After completion of fermentation, it has to be stopped as soon as possible to avoid the over fermentation. Drying of the mass with hot air stop fermentation process. Drying reduce the moisture content of fermented leaves from 50 – 70 % moisture content to 3 %. In dry black tea, it also allows development of tea aroma. Drying is physically achieved by blowing hot air through fermented leaves as they are conveyed in chains. The temperature of hot air in the inlet is 100 – 120°C conveyed while that at the outlet is 56°C. Drying process takes about 15 to 30 minutes. The principle biochemical process involves conversion of chlorophyll to pheophytin and pheophorbide responsible for the black appearance in the manufactured tea. The high temperature during drying also cause loss of low volatile compounds, continuation of few reaction of fermentation stage and the formation of new compounds like β-ion and theaspirone which play an important role in the formation of aroma.
Objective of drying:
1. To arrest fermentation
2. To fix the desired properties
3. To reduce the moisture content
4. To make the finished product stable
5. To handle the finished product conveniently and store and transport conveniently
Generally 2 types of drier are used in drying
1. Conventional drier: It is basically rectangular chamber. Tea to be dried is sprayed on perforated tray made of metal strips. These drier are driven by trays carrying chains and sockets running on angle iron runners. The movement of the tray with the leaf is counter current to the draft of hot air.
2. Fluid bed drier: It has grid through which hot air is forced out like a jet. Fermented leaf is blown off by these air. These air streams fluidize the leaf and at the same time leaf is dried. The jet are so arranged that bed of leaves also move forward and therefore the leaf after drying reach the other end of the grid where the dried leaf is discharged. Fermented leaf with initial high moisture content normally cannot be fluidized without mechanical help. Therefore the grid is either vibrated as vibratory fluid drier or the material is racked as in most other cases.
Cleaning, Sorting and grading:
After drying, cleaning, sorting and grading of tea particles is done before final packing to dispatch is made. Tea particles are graded into various shape and size. Grading and sorting are traditionally done with mechanically oscillated sieves fitted with meshes of appropriate aperture. Sorting is performed by hand sieves or by winnowing and stalks are picked by hands. Such work need more hard work, skill and attention. Sorting procedure may vary from factory to factory. The percentage of stalks and fiber in tea is dependent on plucking standard, as well as climatic condition prevailing at the time of plucking.
The grades such as broken orange pekoe (BOP), broken pekoe (BP), fannings, and dust are generally based on the relative proportion of particle size. Though each grade carries a certain amount of typical particle pieces of stalk. Recently, the possibility of dielectric separation of tea leaves and stalk is possible. The dielectric medium separation employs the difference in dielectric constant of the constituent (leaf and stalk) in the mixture. As the mixture is exposed to the variation in humidity, the dielectric of the two constituent behaves differently.
Grading of leaves is the last stage of production. Tea can be graded as whole leaf, broken leaves, crushed leaf, fannings and dust. Fannings and dust are mainly used for large scale commercial tea industry. Fannings are used especially in tea bags. Dust is considered as low grade because dust of air, floor, machine, leaves etc. contaminate the tea dust and it is almost impossible to remove.
Each of the above grades has subgrades as super fine tippy golden flowery orange pekoe (SFTGFOP), fine tippy golden flowery orange pekoe (FTGFOP), golden flowery orange pekoe (GFOP) and flowery orange pekoe (FOP).
For broken or crushed leaves, the subgrades are tippy golden broken orange pekoe (TGBOP), golden broken orange pekoe (GBOP), broken orange pekoe (BOP) and broken pekoe (BP). Fannings are sub classified as orthodox fannings (OF) and pekoe fannings (PF). Similarly, orthodox powdered dust (OPD) or orthodox dust (OD) are sub classification of dust.
Name : Pratiksha Shrestha
Ms. Shrestha holds masters degree in food engineering and bioprocess technology from Asian Institute of Technology (AIT) Thailand. She is currently working for Government of Nepal at Department of Food Technology and Quality Control (DFTQC), Kathmandu. She is also a teaching faculty in College of Applied food and Dairy Technology (CAFODAT) affiliated to Purbanchal university, Nepal.