indicator of their tenderness. Caffeine levels also vary seasonally, with summer teas generally having higher caffeine content than spring teas.
Caffeine imparts a bitter taste and is highly soluble in water, with over 80% of it dissolving during tea brewing, making it an important component contributing to the taste of tea. Interactions between caffeine and catechins show that caffeine significantly enhances the bitterness and sweet aftertaste characteristics of simple catechins EGC and EC, but has little effect on astringency. Caffeine can form complexes with catechins, altering their taste characteristics. In black tea infusions, the complex formed by caffeine and theaflavins has a fresh and brisk taste, enhancing the overall freshness of the tea.
Therefore, the caffeine content is an important factor affecting tea quality. According to research by Liu Benying et al., the caffeine content in Yunnan large-leaf tea is higher than the typical range of 2 to 4% found in other teas, reaching 4 to 5%. Compared to small- and medium-leaf teas, Yunnan large-leaf tea has a significant advantage in caffeine content.
Current research suggests that low doses of caffeine have stimulating and refreshing effects and can dilate renal blood vessels, increase blood flow in the kidneys, and enhance glomerular filtration rate, leading to diuretic effects within a certain concentration range. Caffeine also has fat-decomposing properties, which may explain why regularly drinking Pu'er tea, made from the fresh leaves of Yunnan large-leaf tea, can help prevent obesity and promote weight loss. Additionally, caffeine has the effects of strengthening blood vessels, strengthening the heart, relaxing smooth muscles, relieving bronchial and bile duct spasms, regulating body temperature, lowering cholesterol, and preventing atherosclerosis. Research at Rutgers University in the United States has confirmed that, relative to tea polyphenols, caffeine has a synergistic effect in cancer prevention and treatment in humans.
Advantages of Catechin Content and Composition Ratio in Yunnan Large-Leaf Tea
Catechins are the main components of flavonoids in tea, belonging to the flavan-3-ol class of compounds and are derivatives of 2-phenylbenzopyran. They make up 12% to 24% of the dry weight of fresh tea leaves. Four catechins significantly influence tea quality and functionality: epigallocatechin gallate (EGCG), epigallocatechin (EGC), epicatechin gallate (ECG), and epicatechin (EC). Additionally, there are small amounts of gallocatechin (GC), catechin (C), catechin gallate (CG), and gallocatechin gallate (GCG).
The advantages of catechins in Yunnan large-leaf tea mainly lie in the following three aspects:
Firstly, the total content is higher than that in small-leaf varieties. According to research from the Chinese Academy of Agricultural Sciences' Tea Research Institute, the total catechin content in Yunnan large-leaf tea is 17.7%, while in small-leaf teas, it ranges between 12% and 14%.
Secondly, the proportion of simple catechins in the total catechin content is higher than in small- and medium-leaf teas, giving them an advantage in terms of sensory quality and suitability for black tea production. Research from the Chinese Academy of Agricultural Sciences' Tea Research Institute shows that in Longjing (small-leaf variety) tea, the proportions of simple catechins