Withering is the first processing step in the factory and is a process in which freshly plucked leaf is conditioned physically, as well as, chemically for subsequent processing stages. Indeed, withering is one of the most important tea processing steps and can be said to constitute the foundation for achieving quality in tea manufacture. Based on achieving the desired level of withering, one can make better quality teas and, on neglect, can invite serious problems in subsequent steps of manufacture. As a matter of fact, in planter’s perception, “Withering makes or mars the tea”.
The process objectives to be achieved during withering are as follows:
- To breakdown complex chemical compounds in the cells to simpler compounds which along with other simpler molecules then recombine to contribute to quality attributes of tea like the ‘body’ and ‘flavour’ at a later stage. This is known as the Chemical Withering of the leaf.
- To reduce the moisture content of the fresh leaf which ranges between 74 – 83%
- To make the leaf `flaccid’ or `rubbery’ which is essential for the subsequent step of processing (maceration) or rather for ‘twisting’ or ‘curling’ etc.
- Both these constitute the Physical Withering of the leaf.
Chemical wither starts immediately after plucking. It is independent of the rate of loss of moisture and is a function of time and temperature. Although the desired moisture level may be reached in a few hours, the catabolic changes, which had been initiated at the time of plucking, will take time. The chemical composition of the leaf will thus be unsuitable for manufacture after the leaf has been desiccated for a few hours. It is, therefore, necessary to continue to supply sufficient air and wait for breakdown of large organic molecules to simpler structures. The following chemical changes occur during withering:
- Release of carbon dioxide and water due to break down of larger molecules.
- Changes in enzyme activity.
- Partial break down of proteins to amino acids which act as precursors for aroma.
- Increase in caffeine content – this contributes towards briskness.
- Production of Volatile Flavour Components (VFC): Some of these compounds contribute to the grassy odour and others are responsible for the flowery aroma.
- Reduction in chlorophyll content.
The above chemical changes are all intrinsic of the biochemical structure of the leaf, but the range and the extent of the reactions depend on the jat, cultural practices and physical parameters like temperature, humidity etc. This process normally takes about 12 – 16 hrs. and cannot be hastened.
Physical withering reduces the moisture content in the leaf and correct withering is essential for quality, although, it has always been a difficult task to determine the end-point of wither. The same reduction in moisture percentage and increase of flaccidity of leaf to the desired level can be achieved in a shorter period; a longer period is necessary for chemical wither. Therefore, physical wither is regulated at a slower rate, so as to reach the desired physical withering in the same interval as required for the chemical wither. The objectives are achieved by passing air through the leaves.
Percentage of Wither (% wither)
Percentage of wither is defined as the weight to which 100 kg of leaf is reduced at the end of the withering process. In conventional parlance, 70% withering signifies that 100 kg of fresh green leaf has been reduced to 70 kg after withering and 30 kg of water has been removed. This method of expressing wither is irrespective of the initial moisture content of the leaf. Thus, depending on initial moisture content, the same 70% withered leaf, would lead to various moisture contents. This is an anomalous situation and was introduced when it was not possible to measure the moisture content of green and withered leaf accurately. The following table illustrates how the variability in the moisture content of the withered leaf, depending on the initial moisture content of the leaf, vary even though the leaf has been subjected to the same 70% wither.
Variation in moisture content of withered leaf
It is, therefore, not desirable to wither the leaf throughout the season to one and the same weight as this may require elimination of different quantities of water in different periods. Besides, during the withering process though there is carbohydrate degradation and consequent respiration leading to loss of 4% soluble solids, there is some increase in caffeine and other constituents. Thus, for practical purposes the loss is compensated. It would be better to ascertain the degree of wither by use of the ratio of dry matter to moisture.
The ratio of made tea to green leaf is termed as “recovery percentage” alternatively, as “out turn”. This ratio varies depending on the initial moisture content of the leaf. Leaf with moisture content of about 83% during wet period produces 16.5 kg black tea from 100 kg of green leaf. Similarly, leaf with moisture content of 72% during dry period produces as much as 27.5 kg of tea from 100 kg of green leaf. Taking an average of 77-78% moisture for the whole season, 22.5 kg of made tea is expected from every 100 kg of green leaf.
Hygrometry and Relative Humidity
Usually, Wet and Dry Bulb Thermometers are used to measure RH. The difference between the wet and dry bulb readings is known as Hygrometric Difference or HD. A difference of 40C(70F) has been found to be optimum under normal conditions, but with wet leaf a difference of 6-8OC (10-14.5OF) is used initially to drive off the surface moisture after completion of which the hygrometric difference is brought down to 4O C.
Various ingenious systems viz., Tat withering, Tunnel withering, Drum withering and Trough withering have been in use. Of these the Trough withering is the most popular system currently being used all over the world. The advantages of a Trough withering system are: a) economy, b) greater flexibility with respect to capacity and degree of wither, c) flexibility of construction, d) saving in space and e) economy of labour and easier operation.
Withering Trough is a rectangular duct with one opening for letting the air in and the leaf bed acts as an outlet or exhaust. Air is blown into the trough by an axial flow fan fitted at the inlet with the help of a tapered expansion hood, termed as the diffuser, which smoothens the airflow and reduces the turbulence. The escaping air thus carries moisture in the leaf away.
Due to long duration of withering, the trough cannot normally be used more than once a day. Therefore, sufficient capacity must be provided to hold the entire quantity of a day’s leaf input in the factory. Capacity to hold green leaf equivalent to 1% of the annual crop is considered as a reasonable provision for the purpose, since this capacity exceeds only on 2 or 3 occasions in a year. Withering troughs can be of various sizes. However, for convenience of loading and unloading, a width of about 1.8 m is preferred in which case the length is 23 to 24 meters.
The troughs are of two types – Open trough and Enclosed trough.
In the open type of trough, leaf is spread at a given thickness and air is blown upwards from the bottom of the perforated bed. This results in the wither of the bottom layer of the leaf first. Therefore, fans need to be periodically reversed so that the leaf in the upper layer can also get withered as air is drawn into the leaf bed. Therefore, fan motor in case of open troughs is provided with facility for reversing the direction of rotation by means of a reversing switch. However, during the reverse rotation or the suction mode of fan operation, the fan efficiency is drastically reduced to about 60%, thus consuming more power with respect to the work done as compared to that of the forward or blowing mode of operation. Moreover, in the reverse mode it is not possible to draw hot air through the leaf bed. The dual direction of movement of air is, however, expected to furnish more uniform wither, but very often sandwich effect results, whereby, leaf at the middle layer does not get the desired wither. It is, therefore, necessary at some time to turn the leaf to achieve more uniform wither. One has to however, ensure cleanliness of hand or use clean gloves for turning the leaf.
Schematic diagram of Open Trough
Modification of Open troughs
To achieve more uniform wither, TRA has introduced modification in existing open troughs by using streamlining vanes and honeycomb chamber in the downstream of the fan. The air distribution in the leaf bed can also be improved by modifying the leaf bed structures.
In the enclosed troughs the leaf bed is kept in an enclosed environment by raising the sides of the withering troughs and using a cover on top of the bed. This is designed to create a plenum chamber on top of the leaf bed as well. In this case the fan is always made to blow air only in the forward direction and air can be made to pass either from top to the bottom or from bottom to the top with damper and shutter control at the air entry and exit points respectively without reversing the direction of the fan. Since handling of leaf is less, the chance of leaf damage in enclosed trough is much lower. The leaf also enjoys some isolation from the sudden variations in ambient atmospheric conditions. It is, however, more convenient to load and unload leaf in the open troughs and it is easier to check the progress of wither. Therefore, open troughs are still popular in the industry.
In some models of the enclosed trough, air is continuously drawn in instead of being blown with same effects on the leaf. In this configuration, heating of air is to be done at the front end of trough and not at the fan end.
Schematic diagram of Enclosed Trough
Green Leaf Storage System (GLSS)
The GLSS developed by TRA keeps the plucked shoots in near fresh condition up to 48 hours. The system consists of a rectangular chamber with the two ends flaring downwards within which a number of framed wire mesh partitions spaced at regular intervals hang from the top of the chamber. The partitions can be kept in inclined position by using a locking device fitted to the walls. The gap between the two adjacent partitions provides the space for loading the bulk green leaf from the top of the chamber. Air is made to percolate through the leaf from the bottom to keep the leaf cool. The leaf can be unloaded from the device by releasing the partitions one after another resulting in the dropping of the leaf on to the floor from where it is transferred to the withering trough.
Fan is one of the most important components of withering trough. For the purpose of withering axial flow fans are used. The direction of airflow in axial fans is essentially parallel to the axis of the impeller. These fans operate at a static pressure of 12 mm (½ in) water gauge and can handle large volumes of air. One advantage of axial flow fans is that their direction of rotation can be reversed to make the air flow in the reverse direction. But with reversal, capacity of the fans reduces nearly to half. Even if the blades are specially designed to give same air flow in both directions the power required will be 40-60% higher than the power required for normal blades.
Leaf Loading and Thickness of spread
The spread of leaves actually act as a valve and create the necessary system resistance for the airflow helping to even out the static pressure in the plenum chamber.
Leaf is generally spread uniformly on the trough at the rate of 23 kg m-2 for CTC or 13-16 kg m-2 for orthodox tea manufacture in the plains and 8-10 kg m-2 for orthodox tea manufactured in the hills.
Period of Wither
The period of wither is ascertained by taking both physical and chemical wither; physical wither can be achieved in 3-4 hours but chemical wither requires 12-16 hours. In case the leaf is under withered, the following problems are envisaged:
- Rolling of unwithered leaf leads to breaking up into small flakes, which would not respond to the subsequent processing steps and produce unacceptable teas.
- If the leaf were under withered valuable water-soluble solids would be lost during the leaf conditioning process.
- Under-withered leaf when rolled turns into a wet watersogged mass; the sogginess restricts supply of oxygen and hinders uniformity in the subsequent oxidation reaction (fermentation).
- Maceration of under withered leaf also leads to formation of lumps during fermentation.
- At temperature more than 25°C with under withered leaf, chances of bacterial contamination increase.
- Proper physical wither reduces load on the dryers.
Heating of air for withering
The following methods are employed for heating of air for withering:
- Individual gas burners for each trough, where gas is available
- Hot air ducting to each trough from a separate heater.
- Direct use of hot air from the drier when it is empty.
- Using exhaust air from drier.
The last two methods are used extensively in Darjeeling, for which bulking chambers are used. But the fourth method may give humid hot air. The first method is used only where natural gas is available. The second method is used almost everywhere. As a thumb rule, hot air is mixed at the rate of 1/10th of the volume of ambient air.
For heating of natural air either a DF oil heater or a onventional coal heater can be used. But since the temperature of air obtained from the above sources is much higher, it is to be bulked with a certain quantity of cold air to attain the requisite temperature. The following examples show how to calculate the temperature of the mixed air, quantity of hot air to be used.
Withering – considerations in relation to manufacture
For orthodox manufacture in the plains of N.E. India,wither should be aimed to achieve 60-65% moisture in the withered leaf. For Darjeeling manufacture the wither is rather hard, moisture content of the withered leaf being around 30%.
For CTC manufacture, moisture content of 70% in the withered leaf should be adequate. Use as many troughs as possible. The thickness of spread should not come below 10 cm (4″) in the plains. However, in the hills a minimum thickness of 7.5 cm (3″) is used. Subject to this minimum thickness, thinner the spread, the better.
While loading the trough, a small volume of ambient air (by use of damper) should be passed through the leaf in order to cool it. This will prevent further damage of leaf particularly during the summer, when the temperature of the leaf brought for wither may touch 45°C (113°F).
- Spreading of leaf should be uniform across the length and breadth of the trough. Bunching of wet leaf should be avoided.
- Air velocity should be such that the leaves are not lifted up.
- Leaf should be handled carefully. Drainage during spreading must be avoided and the labourers must not be allowed to walk on the leaf. Withered leaf bruisesmore easily.
- The duration and temperature of withering influence the character of made tea.While low temperature favours development of quality, high temperature may develop colour at the expense of quality. Unwithered teas are flaky – they may be brisk but with poor quality.
- A period of 12-14 hours of wither is essential for completion of chemical wither.
- Proper monitoring of withering through use of hygrometers, dry and wet bulb thermometers, moisture meter, weighment before and after wither, will help in producing better quality tea.
- Effort should be made to wither evenly over the entire period of withering, so that the rate of loss of moisture proceeds at a uniform rate. This will to allow the chemical reaction to proceed in the desired manner.
- At the end of withering ambient air should be blown to cool down the leaf.
- Leaf from the troughs should be taken as and when required and one trough at a time should be emptied.
- Withered leaf must be loosely packed in the basket.
Electronic monitoring and control system (EMCS)
An Electronic monitoring and control of withering process has been developed by a TRA-CEERI joint R&D programme and is in operation in some factories. This facilitates total monitoring and exercising of necessary controls in airflow, heat application, period of withering, rate of evaporation etc. by a Computer once the process parameters are fed into the system .The system takes the fan to off mode as and when required and thus saves considerable energy. Consistency in withering and quality improvement is also achieved.