Article
Black Tea processing
Market view of Tea processing
Tea is generally categorized into three different types black, green and oolong. Black tea is consumed in Western hemisphere, while consumption of green tea is confined mainly to Asia and Middle East. The worldwide black tea production is about 78%, whereas green tea constitutes about 20% and rest about 2% is oolong tea. Oolong is mainly produced in China and Taiwan. Small quantities of green and oolong are also produced in India and Sri Lanka.
How Black Tea is Processed?
Black tea is nothing more than the leaves of the Camellia sinensis that have been processed a certain way. Black teas are the most consumed of the four types of teas. They are the highest in caffeine, but still have antioxidant properties, just not quite as much as others.
Withering
It is the first stage of tea manufacture. During withering harvested tea flush is evenly spread on the withering troughs which are normally 4 to 5 feet wide and 50 to 70 ft long. During the process hot and cold air is blown through to remove the moisture of fresh leaf which is achieved during 16 to 24 hours of period.
Rolling
This process has two objectives: to reduce the leaves into small pieces and to disrupt the leaf structure, so that the polyphenol oxidase is brought into contact with the flavonols and the fermentation can proceed efficiently.
In case of orthodox type of tea, the withered leaf is charged in the rollers which vary in size. Normally they are 36" and 48" rollers capable of rolling 70 to 120 kg of withered leaf. The pressure is exerted on the leaf in 'open and pressure' sequence. This operation lasts for about 45 minutes. During this process tea leaf is twisted as a result of repeated rolling action and simultaneously leaf tissue ruptures thus forcing out the cell-sap which spreads as a this film on the surface of theleaf. Various enzymes and their substrates come in contact and enzymatic oxidation starts at this stage and imparts liquoring properties to the leaves i.e., briskness, brightness, colour, strength, and quality to the made tea. Moreover in case of orthodox tea manufacture, this process imparts twist or style to the made tea. The green colour of leaf starts diminishing and brown coppery colour of leaf begins to appear.
The CTC (cut, tear and curl) machine has two rollers, carrying teeth, which rotate in opposite directions at different speeds. The teeth mesh in with one another, leaving a gap that is variable. Leaves are drawn through the gap and are cut and curled. It is common practice to reduce the size of the leaves fed to a CTC machine by prior passage through a rotorvane. The cut leaves often pass through a second CTC machine. These machines reduce the leaves to very much smaller pieces than those described earlier. This is important for the fine tea required for tea bags. The tea leaves are moved up to the top of the machine by the inclined moving belt. The cutters are behind the tops of the inclined belt guards. The tea falls between the pair of cutters. The first machine on the right discharges cut leaf on to the feed belt of the second machine.
Fermentation (Oxidation)
It is mainly an enzymatic process during which polyphenols are oxidized with the help of enzyme and oxygen from air. The main object is to complete the chemical change up to the required degree so that the best balance of liquoring properties is obtained to make the tea liquor pleasant to taste. Rolled leaf is put on racks specially made of tiles, cement or aluminium. Oxidation rooms should be kept clean and cool. In Darjeeling, oxidation is carried out at low temperature. During hot season the air is kept humid with the help of humidifiers with less than 200c hygrometric difference. Optimum oxidation is adjudged by the subjective assessment of the tea maker. Oxidation period varies from 2 to 3 hrs depending on the type of leaf, degree of wither, temperature, availability of oxygen and oxidizing ability of the tea flush. The time of oxidation is reckoned from the time at which rolling begins. Oxidation is a very sensitive process. Even little delay in adjusting the correct oxidation period is likely to deteriorate flavour appreciably. During oxidation the leaf changes colour and becomes dark coppery. A typical aroma develops at this stage. Oxidation of catechin / polyphenols by the enzyme polyphenol oxidase is the main reaction of the process that imparts 'tea character' to the black tea.
Firing / Drying
The main objects of drying tea are to stop the enzymatic mellowness of fermented leaf and to obtain the desirable mellowness of resultant tea, and to reduce the moisture of fermented leaf to 2.5 to 3 percent. Thickness of spread, speed of trays and volume of air blown through are regulated meticulously so as to achieve the correct drying. The whole process of drying is completed within 20 to 30 minutes. Darjeeling teas are normally fired at high temperature which is considered essential for squeezing the correct balance of volatile flavoury compounds in high quality teas.
Sorting and grading
At the end of the manufacturing process, the leaf particles are sorted into different sizes or grades. Bulk tea is sorted on mechanically oscillated sieves. One of the objectives of sorting is to classify the tea as per the size of the particles. The sieve size decreases gradually from top to bottom. Thus, the bigger size teas are retained on the top while the broken leaves fall towards the floor. The modern sorting machines employ different sieves having meshed nos. 8, 10, 12, 16, 24, 30 or more. Teas are passes through electrostatically charged rollers which attract the fibre and stock preferentially. Thus a series of clean teas, free of stalk and fibre are produced. The tea passing through the meshes are classified into various grades. In general there are four grades of tea. They are Whole leaf, Broken leaf, Fannings and Dust. Each of these grades are again classified into various sub-grades. The key grades of black tea are: CTC grades BP1, BP (Broken Pekoe), PF1, PF (Pekoe Fannings), PD (Pekoe Dust), D1, D; Orthodox grades TGFOP (Tippy Golden Flowery Orange Pekoe), GFOP, FOP, OP; Brokens TGBOP (Tippy Golden Broken Orange Pekoe), GBOP, FBOP (Flowery Broken Orange Pekoe); Fannings BOPF, GOF, Dust PD, D1 etc.