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.
Chemical composition in relation to plucking
Quality of tea depends to a large extent on the standard of plucking because, while the quantity of essential chemical constituents gradually decrease in the older leaf, fibre and other insoluble materials increase. Thus, the manufacture starts with heterogeneous raw materials having different levels of chemical constituents and physical characteristics. When such materials are subjected to subsequent steps of processing each category responds differently resulting in some over-processed and some under-processed mass. The large volume of harvested shoot limits the technology to separate two and a bud and three and a bud, otherwise it would be ideal to separate them to different bays and perform manufacture separately.
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 –|
The membranes separate the chemical constituents in different components in a normal leaf and therefore these constituents cannot come in contact and react. During processing the increase in cell permeability facilitates the intermixing of the constituents, which intensifies during later part of processing through cell disruption. When the cells are macerated the chemical constituents come in contact and instant oxidation reaction takes place leading to formation of larger phenolic compounds called theaflavins (TF) and polymeric thearubigin (TR). Depending upon the extent of the reaction of the phenolic compounds in presence of enzymes Black tea, Oolong tea or Green tea are produced.
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.
For manufacture of black teas, the shoots pass through the following six distinct phases of processing: