Purpose: Tea is the most frequently consumed drink worldwide, next to water. About 75% of the total world tea production includes black tea, and withering is one of the major processing steps critical for the quality of black tea. There are two types of tea withering methods: physical and chemical withering. Withering can be achieved by using tat, tunnel, drum, and trough withering systems. Of these, the trough withering system is the most commonly used. This study focuses on the different types of withering, their effect on the various quality attributes of tea, and other aspects of withering methods that affect superior quality tea. Results: During physical withering, tea shoots loose moisture content that drops from approximately 70-80% to 60-70% (wet basis). This leads to increased sap concentration in tea leaf cells, and turgid leaves become flaccid. It also prevents tea shoots from damage during maceration or rolling. During chemical withering, complex chemical compounds break down into simpler ones volatile flavor compounds, amino acids, and simple sugars are formed. Withering increases enzymatic activities as well as the concentration of caffeine. Research indicates that about 15% of chlorophyll degradation occurs during withering. It is also reported that during withering lipids break down into simpler compounds and catechin levels decrease. Improper withering can cause adverse effects on subsequent manufacturing operations, such as maceration, rolling, fermentation, drying, and tea storage. Conclusion: Freshly harvested leaves are conditioned physically and chemically for subsequent processing. There is no specified withering duration, but 14-18 h is generally considered the optimum period. Proper and even withering of tea shoots greatly depends on the standards of plucking, handling, transportation, environmental conditions, time, and temperature. Thus, to ensure consumption of high quality tea, the withering step must be monitored carefully.

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Amino-acids and amides of fresh and withered tea shoots have been estimated by a spectrophotometric method following separation on chromatograms. During withering, asparagine, glutamine, aspartic acid, glutamic acid, leucine (isoleucine), phenylalanine, valine and serine accumulate as a result of protein breakdown. The amino-acid content of leaf plucked in the evening after an active period of photosynthesis is generally higher than that of leaf plucked in the morning. When leaf is stored under reduced pressure, glutamine content decreases ; asparagine does not accumulate and relatively smaller amounts of other amino-acids are formed. Amino-acids are partially used up during the manufacture of black tea.

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Data are presented showing the relationship of the nitrogen content of tea leaves to the level of nitrogenous manuring, to the position in plucked and unplucked shoots, and to various genetic and environmental factors. In carrying out comparative studies of the chemical constituents of tea leaves it is suggested that sampling should be confined to a particular leaf in a specified position on the stem. Samples drawn from plucked shoots should consist of the second leaves of comparable shoots taken from bushes growing in the inner rows of a plot.

COMPARATIVELY little work has been carried out on the respiration of leaf tissue using manometric methods.-Kempner [1936] measured rates of respiration of whole leaves from pine, tobacco, plum and oleander, in this manner, and Allen & Goddard [1938] recorded respiration rates of strips of leaf from wheat floated in water, using the Fenn micro-respirometer. Later, Marsh & Goddard [1939, 1], using this same apparatus, investigated the respiration of carrot leaf with 100 mg. samples floated on phosphate buffer. The success of these workers encouraged us to make similar observations on the rate of respiration of tea leaf.These investigations into the respiration and anaerobic fermentation1 of tea leaf, obtained both by the manometric and by the standard Pettenkofer methods, together with manometric measurements of the rate of tea fermentatian and of simpler enzyme-substrate systems have enabled us to draw fairly definite conclusions as to the presence of the cytochrome system in tea leaf, and the almost complete inactivation of the H-carrying coenzymes (cozymase) on extensive mechanical damage to the leaf. Methods(1) Manometric method of determination of respiration rate. Disks, I in. in diameter, are punched in the centres of the second leaf of shoots consisting of two leaves and a bud. 10-15 such disks, weighing approximately 250-400 mg., constitute a fairly representative sample; in fact in only one experiment out of 20 was there any serious variation between replicates.The disks were suspended in 3 ml. water or M/15 p1iosphate buffer pH 5-8, contained in Warburg vessels. KOH, where necessary, was contained in the inner cups, rolls of filter paper being employed to ensure an adequate surface of the C02 absorbent. C02 outputs were obtained by difference between the mean uptakes of four vessels with KOH, and of another four without KOH, using the appropriate constants of the vessels for 02 and C02. For plant tissues in a medium of water this method appears to give results quite as accurate as those obtaine,d by the method of Dickens & 8tmer [Turner, 1938].In these determinations the Warburg apparatus was covered by a black cloth, thus preventing any disturbance of the 02-CO2 interchanges by photosynthetic processes.1 A careful distinction must be drawn between the two us,es of the word fermentati6n. Tea fermentation is not an anaerobic process, and to avoid confusion is always referred to as tea fermentation.1507