Fermentation of tea is the most significant step in tea manufacturing since in this step the most important properties of tea i.e. liquor characteristic develops. As you may already know, the term 'fermentation' is rather historical and does not accurately describe the process that occurs during the manufacture of black tea. This process involves enzymic oxidation of polyphenols, lipids, carotenoids and tarpene- glycosides, and their subsequent condensation/degradation leading to formation of coloured polymers, aroma and flavor compounds. Fermentation of leaf begins with its rolling to bring about the necessary changes to make tea liquor palatable. The complex changes occurring during fermentation, in which the polyphenols are oxidized and other associated chemicals also undergo some changes, make the liquor develop mellow character. Under optimum condition of fermentation, the liquor becomes bright and brisk with adequate colour and strength. These attributes of quality develop only up to a certain stage of fermentation beyond which the quality begins to decline.
Various liquoring qualities of tea are mainly derived from the same group of chemical compounds. Therefore, the excessive production of one property will naturally take place at the expense of another. Briskness, quality, strength and colour change with time and temperature during fermentation and each character is at its best at different times. In under-fermented tea, the leaf yields poor liquor and hence less quality, but well fermented leaf gives good colour. Over-fermented leaf may produce coloury liquor, no briskness and very poor quality. Only optimum fermentation ensures strength, brightness, briskness and quality of the liquor.
The common fermentation systems practiced in tea manufacture are as follows:
(a) Floor Fermentation
Fermentation on cement floor is the oldest and most popular method. Leaf is spread over cement floor or racks at 2.5-3.75 cm thickness for orthodox and 1.25 cm in case of CTC tea. The floor should not be wet when the leaf is being spread and there should never be any stale juice deposits. This will help to keep the bacteria away. Washing of the floor daily with a suitable detergent is absolutely necessary. Floors with glazed marble tiles are also used but care should be taken that the joints between tiles do not become the source of bacterial contamination. Aluminium and plastics trays are sometimes used for fermentation. Leaf spread over the trays should be sprayed thin so that proper aeration can take place. Only plain aluminium or plastic sheets should be used to avoid bacterial contamination
In trough fermentation, troughs made of aluminium are placed on saddle to facilitate uniform distribution of air. Rubber or fads lining are used as pads on the fermenting units to prevent air leakage. Two types of gumlas (shallow large container without lid) are used, one with valves and the other with four holes at the bottom. The air pressure should be maintained at 2 inch water gauge. Fermenting containers do not work well with the under withered leaf. Trough fermentation is more controllable because the quantity of air flow and the pressure can be adjusted. It also cuts down the total surface area required. A 15 cm deep container can hold up to 16 kg of pressed leaves when filled to the top. However, the containers are not filled to the top level, so as to facilitate turning of leaves as and when required. In trough fermentation, the temperature encountered is high but even at temperature up to 43° C is not harmful for the quality if adequate amount of humidified air is used to pass through the leaf bed and the fermentation time is cut down.
(c) Continuous Fermenting Machine
The Continuous Fermenting Machine (CFM) is the latest technology and can be termed as moving floor with controlled conditioning. CFM helps to eliminate microbial contamination in tea. Micro organisms occur as contaminants during tea processing due to the presence of a layer of fermented juice on the processing machines and other equipments. The CFMs consist of a tray made up of conveyor racks with three to four tier systems like “quality” drier arrangements. The fermented leaves travel in a thin layer on the conveyor rack. Above or below the tray, UV lamps are fitted which are used to kill the external bacteria and also triggers the activity of polyphenol oxidase, thereby hastening the biochemical reaction. Bright infusions are obtained in the continuous fermenting machine.