Tightly packed leaf, gets heated easily as respiration continues after plucking and heat cannot escape. Unless the heat generated due to the exothermic reaction caused by respiration is allowed to dissipate, temperature of the leaf mass continue to increase and the rise may even be beyond 10°C. Storage of leaf in heaps in excess of 8 kg/m2 of floor area has been found to lead to heat development that affects quality.
Ramming or compression of leaf into the basket results in ripping of leaves and generation of excess heat leading to oxidation (fermentation) even before the arrival of the leaf in the factory. The rough interior of the baskets further aggravates the situation.
Contact with foreign matter viz., sand, oil etc., also may damage the leaf.
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".
Withering: Loss of moisture, increase in cell membrane permeability, initiation of polyphenol oxidase/ peroxidase activity, breakdown of chlorophylls, breakdown of proteins releasing amino acids, degradation of carbohydrates, increase in caffeine and inorganic phosphate contents and the development of flavour volatiles.
Rolling: Enhanced levels of polyphenols oxidase/Peroxidase activity, production of flavour compounds and degradation of chlorophylls.
Fermentation: Polyphenols are oxidized and then condensed to form theaflavins, thearubigins etc.; chlorophylls are degraded and some of flavour components are produced from lipids, amino acids, carotenoids and terpenoids.
Drying: Loss of moisture, inactivation of enzymes, loss of soluble solids, development/loss of some flavour components and phaeophytin produced from chlorophylls.
Process objectives
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.
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