Manufacture

The manufacturing process converts freshly harvested leaf to products of commerce. Black tea is the most widely consumed form of tea. It is produced by promoting the aerobic oxidation of fresh leaf catechins in reactions catalyzed by tea polyphenol oxidase. Green tea, consumed mostly in China, Japan, the Middle East, and North Africa, is processed so as to prevent the oxidation of catechins. Oolong tea is partially oxidized. It is manufactured and consumed primarily in China and Taiwan.

Black Tea

The most significant change that occurs during the manufacture of black tea is the conversion of the colorless catechins to a complex mixture of yellow-orange to red-brown substances accompanied by the development of a large number of volatile compounds. The process was historically referred to as fermentation, but no microbiological processes take place. The changes result in a dark-colored leaf that produces an amber-colored beverage, less astringent than that derived from fresh leaf. The product possesses an exceedingly complex flavor profile.

Catechin Oxidation

The first step in catechin transformation is oxidation to highly reactive quinones (Fig. 5) (28). All catechins undergo this reaction, but the gallocatechins (3) and (4) are preferentially oxidized. Quinone formation is the primary driving force for the transformation of catechins to black-tea components.

Theaflavins

During fermentation, quinones derived from a simple catechin (1), (2), or (5) react with quinones derived from gallocatechins (3), (4), or (6) to form compounds with a seven-membered benztropolone ring known as theaflavins (Fig. 6). The predicted theaflavins derivable from each of the possible quinone combinations have been isolated (29,30). In solution, theaflavins are bright orange-red in color and provide "brightness," a desirable quality, to the beverage. (Many of the named attributes of tea arise from the tea taster's lexicon and are difficult to define objectively (31).

The distinctively colored theaflavins are determined spectrophotometrically in a tea brew (32). Total theaflavin concentration in black tea does not exceed 2.5% and is sometimes as low as 0.3% (33). Prolongation of the oxidation period and high fermentation temperatures decrease theaflavin content (34). Theaflavins are oxidized by epi-catechin quinone, formed late in the oxidation process because of its high oxidation potential (35). Low oxygen tension also results in decreased theaflavin formation (36). At most, only 10% of the original catechin content of tea flush is accounted for as theaflavins in black tea.

Theaflavic Acids

Theaflavic acids are formed by the reaction between quinones derived from (1), (2), or (5) and gallic acid quinone (Fig. 7). It is postulated that gallic acid is formed by de-gallation during processing. Although it is not directly oxidized by tea polyphenol oxidase, it is converted to a quinone by the catechin quinones that possess a higher oxidation potential, such as that of epicatechin (37,38). The theaflavic acids are bright red, acidic substances present in black tea only in very small quantities because of their reactivity (39). Theaflagallins originate through a similar route but involve the gallocatechins (3), (4), or (6). They are also present in very small quantities (40).

Bisflavanols

Coupling of quinones derived from (3) and (4) produces a series of colorless substances known as bisflavanols (35). They are highly reactive and occur only at low levels in black tea (Fig. 8). The bisflavanols are also known as theasinensins. There have been some reports of small amounts of theasinensins in fresh leaf (12).

Thearubigins

In the course of black-tea formation, ca 10 to 15% of the leaf catechin fraction remains unchanged and ca 10% is accounted for by theaflavin, theaflavic acid, and bisfla-vanol formation. Seventy five to 80% of the original catechin fraction is converted to a complex mixture, incompletely defined and largely intractable to resolution. This red-brown fraction was named thearubigen when first described in 1957 (42). Investigations of its chemistry over a period of four decades have resulted in conflicting data and conclusions, attesting to the complexity of the mixture. Molecular weight determinations have indicated a range of 700 to 40,000 (43), but this must be considered in context with the known increase in apparent molecular weight

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