Rolling:
The aim of rolling is to establish proper condition for an enzymatic oxidation (fermentation) of catchin and polyphenols by polyphenolase in the presence of atmospheric oxygen. Rolling twist the leaf, break the cell structure and expel the juices, catchin and enzymes. Catchin and enzymes are originally separated in fresh leaf by cell compartment. During rolling, both the polyphenols and enzymes polyphenolases are brought together and are exposed to the atmospheric condition that begins the oxidation process.
Objective of rolling:
1. To rupture the cell wall and expose their contents
2. To bring the contents of leaf cells in contact of air to start the process of oxidation (fermentation)
3. To twist the leaf and give the desired shape, size and appearance
4. To break the larger twisted leaf into smaller particles or granules, dust
5. To macerate leaf for good mixing
Method or rolling:
Rolling is achieved by mechanical process such as rolling only (in orthodox process) or as crushing, tearing and curling (in CTC process). The machines used are in various size and designs but their principle are alike. They compress and turn over the leaf in continuous motion.
Orthodox method: The traditional method is to roll the bunch of leaf between the hands or by hands on the table until the leaf is twisted and evenly coated with juice and finally cut into pieces. Leaves are forced through an aperture of leg cutter and cut into strips. Mechanized method for the preparation of orthodox tea uses rotor van that consists of horizontal barrel with a feed hopper at one end and perforated plate at another. Tea leaves are forced through barrel by a screw type rotating shaft fitted with vanes at the Center. The leaf is distorted by resistor plates on the inner surface of the barrel and cut at end plate.
The rupture of cell is completed faster in fine leaves of small and tender shoots while the leaf require a little more time than fine leaf. Also the leaf which has already gone size reduction and twisted after some amount of rolling impedes twisting of larger leaves which may extend the period of rolling. Segregation of such leaf is carried out after an appropriate period of rolling which is usually 30 – 45 minutes.
The leaf is then discharged and passed through “googy shifter”. The “googy shifter” have perforations of different size. The particles passing through the perforations are called “fines” and the spills are known as coarse leaf which is sent for further rolling. The fine leaf is then taken for oxidation (fermentation). The coarse leaf after second roll is subjected to similar process to get second fine and needed particle size.
CTC method: CTC machine achieve the three actions of crushing, tearing and curling in the same machine at once. Instead of working on large volume of leaf, CTC machine takes fast thin but steady stream of leaf to pave the way to continuous processing unlike batch process of orthodox roller.
Fermentation:
Fermentation is the process in tea leaves where polyphenols are oxidized in presence of enzymes called polyphenolase. The term of fermentation is actually a misnomer for tea, which is technically an oxidation process. Under optimum condition of fermentation, liquid becomes brisk and bright with adequate color and strength. This attribute of quality develop only up to a certain stage of fermentation beyond which quality begin to decline.
Objective of fermentation:
1. To convert catechin into chemically oxidized compound
2. To activate enzymatic action
3. To ensure correct ratio between theoflavin (TF) and theorubigin (TR)
4. To maximize TF content of tea
Fermentation is the most important chemical process which involves oxidation of polyphenols with the help of an oxidase. The mechanical aspects involves spreading out of leaves in a layer 5 -8 cm thickness and maceration by rolling. The fermentation time is generally 45 minutes to 3 hours depending on quality of leaf. Fermentation vessel can makes the process continuous. However, principle biochemical event in fermentation is the production of O-benzoquinone derivative from catechin but the reaction involves development of color, strength, quality and aroma.
Development of color: Fermentation of Theaflavin (TF) and Thearubigins (TR) is centered to the biochemical reaction during fermentation but this fermentation occurs only with appropriate combination of flavonols. The polyphoenols present in tea leaves which undergo change during fermentaion are catechin, epigalocatechin (EGC), galocatechin (GC), epicatechin(EC), epicatechingallate (ECG) and epigallocatehingallate (EGCG).
Only epigalllocatechin and its gallate are readily oxidized than epicatechin. The rate of conversion of these polyphenols are dependent on temperature and oxygen concentration. Following the oxidation of these orthoquinone derivatives, structurally rearranged variant are formed either out of a pair of epigallocatechin molecules or a hybrid consisting a molecule consisting each of EGC and EGCG. The rearranged pairs are known as the bis-flavonols which dirrerently reacts with orthoquinone to from a theaflavin and additional oxidation transforms TF to TR.
Theaflavin (TF) play important role in determining the quality and brightness of black tea. It was shown that all catechin and epicatechin gallate decrease during fermentation. The approximate proportion of different fractions being theaflavin gallate ( 8 %), theaflavin epicatechin gallate (20 %) theaflavin digallate (40 %) isotheaflavin (4%) and others. Total theaflavin in tea liquor constitute about 0.3 to 1.8 % of hte weight of black tea. Thearubigin constitute about 30 – 60 % of the total solid in tea liquors affecting color, strength and briskness of brewed tea.
Development of aroma: The formation of volatile flavor constituent start with the degradation of lipids during withering but it gains momentum during rolling and particularly during fermentation. Lowering the fermentation temperature, it is possible to reduce the rate of polyphenolic oxidation reaction. This allows volatile flavor constituents from the flavor producing substrates that survive rolling. The amount of volatile compound vary also in fermentation time.
Factors affecting tea fermentation:
1. Quality of raw materials
2. Degree of withering
3. Temperature
4. Cell damage during processing
5. Rate of air supply
Assessment of fermentation:
1. Change of color from dull green to coppery
2. Development of sweet aroma
3. Development of color, briskness and quality
Type of fermentation:
For production of tea by CTC process, fermentation is carried out by floor fermentation or drum fermentation. For production of tea by orthodox process, fermentation is carried out by fermenting machine (continuous process), trolley fermentation system, floor fermentation system and drum fermentation system. Fermentation time is generally 2 – 3 hours but varies according to different seasons of the year. Fermentation temperature ranges from 25 – 27°C for 95 % RH. It should be taken into consideration that below 15°C enzymes are not activated and above 45°C enzymes are destroyed.
About Author
Name : Pratiksha Shrestha
pratiksha.shrestha2001@gmail.com
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.