Withering

Throughout this first stage of manufacturing the harvested teas are handled in a manner to prevent bruising and to promote dissipation of heat generated during continued respiration. The moisture content is reduced from 75% to 55% (approx.) by a flow of cool air. This process is carried out for a period of 6 to 18 hours depending on the factory equipment and weather conditions. During withering significant chemical changes occur. The cell membrane permeability is increased which allows for disruption of cell structure. Amino acid, caffeine and organic acid levels are increased and polyphenolase activity is also increased.

For withering (wilting), the shoots are spread out evenly on trays, nylon nets, fine meshed screen trays, racks or shelves. Withering may be conducted in open sheds by utilizing the effects of natural breezes and air currents to wilt the leaves or, in special facilities with controlled heating and ventilating equipment. Regardless of the process used, the leaves are withered so that moisture content is reduced from 70-80% to 50% and leaf conditions become limp and flaccid, suitable for rolling. Normally withering is carried out by spreading the leaves thinly on banks of trays or "tats" made of tightly stretched jute hessian or wire netting. The tats are kept 12 to 15 centimeters apart, to allow of free access of air.

There are two types of withering except by thickness of spread, and the length of time of the withering phase. This constitutes "natural withering" in its simplest form.

In closed lofts natural withering can be carried out by opening the windows and in addition air movement can be provided by means of fans.

The degree of withering depends upon the type of tea to be produced and the method of manufacture. The degree is derived from the weight of the withered leaves expressed as a percentage of the weight of fresh leaf. Light withers (40-42 percent out turn) are usual in Assam and East Africa, and medium to hard withering (45-50 percent out turn) are the norm in Ceylon. In through withering , air is forced through  a thick layer of leaf on a mesh in a through. Mechanized system greatly reduced withering time, but they also lower the quality of final product by reducing the time for chemical withering, during which proteins and carbohydrates break down into simpler amino acids and sugars, and the concentration of caffeine and ployphenols increases.

Rolling

The aim of rolling is to establish proper conditions for an enzymic oxidation for an enzymic oxidation of catechin polyphenols (flavanols) by atmospheric oxygen. Rolling twists the leaf, break the cells structure and expresses the juices (catechins and enzymes). The result is that the catechins and enzymes, originally separated in the leaf, are mixed. Both are exposed to the atmosphere and the enzymic oxidation of the catechin plyphenols begins.

Methods

This is achieved by a mechanical process such as rolling (the orthodox method) or crushing tearing or curling with contrarotating rollers (the CTC method). The machines used vary in size and design but their principles are alike, they compress turn the leaf over, keeping it in continual motion.


The traditional method is to roll bunches of leaves between the hands, or by hands on a table, until the leaf is twisted, evenly coated with juices, and finally broken into pieces.

The Leg cutter (actually a tobacco-cutting machine), the leaf is forced through an aperture and cut into strips.

The crushing tearing and curling (CTC) machine consists of two serrated metal rollers, placed close together and revolving at unequal speeds, which cut, tear, and twist the leaf.

The Rotorvane consists of a horizontal barrel with a feed hopper at once end and a perforated plate at the other. Forced through the 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 is cut at the end plate.

This nontraditional distorting machines can burst leaf cells so thoroughly that in many cases the withering stage is unnecessary. However unlike traditional rolling they do not produce the larger leafy grades of tea.
CTC manufacture delivers three grades - Broken Pekoe (BP), Pekoe Fanning (PF) and Pekoe Dust (PD).

Roll-breaking and Sifting

On discharge from the roller the leaf mass is more or less compressed into lumps. These are broken up in the sifting process by the machine which usually combines the operation of roll-breaker and sifter perform three essential functions. In the first instance it cools the leaf, secondly it aerates the mass, and thirdly by sieving out particles of small size it separates leaf into portions that will be reasonably uniform in their rates of fermentation.

Oxidation (Fermentation)

During this process the green leaf is converted to black tea. History Although this is refereed to as fermentation it became around 1901 as an oxidation process initiated by the tea enzymes.

The Process

Oxidation is the most important stage in the  manufacture of black tea , and this process makes it uniquely different from all other teas. The oxidation is carried out in a custom designed fermentation room maintained at high humidity at the lowest feasible temperature. The shifted leaves are spread out in thin layers on tables, perforated aluminum or racked trays, in order to continue the oxidative process. The thickness of spread controls three important factor namely the temperature of the leaf , the rate of moisture loss and the access of oxygen. Some factories install air conditioners to ensure controlled conditions. Depending on the ambient temperature, maceration technique and the style of tea desired the oxidation time range from 45 minutes to 3 hours. This time can be halved if the leaves are broken by a legg cutter, CTC machine, and Rotorvane. Temperature of the leaf is maintained around 24-30^oC. However, lower temperatures have been shown to improve flavor characteristics coppery color and fermented tea aroma. When fermentation is judged to be sufficient the leaves are transferred to the drier.

Chemistry

A large number of chemical changes take place during the fermentation process. Briefly it can be described as the changes that take place to the polyphenols due to mixing with oxidative enzymes (polyphenol oxidizes) liberated by rolling. The polyphenols are converted by the enzyme-controlled oxidation, into the flavins and the arubigins.
 
Firing

At this stage of processing, oxidation enzymes are deactivated by heat, Traditionally, fermented leaf was dried on large pans or screens over fire but since the 19 Th. century, heated forced air has been used. Firing also reduces the moisture level in the leaves  to 2-3%. The process is critical as incomplete inactivation can cause accelerated deterioration during storage. A mechanized drier consists of a large chamber into the bottom of which hot air is blown as the leaf is fed from the top on a series of descending conveyors, The dried leaf is then cooled quickly to prevent over dying and loss of quality. The fermented trays goes through the dryers counter current to the air flow. Incoming air temperatures is 900^oC and emergent air is approximately 500^oC. Time temperature profiles are controlled for effective drying in 18-20 minutes. Firing plays an important role in determining the quality of the tea.

The hot-feed drier and the fluid-bed drier

Modern innovations on the drier are the hot-feed drier, where hot air is supplied separately to the feeder to arrest fermentation immediately as the leaf is fed, and the fluid-bed drier, where the leaf moves from one end of the chamber to the other over a perforated plate in a liquid fashion. After the firing the tea is spread out to cool and then temporarily stored to await sorting perforated plate in a liquid fashion. After the firing the tea is spread out to cool and then temporarily stored to await sorting.

Grading and Sorting

Sorting, or Grading is the final stage in the tea process. Here leaves are shifted into different sizes, then classified according to appearance and type. When it comes to blending each packet must contain particles of similar size, to prevent larger pieces from sinking to the pack bottom and spoiling the balance of the blend. The teas should be sorted daily and not left exposed to the air.
The dried teas are sorted into different grades by passing it over mechanically oscillated sieves, similar to those used in the green stage, and fitted with meshes of different sizes. Electrostatically charged preferentially attract and remove stalks and fibre. The passage of teas through this system gives a number of grades with a more or less evenly sized particles. These grades fall into four main groups : leaf, brokens, fannings and dust in descending order of particles size. They are traded under a wide variety of traditional names: