Tea Manufacturing

Black tea manufacturing technology essentially involves disruption of the cellular integrity of tea shoots, thereby enabling the mixing up of substrates (polyphenols) and the enzymes (polyphenol oxidases). This results in the initiation of a series of biochemical and chemical reactions with the uptake of atmospheric oxygen and formation of oxidized polyphenolic compounds that are characteristic of tea along with volatile flavor compounds that impart characteristic aroma to tea.

Tea manufacturing is normally carried out in two ways, (i) CTC and (ii) orthodox. CTC refers to the Crush, Tear & Curl process where the withered green leaves are passed in-between two rollers rotating in opposite directions. There is complete maceration of the leaves and the resulting powdery material is referred to as “cut dhool“. Enzymatic action is higher in the CTC type of manufacture. In orthodox type of manufacture, the withered leaves are rolled on specially designed orthodox rollers which twist and crush the leaves thereby rupturing the cells. The maceration is less as against CTC processing. But this process results in teas with good flavor and aroma. Steps in CTC tea manufacture includes, withering of harvested crop, green leaf shifting, reconditioning, rolling, fermentation, drying, grading & sorting and packing.

WITHERING is the first and fore most steps involved in tea manufacture. The evaporation of moisture in the green leaf is brought about by blowing or moving air over the leaf in the withering trough. The current of air performs a two functions viz., conveying heat from the leaf as well as carrying away the water vapor through a bed of green leaves to achieve physical withering. Whenever the hygrometric difference is below 3°C, hot air is mixed in suitable proportion or heat energy is supplied to increase the hygrometric difference with the concomitant rise in the dry bulb temperature of air. But the dry bulb temperature of air after mixing should not exceed 35°C.

Currently in most of the south India tea factories trough withering is practiced. The dimensions of the trough in most of the factories vary considerably. The width of the standard (conventional) trough is 6′ and its length varies between 60′ and 120′. However, nowadays wider troughs with two axial fans are preferred. Sizes vary between 12′ and 15′ of width and 60′ and 120′ of length. Reasonably even – wither is achieved in wider troughs. Conservation of electrical energy is also possible by switching “off” one of the fans after the required degree of physical wither is achieved. The other fan supplies air just to control any heat developed.

So, in a well designed, balanced factory, an optimum load of 30kg per meter square for a peak crop anticipated in a single day has been the basis for the design of trough capacity. The good essence of withering is well ventilated withering lofts and access of drawing large volumes of air by the trough fans. The ideal qualities of air required for withering are low dry bulb temperatures and high hygrometric differences with ample supply. The upward passage of air through the bed of leaves usually results in the bottom of the bed being withered first and the upper leaves last. To achieve a more even wither turning over the leaf carefully once or twice is suggested. However, turning over may be practically difficult in wider open troughs. To achieve a more even wither turning over, reversible air flow systems have been practiced.

Many of these difficulties have been overcome by using enclosed troughs. In this system, the direction of air flow can be changed by a movable baffle plate either below or above the trough. Air space above the trough is totally enclosed. There are two sets of exit doors for the spent air; one set above the level of leaf and the other below. The system makes it possible to maintain fan efficiency and dispenses with the need for reversible fans. Air heating system is also available to warm the air irrespective of the direction of air flow through the system. The other advantage is that a more uniform wither is achieved without the necessity of turning over and thereby the withered leaf is intact.

It is important that pressure inside the plenum chamber should be constant throughout the length to have an uniform air flow rate. However, in the conventional troughs the pressure varies over the length for constant thick spreading. A tapering cross section decreases the area towards the end and equalizes the pressure inside the plenum chamber. The method commonly employed to heat the air for withering are as follows: 1) direct use of hot air from the drier when it is empty, 2) hot air ducting to each trough with damper control from a separate heater, 3) hot water or steam based insitu radiators in each withering trough and 4) using exhaust air from the drier.

During normal conditions, it is always preferable to use ambient air as long as it has drying properties. If at all hot air is required, it would be ideal to use it during the early part of withering when the leaf is still turgid. Warm air should not be used to wither during later stages, since the leaf takes a temperature nearing the dry bulb temperature.

It would be better and more accurate to express the status of wither in terms of percentage of moisture content in the withered leaf. If the percentage of moisture content at the end of withering is the same irrespective of moisture content of the green leaf, then by and large the physical condition will remain the same. Withering to constant moisture content every day is impractical even with the greatest care. The following two conditions are essential for good withering: storage of fresh leaf for a minimum period of nine hours is absolutely essential to allow chemical changes to take place whether a physical wither is desired or not, to make a product with required characteristics, this is referred to as chemical wither. Physical wither is necessary for good fermentation.

Green Leaf Sifting Extraneous matter such as stones, sand or metal pieces may find their way in the leaves brought into the factory; if such materials are fed into the fine-tuned, continuous machines, the moving parts will be severely damaged. Similarly if the leaves were not fed evenly into these machines, they could become jammed or would not function efficiently. Hence green leaf sifting is essential prior to processing. The green leaf sifter is essentially a device for introducing a continuous even flow of withered leaf to the CTC processing section. It is a vibrating tray, which is perforated with holes or is of a wire mesh. Powerful magnets have been provided in the green leaf sifter to remove any iron pieces present along with the leaf.

Reconditioning: In South India, secondary grades and other residues which are obtained while cleaning the primary grades are ground and recycled with the withered leaf. This process is known as reconditioning. The primary objective of this practice is to produce grainy grades as well as tea of high density. It also helps to minimize or eliminate secondary grade teas. The quantity of recycled material, known as recondition dust, varies from factory to factory, it depends on the quality of green leaf, the moisture content of the withered leaf and the standard of machinery available. The percentage of RC is mostly expressed on the weight of green leaf or made tea basis. However, there is a wide variation in the moisture content of green leaf and thereby the quantity of made tea produced. So quantifying the amount of RC material for the made tea to be produced depending upon the green leaf conditions is difficult. The best practice is that the amount of RC should be calculated on the withered leaf weight basis. However, as already mentioned variation in the moisture content of the withered leaf should be controlled to a narrow range say 2 to 3%.

Leaf Conditioning: The leaf shredder and rotor vane combination has been found to be ideal to pre-condition the leaf for CTC processing. The output of both these machines should match with the CTC as well as drier capacities. Shredding of withered leaf into fine particles increases the rotorvane capacity and helps efficient mixing of the leaf with reconditioned dust in the rotorvane. The macerated leaf from the shredder has to be in the form of ‘chutney’. To ensure this the shredder blades have to be changed every week. It is important that the weight of each blade should be less than one Kg. and properly balanced on the knife edge. Reduction of gap between two blades progressively helps to achieve better results.

Rotorvane is essentially a large mincing machine and is imperative to precondition the leaf suitable to be fed into the CTC machine as well as for better fusion of RC material with withered leaf. There are two sizes, one with cylinder of 20.3 cm (8″) diameter and the other large machine with a cylinder dia of 38.1 cm (15″). The leaf is processed in this cylinder in which a rotor provided with vanes rotates between resistors thereby propelling the leaf forward and discharging through an end plate. The leaf is distorted and shredded as it moves along the cylinder and cut into small pieces by the revolving cutter through which it must pass before it can leave through the apertures of an iris diaphragm. For good results the rotorvane should crush the leaf along with the RC dust at the maximum possible pressure. The 8″ rotorvane exerts a much higher pressure on the leaf than does the larger machine, consequently the leaf is much more damaged when passing through it. In the larger rotorvane, a cone end plate is attached at the discharge end to increase the pressure; the leaf is discharged between the gap of the cone and the cylinder; the inner of the cylinder and cone are provided with battens in order to increase the efficiency of crushing.

Rolling After preconditioning, the leaf is passed through four or five CTC machines arranged in tandem. The CTC machine essentially consists of two contra-rotating toothed rollers of equal diameters (20.3 cm or 8″). Depending upon the processing capacity required, rollers with different width are used i.e., 61 cm (24″), 76.2 cm (30″), 91.4 cm (36″). The two rollers rotate at different speeds. A slow speed roller; high speed roller ratio of 1:10 with speeds between 70:700 rpm and 100:1000 rpm have good effect. The slow speed roller acts initially as a conveyor apart from providing a surface for cutting. In order to derive the maximum benefit of a good cut, the drop point should be adjusted behind the crown of the slow speed roller, so that the leaf is conveyed into the cutting area. Otherwise, a portion of the leaf gets thrown over the high speed roller, thereby, losing the benefit of cut.

A number of hollow segments of 2″ width are mounted side by side on a mandrel to form a roller. Even spaced, helical grooves are formed along the circumference by a standard angular milling cutter. The teeth are formed by cutting circumferential grooves on the roller which has the helical grooves. Each tooth has two longitudinal characteristics, the shoulder and the back slope. The ratio of the length of the shoulder to the back slope projection is known as the profile or style ratio which influences quality. As a general rule, a style ratio of 5:3 will produce a grainy tea with higher dust percentage.

Precision in sharpening and machining the CTC roller surfaces are the keys to good CTC manufacture. Quality CTC teas cannot be made if roller teeth are worn out or damaged. It is, therefore, imperative that sharpening of segments is done precisely and on schedule. The speed of the high speed and low speed roller in conventional CTC roller will be 700 to 750 and 70 to 75 RPM, respectively. For Senova (13″ dia) roller, the speed will be 560 to 600 and 56 to 60 RPM. The deviation in the speed of a few rollers will result in erratic High Speed Roller (HSR), Low Speed Roller (LSR) ratio. The linear speed difference between the rollers should be checked periodically to enhance the appearance of made tea and to improve the recovery percentage. Difference in the diameter of rollers leads to different speed in rollers. The pulley size also influences the speed. To achieve 10:1 ratio, proper matching of equal diameter rollers is essential.

Fermentation It is the practice in south Indian CTC factories to pass the CTC, ‘dhool’ through a large revolving drum for 60-90 minutes with conditioned air. Rotation of the fermentation drum facilitates granulation of the tea particles and increases the bulk density which is desirable for south Indian CTC teas. In drum fermentation, the whole process is dynamic and the leaves are constantly rotating. Every bit of tea that is being fermented is constantly layered and exposed to the fresh air or conditioned air. Rubbing of leaf against leaf takes place and the juices present in the micro cells of leaf are evenly coated on the exterior of the tea leaf. Drum fermentation produces blacker teas as compared to floor fermentation. These teas are usually brisker due to better aeration.

Since most of the biochemical reactions occurring during fermentation are oxidative in nature, mass transfer of oxygen to the tea particle is a critical parameter in the design of any fermentor. The fermenting drums are equipped with spiral flights on the interior for lifting and showering the solids through the air stream and to accelerate the forward flow in the drum. Good ventilation and access to fresh air are paramount to proper fermentation. In most of the factories, the air required for fermentation is drawn from the rolling room. This results in recycling of spent air for fermentation which is not advisable. It is suggested to have a co-current air flow arrangement in the fermenting drum; the blower type is preferable to the suction type of fan. Co-current air is required to supply more oxygen during the initial stages of enzymic fermentation when it is needed at maximum levels. This air should be fresh, cool and saturated with moisture.

Another new and promising aid to fermentation is ultraviolet radiation. Ultraviolet rays have two functions viz., (1) it kills external bacteria and other micro organisms and (2) it triggers the activity of polyphenol oxidase and thereby hastens the biochemical reactions. Bright infusions are obtained by passing conveyor racks which contain fermented dhool through a UV chamber. It is practically impossible to fix UV bulbs inside the fermenting drum. However, UV lamps could be easily installed in continuous fermenting machines (CFM). CFM gives closer control of the entire fermentation process resulting in tea with improved quality. Leaf spread and run through time are infinitely variable within a selected range to ensure optimum fermentation under different climatic conditions. A post fermentation ball breaker is essential. While ensuring minimum ball formation will also increase in the percentage of dust grades.

Drying is the most expensive process in the manufacture of tea. The capital investment on the driers is also the highest among the different processing machines. Objective of drying are to arrest the fermentation process and to remove the moisture and to produce tea with good keeping qualities.

Before going to the different types of drying systems it is essential to know the basis of drying. Drying a solid matter indicates removal of water from the solid materials by evaporation. During the early stages of drying, the solid is so wet that a continuous film of water exists over the entire surface. The water removed during in this period is mainly superficial water. During this period the rate of drying under a given set of air conditions is constant and independent of the moisture content. This period is known as the constant rate of drying and the temperature of the solid during this period approaches the wet bulb temperature of the air. The magnitude of the constant rate depends on the area exposed to the drying medium, the difference in temperature between the gas stream and the wet surface of the solid and the air velocity.

Conventional Drying: The principle involved in the conventional driers is that fermented leaf is subjected to a blast of hot air in such a manner that the hottest air first comes in contact with the tea having the least moisture content. In these driers, the fermented leaf falls on a series of moving perforated trays on which it is passed and repassed through a moving stream of hot air.

The perforated trays are mounted on an endless chain and arranged in a tier of six or eight units which alternate in the direction of motion. The Design is such that at each stage of the drying operation, the leaf is subjected to a different temperature. As the leaf passes from tray to tray, it progressively comes into contact with higher temperatures. When the air takes up moisture, the dry bulb temperature falls. A final moisture content of between 2.5 and 3.0% should be the aim. If the tea is dried below 1.0%, it loses some quality. Tea dried to 3.5% moisture content and above does not keep well.

The optimal inlet temperature for CTC processed leaf is 100 ± 5°C. The exhaust temperature should be maintained at 54.4 ± 2.7°C (130 ± 5°F). If the exhaust temperature is less than 49°C (120°F), the post fermentation process will continue for a considerable time and will soften the liquor. This condition is referred to as “stewing”. If the exhaust temperature is greater than 57.2°C (135°F) the rate of moisture removal is too rapid and results in case hardened tea in which the particles are hard on the outside but incompletely dried within; such teas yield harsh liquors and do not keep well. So it is of paramount importance to ensure that temperatures are kept under control to the extent possible.

Fluidized Bed Drying: Tea industry presently enjoys a variety of fluidized bed drying equipments like vibrobed, five zones and three zones cross flow fluid bed driers. All of them strive to get increased fuel economy without affecting quality.

When a fluid flows upwards through a bed of granular particles, the pressure drop is initially proportional to the rate of flow: At a certain increased air velocity, the frictional drag on the particles becomes equivalent to the apparent weight and the bed begins to expand. This stage is known as the onset of fluidization or incipient fluidization. Further increase in velocity causes the individual particles to separate from one another and float. Under these conditions the system is said to be fluidized. In fact the relative movements of the individual particles in the air stream acquire many properties of liquid and have analogous flow characteristics. Hence the term ‘fluidized bed’.

One of the virtues of fluidized systems is that they have high rates of heat and mass transfer while maintaining uniform temperature characteristics on the bed. Consequently conditions such as case-hardening are seldom encountered with fluidized systems. Good thermal contact between the tea particles and the drying medium results in improved fuel efficiency. Particle to particle attraction in a fluidized drier is minimized because each particle is surrounded by its own fluid cushion. In practice, too, this expectation is realized by the production of blacker teas with better appearance and bloom.

The fluid bed drier essentially consists of a drying chamber, plenum chamber, dust collectors and flow control dampers. The drying chamber normally consists of three drying zones and one cooling zone. Fermented leaf is loaded on a grid plate of the drying chamber. The top of the drying chamber is totally closed and two sets of centrifugal exhaust fans are provided with cyclones; one for refiring and the other for dust extraction. Beneath the drying chamber is a plenum chamber where the air pressure is equalized. The direction of the hot air entering into a grid plate is controlled by the flow control dampers which can be operated independently. The flow control dampers have dual purposes – during the operation their direction determines the residence time of tea particles in the drier and at the end of manufacture, they serve to evacuate the drier completely. In each zone, the required volume and pressure of air is maintained by independent air valves. In some commercial driers, a blow-hole suppressor is provided in the drying chamber to facilitate easy cleaning of the grid plate.

When the fermented leaf enters the drying chamber, it has very high moisture content which is rapidly reduced in the first zone. At this point, maximum volume of air is introduced since rapid evaporation is required. As the moisture loss takes place, density of the material is reduced. This material tends to move away from the feed end as it is being displaced by fresh materials which contain more moisture and hence have high density. The movement of the tea particles within the drying chamber is governed by the principle of displacement. When the material is fully dried, it is expelled into a cooling chamber wherein ambient air is introduced by a forced draft fan.

The desirable inlet temperature ranges from 140° to 150°C. Firing at this temperature resulted in improved leaf appearance and better bloom. The exhaust temperature has to be maintained at 71.1°C(160°F) to 76.7°C(170°F) in the third section. In some driers, exhaust temperature is measured at the centre of the drying zone along the length, and kept at 57.2 ±2.8°C(135± 5°F).

Air Heater: Air heater basically exchanges the heat, released from the combustion of fuels, indirectly to raise the temperature of ambient air for drying purposes. Tea drying is a high thermal energy consuming operation. Hence, it is essential to know the basis of combustion for the efficient operation of heater or stove. Air heaters commonly used in South India are of two types. In the first type, the hot flue gas from the combustion chamber passes through the tubes of a heat exchanger. In the other type, it flows outside the multitubular heat exchanger. The former is most common and suitable for fluidized bed driers. The selection of the stove should be based on the compatibility with the drier in regard to heat requirement as decided by the fan characteristics such as air volume and total pressure. Any under rating of the air heater implies burning more fuel than the stipulated quantity and results in higher flue temperatures. The efficiency of the heater is mainly determined by the heat transfer area, insulation, type of fuel used, combustion control and design of the furnace itself.

Grading And Sorting Sorting is the operation in which tea particles of the bulk are separated into various grades of different sizes and forms confirming to trade requirements. In other words, it basically converts the bulk into finished products. The process of sorting has two objectives (i) to enhance the value (ii) to impart quality. Grading of the manufactured bulk is therefore, undertaken to improve its marketability and to obtain the premium that different buyers are willing to pay for the size of their preference. Cleaning of fiber is also part of the sorting procedure which is directly related to value enhancement.

Sorting enhances the appearance and quality of liquor; at the same time it can also deteriorate the quality. The presence of fibre or flakes of coarse leaf in a primary grade causes harshness and their removal makes the liquor mellow. The cleaning of fibre also improves the black appearance of tea which is desirable. Bloom is indicative of liquor character; over sorting and over cleaning can result in loss of bloom. Usually a tea which has not been well fired, loses bloom more quickly. If tea absorbs moisture during the cleaning process, liquors can deteriorate and its keeping quality reduces. Sorting of bulk has to be done in three stages, viz., cleaning of fibre, grading and winnowing.

Currently, PVC rollers are being widely used to remove the fibres as well as flaky teas from the rest of the bulk. The principle involved here is that PVC rollers are (static) electrically charged by the contact of a sponge like material known as felt. Fibre and flaky teas differ in many characters like moisture content and density from the rest of the tea. These electrically charged rollers preferentially attract the fibre and flaky teas which are higher in moisture content and thereby, they are removed from the bulk. If teas are exposed for longer time in the humid conditions, the difference of moisture content between fibre and rest of bulk narrows down; this reduces the efficiency of the removal from the bulk.