Why Wither Tea Leaves?
Tea is the most frequently consumed drink worldwide, next to the 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. Plucking the leaf initiates the withering stage, in which the leaf becomes flaccid and loses water until, from a fresh moisture content of 74 to 83(seasonal variations will be there) percentage by weight; it arrives at a withered content of 55 to 70 percentage, depending upon the type of processing. In the traditional process, the fresh leaf is spread by hand in thin layers onto trays or sections of coarse fabric called tats. It is then allowed to wither for 12 to 20 hours, depending upon several factors that include the temperature and humidity of the air and the size and moisture content of the leaf. Withering in the open air has been replaced by various mechanized systems. In trough withering, air is forced through a thick layer of leaf on a mesh in a trough. Continuous withering machines move the leaf on conveyor belts and subject it to hot air in an enclosed chamber, discharging withered leaf while the fresh leaf is simultaneously loaded. the trough withering system is the most commonly used. Mainly two types of troughs, open trough and enclosed trough, are commonly used respectively.
In open troughs, leaves are spread at a particular thickness and air is blown in an upward direction from the bottom to the top layers, and in this way, the bottom layers of the leaves become withered. In order to minimize leaves handling and to reduce leaves damage by avoiding the leaves turning operation, the provision is made to move the fan both in forward and reverse directions. Therefore, the leaves in the upper layer also become withered as air is drawn into the leaf bed. However, during the reverse rotation, the fan efficiency is drastically reduced to about 60% as compared to the efficiency in the forward direction, thus consuming more power.In the case of using an enclosed trough, leaves are spread enclosed by the raised sides of the trough and a cover on top of the bed is used. In this system, a fan is forcing air only in the forward direction, and simultaneously air is passing through both the top and bottom of the leaves using a damper. As the handling of the leaves is less, the chances of leaves damage in enclosed troughs are much lower. In addition, the leaves do not get affected by the sudden variation in conditions of the outside environment. However, open troughs are still more popular in tea processing factories because they are more convenient for loading and unloading, and it is easier to inspect the progress of the withering process.
There are two types of tea withering methods: physical and chemical withering. During physical withering, tea shoots loose moisture content that drops from approximately 70-80% to 55-70% (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 one’s volatile flavour 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/degradation into simpler compounds( if the degradation is not complete, it results in “Pacha taint”). Improper withering can cause adverse effects on subsequent manufacturing operations, such as maceration, rolling, Oxidation, drying, and tea storage. Withering increases the hydrolytic activity due to water loss, which increases hydrolysis and tea extract activity. Ultimately, this activity catalyzes the oxidative reaction, which is mainly responsible for the quality of the product
We can conclude withering as freshly harvested leaves are conditioned physically and chemically for subsequent processing. There is no specified withering duration, but 12-20 hr 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.
If attempts are made to roll fresh leaf in the conventional tea roller without the normal withering preparation, it breaks up into small flakes and the resultant tea is unacceptable to the tea trade, or at least, they are not prepared to pay normal prices for such product. The juice from the broken leaf which contains the full content of moisture is diluted and drained away from the broken leaf very freely, introducing a practical difficulty and losing the potential strength of liquor. In spite of the mess and the loss of valuable juice the cells in the pieces of flake are not properly bruised and the flake does not ferment evenly. It is obvious, therefore, that since the conventional tea roller is designed to twist or wring juice out of tea leaf, the leaf must be conditioned to stand up to such treatment. The main objects of withering are, therefore, to reduce the moisture content of the leaf, to concentrate the juice, and bring the physical condition to a “rubbery” state in which it will stand twisting without breaking up into flakes.
One of the first essentials of withering is, therefore, to get a correct physical condition which will not only allow the leaf to be rolled without breaking up to quickly but which will also prevent the juice from running out of the leaf. The feel of a well-withered leaf is something like that of a crumpled soft handkerchief. It has a drooping appearance and lacks the lustre of fresh leaf. The stalks can also be bent without their breaking, and if the fingers are pressed into the leaf, the impression is clearly visible. The smell of the leaf is also a guide for judging the wither. These empirical methods are the best means of judging the extent of wither, cannot always be entirely depended upon.
The Main Factors that influence withering are:-
(a) The extent of damage to green leaf – damage tends to cause premature critical changes and hence uneven withering. As a result of the cell constituents being mixed with the enzyme, premature Oxidation is started. The leaf which is damaged also dries out during withering and causes loss of both appearance and potential liquoring quality. A soft wither will help to minimize the effect, but it is virtually impossible to prevent it altogether. Blister blight causes the same mechanical damage and badly blistered leaf cannot be hard withered. Damaged leaf is easily recognized after withering by its discoloured appearance. The brownishness it acquires sometimes gives the impression that it is caused by too high a temperature, but that is generally not the case. If the leaf that is damaged is carefully examined it will be found that only those parts which have been bruised are affected. Undamaged leaf retains its green colour.
(b) The condition of the leaf received – coarse plucking results to slow withering leaves than fine plucking. surface water content determines the degree of withering or time. Presence of moisture encourages bacterial growth which adversely affects final tea quality thus, the rapid removal of surface moisture is strongly recommended for wet leaf.
(c) Type of Leaf – Leaves from different fields may have different sizes and composition which may influence the rate of withering(Type of leaf or “Jat”, size of the leaf, and its general composition).
(d) The thickness of spread – Air flow rate depends on the thickness of spread of leaf, higher the thickness withering is slowed down (time to achieve the desired wither will be increased). Thick spreading also increases the unevenness of wither. Extremely thin spreading is wasteful in space and should be restored to only if conditions are unfavourable to the procuring of a wither in a reasonable time(during wet days it is sometimes necessitated). The deciding factor should be the drying capacity of the air used. The whole question of how thickly leaf should be spread depends on this. A fixed spreading rate cannot be adopted in any factory. Apart from the wide variation in the conditions of the leaf that is received, the lofts themselves do not provide completely uniform conditions for withering. Their position in relation to the direction of the wind and bulking chambers has to be carefully noted. Besides, airspeeds in Lofts are seldom uniform.
(e) Period of wither – This is governed to a large extent by the amount of crop, equipment, type of tea required and working hours. There is no such thing as a normal withering period. What is normal to one estate may be abnormal to another. Long withers give more coloury teas than short withers, and this colour is gained at the expense of quality. This is an established fact. The longer leaf is withered, the more quality is lost. Flavour reacts in similar manner. It is thus a simple matter in so far as colour, quality and flavour are concerned, to gain some control over these characteristics by varying the period of wither. It must be remembered, however, that temperature is inter-linked with the period. If quick withers are obtained by employing high temperatures, colour may be developed at the expenses of quality, the Low temperature in association with long periods of wither may have the same effect. The optimum period to be decided on must, therefore, be related to withering conditions and their influence on the finished product. When quality and flavour are at high level, there is no advantage in holding back a wither, but it should not be rushed. Flavoury teas can be produced in a withering period of as long as 20 hours. If it could be shortened without the use of heated air so much the better. When these to characteristics are virtually absent, withering can be prolonged, but should not exceed 48 hours because this may lead to sourness. If neither of these characteristics is marked a normal withering period of about 20 hours will probably give optimum results unless experience has shown that either extreme has been found to give the most satisfactory results.
(f) Drying capacity of Air – This factor embraces temperature, hygrometric difference, the volume of air and movement of air. If large volumes of moving air are supplied for withering, the necessity for very high temperatures does not arise, providing the hygrometric difference is large enough to wither the leaf to the degree necessary in manufacture. High temperature in withering are harmful to quality, This is another established fact, but the statement may be misleading unless the effect of both the dry and wet bulb temperatures is clearly understood.
Withering brings about biochemical changes in tea leaves. To acquire the improved aroma, flavour, and other benefits in black tea, the proper withering of leaves is exceedingly important.
Suggestions and comments are welcome to improve the article further.
Manoj. D. Archibald
Ref: (I). Monographs on tea production by E.L Keegel,1958,
(2). (Changes in enzyme activities (polyphenol oxidase and phenylalanine ammonia lyase) with type of tea leaf and during black tea manufacture and the effect of enzyme supplementation of dhool on black tea quality Author links open overlay panelRamaswamyRavichandran RamaswamyParthiban