Tea University – Processing

Based on the type of manufacturing process, teas in general can be classified into three major types: green tea (un-oxidized), oolong tea (partially oxidized) and black tea (fully oxidized). Apart from the above three types, a specialty tea called ‘white tea’ is produced in lesser quantities (which is correspondingly more expensive than tea from the sample plant processed by other methods) and instant tea (readily soluble in water). In the process of black tea manufacture, the flavanols present in the cell are allowed to ferment fully (enzymatic oxidation in the true sense and no microbes are involved) to get the characteristic aroma, color and taste of black tea. Of all the above types, black tea is produced to the maximum extent, which accounts for about 75% of world tea production.

White Tea:

White tea, the best form of green tea in general is produced from special varieties of tea clones with large buds and dense pubescence. The unopened buds are harvested carefully and subjected to minimum processing (not oxidized or rolled, but simply withered and dried slowly under controlled conditions) to produce ‘white tea’ or ’silver tips tea’. White tea is a very rare, expensive connoisseurs tea that is only produced in certain pockets of the world. White tea is the most subtle of all the varieties of tea. One of the best/full effects of white tea can only be achieved by chewing the leaves themselves.

Green Tea:

In contrast to black tea, during green tea manufacture, freshly picked leaves are directly subjected to high temperature to inactivate the oxidizing enzymes using steam (steamed green tea) or by roasting in a hot pan (pan fired for a few minutes. After complete inactivation of oxidizing enzymes present in the leaves, it is partially dehydrated, rolled and dried to final moisture of around 2.5 – 3.0%. The tea thus obtained will be a rich source for antioxidants with many health benefits.

Stages of Black Tea Manufacturing:

Withering: Storage of green shoots in a trough for about 12 to 16 hours with the partial removal of moisture by blowing air is called withering, which is earned out to prepare the green tea shoots biochemically and physically for subsequent manufacturing operations. During withering the leaf becomes flaccid and is reduced from its turgid conditions to pliable form by uniform and gradual loss of moisture. The sap gets concentrated. Chemically the enzymes get activated, and the levels of proteins, free amino acids and sugars increase during withering.

Orthodox Processing: In the orthodox or conventional method of tea process, rolling of teashoots is done in rollers, consisting of three principal parts – a table fitted with cones and battens, a bottomless receptacle called jacket and pressure cap fitted with an adjusting screw to enable pressure to be applied to any desired extent. It is as if the leaf is rolled between the palms of two hands or as in the domestic hand grinders of yesteryears. After rolling for about 45 minutes, the leaf is passed over a sifter to remove the well-rolled leaf called fines. The sifter may be rotary or reciprocating. The first ‘dhool’ (the macerated leaves) will always contain a larger proportion of tender shoots from fine leaf. After roll breaking, the fmes are taken for fermentation while the residue left after sifting is put back into the roller and the rolling and roll breaking process is repeated. The rolling followed by roll-breaking is carried out about four times.

CTC (Crush, Tear and Curl) Processing: During 1930, William McKercher designed the first CTC machine at Tocklai (Assam). In this process, the withered leaves are preconditioned by macerating lightly in a shredder, followed by rotorvane machine, in shredder, a rod fitted with sharp knives rotates in the axis of a cylinder, and facilitates fragmentation of withered leaves. The macerated leaf from the shredder has to be in the form of ‘Chutney’, which is further crushed in the rotorvane, which essentially consists of a vaned cylinder equipped with a rotor shaft and a pressure cap at the end that determines the residence time and degree of maceration. The rotorvaned tea ‘dhool’ (the macerated leaves) is then passed through 4 or 5 CTC machines arranged in tandem for adequate maceration. The CTC machine consists of two cylindrical rollers, having stainless steel segments with fine tooth-like sharp ridges and are fixed in parallel with only marginal clearance between them. The two rollers rotate in opposite direction at different speeds. A ratio of 1:10 with speeds between 70:700 mm and 100:1000 mm is nonnally employed. The slow speed roller acts as a conveyer apart from providing a surface for cutting. The CTC type of maceration take hardly a few minutes.

Fermentation:

Tea fermentation is an important stage in the black tea processing, because during this stage, the most important properties of tea are produced. Conventionally the completion of fermentation is judged by the change in colour (green to coppery) and the pleasant aroma that develops. In the orthodox process, fermentation is carried out by spreading the rolled leaf in a layer of 1″ to 3″ thickness on floor. The duration varies between 2 to 3 hours. During fermentation low temperature (between 20 to 25 °C) and high humidity (above 90 % RH) are desirable. In the CTC process, the macerated tea ‘dhool’ is fermented in a revolving large fermenting dmm with conditioned air for 60 to 90 minutes or in continuous fermenting machines.

Drying

The objectives of drying are to arrest fermentation and to remove moisture and produce tea with good keeping qualities. Drying is the most expensive process in the manufacture of tea. The most important factors, which influence the drying process, are temperature, volume of air, loading time and drying time. The normal drying period is 20 to 25 minutes in conventional driers (ECP – Endless Chain Pressure) and 10 to 12 minutes in modern driers (VFBD – Vibrating Fluid Bed Driers). Sorting: Sorting is the operation in which tea particles of the bulk are separated into various grades of different sizes and forms confirming to trade requirements. Sorting of bulk tea is done in three stages viz., a) Cleaning of fibre, b) Grading and c) Winnowing. After removing the fibres as well as flaky teas from the rest of the bulk by using electrically charged rollers, grading is done by passing the tea through sieves of different sizes. After sifting, each grade is finally cleaned by winnowing during which fibres and flaky teas are removed.