Dr. B.L. Wedzicha of t h e Procter D e p a r t m e n t of Food Science, U n i v e r s i t y of Leeds, describes t h e c h e m i s t r y of black t e a manufacture Tea is the m o s t w i d e l y c o n s u m e d beverage in the w o r l d . The economic importance of an annual w o r l d production of tea estimated to be in the region of 1-1·5 million tonnes has resulted in considerable attention being paid to the understanding of the chemical and physical changes w h i c h take place during tea manufacture. The three main types of tea, black, green and instant tea, are made by processing the y o u n g shoot or flush, comprising the terminal b u d and t w o adjacent leaves of the tea plant (Camellia sinesis), s h o w n opposite. Of these types of processed tea the most important is the familiar black tea, w h i c h is a fermented product, the colouring matter arising f r o m enzymic oxidation of phenolic components of the tea leaf. Green tea, o n the other hand, resembles m o r e closely the dehydrated leaf, any chemical changes being non-enzymic and its brews do not contain highly coloured products. Green tea is the m o s t popular f o r m of tea in a number of countries including China and Japan. Instant tea may be prepared f r o m both black and green tea, the process essentially involving extraction w i t h water, concentration and dehydration. The w o r l d market for instant tea, however, is small (some 5% of w o r l d tea production), indicating perhaps that satisfactory products have not yet reached the customer. The market has been further affected by the introduction of tea bags. In view of the commercial importance of black tea and the intricacy of the mechanisms of its manufacture, this product has received by far the most attention and the purpose of the present article is to outline some findings in this field. The black tea process 1 The freshly plucked tea flush is allowed to wither in air for some 18-20 hours, or for shorter periods when heated air is circulated, when it loses water and acquires a kid-glove feel. Important chemical changes have already begun to take place 2 . For example, amino acids are formed as precursors of compounds ultimately leading to the production of flavour and non-enzymic browning, the formation of keto compounds as flavour precursors and the 2
formation of caffeine. The leaf also becomes capable of acquiring a twist, rather than breaking up, when it is subsequently rolled. Fermentation is initiated by rolling when the enzyme, normally located in the chloroplast, and the phenolic substrate, found in the cell vacuoles, are mixed in the presence of oxygen, without extensive damage to the outer cell wall. A three hour fermentation results in less than 10% of unchanged substrate remaining 3 . Fermentation is arrested by firing in a stream of hot air which also dries the product to some 3% moisture content. The final stage is grading. Enzymic oxidation Phenolases or polyphenoloxidases are enzymes which mediate in the oxidation of o-diphenols to o-quinones in the presence of
oxygen but most of these enzymes are also capable of oxidising monophenols to o-quinones. The tea enzyme is a polyphenoloxidase but, unlike the ordinary phenolases, it does not mediate in the oxidation of monophenols and the substrates for the so called fermentation are flavanol components of the tea leaf3. These are based on the flavan structure, figure 1. Polyphenolic components comprise some 25-35% of the tea flush on a dry weight basis, of which some 20% may be found as flavanol 4 . Specific flavanol structures are shown in figure 2. They may clearly be divided into two groups — the catechins and the gallocatechins according to whether there are two or three hydroxyl (OH) groups in the right hand phenolic ring. In fact, each group of compounds may be further distinguished according to the arrangements of groups around carbon atoms 2 and 3, resulting in four possible isomers. For example, the isomers of...
References: 2,3,4 and 7 are reviews and are ideal for further reading. The reader should, however, note that some structures shown in the earlier publications were amended as a result of further experimental work. Other suggestions for further reading are: M.A Bokuchava and N.I. Skoboleva, Adv. Fd Res., 1969,17,215; and E.A.H. Roberts in The Chemistry of Flavanoid Substances, Pergamon, 1962, pp468-512.
Nutrition and Food Science
RCHO + CO2 + NH 3 where R is an alkyl group and this
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