Chemical Wither
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 Wither
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.
