Green Tea

Leeds, describes t h sec h e m is t ray of black tea manufacture Tea is the m so two I d e I yes o NSA u m De beverage in the w o r old . The economic importance of an annual w o r I d production of tea estimated to be in the region of 1-15 million tones 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 n g shoot or flush, comprising the terminal b u d and t w o adjacent leaves of the tea plant (Camellia genesis), 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 coloring matter arising f r o m enzymes oxidation of phenol 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-enzymes and its brews do not contain highly colored pro ducts.Green tea is the m so t popular of 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 I d market for instant tea, however, is small (some 5% of w o r I 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 flavor and non-enzymes browning, the formation of kite compounds as flavor 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 phenol 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. Enzymes oxidation Phenols or polysaccharides are enzymes which mediate in the oxidation of o-depletion to o-quinine's in the presence of oxygen but most of these enzymes are also capable of oxidation monopoles to o- quinine's. The tea enzyme is a polysaccharides but, unlike the ordinary for the so called fermentation are flavor components of the tea leaf.These are based on the flan structure, figure 1 . Polyphonic components comprise some 25-35% of the tea flush on a dry weight basis, of which some 20% may be found as flavor 4 . Specific flavor structures are shown in figure 2. They may clearly be divided into two groups ? the catechist and the collocations according to whether there are two or three hydroxyl (OH) groups in the right hand phenol 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 the catechisms are: (-) catechist, (+) catechist, (-) peachiness and (+) peachiness. In addition, these compounds exist as esters with Gaelic acid, figure 3. The most abundant are the collocations and specifically (-) epistemolo gically and its gallant ester (ca. 10% dry weight). In order of abundance, this is followed by (-) peachiness and its gallant (ca. 5 by weight) 4 . It is reasonable to assume that the first stage of oxidation involves conversion of Nutrition and Food Science these substrates to o-quinine's and is followed by condensation of these quinine's to dimmers and polymers.Flavor derived products in black tea The oxidation of flavors by way of quinine leads to the formation of dimmers by meaner of bonds between adjacent molecules, such that the 2†² position on one molecule, figure 2, links to either the 6 or 8 position on another in the case of catechist (ahead to tail' dimmers), and in the case of collocations the 2†² position on one molecule becomes linked to the 2†² position on another (tail to tail' dimmers). These tail to tail dimmers have been identified in black tea and are found to be derived from (-) epistemologically and its gallant as expected 5 .During fermentation carbon dioxide is evolved and this is believed to arise from an unusual but most important reaction leading to the formation of a seven member ring. Carbon rings of this size are infrequently found in organic chemistry but the essentials of this reaction are illustrated by the oxidation of paroxysmal to form purloining, figure 4. Gaelic acid, found extensively in fermented tea, can undergo a similar reaction to form purpurogallincarboxylic acid.The thyroxin grouping of the collocations can react in a similar manner to paroxysmal and it is therefore, not surprising that compounds such as paleontologist, figure 5, are found to be present in black tea 6 . It is also found that the catechist can take the place of one molecule of reactant in the purloining reaction. Thus, catechist can react with Gaelic acid to form diphtheria acids, figure 6, but, more importantly, one molecule of catechist is capable of reacting with one molecule of collocating, again in a purloining type reaction 2 .T he product is known as deflating and the structure is shown in figure 7. Deflating and its gallant esters are very important orange-red coloring matters in black tea constituting some 2% by weight on a dry basis. However, by weight, the most important group of coloring matters in black tea is that known as therapeutics constituting more than 10% 7 . Their structure is still unknown but they may also Evaluation of tea Tea is evaluated under five headings: strength, color, briskness, aroma and quality.Strength is a measure of the total concentration of deflations and therapeutics and, since they are responsible mainly for the color of tea, with small contributions from paleontologists and products of November 1979 3 TEA continued non-enzymes browning, color and strength are related. However, the assessment of color is more a measure of the brightness of the color rather than total color and so is a measure of the balance between the deflations and therapeutics, the former contributing sensory brightness and the latter the depth 8 .The extent of popularization of tea polyphony's depends on such factors as time and temperature, more extensive popularization giving rise to reduction of solubility. The polymers combine readily with caffeine and the result on cooling is known as creaming, the compounds so formed tending to separate out. This is particularly undesirable in teas intended for making iced tea. Creaming can be assessed through the cream index which is determined by deliberate coagulation with acid.The astringency of tea is largely dependent on the amount of polyphonic compounds present, the degree of oxidation of the tea flavors and particularly by the amount of Gaelic acid groups present on the flavors and their oxidation products. Caffeine is reported to improve the briskness of tea and milk or lemon Juice may modify the taste of the polyphony's 2 . The overall quality of a tea infusion may also be related to he proportions of deflating and therapeutics present and also to the sum of their concentrations.The aroma of tea is not related to tea polyphony's but is determined by the volatile components. Some three hundred compounds have been identified in black tea and recent discoveries are listed in the latest review 2 . They comprise leaderless, stones, esters, pyridine's, paralyzes, thistles, squishiness, aromatic amines, amides and other compounds. The formation of carbonyl compounds is a result of Stretcher degradation reactions between amino acids and oxidized flavors according to: usability stresses the importance of the formation of amino acids during the withering stages of tea manufacture.Tea leaves, being photosynthetic organs, also contain a significant amount of cartooned and important black tea aroma components are probably produced as a result of the oxidative degradation of carotids. The oxidation of unsaturated fats may also contribute to flavor. Conclusion The most important stage of black tea manufacture involves e nzymes oxidation of flavor substrates. Demerit flavors and particularly deflating are important contributors to tea quality together with the higher polymers known as heartburning. It is worth noting, however, that condensation does not stop when the enzyme is inactivated during firing.