Withering is the first process in black tea manufacture. The primary aims of withering are to reduce the moisture content of the leaf and to soften it, so it becomes pliable and can withstand the subsequent process of ‘Rolling’, without breaking into flakes. Or in other words, it is carried out in order to condition the leaf physically for subsequent processing. The fresh plucked leaves after sorting are placed on the withering troughs and air is allowed to pass over the leaves.
In olden days, leaves were withered in “Chung House” and that what we called “Natural Withering”. In this type of withering, leaves were spread over the hessian cloth in the “Chung house” overnight to wither the leaves. However, now leaves are withered artificially in troughs.
During withering, some chemical changes also take place. When you take a walk around a tea factory, you will get a pleasant smell which is due to aroma and flavor compounds that are formed. Thus, the process of withering involves both Physical Wither and Chemical Wither.
(a) Physical Wither
During physical wither the leaf loses its moisture which results in makeing the leaves flaccid or rubbery, a most desirable condition to help in its rolling and for obtaining the desired style and appearance. The extent to which the wither (percent of wither) is to be carried out will depend upon the method of manufacture. The percent of wither denotes the weight of 100 kg fresh leaves at the end of the withering process. For example, if 100 kg leaves become 65 kg at the end of withering period, we say the leaves are 65 % wither. Under the North East Indian conditions usually 70-72 % wither is adopted for CTC and 65-68% for orthodox manufacture.
(b) Chemical Wither
Chemical wither starts immediately after leaf is plucked from the bush. Following chemical changes occur within the leaf cells during withering:
i) Larger molecules are broken down to smaller ones that results in the increase of contents of amino acids and flavor compound
ii) Increase in caffeine which is responsible for the stimulant effect of tea.
iii) Increase in permeability of cell membranes which has a great effect on the mixing of polyphenols, enzymes and oxygen for even fermentation.
To achieve the chemical wither, therefore, the withering period should be longer. A short wither allows the leaves to retain a greenish appearance and grassy flavors while a longer wither darkens the leaf and intensifies the aromatic compounds.
The longer the wither, the more aroma and flavor compounds develop in the leaves. This is because during the withering process, many of the chemical compounds in the leaves degrade into volatile compounds. In fact, many tea makers use their sense of smell to tell when the withering process is complete. If the leaves are withered too long, polyphenol and peroxidase activity ceases due to dehydration. Once withering is deemed complete, processing continues