Black tea manufacturing technology essentially involves disruption of the cellular integrity of tea shoots, thereby enabling the mixing up of substrates ( polyphenols) and the enzymes (polyphenol oxidases). This results in the initiation of a series of biochemical and chemical reactions with the uptake of atmospheric oxygen and formation of oxidized polyphenolic compounds that are characteristic of tea along with volatile flavor compounds that impart characteristic aroma to tea.
Tea manufacturing is normally carried out in two ways, (i) CTC and (ii) orthodox. CTC refers to the Crush, Tear & Curl process where the withered green leaves are passed in-between two rollers rotating in opposite directions. There is complete maceration of the leaves and the resulting powdery material is referred to as "cut dhool". Enzymatic action is maximum in the CTC type of manufacture. In orthodox type of manufacturer, the withered leaves are rolled on specially designed orthodox rollers which twists and crushes the leaves thereby rupturing the cells. The maceration is less as against CTC processing. But this process results in teas with good flavor and aroma.

STEPS IN CTC TEA MANUFACTURE
	Withering
	Green leaf sifting
	Reconditioning
	Rolling
	Fermentation
	Drying
	Grading and sorting
	Packing

WITHERING is the first and fore most step involved in tea manufacture. The evaporation of moisture in the green leaf is brought about by blowing or moving air over the leaf in the withering trough. The current of air performs a two functions viz., Conveying heat from the leaf as well as carrying away the water vapor through a bed of green leaves to achieve physical withering. Whenever the hygrometric difference is below 3° C, hot air is mixed in suitable proportion or heat energy is supplied to increase the hygrometric difference with the concomitant rise in the dry bulb temperature of air. But the dry bulb temperature of air after mixing should not exceed 35° C.

Currently, in most of the South India tea factories trough withering is practiced. The dimensions of the trough in most of the factories vary considerably. The width of the standard (conventional) trough is 6' and its length varies between 60' and 120'. However, nowadays wider troughs with two axial fans are preferred. Sizes vary between 12' and 15' of width and 60' and 120' of length. Reasonably even - wither is achieved in wider troughs. Conservation of electrical energy is also possible by switching “off” one of the fans after the required degree of physical wither is achieved. The other fan supplies air just to control any heat developed.

So, in a well designed, balanced factory, an optimum load of 30kg. per meter square for a peak crop anticipated in a single day has been the basis for the design of trough capacity. The good essence of withering is well ventilated withering lofts and access of drawing large volumes of air by the trough fans. The ideal qualities of air required for withering are low dry bulb temperatures and high hygrometric differences with ample supply. The upward passage of air through the bed of leaves usually results in the bottom of the bed being withered first and the upper leaves last. To achieve a more even wither turning over the leaf carefully once or twice is suggested. However, turning over maybe practically difficult in wider open troughs. To achieve a more even wither turning over, reversible air flow systems have been practiced.

Many of these difficulties have been overcome by using enclosed troughs. In this system, the direction of air flow can be changed by a movable baffle plate either below or above the trough. The air space above the trough is totally enclosed. There are two sets of exit doors for the spent air; one set above the level of leaf and the other below. The system makes it possible to maintain fan efficiency and dispenses with the need for reversible fans. Air heating system is also available to warm the air irrespective of the direction of air flow through the system. The other advantage is that a more uniform wither is achieved without the necessity of turning over and thereby the withered leaf is intact.

It is important that pressure inside the plenum chamber should be constant throughout the length to have an uniform air flow rate. However, in the conventional troughs the pressure varies over the length for constant thick spreading. A tapering cross section decreases the area towards the end and equalizes the pressure inside the plenum chamber.

The method commonly employed to heat the air for withering are as follows :
	Direct use of hot air from the drier when it is empty
	Hot air ducting to each trough with damper control from a separate heater
	Hot water or steam based insitu radiators in each withering trough
	Using exhaust air from the drier