OXIDATION
Finally the leaf is ready for the main event—oxidation, the process so often mislabeled as fermentation. By definition, fermentation can only occur in the absence of oxygen. Oxidation is a chemical process that requires an abundance of moist, oxygen-rich air. Oxidation rooms, or chambers, for tea production must have fifteen to twenty exchanges of humidified air per hour to guarantee complete oxidation. The catechins (polyphenols) in the leaf absorb a significant quantity of oxygen, particularly during the early stages of oxidation. Oxidation officially begins during the withering stage and accelerates gradually during the subsequent steps. Within the controlled environment of the official oxidation process, the several chemical reactions that together comprise oxidation now take center stage. In traditional oxidation the sieved leaf is spread out in a thin layer (maximum 2 to 3 inches, or 5 to 8 centimeters) on the floor of the factory, on tables, or on perforated trays that are similar to the withering troughs.
A typical oxidation table (Darjeeling, India). Photo courtesy of Eliot Jordan.
The oxygenation of the catechins stimulates them to start the series of chemical reactions that will yield the flavor components and cup characteristics that we expect in black tea. During the first and most important of the enzymic oxidations the enzymes polyphenol oxidase and peroxidase act on other polyphenols to produce theaflavins. These red-orange compounds then react with more polyphenols to produce thearubigins, the chemicals responsible for changing the leaf’s color from green to golden, coppery, or chocolate brown. The thearubigins, meanwhile, are also busy reacting with some of the amino acids and sugars in the leaf to create such distinctive flavor components as the highly polymerized substances. In general, theaflavins contribute to the brisk and bright taste of black tea, while the thearubigins are what provide strength (depth or body) and color. Carbon dioxide is also being driven out of the leaf, and heat is exchanged. If the temperature of the leaf rises too high, oxidation will rage out of control; if it falls too low, oxidation will cease.
At this point the oxidizing leaf takes on a new moniker: dhool. Oxidation requires two to four hours and is controlled by experience, not by science. Although there may be technical markers for determining a prospective end to the process, so many variables come into play that the best method for ending oxidation is the nose and eye of the expert monitoring the process. The tea master must control the thickness and raking of the leaf, which determines the exposure of the surface area of the dhool to the air; the ideal ambient temperature (85°F, or 29°C) and relative humidity (98 percent); and the ventilation (fifteen to twenty complete changes of air per hour). Also, the environment must be completely hygienic; bacteria must be prevented from ruining the dhool.
During oxidation the dhool goes through a predictable series of flavor profiles: brisk, high color, and overall strength. The tea master can direct the dhool into a particular style by adjusting the length of time allowed in oxidation in combination with regulating the temperature/humidity of the oxidation chamber. Most tea is manufactured to a balanced cup showing bright liquor, good brightness in the nose, and a solid full body. When the tea master has determined that the dhool is fully oxidized to the desired level, oxidation is halted by the final and also critical process of black tea manufacture: drying.
DRYING AND COOLING
The purpose of drying is to stop all biochemical enzymatic activity (which then halts oxidation) by exposing the dhool to hot air (heated to a minimum 130°F, or 54°C). Drying also reduces the moisture content of the dhool from the mid-60s percent to a mere 3 percent. Residual sugars are also caramelized during drying, changing the color of the dhool from its coppery-red color to a rich brown-black. While all the other processes in the manufacture of black tea have a fairly large window for success, drying happens quickly (fifteen to twenty minutes). Many experts refer to it as the do-or-die moment, because all the work that has gone into bringing the fresh leaf to the dhool stage will be for naught if the drying fails.
The dhool is dried by one of many methods. Originally finished in large pans or baskets over an open fire or charcoal, as artisanal green tea is still final-fired, virtually all black tea today is dried using mechanically forced hot air. Some of the machines that are used to finish dhool into black tea include drum-dryers, tumble-dryers, fluid-bed dryers (similar to an enclosed withering trough), endless-chain-pressure-type dryers (vertically angled conveyors that tumble the dhool down from level to level, increasing the heat as it travels through the machine), and tempest-dryers. Orthodox leaf tea is then cooled quickly and thoroughly to prevent overheating and loss of flavor. When it is cool and shelf-stable, the finished “made tea” is sorted one last time before final grading and being packed for storage and eventual sale.
PROCESSING CTC-MANUFACTURED BLACK TEA
Because CTC manufacture requires chopping the withered leaf, extra care must be taken to eliminate unwanted material before the leaf goes into the cutting machinery. The cutters used are extremely sharp and fast-moving, so sticks, stones, and other miscellaneous objects must not enter this machinery. Leaf destined to be CTC-grade black tea is thus put through what is known as preconditioning. This is a combination sifting-and-shredding procedure. The first step is accomplished by a machine called a green leaf sifter, a perforated vibrating tray that guarantees that a free flow of withered leaf moves on to the next step, free of such extraneous matter as sticks, stones, and sand. Strong magnets remove iron or steel particles that may have found their way into the mix.
Moving quickly, the sifted leaf now goes into the green leaf shredder, which precuts the leaf into small pieces before it goes on to the Rotorvane. The green leaf shredder is a cylinder with a main shaft to which are fastened extremely sharp, lightweight, and well-balanced knives. These knives rotate on their shaft at 2,500 rpm, shredding the leaf as it passes through. The spacing between the knives decreases along the length of the shaft, increasing the machine’s cutting ability and ensuring that the leaf particle size is uniformly small when it emerges from the cylinder.
Now that the withered leaf is preconditioned, it is sent to a series of two Rotorvanes for final conditioning. The first Rotorvane has an open exit end but is only 8 inches (20 centimeters) in diameter, so it really crushes and compacts the leaf. Because the size of the leaf entering the second, larger-diameter (15 inches, or 38 centimeters) Rotorvane is so much smaller than that used for orthodox tea, this Rotorvane is set up with a sieve plate or screening at the end, to really scrunch the leaf as it exits the machine. This minced leaf is now called chutney, a clear reflection of the English influence on tea production.
ROLLING TO BECOME CTC LEAF
Now that the withered leaf has been meticulously conditioned, it is time for CTC rolling, which means that the chutney is passed through a succession of four or five CTC rollers. A CTC rolling machine consists of two stainless-steel rollers with many sharp cutting teeth. The two rollers are the same size diameter (8 or 13 inches, or 20 or 33 centimeters) and width (variously 24, 30, and 36 inches, or 60, 75, and 90 centimeters). They are mounted parallel to each other and are fixed horizontally. The rollers rotate at different speeds, in opposite directions to one another. The first is the slow roller and the second the fast. The ratio is 1 to 10, with speeds of 70 to 700 rpm to 100 to 1,000 rpm. The chutney, fed from the rear, is let to fall just behind the crown of the slow roller, which rotates forward.
The chutney is drawn between the rollers by the action of the fast roller, which is rotating back, at the faster speed. The slow roller actually performs more as the cutting surface and the fast roller as the cutting knife. Both are kept extremely sharp, however, and together they chop or cut the chutney into the uniformly small particles that will now be oxidized and soon become finished CTC leaf tea. Because the particles are so small, clumping is not a big problem going into the oxidation phase of CTC production, so rolled CTC leaf is not put through the roll-breaking and sifting process that orthodox leaf would be at this stage.
Due to the small particle size, good air circulation is the critical element now, so floor oxidation and trough oxidation are not good choices for the oxidation phase of CTC production. To increase the ventilation of the chutney, CTC production uses drum oxidation and continuous oxidation machinery to facilitate the necessary oxidation.
CTC DRUM OXIDATION
In southern India oxidation drums are used most commonly for this next step. A standard drum has a 16-foot-long (5 meters) cylindrical middle section with a 2-foot (60-centimeter) conical end at each extremity. Oxidation drums are mounted horizontally, in groups, and are perforated to aid air circulation. The rolled chutney is placed into these large revolving drums for sixty to ninety minutes. Fresh or conditioned air is pumped through the drum to assure proper aeration of the dhool. Because it is the transfer of oxygen to the leaf particle that causes oxidation, it is critical that fresh air is brought into the oxidation room, which is sometimes saturated with moisture to assure proper oxidation.
The distinctive movement of dhool within the drum guarantees that all the bits of leaf and every drop of the internal juices are spread throughout the dhool repeatedly, completely, and evenly. Drum oxidation also encourages even granulation, increases bulk density, and assures that the finished leaf is consistently blacker in color. CTC leaf processed by drum oxidation often does not require roll-ball breaking as a finishing step, as the tumbling action accomplishes that during the oxidation stage.
CTC CONTINUOUS OXIDATION, ROLL-BALL BREAKING, AND FINISHING
Continuous oxidation machines are the other choice for mechanical oxidation in CTC manufacture. This equipment is fairly straightforward and consists of a conveyor made of interwoven racks on which the rolled chutney is placed. The conveyor racks, or trays, pass under ultraviolet lamps; this activates the polyphenol oxidase and stimulates the oxidation process. The use of ultraviolet light also kills unwanted bacteria and microbial contaminants. This processing is used where humidity is high and conditions necessitate a super-clean environment for a faster, high-volume production of average-grade CTC leaf. CTC dhool processed using continuous oxidation machinery requires roll-ball breaking as a finishing step, as the conveyor system tends to encourage the creation of small clumps during the oxidation stage.
Following the completion of the oxidation stage, CTC leaf manufacture mimics that of orthodox leaf manufacture. The dhool is cooled quickly and thoroughly to prevent overheating and loss of flavor. When cool and shelf-stable, the finished “made tea” is sorted one last time before final grading and being packed for storage and eventual sale.
Pu-erh Tea
Pu-erh, known in old Chinese as “black tea,” is tea that is actually fermented. Depending on which type of pu-erh is being manufactured, the leaf may or may not oxidize; however, microbial activity involving several different bacteria takes place both in and on the leaf—this is true fermentation. According to the Pu-erh Yunnan Local Standard, there are two main types of pu-erh, with two styles within each type.
Sheng pu-erh (“raw” or “green” pu-erh) Mao Cha. “Young green” (good for investment, not immediate consumption!), this tea requires proper storage and aging, and then it becomes 1B).
Dry storage, naturally aged. The best of all four choices, both for current consumption and for additional keeping, this is authentic, compressed, caked, or bricked pu-erh.
Shou pu-erh (“cooked” or “black” pu-erh) Wo Dui (“wet-pile fermented”). This is almost always loose-leaf, but it is sometimes packed in bamboo or citrus peel or compressed into interesting shapes, generally ready to be consumed.
Wet storage, quickly aged. This is usually compressed into a beeng cha. Purists disdain this style of pu-erh, just as traditionalists scorn wine vinified in the modern style for drinking without cellaring, but much pu-erh is manufactured in this style and enjoyed worldwide.
I received a small brick of tea,
And sipping it, felt cool; I can do
with the wind as I will.
Why should I need paradise?
My whole body is floating amid
the white clouds.
—GIDŌ SHUSHIN (1325–1388)
PU-ERH TEA, DEFINED
Pu-erh is a particularly large, richly colored leaf tea that can be made from either oxidized (“cooked”) or nonoxidized (“raw”) leaf. Pu-erh can be loose-leaf or compressed into the traditional round cakes (known as beeng cha) and such other interesting shapes as mushrooms, spheres, cubes, pyramids, coins, and more.
This store in Dali specializes in both traditional and fanciful pu-erh in a multitude of shapes and forms (Yunnan Province, China).
The large leaves used in the manufacture of highest-quality pu-erh are plucked from the long-lived tea bushes that grow abundantly in southern Yunnan Province (see “China” in chapter 4).
The most famous region in Yunnan Province for tea plants of this type is the Six Famous Mountains region in the Xishuangbanna district, bordered by Myanmar to the west and Laos to the south. This lush, subtropical, and mountainous region provides one of the most perfect tea-growing areas in the world; as such, it is one of the locations where “wild” tea may have originated. Many of the tea plants here qualify as being ancient because they are more than a hundred years old (see chapter 1 for a full discussion on tea’s origins)
Whereas China primarily grows Camellia sinensis var. sinensis (the China bush), which is kept pruned and is picked seasonally, the tea plants that grow in subtropical Xishuangbanna are Camellia sinensis var. assamica (the Assam bush) or minor subvarieties indigenous to the region, known locally as Yunnan Big Leaf, or dayeh. These plant varieties grow quite large and are picked continually, allowing rest periods only for the plants to produce and grow new leaf. In a few growing areas, bushes are semicultivated, but because these tea plants are more treelike than bushlike, for the most part they are found growing randomly and sporadically throughout the hillsides and mountain slopes. Gathering their leaf is thus difficult and time-consuming, and transport is complicated.
Ancient tea trees in Xishuangbanna (Yunnan Province, China).
SHENG PU-ERH MANUFACTURE
Classical, traditionally made Sheng pu-erh is manufactured as follows: Initially, the plucked large leaves are processed by standard tea methodology—following a quick sort and cleaning (primary-leaf pu-erh is easy to sort because of the large leaf size), the leaf is left to air-dry (traditionally in the sun) for a brief period. Then it is fired quickly, to remove the excess surface moisture that large tea leaves often carry. This is a critical step in the processing, because for successful fermentation, the leaf must retain a different moisture content depending on which type of pu-erh it is to become.
Now the decision must be made as to whether the leaf will become a raw or a cooked pu-erh. If it is to become “raw” pu-erh, it is fired almost completely, to stop all enzymatic processes and prevent oxidation, but not so much as to eliminate all internal moisture, which would inhibit internal fermentation. Similar to the phenomenon that occurs when using heirloom apple varieties for cider production (and because the big leaf for making pu-erh tea is harvested from Yunnan’s large, older, dayeh tea plants), the leaf has plenty of natural bacterium present on its surface to begin the chemical transformation that is necessary for pu-erh to develop.
The leaf is now gathered, usually on the floor (as dhool is for black tea oxidation). Careful supervision is required during the next phase, while the leaf is turned regularly and allowed to ferment. The practical differences between this fermentation stage and the oxidation phase of black tea manufacture are that these piles are much greater in size, the leaf is much drier, and higher heat is generated. So while the exposed portion of the pile permits oxygen absorption as a catalyst for surface oxidation, the core of the pile is robbed of oxygen but becomes very hot for short periods, encouraging the true microbial activity of fermentation.
For the leaf to ferment evenly, the pile must be turned at regular intervals, so that all the leaf has a
n opportunity to be at the core of the pile—encouraging the crucial microbial activity. In this way the heat generated by the fermentation process can be dispersed and the leaf remains raw. Because the leaf has been dried so much more than dhool, the level of oxidation that occurs is limited. The leaf at this stage is called Mao Cha.
All pu-erh beeng cha are individually wrapped for market (Yunnan Province, China).
FROM MAO CHA TO SHENG PU-ERH
At this point in the manufacture of raw pu-erh, the Mao Cha is packed into either the traditional or the modern compressed shapes. The goal in making raw pu-erh is to cause the leaf to begin to ferment internally, to reduce the moisture content to a very low level, and to limit exposure to fresh oxygen. The Mao Cha is then packed into a compressed form, where it remains, undergoing postfermentation (aging) for the next ten years. It will continue its gradual development as long as conditions are correct for the chemical process to continue.
Years ago, postfermentation was accomplished in caves, each cave having a unique environment that individualized the production of the particular pu-erh artisan. Today, however, the proper conditions are climate-controlled storage facilities where the humidity is maintained at less than 80 percent, and calm but good air circulation must be maintained. The discs are paper-wrapped to allow the tea to breathe, to mark the place and date of manufacture, and to record important storage information (the future provenance). After some time (about six years) the compressed Mao Cha’s changed balance of bacteria and moisture slows the postfermentation process. From this point on the beeng cha will simply age and become a dry storage naturally aged Sheng pu-erh, ready to be tasted.
The Story of Tea Page 12
