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Tea is the second most consumed beverage in the world next to water. Tea a popular beverage

made from the leaves of evergreen shrub or tree Camellia sinensis, family Theaceae. Tea

processing is the method in which the leaves and flushes from Camellia sinensis are transformed

into the dried leaves for brewing tea. Tea leaves are processed in the tea factories to produce an

acceptable product for human consumption. There are various kinds of, such as, Black tea, Green

tea, Oolong tea, flavored tea etc. in the world tea market. All are processed in different method

and style. The types of tea are distinguished by the processing they undergo. In its most general

form, tea processing involves oxidizing the leaves, stopping the oxidation, forming the tea and

drying it. Of these steps, the degree of oxidation plays a significant role of determining the final

flavour of the tea, with curing and leaf breakage contributing to flavour by a lesser amount.

What is Tea?

Tea is a cross-pollinated plant. Tea plant is a small tree or shrub which may grow to a height of

about 9m. in nature. The botanical name is Camellia sinensis. Leaves are alternate, elliptical,

lanceolate or ovate, leathery and smooth, 1.5-2.1 cm. in length. Young leaves are more or less

pubescent with point buds. Flower buds originate singly or in cluster from leaf axils. Flower

white and fragrant with 5-7 leathery permanent sepals forming a ring, 5-7 white obviate,

emarginated, internally concave Patel. Stamen is numerous, long, with yellow 2-celled anther.

Manufacture of Tea

Tea manufacture is the process of converting young fresh tea shoots into dry black tea. This

involves a number of processes from plucking to packing. At the plucking stage, only the top leaf

tips are picked every 6 to 7 days. The tip leaves are younger and finer which produce a better

quality tea. The fresh green leaves now need to have the moisture removed from them. This is

done by blowing air through the leaves for up to 14 hours, leaving a soft and pliable leaf. There

are then two ways of treating the tea. Tea which is to be used as loose leaf, will normally be

rolled gently to create a twisted appearance.

In contrast, tea which is to be used for tea bags, is shredded and crushed to produce a small

granular product. Rolling and crushing the leaves, results in the rupturing of the leaf cells which

allows oxidation to occur. This gives the tea its distinctive black colour and flavour. The tea is

then dried at high temperatures to achieve the correct taste. When it has been dried, the leaf tea is

of differing sizes and will also contain pieces of fibre and stalk. At this point it is processed to

remove pieces of stalk which will then leave tea suitable to be sold as loose tea. The tea is passed

through varying sizes of meshes to sort it and has to be passed through very fine ones in order to

produce tea fine enough for tea bag production. This process of sorting is a harsh one and it can

cause the tea to lose some of its flavour. That is why loose tea usually has a better flavour than

the tea in a tea bag.

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http://en.wikipedia.org/wiki/Camellia_sinensis

http://en.wikipedia.org/wiki/Tea





Fig: Manufacturing process of different types of tea

Types of Tea

One of the key steps in the tea manufacturing process, that is a factor in determining the type of

tea that is produced, is the degree of fermentation the tea leaves are allowed to undergo. One

method of classifying teas are is based on the degree of fermentation:

(a) Non-fermented and Very Light Fermentation: These teas retain quite a bit of their

original flavor. Green teas fall in this category. White teas undergo very light fermentation

during the withering process. Sometimes these non-fermented and very light fermented teas will

be scented with Jasmine petals to give the tea an aroma of Jasmine. Examples of Non-fermented

and very light fermented teas: Green Tea, Jasmine scented Green tea, Yellow Tea, White Tea.

(b) Semi-fermented: Tea which are allowed to undergo 10% to 80% fermentation fall into the

broad category of semi-fermented teas. Tea brewed from semi-fermented tea leaves have a slightyellow to brown hue and possess a subtle fragrant aroma. These teas can be further classified

into three categories based on their levels of fermentation:

Light (10% - 20%): Jasmine Tea

Medium (20% - 50%): Oolong Tea.

Heavy (50% - 80%): Champagne Oolong.

(c) Fully-fermented: Black teas are fully fermented. Tea from Black tea leaves have a dark red

hue and a sweet aroma of malt sugar. Example: Black Tea.

(d) Post-fermented: Teas which are allowed to ferment and then have the processed stopped and

later fermented again are known as post-fermented tea. Example: Pu-Erh Tea.

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Black Tea

Black Tea is the most common form of tea. Black tea is a dark tea prepared from fresh tea

leaves. Leaves of black tea are more heavily oxidized. Black tea is fully fermented before being

dried. Black tea contains caffeine, which makes its flavor stronger. One of the important facts

about black tea is that it preserves its flavor for several years and other teas usually loose flavor

within 8 to 9 months. About 80 to 90 percent of people around the world drink black tea.

Types of black tea

Orthodox tea

CTC tea

Orthodox tea is rolled slowly using traditional rolling tables that mimic the action of hand

rolling. This unhurried disruption of the leaf cells causes a slow oxidation of the catechins, due to

the realease of a natural leaf enzyme that causes the green catechins to change to orange red

polyphenols. Slow oxidation favours aroma and flavour at the expense of intense colour.

Production of CTC teas: CTC is an acronym for crush, tear and curl. CTC teas are highly popular

on the Indian subcontinent for they produce the strongest brew that is consumed with tea in the

traditional method of brewing tea. In the CTC production process, the tea is passed through

mechanical rollers that crush and tear the leaves and finally roll them into little balls. The cell sap

collected in the process is added back to the teas to give them additional flavor.

Table 01: Chemical composition of black tea Composition (70% Methanol Extract) of Black

Teas by Manufacturing Method and Size Fraction

Manufacturing method grade % caffeine % catechins

orthodox GFBOP 2.00 1.44

orthodox BOPF 2.00 1.01

orthodox PD 2.04 1.06

CTC BP 2.12 0.71

CTC PF 2.20 0.43

CTC PD 2.17 0.48

Specification for Black tea – ISO standard 3720

ISO defines black tea as “Tea derived solely & exclusively & produced by acceptable processes,

notably fermentation & drying from the leaves, buds & tender stems of varieties of the species

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Camellia sinensis”. The tea shall be clean & reasonably free from extratenious matters & that

should conply with the requirements specified in the table below. Tea exporting countries also

have their own standard specification for tea which in most cases conform with the ISO standard.

Table 02: Chemical requirements for black tea

Characteristics ISO requirement In Bangladesh

Water extract, % (m/m) Minimum 32 32

Total ash, % (m/m) Maximum 8 8

Minimum 4 4

Water soluble ash, as % of

total ash

Minimum 45 42

Alkalinity of water solubleash(as KOH), % (m/m)

Minimum 1 1

Maximum 3 3

Acid insoluble ash, %(m/m)

Maximum 1 1

Crude fibers, % (m/m) Maximum 16.5 16.5

Caffeine, % (m/m) Minimum - 2

Tannin, % (m/m) Minimum - 10

Quality analysis of green leaf

The commercial part of plant comprises bud, first, second and the third leaves and the stalk. It

may also contain single or double banjhi (a dormant terminal shoot). The banjhi is smaller than

normal growing bud. if a shoot of two leaves and a banjhi bud are plucked, the leaves usually

become coarser than the normal shoot of the leaves and a bud. In fine or medium fine plucking,

the banjhi shoot is plucked with one leaf only,

If it is soft. Otherwise, it is thrown away if it is double banjhi shoot, consisting of a banjhi and

two leaves. For the quality analysis of green leaves, the plucked leaves are classified as follows-

One leaf and a bud.

Two leaves and a bud.

Small three leaves and a bud.

Single soft banjhi.

Hard and double banjhi.In our country two leaves and a bud is considered as the best shoot quality that gives better

quality of tea.

The quality of green plucked leaves can be determined by two methods as follows-

Ballometer method

Leaf count

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Ballometer count

In this method, the percentage of hard or soft part of the plucked leaves is determined. If the

plucked leaves contain 75% or more soft leaves, it is usually considered as fine quality raw

materials. At first 100 coins or ball bearings of equal weight are taken for weighing the leaves of

the representative samples. It means the weight of each representative sample is equal to the

weight of ball bearings. After that the leaves are broken down by hand into hard and soft leaves.

Then the soft leaves are separated from the sample. Finally the soft leaves are weighted with the

100 ball bearings. The number of ball indicates the percentage of soft leaves. For examples,

suppose the weight of the soft leaves is equal to the weight of the 63 ball bearings. Then the

percentage of the soft leaves will be 63%. Thus 10 representative samples are taken and the soft

leaves percentages are determined. At last, the average of these Percentages is accepted as the

quality of leaf.

Leaf count

In this method, 100 plucked leaves are selected randomly from the representative sample. Thenthe leaves are sorted and put the number in the following table where the leaf number represents

as the percentage of the quality of the leaves.

Types of shoots Number of shoots Percentage%

One leaf + a bub - -

Two leaves + a bud - -

Three leaves + a bud - -

Three or more leaves + bud - -

1 banjhi - -

2 banjhi

Principle stages of processing

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.

The starting material in black tea processing is the young shoot, the terminal bud and the two

adjacent leaves plucked from the tea plant. The salient feature of black tea manufacture involves

following operations, viz. plucking and leaf handling of tea shoots, withering, processing,

fermentation, drying/firing, sorting or packaging. Each stage involves characteristic changes in

the physical and biochemical composition of the leaves and the cumulative effect of these

changes are ultimately reflected in the quality of the finished product, namely the black tea. All

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these operations call for the necessity of meticulous care and technological manoeuvres to

produce a better quality of tea. After the drying is over, the leaves are sorted, that is, divided into

different grades and made ready for the market. Tea quality is assessed by the appearance of the

leaf before and after infusion with boiling and overall organoleptic impact of the resultant liquor.

The principal stage of Black tea processing following is:

Plucking & handling

The plucking of the two leaves and a bud involves a number of systems - Janam plucking, fish-

leaf plucking, step-up plucking etc. Shear plucking is done when there is a scarcity of pluckers

during July to September. Puckers' productivity is found to be maximum in unpruned teas. The

plucked leaves are processed to produce the black tea.

Withering

withering is a process to reduce the moisture in the tea leaf by up to 70%, depending on theregion in which it's been grown. The tea leaves are laid out on a wire mesh in troughs. Air is then

passed through these troughs so that the moisture is removed in a uniform way. This process

takes between 12 and 17 hours, until the leaves are limp and pliable and will roll well.

Rolling

The tea leaves are then placed into a rolling machine, which rotates horizontally on a rolling

table. This action creates the twisted wiry looking tea leaves. The importance of rolling is that it

mixes the polyphenols and enzyme (polyphenol oxidase) in presence of oxygen. When the abovethree elements are mixed then fermentation starts. Before rolling, the polyphenol oxidase is

located in the vacuole and polyphenols are present in the cytoplasm.

Fermentation

Fermentation is one of the most fundamental processes and that is one of the most recognizable

determining factor for the quality of made-tea. The stages prior to this stage merely condition the

leaves for reactions to take place during fermentation. A series of chemical reactions take place

during this process due to the severe damage to the leaf cells. Although heat, light and pH effect

the degradation of carotenoids, which is mostly influenced by oxidized flavonols formed duringfermentation. Some non-volatile compound such as theaflavins (TF) and thearubigins (TR) are

produced in fermentation stage. These compounds together impart to tea liquor and taste. In

addition to the formation of TF and TR, some other chemical changes also take place in the leaf

tissues during fermentation process. Proteins get degraded, caffeine content goes up, the

chlorophyll are transformed into pheophytins and some volatile compounds are generated due to

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transformations of certain aroma precursors present in tea leaf. These changes also contribute

towards the colour and flavour of made-tea.

The mechanical aspect involves spreading out of the leaves macerated by rolling a layer 5-8 cms

thick, for 45 minutes to 3 hours, depending on the quality of the leaves. Fermenting machines

make the process continuous, that is, every unit of macerated leaf has to be spread out for

individual treatment.

Drying

Drying is the most expensive process in the manufacture of tea. The capital investment on the

driers also the highest among the different processing machines. 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

in the period is mainly superficial water. During the 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.

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

• To enhance the value

• 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.

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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 fiber 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.

• Cleaning of fibre

• Grading

• Winnowing

Currently, PVC rollers are being widely used to remove the fibres as well as flaky teas from therest 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 are differ in many

characters like moisture content and density from the rest of the tea. These electrically charged

rollers preferentially attracts the fibre and flaky teas which are higher ion 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.

PackingTeas are packed in airtight containers in order to prevent absorption of moisture, which is one of

the main causes for loss of flavour during storage. Packing chests are usually constructed of

plywood, lined with aluminium foil and paper, and sealed with the same material. Corrugated

cardboard boxes lined with aluminum foil and paper sacks lined with plastic are also employed.

Jute bags lined with BOPP liners are extensively used for the packing of tea in the Industries.

Multi wall paper sacks are also commonly used for packing orthodox teas.

Withering

Withering is the first processing step in the factory and is a process in which freshly plucked leaf

is conditioned physically, as well as, chemically for subsequent processing stages. The

evaporation of moisture in the green leaf is brought about by blowing or moving air over it in the

withering trough. The current of air performs a twofold function: Conveying heat to the leaf as

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well as carrying of 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. Indeed, withering is one of the most important tea processing steps and can be said to

constitute the foundation for achieving quality in tea manufacture. Based on achieving the

desired level of withering, one can make better quality teas and, on neglect, can invite serious

problems in subsequent steps of manufacture. As a matter of fact, in planter's perception,

"Withering makes or mars the tea".

Objectives of withering

The process objectives to be achieved during withering are as follows:

• To breakdown complex chemical compounds in the cells to simpler compounds which

along with other simpler molecules then recombine to contribute to quality attributes of

tea like the 'body' and 'flavour' at a later stage. This is known as the Chemical Withering

of the leaf.

• To reduce the moisture content of the fresh leaf which ranges between 74 - 83%

• To make the leaf `flaccid' or `rubbery' which is essential for the subsequent step of

processing (maceration) or rather for 'twisting' or 'curling' etc.

• Both these constitute the Physical Withering of the leaf.

Chemical Wither

Chemical wither starts immediately after plucking. It is independent of the rate of loss of

moisture and is a function of time and temperature. Although the desired moisture level may be

reached in a few hours, the catabolic changes, which had been initiated at the time of plucking,

will take time. The chemical composition of the leaf will thus be unsuitable for manufacture after

the leaf has been desiccated for a few hours. It is, therefore, necessary to continue to supply

sufficient air and wait for breakdown of large organic molecules to simpler structures.

The following chemical changes occur during withering:

• The levels of soluble proteins, free amino acids & simple sugar increase.

• Level of amino acids increase as a result of breakdown of proteins by the enzyme

peptidase which is responsible for the formation of aroma of made tea.

• Caffeine content which is responsible for cup character of black tea increases.

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• Increase in the level of organic acids which are responsible for flavor & improves

polyphenol oxidase activity.

• Release of carbon dioxide and water due to break down of larger molecules.

• Changes in enzyme activity.

• Partial break down of proteins to amino acids which act as precursors for aroma.• Increase in caffeine content - this contributes towards briskness.

• Production of Volatile Flavour Components (VFC): Some of these compounds contribute

to the grassy odour and others are responsible for the flowery aroma.

• Reduction in chlorophyll content.

The above chemical changes are all intrinsic of the biochemical structure of the leaf, but the

range and the extent of the reactions depend on the jat, cultural practices and physical parameters

like temperature, humidity etc. This process normally takes about 12 - 16 hrs. and cannot be

hastened.

Physical Wither

Physical withering reduces the moisture content in the leaf and correct withering is essential for

quality, although, it has always been a difficult task to determine the end-point of withers. The

same reduction in moisture percentage and increase of flaccidity of leaf to the desired level can

be achieved in a shorter period; a longer period is necessary for chemical wither. Therefore,

physical wither is regulated at a slower rate, so as to reach the desired physical withering in the

same interval as required for the chemical wither. The objectives are achieved by passing air

through the leaves.

• Removal of certain amount of water from the plucked leaf : 10-15% moisture removal

from RC tea manufacture & 15-20% from non RC tea manufacture.

• Conditioning of leaf physically & bio-chemically for the subsequent stages of

manufacture.

• Condition of leaf from turgid stage to flaccid stage which facilitates better rolling.

Effect of withering on quality of tea

Quality of tea largely depends on the green leaf withering. Lowering of moisture to about 55

percent; increases in caffeine, soluble sugars and amino acids; changes in proportion of organic

acids and activity of leaf enzymes. If the withering percentage is low, twisting process of leaf

hampers, when rolling, breakdown of leaf is usual. As a result, amount of oxidation is reduced.

In that cases, most of the part of leaf still green before going to drier. If the withering percentage

is low, in processing when leaves twist in rotorven roller, most of the juices come out. Not only

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the juices, but also some chemical compounds also come out & that’s why the quality of tea

decreases. It is clear that the appearance & liquor is not good of tea, if the withering percentage

is low in leaf.

When processing, less withering leaves are attacked easily by bacteria. Here moisture content is

high, so fuel cost is also high & driers take extra pressure. Some problems are observed, whenwithering percentage is high. When withering percentage is high, CTC machine can not cut the

leaf properly. Sometimes machine goes to off mode because of jam. Tea liquor can be very thin

because of over withering.

Percentage of moisture in green leaf

Percentage of wither is defined as the weight to which 100 kg of leaf is reduced at the end of the

withering process. In conventional parlance, 70% withering signifies that 100 kg of fresh green

leaf has been reduced to 70 kg after withering and 30 kg of water has been removed. This

method of expressing wither is irrespective of the initial moisture content of the leaf. Thus,

depending on initial moisture content, the same 70% withered leaf, would lead to various

moisture contents. This is an anomalous situation and was introduced when it was not possible to

measure the moisture content of green and withered leaf accurately. The following table

illustrates how the variability in the moisture content of the withered leaf, depending on the

initial moisture content of the leaf, vary even though the leaf has been subjected to the same 70%

wither.

Table 03: The recommended moisture content in the withered leaf is shows as follows.

Type of Manufacture Required physical

wither (%)

Moisture (%)in

withered leaf

Orthodox 64 65

Roll- CTC 70 68

Rotorvane- CTC 75 70

LTP 80 72

Assuming 77.5% moisture in fresh leaf

Table 04: Variation in moisture content of withered leaf

Percent wither Percentwith

Percent moisture

Green leaf Withered leaf

70 82 75.3

70 80 72.6

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70 78 69.9

70 77 68.4

70 74 64.3

It is, therefore, not desirable to wither the leaf throughout the season to one and the same weight

as this may require elimination of different quantities of water in different periods. Besides,

during the withering process though there is carbohydrate degradation and consequent

respiration leading to loss of 4% soluble solids, there is some increase in caffeine and other

constituents. Thus, for practical purposes the loss is compensated. It would be better to ascertain

the degree of wither by use of the ratio of dry matter to moisture.

Recovery Percentage

The ratio of made tea to green leaf is termed as "recovery percentage" alternatively, as "out

turn". This ratio varies depending on the initial moisture content of the leaf. Leaf with moisture

content of about 83% during wet period produces 16.5 kg black tea from 100 kg of green leaf.

Similarly, leaf with moisture content of 72% during dry period produces as much as 27.5 kg of

tea from 100 kg of green leaf. Taking an average of 77-78% moisture for the whole season, 22.5

kg of made tea is expected from every 100 kg of green leaf.

Recovery is very important for the tea production. The ratio of green leaf & made tea is called

Recovery. The ratio of green leaf & made tea and multiplication with 100 is called Recovery percentage.

Recovery percentage can be expressed by the following formula

Recovery percentage = ×100

Table 05: Now we see a table to analysis of recovery importance -

year Green Leaf Made Tea Recovery

1st Year 2212389 500000 22.50%

2nd Year 2278881 480000 21.10%

After observing the, we see 2nd year, the green leaf production is higher 62492kg than 1st

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year. But it is observed made tea is higher 20000kg in 1st year. It was possible only for

the recovery percentage. So we should be careful to recovery percentage for better

amount of made tea.

Factors of Recovery

A. Factors for lower Recovery

1. High rainfall causes the lower recover percentage by increasing the moisture in

green leaf.

2. Improper green leaf weighing causes recovery problem.

3. Weighing of pot can create recovery problem.

4. Losses of leaf can cause recovery problem by transporting, arrangement of leaf

in smoke house, weighing time etc.

5. Wrong weighing in “Drier Mouth” indicate recovery problem.

6. Different processing system can give different recovery like CTC process&

Orthodox process.

7. Pruning less green leaf plucking causes lower Recovery.

8. Spoiling of made tea during machine spillage, drier tray, flask remover etc.

B. Factors for higher recovery

1. Factory should keep clean for decreasing loss of tea which increases recovery.

2. Proper sorting & stoking give better recovery.

3. It is easy to keep good recovery percentage by preventing illegal system.

Calculation of withering percentage

The formula which is used for the calculation of withering percentage is given bellow:

PW =

Or PW=100 - × 100

Here,

PW = Percentage of withering

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MG = Moisture content of green leaves

MW = Moisture content of withered leaves.

During withering about 4% solid substances can be loss. That is why the formula is like so.

Example

Before withering the moisture content of green leaf is 80% & after withering the moisture

content of that leaf is 70%. Determine the withering percentage of that leaf.

Solution

We know

Withering percentage PW = Here,

= MG= 80%

= MW= 70%

=64

So, the withering percentage of the leaf is 64%

Or

We know

Withering percentage PW= 100 - × 100

= 100 - ×100

= 100 - 33.33

= 66.67

So, the withering percentage of the leaf is 66.67%

Withering system

Withering means to lose freshness. It was believed that withering of tea leaves was only to

reduce the moisture content in order to make it sufficiently limp for succeeding leaf distortion

operation. This operation may be carried out naturally or artificially. Withering system of tea is

two types such as

• Natural Withering

• Artificial Withering

Natural withering

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Withering is carried out under natural or controlled conditions. In the process of withering, the

leaf is spread in thin layers to obtain maximum exposure. In natural withering the leaf is spread

on banks of trays or tats consisting of wire netting or hessian cloth in open house without walls.

This natural withering is entirely dependent on the hygrometric condition of the surrounding air,

& is a slow process. The leaf is spread evenly at the rate of about 0.5 kg. per square metre.

Chung Withering

In this system the leaves are spread over tats or chungs at the rate of 0.5-0.75 kg per sq. meter

and withered naturally. Tats are the inclined racks of wire nets, and chungs are the bamboo mat

floors arranged one over the other. As there is no control over the atmospheric air, this type of

withering may lead to either over-wither in a dry day or under-wither in a wet day. This system

requires more space and labor but this is still in use in those factories of Bangladesh where acute

shortage of power and crop exists. To achieve the desired degree of withering, about 16-20hours duration may be necessary. During monsoon, the quality control of withered leaf becomes

difficult. Following tests are made as a general guide……

• A handful of fresh leaves, when squeezed and emit a cracking sound or stem break down,

it indicates unwithered.

• A handful of fresh leaves, when squeezed and released, spring back, it indicates

unwithered; when it retains its shapes as a compact ball, it indicates well-withered.

• Withered leaf has a fell like old rags or silken handkerchief.

Artificial withering

Various ingenious systems viz., Tat withering, Tunnel withering, Drum withering and Trough

withering have been in use. Of these the Trough withering is the most popular system currently being used all over the world.

Trough Withering:

This is the most recent method of withering contrivance which is invented by the Tea Planter,

Deson in Zaire. A trough may be about 14-30 m long, 1.8 m wide and 1.5 m high. The leaf is

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placed on the upper part of the trough which includes a layer of wide mesh wire netting. Below

the trough, there is tunnel into which fresh or warm air is blown by means of axial-flow fan

located at one end of trough. Leaves are stored about 25 cm thick on wire mesh nets which

makes the bed of trough. The optimum volume of air that has to pass through the leaf when

loaded at the rate, 25 kg/m2 of trough area, is about 14 m3/ m2 of trough area against a pressure of

13 mm w.g. The use of heat is to increase the evaporative capacity of air and for this a

hygrometric difference of about 3.5. C. For proper withering, the relative should not be more

than 65%. It is noted that for definite type of manufacture, the percentage of moisture in the

withered leaf has to be constant whatever maybe the moisture content in the fresh leaf.

The troughs are of two types –

• Open trough and

• Enclosed trough.

Open trough

In the open type of trough, leaf is spread at a given thickness and air is blown upwards from the

bottom of the perforated bed. This results in the wither of the bottom layer of the leaf first.

Therefore, fans need to be periodically reversed so that the leaf in the upper layer can also get

withered as air is drawn into the leaf bed. Therefore, fan motor in case of open troughs is

provided with facility for reversing the direction of rotation by means of a reversing switch.

However, during the reverse rotation or the suction mode of fan operation, the fan efficiency is

drastically reduced to about 60%, thus consuming more power with respect to the work done as

compared to that of the forward or blowing mode of operation. Moreover, in the reverse mode itis not possible to draw hot air through the leaf bed. The dual direction of movement of air is,

however, expected to furnish more uniform wither, but very often sandwich effect results,

whereby, leaf at the middle layer does not get the desired wither. It is, therefore, necessary at

some time to turn the leaf to achieve more uniform wither. One has to however, ensure

cleanliness of hand or use clean gloves for turning the leaf.

Fig: Schematic diagram of Open Trough

Modification of Open troughs

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To achieve more uniform wither, TRA has introduced modification in existing open troughs by

using streamlining vanes and honeycomb chamber in the downstream of the fan. The air

distribution in the leaf bed can also be improved by modifying the leaf bed structures.

Enclosed trough

In the enclosed troughs the leaf bed is kept in an enclosed environment by raising the sides of the

withering troughs and using a cover on top of the bed. This is designed to create a plenum

chamber on top of the leaf bed as well. In this case the fan is always made to blow air only in the

forward direction and air can be made to pass either from top to the bottom or from bottom to the

top with damper and shutter control at the air entry and exit points respectively without reversing

the direction of the fan. Since handling of leaf is less, the chance of leaf damage in enclosed

trough is much lower. The leaf also enjoys some isolation from the sudden variations in ambient

atmospheric conditions. It is, however, more convenient to load and unload leaf in the open

troughs and it is easier to check the progress of wither. Therefore, open troughs are still popular in the industry.

In some models of the enclosed trough, air is continuously drawn in instead of being blown with

same effects on the leaf. In this configuration, heating of air is to be done at the front end of

trough and not at the fan end.

Fig: Schematic diagram of Enclosed Trough

The advantages of a Trough withering system are:

• Economy

• Greater flexibility with respect to capacity and degree of wither

• Flexibility of construction

• Saving in space and

• Economy of labour and easier operation.

Green Leaf Storage System (GLSS)

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The GLSS developed by TRA keeps the plucked shoots in near fresh condition up to 48 hours.

The system consists of a rectangular chamber with the two ends flaring downwards within which

a number of framed wire mesh partitions spaced at regular intervals hang from the top of the

chamber. The partitions can be kept in inclined position by using a locking device fitted to the

walls. The gap between the two adjacent partitions provides the space for loading the bulk green

leaf from the top of the chamber. Air is made to percolate through the leaf from the bottom to

keep the leaf cool. The leaf can be unloaded from the device by releasing the partitions one after

another resulting in the dropping of the leaf on to the floor from where it is transferred to the

withering trough.

Electronic monitoring and control system (EMCS)

An Electronic monitoring and control of withering process has been developed by a TRA-CEERI

joint R&D programme and is in operation in some factories. This facilitates total monitoring and

exercising of necessary controls in airflow, heat application, period of withering, rate of

evaporation etc. by a Computer once the process parameters are fed into the system .The system

takes the fan to off mode as and when required and thus saves considerable energy. Consistency

in withering and quality improvement is also achieved.

Withering Equipment

There are lot of equipment are used for withering process those are given bellow

Withering fans

Fan is one of the most important components of withering trough. For the purpose of withering

axial flow fans are used. The direction of airflow in axial fans is essentially parallel to the axis of

the impeller. These fans operate at a static pressure of 12 mm (½ in) water gauge and can handle

large volumes of air. One advantage of axial flow fans is that their direction of rotation can be

reversed to make the air flow in the reverse direction. But with reversal, capacity of the fans

reduces nearly to half. Even if the blades are specially designed to give same air flow in both

directions the power required will be 40-60% higher than the power required for normal blades.

Air heater

Air heater is used to supply hot air in withering process of tea. The following methods are

employed for heating of air for withering:

• Individual gas burners for each trough, where gas is available

• Hot air ducting to each trough from a separate heater.

• Direct use of hot air from the drier when it is empty.

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• Using exhaust air from drier.

The last two methods are used extensively in Darjeeling, for which bulking chambers are used.

But the fourth method may give humid hot air. The first method is used only where natural gas is

available. The second method is used almost everywhere. As a thumb rule, hot air is mixed at the

rate of 1/10th of the volume of ambient air.

For heating of natural air either a DF oil heater or a onventional coal heater can be used. But

since the temperature of air obtained from the above sources is much higher, it is to be bulked

with a certain quantity of cold air to attain the requisite temperature. The following examples

show how to calculate the temperature of the mixed air, quantity of hot air to be used.

Period of Wither

The period of wither is ascertained by taking both physical and chemical wither; physical wither

can be achieved in 3-4 hours but chemical wither requires 12-16 hours. In case the leaf is under

withered, the following problems are envisaged:

• Rolling of unwithered leaf leads to breaking up into small flakes, which would not

respond to the subsequent processing steps and produce unacceptable teas.

• If the leaf were under withered valuable water-soluble solids would be lost during the leaf

conditioning process.

• Under-withered leaf when rolled turns into a wet watersogged mass; the sogginess

restricts supply of oxygen and hinders uniformity in the subsequent oxidation reaction

(fermentation).• Maceration of under withered leaf also leads to formation of lumps during fermentation.

• At temperature more than 25°C with under withered leaf, chances of bacterial

contamination increase.

• Proper physical wither reduces load on the dryers.

Withering - considerations in relation to manufacture

For orthodox manufacture in the plains of N.E. India, wither should be aimed to achieve 60-65%moisture in the withered leaf. For Darjeeling manufacture the wither is rather hard, moisture

content of the withered leaf being around 30%.

For CTC manufacture, moisture content of 70% in the withered leaf should be adequate. Use as

many troughs as possible. The thickness of spread should not come below 10 cm (4") in the

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plains. However, in the hills a minimum thickness of 7.5 cm (3") is used. Subject to this

minimum thickness, thinner the spread, the better.

While loading the trough, a small volume of ambient air (by use of damper) should be passed

through the leaf in order to cool it. This will prevent further damage of leaf particularly during

the summer, when the temperature of the leaf brought for wither may touch 45°C (113°F).

• Spreading of leaf should be uniform across the length and breadth of the trough.

Bunching of wet leaf should be avoided.

• Air velocity should be such that the leaves are not lifted up.

• Leaf should be handled carefully. Drainage during spreading must be avoided and the

labourers must not be allowed to walk on the leaf. Withered leaf bruises more easily.

• The duration and temperature of withering influence the character of made tea. While low

temperature favours development of quality, high temperature may develop colour at the

expense of quality. Unwithered teas are flaky - they may be brisk but with poor quality.

• A period of 12-14 hours of wither is essential for completion of chemical wither.

• Proper monitoring of withering through use of hygrometers, dry and wet bulb

thermometers, moisture meter, weighment before and after wither, will help in producing

better quality tea.

• Effort should be made to wither evenly over the entire period of withering, so that the

rate of loss of moisture proceeds at a uniform rate. This will to allow the chemical

reaction to proceed in the desired manner.

• At the end of withering ambient air should be blown to cool down the leaf.

• Leaf from the troughs should be taken as and when required and one trough at a time

should be emptied.• Withered leaf must be loosely packed in the basket.

Biochemical changes during withering

Now it is well known that many physical and biochemical changes occur during the withering

period. The majority of biochemical changes is completed in 6-10 hours. When tea is plucked the

metabolic activity is deranged rapidly but many processes including respiration continue tofunction. This causes an accumulation of products synthesized by the leaf, while some reactions

are apparently triggered by plucking. Many biochemical changes which have been substantiated

during withering are…

1. Increase of polyphenoloxidase activity.

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2. Increase in Caffeine and permeability of cell membrane.

3. Breakdown of chlorophyll and protein to amino acid

4. Changes in caroteniods and levels of organic acids.

All these biochemical changes may directly or indirectly influence the quality of made tea.

During the withering process some bio chemical change occurs in the chemical component of tea

those are given bellow:

A. Breakdown of protein into amino acids

Peptidase present in tea shoots causes an increase in free amino acid levels, particularly those of

aspartic acid, glutamic acid, serine, glutamine, alanine, tyrosine, phylalanine, valine, thereonine

& lysine. A decline in theanine level is particularly interesting as it is caused primarily by the

degradation of glutamic acid & ethylamine. The total loss of protein amounts to about 1.2%; the

breakdown of protein is of significance because of its influence on tea quality.

B. Caffeine

Withering causes an increase in caffeine content but the quantum of increase is depend upon

temperature & nature of cultivar. The biosynthesis of caffeine involves transfer of methyl group

from S-adenosyl methionine to methyl xanthine to form caffeine.

C. Sugars

Loss of sugar during withering amounts to about 4% & those getting reduced include glucose-6-

phosphate, fructose-6-phosphate, and glucose-1-phosphate. A part of sugar is also metabolizedinto amino acids.

D. Organic Acids

A part of sugar is metabolized into organic acid causing an increase in fumaric acid, citric acid,

succinic acid & oxalic acid. However, there is a decrease in malic acid which possibly happens

due to its utilization for synthesis of amino acids.

E. Polyphenols

Polyphenol oxidase activity generally increase, but the activity is dependent on temperature &

moisture of the stored shoots. The increased polyphenol oxidase activity also helps in fermenting

the leaves which is the next operative stage in tea processing.

F. Chlorophyll

Withering causes degradation of chlorophyll to the extent of about 15%. Breakdown of

chlorophyll affects appearance of made tea a part of the degraded chlorophyll produces

chlorophyllide by the action of chlorophyllase present in the shoots.

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G. Phosphorus

All phosphate esters decrease during withering, but there is evidence that monophosphates

increase at the expense of cytidine triphosphate, uridine triphosphate & adenosine triphosphate.

H. Volatile compound

Some amino acid derived aldehydes like phenyl acetaldehyde, methyl butanol & n-hexanol

increase during withering. This apart, cis-2-pentanol, cis-3-hexanol, trans-2-hexanol, linalool

oxide, nerol, geraniol. 2-phenyl ethanol & phenyl methanol markedly increase during withering

but carotenoids decrease.

I. Increased Permeability of cell membrane

Although not a biochemical phenomenon, permeability of cell membrane increases with the

degree of withering. Apart from enhancing efficient mixing of reactants during fermentation, cell

permeability may also influence the formation of aroma components.

Orthodox Manufacture

Orthodox manufacture begins by selectively picking (often by hand) tea leaves. The leaves are

allowed to wither, reducing their water content and making them soft and pliable. Once withered,

the leaves are gently rolled to break down the cellular structure, beginning the oxidation process.

The oxidation stage is primarily responsible for differentiating tea into its various categories –

white, green, oolong, and black - and determining tea's caffeine content. The longer the oxidation

process is allowed to continue, the darker the leaf becomes, and the more caffeine the tea

contains. White teas are not rolled or oxidized at all. Once the desired level of oxidation is

reached, the leaves are dried to halt the oxidation process and make them suitable for

distribution. The dried leaves are then graded and sorted into various sizes: whole leaf, broken

leaf, fannings (small particles) and dust.

In orthodox method, the rolling machine used for rupturing the leaf tissues vary in size, design

and work on certain principle i.e., compressing and turning the leaf over and individual shoots

are twisted under pressure. It is a batch mode process where withered leaves containing 65%

moisture are charged into the roller, which consisting of a vertical metal cylinder open at both

ends. The lower end of the cylinder rests on a larger circular tube and the upper end is closed by

a cap which compresses the leaf. The roller is then moved ecc

entrically over the rolling table. Both, the pressure and the speed may be adjusted according to

the size of leaf particles required. Each batch is rolled for 20-30 min sifted and coarse material

returned for further rolling. After several cycles the coarse fraction may be passed to a rotorvane

or CTC for further treatment. Generally the following rolling program is adopted along with

specification of loading and speed of various rolling tables.

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Unorthodox Manufacturing process

Un-orthodox manufactures practiced in Bangladesh are below-

• CTC (crush, tear and curl)

• Rotorvane

•

LTP (Lawrie tea processor)Among these method the CTC method is the most familiar and popular method in the tea

manufacturing process.

CTC (crush, tear and curl)

CTC (Crush-Tear-Curl) is the most well known non-orthodox method of manufacture, and was

invented during WWII to increase the weight of the tea that could be packed into a chest. It also

eliminates some of the labor required to produce tea, thus increasing the speed and efficiency of

tea manufacture. In the Crush-Tear-Curl process, tea leaves are plucked and withered. After theyare withered the tea leaves are passed through a series of cylindrical rollers that crush, tear and

roll the tea leaves into tiny, irregular balls that somewhat resemble coffee grounds. Once

completed, the leaves are left to oxidize and are finally fired. After the tea is fired, it is sorted

into different grades. CTC tea is used primarily in mass-market teabags. Given the small particle

size, CTC tea has a greater surface area ratio than whole leaf tea, causing it to brew quickly and

generally have a thicker body with more astringency.

The CTC is used in conjunction with either rolling or rotorvance systems where the leaf has been

conditioned for CTC process. In this machine 70% withered leaf is crushed, torn and curled in

the small gap between the serrated surfaces of the rollers. The cell contents of the leaf are freedand mixed without too much heating up of the leaf mass. However, the CTC Roller segment

must always be kept sharp, otherwise cutting will be poor and primary grades will be reduced

and will result dull liquor.

Rotorvane

This resembles a large meat mincer and consists of a horizontal cylinder fitted with a rotor

assembly which includes a worm to drive the leaf of forward and a series of vanes which squeeze

the leaf against resistors the leaf forward and a series of vanes which squeeze the leaf against

resistors fixed on the inside surface of the barrel of the machine. The discharge end consists of

the either and adjustable plate- an iris end-plate, or a cone fitted against the pressure of a spring.

In either case, the gap through which the leaf the discharged is adjustable, allowing the pressure

applied to the leaf to be varied. For pure rotorvane manufacture, a moiature content of around

65% is desirable. Rotorvane may be used as either for secondary treatment after rolling in

orthodox manufacture or pre-treatment for CTC.

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Fig: Schematic diagram of Rotorvane

The severity of the action is determined by:

• The speed of the rotor

• The clearance between the thrust face of the vanes and resistors

• The number and position of vanes and resistors

• The type, form and adjustment of the end pressure device

• The rate of feed and the size of the machine

LTP (Lawrie Tea Processor):

The machine is a modified hammer-mill. Within its barrel is a shaft carrying a series of knives

and beaters. The shaft rotates at high speed (2200 rpm, while leaf is blown into and discharged

from the machine by centrifugal action. The machines consume 60 hp and its capacity is about

equal to that of 112 cm CTC machine. The appearance of the product is generally inferior to and

less uniform than well made CTC leaf. The LTP, manufacture requires high wither, the moisture

content in the withered leaf being 72%. The resultant product manufactured by different

processing may be different in size of particles and quality of the liquor. The best orthodox

grades are relatively large, black with definite twist. CTC, grades are even and grainy inappearance but are generally smaller particle size. LTP grades are similar to CTC, teas with a

rather broader particle size distribution. Liquors from CTC teas are tend to strong and brightly

colored whereas liquors from orthodox teas are generally thinner, less coloury but often have

superior flavor.

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Leaf Conditioning

The plucked leaves should be pre-conditioned before rolling/ unorthodox manufacture. These are

by follows:

To remove stone, metal pieces and other particulate dust by green leaf filter.

Shred the green leaves into small pieces before passing to rotorvane.

The leaves are primarily crushed in the rotorvane in a continuous mode of operation for

subsequent CTC processing.

Conclusion

Tea is a natural beverage brewed from the leaves of an evergreen plant called Camellia sinensis.

Tea processing is the method in which the leaves and flushes from Camellia sinensis are

transformed into the dried leaves for brewing tea. Tea leaves are processed in the tea factories to

produce an acceptable product for human consumption. Manufacture of tea involves following

operation, viz. plucking and handling of shoot, withering, rolling, fermentation, drying grading &

sorting and packaging. All these operations call for the necessity of meticulous care and

technological maneuvers to produce better quality of tea.

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Reference

1. D.L. Sana, Tea Science

2. Md. Lutfar Rahman, Tea Estate Management

3. M.S.H Chaudhury, Tea Growing

4. Barundeb Banerjee, Tea Production and Processing

5. Mulky M.J. and Sharma V.S, Tea Culture Processing and Marketing

6. Internet:

• www.wikipedia.com

• www.tocklai.net/TeaManufacture/index.aspx

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http://www.wikipedia.com/

http://www.tocklai.net/TeaManufacture/index.aspx

http://www.wikipedia.com/

http://www.tocklai.net/TeaManufacture/index.aspx



• www.teauction.com/home/withering.asp

• www.upasitearesearch.org/teamanufacturing.html

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http://www.teauction.com/home/withering.asp

http://www.teauction.com/home/withering.asp

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