Withering is the first and most important step in tea factory in which freshly plucked leaf is conditioned both chemically as well as physically for subsequent processing and can be considered as first step for achieving quality of black tea.
Traditionally, various manual systems like TAT withering, Tunnel withering, Drum withering and Trough withering have been in use.
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Trough withering is the most popular manual system currently being used all over the world because of following reasons:
- Greater flexibility with respect to capacity and degree of wither
- Flexibility of construction
- Easier operation
Such trough can be used only once in a day due to large holding time for withering of green leaf. After plucking of green leaf, leaf is spread in these rectangular troughs. Lot of manpower is used during spreading of green leaf with holding time of 16 to 18 hours. Axial flow fans are used for blowing atmospheric or conditioned air to get required wither. As atmospheric conditions vary throughout the day, it is difficult to maintain consistency in terms of humidity and temperature across the leaf. Excessive manual handling also causes damage to fresh green leaf resulting in degradation of made tea quality.
KILBURN continuous mechanised withering system eliminates all the problems and deficiencies inherent in trough withering as stated above. The new system envisages adherence to consistent parameters and ensures improved and uniform leaf quality. KILBURN Continuous withering system comprises of two separate systems for chemical and physical withering and has separate controls for residence time, humidity and temperature.
Chemical withering starts immediately after plucking. The objective of chemical withering is to allow desirable chemical reactions and enzymatic activity to take place in green leaf. Air mist humidification system will ensure minimum loss of moisture from green leaf. Chemical Withering Bin is designed to keep the green leaf for required duration with desired parameters of humidity and temperature. By controlling humidity and temperature of leaf, green leaf can be kept for minimum 24 hours without any deterioration of colour and quality.
Chemical changes like production of volatile flavour component, increase in caffeine content, production of organic acid, breakdown of larger organic molecules to simple compounds occur during chemical withering and significantly influence the quality of black tea. The heat generated due to respiration ( a process of breakdown of glucose to carbon dioxide and water in presence of oxygen) of the mass of the leaf would heat up the leaves. Sufficient amount of air is blown though the leaves in order to take away the heat. The blown air would take away the heat generated and cool down the leaves and also provide necessary supply of oxygen which prevents anaerobic reaction (which are not desirable) to take place. Chemical withering normally takes 12-14 hrs. from the time of plucking and cannot be accelerated.
After chemical withering, partial withered leaf moves into another set of wire mesh conveyors for physical withering where the moisture content in leaf is reduced. Reduction in moisture level increases the flaccidity of leaf to the desired level.
Desired wither can be achieved by passing conditioned air with respect to atmospheric temperature and humidity conditions. Residence time in holding bin for chemical wither and removal of moisture during physical withering can be varied to achieve the desired parameters.