Withering — In an evaporation process water escapes in the form of vapour, and this is brought about by heat, the rate at which it is supplied controlling the rate of evaporation. Heat used for such a purpose comes from the air and the wet body, and the changes that take place continuously during the process of water evaporation are cooling of the air, increase in the humidity of the air, and cooling of the wet body.
Rate of evaporation decreases as the air becomes more saturated, and no water will evaporate when the air becomes fully saturated. If evaporation is to continue from a wet body it must have a continuous supply of heat. Green leaf behaves like a wet body during the withering process, but does not lose its water as a free surface does since most of the moisture is evaporated through the stoma or “pores, ” and a certain amount through the epidermis (the outer layer of the leaf). The water from the interior has to travel to the surface before it can evaporate. The rat e at which leaf loses moisture is therefore affected not only by external conditions but by its structure as well. In the case of stalk, a part of its moisture passes to the leaves from where it is evaporated. For a good standard of plucking this is very small. The leaves and buds lose moisture quicker than the stalk, but their rate of withering is not affected by the presence or absence of stalk.
The main factors that influence withering
- Damage to green leaf
- Condition of the leaf, whether wet or dry
- Type of leaf
- Standard of plucking
- Thickness of spread
- Condition of tats
- Period of wither
- Drying capacity of the air,
Damage to Leaf—The effect of bruising has already been referred ~ \ to in connection with the chemical changes that follow. As a result of the cell constituent being mixed with enzymes, premature fermentation 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. The 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 this is generally not the case. If 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.
Condition of the Leaf—Owing to variations in climatic conditions leaf may arrive at the factory in a dry or wet condition, as a result of which, the moisture which has to be evaporated from the leaf varies considerably from day to day. Hence withering is prolonged when leaf is wet, and unduly prolonged if steps are not taken to remove surface moisture as rapidly as possible. Surface moisture on the leaf exists as free water and evaporates more speedily than the moisture contained in the leaf.
Withering really commences after surface moisture is removed. If atmospheric conditions are too humid, recourse to hot air from the driers is necessary. Withering fans should be set going and hotter air than is advisable for surface dry leaf may be used. When the leaf has arrived at the condition when withering may be said to have started, the temperature should be lowered. The rapid removal of surface moisture is strongly recommended for another reason as well. Wet leaf contains more bacteria on its surface than dry leaf, and bacteria are known to be one of the causes of dull infusions and soft liquors.
Type of Leaf—Rate of withering is appreciably influenced by the type of leaf or ‘jat, ” size of leaf, and its general composition. The resistance offered to evaporation of water varies with the amount of water in the leaf. For example, flush from tipping fields, or from shoots allowed to run up have thick stalk but immature leaf, which loses its moisture more readily on withering than the stalk. If an attempt is made to wither such leaf to low moisture content, the leaf is liable to dry out leaving the stalk soft. To evaporate the maximum amount of moisture from the stalk, and at the same time prevent the leaf from getting brittle, a slow and long wither is necessary.
It may be wondered why so much importance is attached to the withering of the stalk. One reason is that extra moisture in stalk makes proper rolling difficult. Another reason is that the rolling of underwithered stalk tends to strip off the outer skin, thus making more prominent the familiar ‘reds’ and ‘stalk’ in the made tea.
Standard of Plucking—The influence of the standard of plucking is partly associated with the texture of the leaf and partly with its moisture content. Stalks contain more moisture than the leaf, and leaves and stalk lose moisture at different rates during withering. Tender leaves wither faster than coarser leaves. A mixed pluck, or a coarse pluck, will thus result in a very uneven wither.
The presence of tough banji and hard, single leaves makes the control of withering more difficult. These leaves hardly wither at all and at the end of the withering period may be so green as to upset one’s judgement when estimating the degree of wither. Therefore, the more uneven or coarser the pluck, the larger the variation in the degree of wither. It is unnecessary to stress how important it is to pay careful attention to plucking when it is realized that even under homogeneous conditions of withering, variations in the moisture content have been found between different pieces of flush from one bush. A good, uniform standard of plucking will minimize the variation in the withered material.
Thickness of spread—Leaf can be withered satisfactorily with a rate of spreading up to as much as 1 pound per 5 square feet, but withering is slowed down. Thick spreading also increases the unevenness of the wither. It is perhaps not realized that by spreading leaf too thinly uneven withers can result as well. Extremely thin spreading is wasteful in space and should be resorted to only if conditons are unfavourable to the procuring of a wither in a reasonable time. It is sometimes necessitated by wet leaf, but even then, a rate of spreading of 1 pound to 20 square feet should be ample. If sufficient air supply is available, thicker spreading is permissible.
The deciding factor should be the drying capacity of the air used. The whole question of how thickly leaf should be spread depend on this. Under normal factory conditions the area on which a pound of green leaf can be spread to give good withers, may vary from 10 square feet to 15 square feet. The prevailing idea that the best results are obtained with a spread of 1 pound to 15 square feet has nothing to support it. The important consideration is the air supply. For estimating withering space requirements, however, this figure may be used to allow for unexpected increases in crop or fickleness in weather.
A fixed spreading rate cannot be adopted in any factory. Apart from the wide variation in the condition 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 wind and bulking chambers has to be carefully noted. Besides, air speeds in lofts are seldom uniform.
Condition of Tats—Sagging tats are the cause of a great deal of trouble in withering. Tats must always be maintained in a taut condition, and should under no circumstances be left slack. When pockets are formed rate of wi thering is appreciably retarded. It is not a laborious business retensioning hessian and the care and attention given to withering tats will undoubtedly be repaid many times over.
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. This is because the period of wither has a direct bearing on the liquoring properties of a tea.
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 a similar manner. It is thus a simple matter in so far as colour, quality and flavour are concerned, to gain same control over these characteristics by varying the period of wither.
It must be remembered, however, that temperature is inter-linked with period. If quick withers are obtained by employing high temperatures, colour may be developed at the expense of quality. Low temperatures 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 a 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 two 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. It may be shorter or longer according to convenience in factory organisation and still not greatly affect the character of the tea.
Drying Capacity of Air—This factor embraces temperature, hygrometric difference, volume of air and movement of air.
It is generally believed that the evaporative capacity of air can only be increased by raising its temperature. This is not the case. The volume and speed, if increased, can be far more effective than a rise of a few degrees in the dry bulb temperature. This is why natural conditions at comparatively lower temperatures than those employed in artificial withering sometimes produce quicker withers. The process is accelerated when the hygrometric difference is larger and air movement is rapid. In estimating the drying capacity of air it is not enough to take note of the dry bulb temperature only. No matter how high the dry bulb may be, if the difference between the dry and wet bulb temperature is small, the relative humidity of the air will be so high as to cause comparatively little evaporation. If the air is stagnant the wither will be extremely slow.
It is thus evident that, 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 temperatures 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 Temperatures—Green leaf at the beginning of the withering process behaves exactly like a wet body and has a temperature approximately equal to the wet bulb temperature of the air. As withering proceeds the leaf becomes physically less wet and its temperature increases above the wet bulb. It will be seen therefore that the temperature of the leaf depends partly on its moisture content and partly on the wet bulb temperature of the air
From this phenomenon it may be concluded that leaf, towards the end of a withering process, is generally at a higher temperature than during the initial stages of its process, all other conditions being equal. It follows therefore that, if the temperature of the leaf is to be kept as low as possible, both wet and dry bulb temperatures should get lower and lower while moisture is being removed from the leaf. This explains why the use of heated air at the beginning of the withering process is preferable to employing it in the later stages.
A fact that must be recognized from what has been just said is that although leaf is a wet body, or pariiy wet body, it tends to follow the wet bulb temperature of the air only so long as water is being evaporated from its surface. If no evaporation takes place, as may very well happen in a badly ventilated loft, the leaf assumes the dry bulb temperature of the air. The rate of evaporation from the leaf is therefore as important as the wet bulb temperature. The greater the evaporation, the cooler the leaf. The ideal conditions for withering are accordingly low wet bulb temperatures and large hygrometric differences with an ample supply of moving air.
At certain times of the year during the height of the flavoury season atmospheric conditions approach this ideal.’ In the day-time the hygrometric difference may be greater than 15°F, and the wet bulb temperature as low as 50°F. At night, though the hygrometric difference is smaller the temperatures are lower. During periods like this, if natural withers only are taken, the temperature of the leaf rarely rises above 50°F. To reproduce these conditions at other times of the year by artificial means is extremely costly. Heated air has perforce to be used when atmospheric conditions are unfavourable. While it is easy to heat air, it is impossible to prevent the wet bulb temperature from rising. The wet bulb temperature rises approximately 1° for every 3 to 4° increase in the dry bulb temperature. This means that big hygrometric differences can be achieved only if the dry bulb temperature is raised unduly high. It is incorrect to assume that an increase of say 20° in the dry bulb will result in a 20° bigger hygrometric difference. The later will be much less.
The following example illustrates the effect heating air.-— Assume air on a very wet night to be practically 100% humid at a temperature of say 65°F (dry); 65°F (wet). If heated to 75°F, the wet bulb temperature will be about 68°F—a7° hygrometric difference. If heated to 85°F, i.e. a 20° rise in the dry bulb, the wet bulb will go up as high as 72°F. If the air can be conditioned cheaply so as to raise the dry bulb temperature without raising the wet bulb, there need not exist any misgiving about the influence of high temperatures in withering on quality. It is because the wet bulb also rises in the ordinary process of heating air that care must be taken not to employ too high a temperature. The temperature above which quality is impaired has not yet been determined, but in view of the considerable risk in withering leaf at high temperatures, particularly if they are maintained too long, the best advice that can be given is to create conditions at night as close as possible to what natural conditions should be in the day-time for procuring a reasonable wither. To keep temperatures as low as possible and still not seriously reducing the drying capacity of the air a large volume is necessary. With adequate fan capacity it is generally feasible to keep the temperatures of heated air down to about 75°F with a satisfactory hygrometric difference as well. Dependent on outside conditions, temperatures even lower than this can be employed without prolonging the wither unduly but only if large volumes of air available.