Graham H N
Prev Med. 1992 May;21(3):334-50. doi: 10.1016/0091-7435(92)90041-f.
Tea is grown in about 30 countries but is consumed worldwide, although at greatly varying levels. It is the most widely consumed beverage aside from water with a per capita worldwide consumption of approximately 0.12 liter per year. Tea is manufactured in three basic forms. Green tea is prepared in such a way as to preclude the oxidation of green leaf polyphenols. During black tea production oxidation is promoted so that most of these substances are oxidized. Oolong tea is a partially oxidized product. Of the approximately 2.5 million metric tons of dried tea manufactured, only 20% is green tea and less than 2% is oolong tea. Green tea is consumed primarily in China, Japan, and a few countries in North Africa and the Middle East. Fresh tea leaf is unusually rich in the flavanol group of polyphenols known as catechins which may constitute up to 30% of the dry leaf weight. Other polyphenols include flavanols and their glycosides, and depsides such as chlorogenic acid, coumarylquinic acid, and one unique to tea, theogallin (3-galloylquinic acid). Caffeine is present at an average level of 3% along with very small amounts of the other common methylxanthines, theobromine and theophylline. The amino acid theanine (5-N-ethylglutamine) is also unique to tea. Tea accumulates aluminum and manganese. In addition to the normal complement of plant cell enzymes, tea leaf contains an active polyphenol oxidase which catalyzes the aerobic oxidation of the catechins when the leaf cell structure is disrupted during black tea manufacture. The various quinones produced by the enzymatic oxidations undergo condensation reactions which result in a series of compounds, including bisflavanols, theaflavins, epitheaflavic acids, and thearubigens, which impart the characteristic taste and color properties of black tea. Most of these compounds readily form complexes with caffeine. There is no tannic acid in tea. Thearubigens constitute the largest mass of the extractable matter in black tea but their composition is not well known. Proanthocyanidins make up part of the complex. Tea peroxidase may be involved in their generation. The catechin quinones also initiate the formation of many of the hundreds of volatile compounds found in the black tea aroma fraction. Green tea composition is very similar to that of the fresh leaf except for a few enzymatically catalyzed changes which occur extremely rapidly following plucking. New volatile substances are produced during the drying stage. Oolong tea is intermediate in composition between green and black teas.
茶叶种植于约30个国家，但在全球范围内被消费，尽管消费水平差异很大。除水之外，茶是消费最为广泛的饮品，全球人均年消费量约为0.12升。茶叶有三种基本加工形式。绿茶的加工方式能防止绿叶多酚氧化。红茶生产过程中则促使氧化发生，这样大部分此类物质都会被氧化。乌龙茶是部分氧化的产品。在约250万吨干茶产量中，只有20%是绿茶，乌龙茶不到2%。绿茶主要在中国、日本以及北非和中东的一些国家消费。新鲜茶叶中富含被称为儿茶素的黄酮醇类多酚，其含量可达干叶重量的30%。其他多酚包括黄烷醇及其糖苷，以及缩酚酸，如绿原酸、香豆酰奎尼酸，还有一种茶特有的成分，没食子酸（3 - 没食子酰奎尼酸）。咖啡因平均含量为3%，还含有极少量其他常见的甲基黄嘌呤，如可可碱和茶碱。氨基酸茶氨酸（5 - N - 乙基谷氨酰胺）也是茶所特有的。茶叶会积累铝和锰。除了植物细胞酶的正常组成外，茶叶还含有一种活性多酚氧化酶，在红茶制作过程中，当叶细胞结构被破坏时，该酶会催化儿茶素的有氧氧化。酶促氧化产生的各种醌类会发生缩合反应，生成一系列化合物，包括双黄酮醇、茶黄素、表茶黄素酸和茶红素，这些赋予了红茶独特的口感和色泽。这些化合物大多能与咖啡因轻易形成复合物。茶中不存在单宁酸。茶红素是红茶中可提取物中含量最大的部分，但其成分尚不明确。原花青素是该复合物的一部分。茶过氧化物酶可能参与其生成过程。儿茶素醌类还引发了红茶香气成分中数百种挥发性化合物中许多化合物的形成。绿茶的成分与鲜叶非常相似，只是采摘后会发生一些极其迅速的酶促催化变化。干燥阶段会产生新的挥发性物质。乌龙茶的成分介于绿茶和红茶之间。
