- About Tea
Quality of the end product in any process industry depends mainly on the raw material and tea is no exception. Apart from plucking standard, age of bush, stage from previous pruning, plucking round etc. are some of the important factors that contribute to the quality of the tea shoots. Besides, in the same shoot, the chemical composition of 1st leaf, 2nd leaf, other leaves, stem, etc. vary widely.
The shoots of tea plants contain large range of chemicals of which the catechins are the most important characteristics for manufacture of black tea. The leaves also contain polyphenol oxidase (PPO) and peroxidase (PO), which are enzymes capable of oxidizing catechins. The overall constituents of the tealeaf as located in different cell compartments are as under:
|Location||Chemical||Dry weight||Water soluble|
|Cell wall||Cellulose Hemi Cellulose||14-22|
|Plastids||Various enzymes –|
Inhibiting the interaction of the enzymes and the catechins produces green tea. Oolong tea on the other hand is produced through partial oxidation of catechins. In the case of orthodox teas the oxidation is prolonged, while, in case of CTC, the oxidation is more intense.
The liquoring quality of a tea is measured in terms of total oxidisable matter (catechins) or primary polyphenols and caffeine. If the plucking is coarse, the percentage weight of two-leaf shoot will be much less. An average plucking standard should have ideally about 75% fine leaf by weight comprising bud and two leaves, soft banji, undeveloped bud plus two leaves. For achieving quality the fine leaf should not be diluted with coarser leaf. Approximate contribution of different shoot components in different plucking standards is given hereunder.
|Shoot component||% in 2 and a bud||% in 3 and a bud|
The figures given above may vary because of climatic and other factors, but are fairly representative. Imperatively increase in fibre content and ashy substances as well as decrease in enzymic activity and soluble solids in the coarser leaf affect quality.
Green leaf shoots equivalent to 100 numbers of balls of equal size are picked up at random from each withering trough and thereafter these are separated into fine leaves and coarse leaves. The fine comprise of two leaves and a bud, smaller shoots and also soft banjis. The separation of fine leaf from coarse leaf is done, where necessary, by breaking back the two and a bud/soft banji from the plucked shoot. In the case of three and a bud shoot, the two and a bud comprises the fine leaf and the rest of the broken stem is coarse leaf. Thereafter, the fine leaf is weighed against the equivalent number of balls/coins and this gives the Ballometric/Paisometric Count. It may be noted that the loading of the trough should be done prune-wise, plucking challan-wise and also section-wise, if possible, for each of the three weighments.
Total hundred shoots are collected at random from each plucking challan for each of the three weighments and thereafter these shoots are separated into one and a bud, two and a bud, three and a bud, single banji, double banji and single/coarse leaf. This count enables tracking of the percentage of different sizes of shoots being plucked.
Total hundred shoots are collected at random and are separated into damaged shoots on one side and undamaged shoots on the other side. The damaged shoots are counted and this gives the Damaged Leaf Count for each of the three weighments.
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.
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.
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.
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.
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.
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.
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”.
The process objectives to be achieved during withering are as follows: