Withering brings about physical and chemical changes in the shoots and leaves to produce quality tea. It also provides flush to tea leaves for rolling in subsequent step by reducing turgor pressure. Withering is generally achieved by thinly spreading the flush on mats or in thicker layer in trough for 8 – 20 hours, depending on condition of leaf. During this period, the moisture content of leaf drop between 60 – 65 % (in soft withering) and 50 – 55 % (in hard withering). The physical part of withering can be achieved quickly by passing blast of hot air (35°C) through the leaves but this may adversely affect quality of tea because of inhibition of full biochemical changes. Moisture evaporation in leaves takes place through stomata and highly through the epidermis. The rate of loss of moisture not only depends on external condition but also on the leaf standard. Leaves and buds loose moisture more easily than the stalk and shoots. Objectives of withering:
1. Breakdown of complex chemical compound in cell t simpler compounds which along with other simpler molecules then combine to contribute quality attribute of tea like the body and flavor at later stage of processing.
2. To reduce moisture content of tea leaves which ranges 50 – 60 %.
3. To make the leaf rubbery or soft which is essential for the subsequent steps of processing i.e. twisting and curling.
Physical changes during withering:
1. Fresh leaf contain about 80 % water and these are turgid. Such leaf if subjected to roller, it will be crushed to paste rather than being rolled. The withered leaves are flaccid which can be rolled without much crushing.
2. Leaf juice is more concentrated due to removal of water. The concentrated juice is expressed out and evenly spread on rolled leaves during rolling. This helps in making attractive black tea with a good bloom.
3. Cell wall are weakened and are easy to rupture during rolling process. The cell in unwithered leaves are less damaged during rolling which causes less expression of polyphenols to react with enzymes and hence give greenish and brassy tea.
4. Withering also reduces contamination and gives better output.
Chemical changes during withering:
1. Peptidase present in tea shoots case an increase in free amino acid level particularly, aspartic acid, glutamic acid, serine, glutamine, alanine, tyrosin, phenylalanine, valine, theronine and lysine. Total loss of protein is evinced up to 1.2 %. Breakdown of protein is important because of its influence in final tea quality.
2. Withering causes increase in caffeine content but the amount of increase is dependent on the temperature and nature of cultivar. The biosynthesis of caffeine involves transfer of methyl group from s-adenosyl methionine to methyl xanthine of caffeine.
3. Withering causes loss of carbohydrate or starch up to 4 %. Carbohydrate is reduced to glucose-6-phosphat’, fructose-6-phosphate and glucose-1-phosphate. A part of sugar is also metabolised into amino acid.
4. During withering, a part of sugar is also metabolised into organic acid such as formic acid, citric acid, oxalic acid and succunic acid.
5. Polyphenols oxidase activity is increased during withering, but the activity is dependent on the temperature and moisture of shoots and leaves. Increased polyphenolase activity helps in fermentation.
6. Withering causes degradation of chlorophyll to about 15 %. This breakdown of chlorophyll affects the final tea appearance. A part of degraded chlorophyll produce chlorophyllate by the action of chlorophyllase present in shoots and leaves.
7. All phosphate ester decrease during withering but there is evidence that monophosphate incease at the expense of cystidine triphosphate.
Types of withering:
Natural withering: In this process, leaves are exposed to maximum dry air, either on floor or any trough like place. If temperature is 25 – 30°C and relative humidity is 50 %, and thickness of spread is 15 cm, surface moisture is freed. For complete withering, it takes about 23 to 25 hours but it requires large area and high labor cost.
Tat withering: It is improved form of natural withering where a fan is adjusted but no heater.
Drum withering: Here, leaves are subjected to perforated rotating drums. It is not desirable means as the enzyme polyphenolase of the leaves get break down and tea leaves get damaged as it comes in contact with wall of drums in excess. The drum is subjected to continuous air circulation.
Trough withering: It is most popular system, currently being used in almost all of tea industry. Among the artificial withering system, trough is mostly used in tea industry because of its simplicity and low cost, greater flexibility, greater capacity and easy operation. A desired degree of wither of whole bulk of leaf can be obtained with this system. For completing this process, it takes about 8 – 18 hours but good uniformity is difficult to achieve. The trough can be made of various size. However, for convenience for loading and unloading a width of trough up to 6 feet is preferred and length should be limited to 75 feet. A conventional withering through allow air to pass upward through leaf bed.
Tunnel withering: It is much better system than other trough and can give a very uniform wither of desired degree of leaves. The withering time is 3 -5 hours and length of tunnel is 5.5 meter. The leaves are loaded in trollies with wheel or rail trucks into the enclosed chamber and are exposed to a hot air blast. The trolley carries bank of tats in which leaves are spread at the thickness of 2.2 kg/m2. Air is heated to hygrometric difference of 8.5 and blown into a tunnel. During hot air passage, the hygrometric differences comes down to 1.6. Advantage of tunnel withering is it requires less time and fuel.
Electronic monitoring and control system (ECMS): This system facilitates total monitoring and exercising of necessary control in air flow, heat operation, period of withering and rate of evaporation by a computer process. In this system, the improvement in quality and consistency in product can be achieved.
Degree of withering: Degree of withering depends on type of tea to be produced and method of manufacture. For processing of cut, tear and curled (CTC) tea soft withering is preferred whereas for orthodox tea hard withering is preferred. In soft withering, moisture content of tea leaves is brought to 60 -65 % whereas in hard withering moisture content is brought to 50 – 55 %. Degree of withering can be measured by following methods
Feel: An experienced person can judge whether the leaves have undergone complete withering or not. Leaves are taken on hand and are pressed. If the leaves form a ball which doesn’t spring apart, withering is supposed to be completed. Stalks and stem of tea leaves do not break easily if leaves are properly withered.
Weight: In this method, weight reduction of withered leaves is observed. If 100 kg of fresh leaf is withered to give 70 kg leaves, it is called 70 % withered. However, weight reduction largely depends on seasons due to moisture content in green leaf.
Name : Pratiksha Shrestha
Ms. Shrestha holds masters degree in food engineering and bioprocess technology from Asian Institute of Technology (AIT) Thailand. She is currently working for Government of Nepal at Department of Food Technology and Quality Control (DFTQC), Kathmandu. She is also a teaching faculty in College of Applied food and Dairy Technology (CAFODAT) affiliated to Purbanchal university, Nepal.