Chemical Wither & Physical Wither

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.

Post harvest care

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.

Red leaf formation and quality of made tea

The green leaf can turn brown/red as a result of physical damage arising from bruising of leaf and heat stress. The red colour is observed to be prominent when the leaf temperature exceeds 35?C. The increase of red leaf percentage has got detrimental effect on quality. The cause of red leaf formation is mainly due to oxidation of polyphenols. Reddening of leaf may occur more rapidly when the leaf temperature exceeds 40?C. With the increase in percentage of red leaf formation there is significant increase in undesirable TRs. It will reduce brightness and briskness of liquor along with reduction in essential volatile flavour oconstituents (VFC) of made tea.

Careless handling and transportation resulting in bruising and tearing, therefore, leads to reduction in quality. Care should be taken to allow the heat to dissipate and avoid friction to save the leaf from bruising and tearing.

It is imperative to remember that the shoots as raw material, determine the value of the end product. During manufacture the value can decrease due to faulty operation, but the inherent quality attributes cannot be enhanced. It is, therefore, essential that the quality attributes be not affected.

Effect of physical damage of leaf on made tea quality

Apart from damage of the quality attributes in green leaf due to heat generation, leaf can be damaged through rough handling, causing bruise and tear in the leaves. Cell damage is the ultimate objective in the manufacture of black tea. However, if the damage is initiated before withering and is indiscriminate, the leaf may undergo uneven oxidation process adversely affecting the desirable chemical constituents in the end product. Also, damaged leaf withers at a faster rate, as the structure of cuticular waxy layer of the leaf is disturbed. This ultimately influences the water holding capacity of the leaf and will lead to uneven wither.

Packing Density in Baskets

With the compactness of packing of the leaf in the basket/bag there is a considerable loss of quality of the made tea. There is a significant decrease in TF content, as the TFs get converted to TRs over the time. This ultimately reduces brightness and briskness of the tea. The rise in temperature disturbs the entire cell component resulting in likely occurrence of enzymic oxidation of catechins prior to maceration, which is undesirable. Thus one must take utmost care while packing the leaf in basket/bag at the field.

Effect of green leaf temperature on made tea quality

The principal precursors for liquor characteristics are primary polyphenols, which through enzymatic processes are converted to large polyphenols like TF and TR. Leaf temperature and storage time have got a significant effect upon the variation of TF and TR profile. The effect is more prominent when the leaf temperature rises beyond 35?C.

Rise of temperature increases the enzymic oxidation of primary polyphenols and the products, which are formed by the oxidation process, overtakes the enzymic hydrolytic reaction desirable for quality.

Green Leaf Plucking

Plucking

Shoots comprising of 1+ bud and 2+bud are detached either by hand, shears or by machine, put normally in a basket, weighed and taken to the factory either manually or in a multi-tired trolley by a tractor.

Chemical changes after plucking

Once the leaf is plucked the anabolic reactions practically cease and catabolic reactions leading to the breakdown of large organic compounds to simpler molecules start. Burning of sugar molecules produced earlier through photosynthesis provides the energy required to run these biochemical reactions in the shoots.

There is rise in temperature in the mass of the plucked leaf during storage and transportation due to respiration as well as the aforesaid reactions. In the process of respiration organic substances in the cells (usually sugar) get oxidised into carbon dioxide and water with release of fairly considerable amount of water as may be seen from the following equation :

C₆H₁₂O₆ (Glucose) + 6O2 (Oxygen) 6CO2 (Carbon dioxide) + 6H2O (Water) + 674 calories (Heat)

After the leaf is plucked, biochemical reactions are initiated towards biological degradation of the shoots. In this direction, large molecules like cellulose and lignin, which are responsible for the rigidness of the shoots degrade to make the shoots more flaccid. Lipids carbohydrates etc., which are also large molecules, degrade. Carbohydrates degrade to produce sugars, which burn in presence of oxygen and produce energy to run various biochemical reactions, which are enzymatic and temperature dependent. If in the plucked shoots sufficient oxygen is not available, anaerobic reactions take place. Those reactions produce much less energy than aerobic reaction mentioned above. Secondly, the results of anaerobic reactions are different and are not desirable for making quality teas. Adequate oxygen availability is, therefore, a must. The lipids degrade to produce smaller molecules, which are quality attributes. Therefore, if before the lipids are degraded, cell rupture and manufacture are initiated, quality attributes will be missing. It is, therefore, essential to make the lipids degrade by providing proper conditions, i.e. temperature and time.