Black Tea Production Process
Tea leaves and flushes, which includes a terminal bud and two young leaves, are picked from Camellia sinensis bushes typically twice a year during early spring and early summer or late spring. Autumn or winter pickings of tea flushes are much less common, though they occur when climate permits. Picking is done by hand when a higher quality tea is needed, or where labour costs are not prohibitive. Depending on the skill of the picker, hand-picking is performed by pulling the flush with a snap of the forearm, arm, or even the shoulders, with the picker grasping the tea shoot using the thumb and forefinger, with the middle finger sometimes used in combination. Tea flushes and leaves can also be picked by machine, though there will be more broken leaves and partial flushes reducing the quality of the tea. However, it has also been shown that machine plucking in correctly timed harvesting periods can produce good leaves for the production of high quality teas.
The tea leaves will begin to wilt soon after picking, with a gradual onset of enzymatic oxidation. Withering is used to remove excess water from the leaves and allows a very slight amount of oxidation. The leaves can be either put under the sun or left in a cool breezy room to pull moisture out from the leaves. The leaves sometimes lose more than a quarter of their weight in water during withering. The process is also important in promoting the breakdown of leaf proteins into free amino acids and increases the availability of freed caffeine, both of which change the taste of the tea.
Known in the Western tea industry as “disruption” or “leaf maceration”, the teas are bruised or torn in order to promote and quicken oxidation. The leaves may be lightly bruised on their edges by shaking and tossing in a bamboo tray or tumbling in baskets. More extensive leaf disruption can be done by kneading, rolling, tearing, and crushing, usually by machinery. The bruising breaks down the structures inside and outside of the leaf cells and allows from the co-mingling of oxidative enzymes with various substrates, which allows for the beginning of oxidation. This also releases some of the leaf juices, which may aid in oxidation and change the taste profile of the tea.
04.Oxidation / Fermentation
For teas that require oxidation, the leaves are left on their own in a climate-controlled room where they turn progressively darker. This is accompanied by agitation in some cases. In this process the chlorophyll in the leaves is enzymatically broken down, and its tannins are released or transformed. This process is sometimes referred to as “fermentation” in the tea industry. The tea producer may choose when the oxidation should be stopped, which depends on the desired qualities in the final tea as well as the weather conditions (heat and humidity). For light oolong teas this may be anywhere from 5-40% oxidation, in darker oolong teas 60-70%, and in black teas 100% oxidation. Oxidation is highly important in the formation of many taste and aroma compounds, which give a tea its liquor colour, strength, and briskness. Depending on the type of tea desired, under or over-oxidation/fermentation can result in grassy flavours, or overly thick winey flavours.
During this stage, a gentle heat is applied to the leaf to stop the oxidation process. Firing may be done by blowing hot air over the leaf or running the leaf through heat tunnels. The temperature for firing is between 140 and 170 degrees Fahrenheit and lasts from 10 minutes to no more than an hour. This heating, or “firing” processes destroys the specific shapes of the enzyme proteins in the leaf, killing the enzymes so the leaf is stable and does not mold or break down.
The machine operates on the principle of identifying different types of tea particles based on color and separating them by activating a pneumatic actuator system. The system captures images of moving tea particles through a Charged Coupled Device (CCD) high-speed camera and processes that image data using Field Programmable Gate Array (FPGA) and advanced electronic systems.
Sorting is the process to determine the various leaf grades. The dried leaves are moved over vibrating wire mesh trays. This simple apparatus sorts the leaf into whole leaf, broken leaf, fannings and dust grades.
Packing is the last step in the manufacturing process where different grades of tea are packed into plywood chests or paper sacks. Each of these packages is marked with the grade name, garden, invoice number, chest number, year of manufacture, etc.