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are catalyzed by polyphenol oxidase and/or peroxid-

ase in the presence of oxygen and catechins. Some of

the formed aldehydes are further oxidized to carbox-

ylic acids during firing (as the concentration of acids

is higher in fired teas) or storage, although some of

the aldehydes are certainly reduced to their respective

primary alcohols.

Lipids

0019 Lipids make up between 4 and 9% dry weight of the

fresh tea leaf and are composed mainly of free fatty

acids and fatty acid esters. Linolenic acid is the major

fatty acid in tea, but variations in this observation

have been noted. The fatty acid profile changes with

the geographical area of tea production, agronomic

practices, and variety. The levels of fatty acids change

throughout tea manufacture. During withering, the

fatty acid esters are hydrolyzed to free fatty acids. The

unsaturated free fatty acids degrade to form aroma

compounds, but the fate of saturated fatty acids

during tea processing is unknown.

0020The mechanism for the degradation of unsaturated

fatty acids to aroma compounds is outlined in Figure 5.

0021Linoleic and linolenic acids produce hexanal and

E-2-hexenal, respectively, when the acids are added

to tea leaf extracts. Z-3-Hexenal formed from lino-

lenic acid easily isomerizes to E-2-hexenal, and also is

the precursor of Z-3-hexenol in macerated tea leaves.

Alcohol dehydrogenase reduces the aldehydes to

alcohols.

CHO

Peroxidase

or polyphenol

oxidase

Aldehyde

Alcohol

dehydrogenase

RCH

alcohol

Schiff base II

Schiff base I

RCO

Acid

Heat and/or

autooxidation

N CH

−NH

Polymerized

polyphenol

−H

Quinone

Amino acid

Peptidase

Proteins

Polyphenol oxidase

or peroxidase

(−)-Epicatechin

fig0004 Figure 4 Formation of aldehydes, alcohols, and carboxylic acids from amino acids.

5748 TEA/Chemistry

0022 The linoleic acid forms 13-hydroperoxy acid,

which is an intermediate in the production of C

aldehydes and alcohols in tea leaves. The hydro-

peroxidation of the acid occurs in the presence of

lipoxygenase enzyme in a highly stereospecific

manner forming only l-hydroperoxy acid. Hydroper-

oxide lyase breaks down the 13-hydroperoxide acid

to C

aldehyde and 12-oxo-acid. The action of

this enzyme is enentioselective, breaking down only

the l-hydroperoxide acids.

0023 The formation of 9-oxo-nonanoic acid from lino-

lenic acid in tea chloroplasts, by cleavage at C-10,

suggests that Z-3,Z-6-nonadienal, Z-3,Z-6-nonadie-

nol, E-2,Z-6-nonadienal, and E-2,Z-6-nonadienol

may also be derived from linolenic acid via a similar

intermediate. Similarly, cleavage at the C-10 carbon

of linoleic acid might be expected to produce Z-3-

nonenal, E-2-nonenal and E-2-nonenol. However,

only minor amounts of E-2,Z-6-nonadienal and E-2

nonenal have been detected in tea, implying that the

hydroperoxidation of linoleic and linolenic acids

occurs predominantly at the C-13 carbon to produce

the C6 aldehydes and alcohols.

0024Low levels of palmitoleic and oleic acids have been

detected in fresh tea leaves. These fatty acids break

down to form heptanal and heptanol, nonanal, and

nonanol, respectively, during tea processing. The re-

lationship between precursor fatty acid in fresh leaf

Free falty acids

and lipids

Lipolytic acylhydrolase

Linolenic acid

Lipoxygenase

H + other free

fatty acids

OOH

13-Hydroperoxylinolenic acid

13-hydroperoxylinolenic acid

Hydroperoxide lyase

Hydroperoxide

lyase

OHC

E-2-Hexenoic acid

Z-3-hexenoic acid

Pyrolytic

autooxidation

Pyrolytic

autooxidation

Pyrolytic

autooxidation

Pyrolytic

autooxidation

Pyrolytic

autooxidation

CHO

ADH

12-OXO-(9Z )

12-OXO-(10E)

-dodecenoic acid

CHO

ADH

Hexanal

Hexenal

OHC

OHCO

Z-3-hexenal

Diacid

Hexanoic acid

ADH

E-2-hexenal

E-2-hexenoic acid

E-3-hexenoic acid

E-3-hexenal

fig0005 Figure 5 Production of volatile flavor compounds from linoleic and linolenic acids.

TEA/Chemistry 5749

and derived aroma compound in the processed prod-

uct is rarely linear due to the various interactions that

take place during processing. Linolenic acid and 13-

hydroperoxylinolenic acid, for example, inhibit the

formation of n-hexanal from linoleic acid during

tea manufacture. In addition, the aroma compounds

formed have different boiling points, and more of the

lower boiling compounds are lost by volatilization

during processing.

Terpene Glycosides

0025 There has been considerable speculation on the mech-

anism of formation of monoterpene alcohols during

tea manufacture. It was originally thought that lina-

lool was a product of carotene degradation. Later, it

was suggested that terpene alcohols were produced

from oxygenated isoprenoid hydrocarbons. However,

it has now been demonstrated that linalool and

geraniol are hydrolytic breakdown products of b-

d-terpene glycosides during tea manufacture. Indeed,

many alcohols in the tea aroma are products of glyco-

side hydrolysis. In recent studies, several alcohol

glycosides have been isolated from tea leaves. These

include identified glycosides of 2-phenylethanol, all

the four isomers of linalool, geraniol, benzyl alcohol,

nerolidol, etc. These glycosides are hydrolyzed during

tea manufacture to form their respective alcohols.

Pigments

0026 Fresh tea leaves contain appreciable amounts of pig-

ments, mainly chlorophyls and carotenes. Fresh tea

leaves contain about 1.4 mg g

1

dry weight chloro-

phyls a and b. During tea processing, the chlorophyls

degrade to pheophytins and pheophorbides. These

compounds play an important role in giving black

tea its shade of color. A number of breakdown

products from the phytol side-chain contribute to

the aroma complex of tea.

0027 More than 15 carotenoid pigments, dominated

by neoxanthin, violaxanthin, lutein and b-carotene,

have been identified in fresh tea leaf. These caroten-

oid compounds account for about 0.5% dry weight of

tea leaves. The carotenes decrease during tea process-

ing with the resultant production of various aroma

compounds. b-Carotene degrades to b-ionone,

whilst b-ionone, a-ionone, 3-hydroxy-b-ionone, 3-

hydroxy-5,6-epoxyionone, 3,5-dihydroxy-4,5-dihy-

dro-6,7-didehydro-a-ionone, and other terpenoid

aldehydes and ketones are degradation products

of other carotenes present in tea leaves. Dihy-

droactinidiolide, 2,2,6-trimethylcyclohexanone, 5,6-

epoxyionone, 2,2,6-trimethyl-6-hydroxycylohexa-

none, and theaspirone and possibly formed

form the primary oxidation products of carotenes,

i.e., b-ionone. b-Damascenone, a-damascone, b-

damascone, 3-oxo-b-ionone, 1,2-epoxy-1

dihydro-

b-ionone, loliolide, dehydrovomifoliol, and 3,

7-dimethyl-1,5-octadien-3,7-diol are speculated to be

derived from carotenes via oxidative enzymatic reac-

tions that take place during withering and fermenta-

tion, and pyrolytic reactions during firing. The

mechanisms of these reactions have, however, not

been fully worked out. The formation of these com-

pounds is affected by the amounts of catechins present,

oxidase activity, degree of mixing of the cell contents,

and concentrations of the reactants. These factors

change with degree of wither. Loss of carotenes has

been demonstrated to increase with physical withering

and fermentation process. Further pyrolytic and

photo- and/or autooxidative reactions of carotenes

occur during firing to produce more aroma com-

pounds. The compounds produced from carotenes

have a major effect on the aroma of tea. Flavory teas

are normally produced from green leaf with high car-

otene contents.

0028As research on tea aroma continues, it is inevitable

that more mechanisms and pathways for the forma-

tion of tea aroma compounds will be identified. These

will likely involve nonvolatile precursors, which cur-

rently are largely ignored with respect to tea aroma

and quality. For example, it is known that chlorophyl

degrades to phytol and other products, but the con-

tribution of chlorophyl degradation products to tea

aroma is not known.

0029Considerable research has been directed into deter-

mining how the aroma complex changes with vari-

ations in agronomic, cultural, and manufacturing

practices. Many studies have indicated the changes

that occur in aroma composition by varying one par-

ameter or the other without any attempts to quantify

and classify the contribution of the aroma com-

pounds to quality. Generally, the aroma compounds

can be classified into two groups, i.e., those although

important for the characteristic black tea smell, are

deleterious to black tea quality when present at higher

concentrations (group I compounds), and those that

impart a sweet flowery aroma to tea, the presence of

which is considered to be highly desirable (group II

compounds). The classification of aroma compounds

in group I and group II compounds has been based on

either the odor characteristics or the retention time of

the aroma compounds during gas chromatographic

analysis. The ratio of group II to group I aroma

compounds has been used to classify teas in order of

flavor quality. In other studies, the ratio of terpenoid

to nonterpenoid compounds has been used.

0030Although these ratios provide the basis of a semi-

quantitative method for classifying teas in order of

their aroma quality, the ratios must be used with

5750 TEA/Chemistry

caution as the olfactory perception limits of the

aroma compounds differ widely. Some compounds

may be present at very low levels yet have a large

impact on aroma and vice versa. For example, methyl

epijasmonate has an aroma that is 400 times stronger

than that of methyl jasmonate at the same concen-

tration. In addition, some of the compounds con-

sidered deleterious to black tea quality are important

for green tea quality.

Chemistry of Tea Manufacture

0031 Several tea beverages exist. These beverages include

green teas, several semifermented teas, and black

teas. The chemistry occurring during their processing

varies depending on the desired final product. In

green tea processing, oxidative reactions, discussed

above, are completely discouraged. In black tea

processing, there is more extensive oxidation of the

catechins, other polyphenols, amino acids, and unsat-

urated fatty acids. Fewer oxidative reactions occur in

the processing of the semifermented teas.

Black Tea Processing

Withering

0032 Processing of tea beverages starts as soon as the

leaf is detached (plucked or harvested) from the

plant. The polyphenol oxidase activity decreases,

while the catechins levels vary. ()-Epicatechin,

()-epigallocatechin gallate, and ()-epicatechin

gallate levels decline. This decline is associated

with oxidative transformations. Caffeine levels

rise, while protein levels decline. This decline is

caused by an increase in the activity of proteolytic

enzyme activity, which hydrolyzes the proteins to

amino acids, with a concomitant increase in the

level of free amino acids. Carotenoid compounds

degrade due to photoisomerization to volatile flavor

compounds. Fatty acid esters are hydrolyzed to free

fatty acids that oxidize during fermentation through

a lipoxygenase-initiated reaction to volatile flavor

compounds associated with the green notes in tea.

Terpene and other alcohol glycosides hydrolyze to

simple alcohols that contribute to tea aroma. These

transformations, which continue up to the point at

which the leaf is macerated, are collectively called

’chemical wither.’ Usually, the leaf is subjected to

moisture loss to make it more flaccid, so that ma-

ceration is easy. Moisture loss, the most visible

change in the leaf before maceration, is referred to

as ’physical wither.’ Chemical wither benefits

mostly flavory black teas, as it improves the black

tea aroma and to some extent benefits plain

black tea as it reduces the level of green taste.

Physical wither benefits both flavory and plain

black tea quality. Hard physically withered leaves

are easier to macerate and make more aromatic

black teas. Withering therefore plays an important

role in black tea processing.

Maceration

0033Maceration ruptures the leaf cell structure, exposing

the chemical constituents of the cells, mainly poly-

phenols, oxidative and degradative enzymes, lipids,

amino acids, etc., to oxygen. Most importantly, cate-

chins come into contact with the polyphenol oxidase

enzyme, initiating ’tea oxidation,’ which is errone-

ously referred to as ’fermentation.’ Several methods

are used, but the most common are the orthodox

rolling, crush, tear, and curl (CTC), and Laurie tea

processor (LTP) methods. The method used has a

significant effect on the resultant black tea. There is

less cell matrix destruction with the use of orthodox

rollers than the other two. The orthodox maceration

therefore leads to fewer oxidative reactions, and fer-

mentation is slow. Cell matrix destruction is greater

in the CTC and LTP maceration methods, leading to

more extensive oxidation, but these teas are less aro-

matic, with higher plain tea quality parameters. LTP

manufacture requires a softer physical wither than

CTC processing.

Fermentation

0034Most chemical transformations occur during the fer-

mentation phase of black tea processing. These trans-

formations are responsible for the characteristic taste

and aroma products of black tea. As illustrated in

Figures 2 and 3, the catechins and polyphenol oxidase

form the theaflavins and thearubigins. The amino

acids and fatty acids also oxidize to various volatile

flavor components. For LTP and CTC manufacture,

the process is complete after 60–120 min.

Firing (Drying)

0035Firing is necessary to terminate fermentation and to

dry tea for storage and transport. As a result of the

temperature rise, some reactions are accelerated until

the rise is adequate to denature the enzymes or mois-

ture has been adequately removed to prevent reac-

tions occurring, but a lot of changes occur to give

black tea its character.

0036As a result of firing, the color changes as a result

of the transformation of chlorophyls to pheophytins

and pheophorbides. Some caffeine is lost while the

amount of volatile flavor compounds is reduced. The

volatile flavor compounds that result from various

pyrolytic reactions are formed during firing.

TEA/Chemistry 5751

Sorting

0037 The fired black tea is sorted first by removal of fiber

then by separation into different particle sizes. The

various particle sizes define the various grades. Gen-

erally, although the grades have different chemical

compositions, quality is not solely dictated by grade.

Other Tea

Green and Semifermented Teas

0038 There are fewer chemical transformations in the pro-

cessing of the other teas compared with black teas. In

green tea processing, attempts are made to insure that

there is no oxidation, especially that of the catechins.

The process therefore starts with steaming or roasting

to deactivate polyphenol oxidase activity. The green

teas are made from tea plant varieties with a lower

catechin content than those for black tea, but this

level is sufficient to create astringency. The volatile

components of green tea are basically those of the

primary products.

0039 Partially fermented teas undergo incomplete fer-

mentation. Several types exist. Some are processed

by roasting, whereas others are subjected to high-

temperature rolling.

See also: Amino Acids: Properties and Occurrence;

Caffeine; Carotenoids: Occurrence, Properties, and

Determination; Chlorophyl; Sensory Evaluation:

Aroma; Tannins and Polyphenols

Further Reading

Owuor PO (1992) A comparison of gas chromatographic

volatile profiling methods for assessing the flavour qual-

ity of Kenyan black teas. Journal of the Science of Food

and Agriculture 59: 189–197.

Owuor PO (1995) Results from factory processing and

black tea quality research in Kenya (1982–1994). Tea

16: 62–69.

Robertson A (1992) The chemistry and biochemistry of

black tea production – the non-volatiles. In: Willson

KC and Clifford MN (eds) Tea: Cultivation to Con-

sumption, 1st edn, pp. 555–601. London: Chapman &

Hall.

Robinson JM and Owuor PO (1992) Tea aroma. In: Will-

son KC and Clifford MN (eds) Tea: Cultivation to Con-

sumption, 1st edn, pp. 603–647. London: Chapman &

Hall.

Sakata K, Gou W and Moon JH (1999) Tea chemistry with

special reference to tea aroma precursors. In: Jain NK

(ed.) Global Advances in Tea Science, pp. 693–706. New

Delhi: Aravali Books International.

Takino Y, Imagawa H, Horikawa H and Tanaka A (1964)

Studies on the mechanism of oxidation of tea leaf

catechins – formation of a reddish/orange pigment and

its spectral relationship to some benzotropolone deriva-

tives. Agriculture and Biological Chemistry 28: 64–71.

Yamanishi T (1999) Tea flavour. In: Jain NK (ed.) Global

Advances in Tea Science, pp. 707–722. New Delhi:

Aravali Books.

Processing

P O Owuor, Tea Research Foundation of Kenya,

Kericho, Kenya

Tea

0001Beverages produced from tea leaves include black tea,

green tea, and various partially fermented teas such as

oolong and pouchong. All of these products are culti-

vated and harvested using similar procedures, but

variations in manufacturing methods determine the

final product. This article deals with tea processing

from cultivation through to packaging.

Cultivation

0002Tea was introduced into many countries of the world

from South-east Asia and grows in climates ranging

from the Mediterranean to the hot humid tropics.

Commercially viable plantations have been estab-

lished between as far north as Turkey and Georgia

(24



N), and as far south as Argentina (27



S), at

altitudes ranging from sea level to 2700 m.

0003Successful commercial cultivation of tea requires

a minimum annual rainfall of about 1400 mm when

irrigation is not carried out. Rainfall needs to be well

distributed with at least 120 mm per month. Pro-

longed drought adversely affects tea growth, and in

such conditions, irrigation is advocated. The inter-

action of soil texture and rainfall distribution is an

important factor to be considered when assessing the

suitability of an area for tea. Tea does not tolerate

water-logged conditions.

0004Ambient temperatures of 12–30



C are considered

ideal for growing tea. Temperatures above 30



accompanied by low humidity, have been shown to

inhibit active growth. The optimum soil temperature

for active growth within the feeder root depth is

20–25



C. Using long-term average yield data from

Kenyan tea estates, situated at altitudes between 1500

and 2250 m, it has been shown that annual tea pro-

duction falls by 200 kg of black tea per hectare

for every 100 m rise in altitude. This has been attrib-

uted to decreases in air and soil temperature. The

tea plant cannot withstand frost conditions. Night

5752 TEA/Processing

temperatures below 6



C require that measures be put

in place to mitigate frost, which could lead to the

wilting of tea leaves.

0005 Tea grows most successfully in acidic soils at pH

values ranging from 4.0 to 5.8. A wide range of soil

types are suitable for cultivation, including old

sedimentary soils derived from gneiss or granite,

soils formed from the wash of old sandstone, alluvial,

peat, and soils derived from volcanic rock.

0006 Propagation is carried out either from stumps gen-

erated from seeds or from cuttings. Propagation from

seeds takes at least 3 years from seed planting to the

production of viable plants capable of transplanting

to the field, whereas plants generated vegetatively

require a minimum of 6 months before they are

ready for transplanting. However, when the plants

are vegetatively produced under very cold conditions,

the period needs to be lengthened. Vegetatively pro-

duced plants have the additional asset that they are

usually of predictable yield and quality. Grafting

plants with other desirable characteristics can further

enhance these attributes.

0007 After transplanting, the young plants are left to

establish for at least 18 months in the field before

commercial plucking (harvesting) can commence.

During this time, the plant is manipulated through a

process described as ’bringing into bearing’ to form a

plucking table. This is achieved by tipping and peg-

ging. The tea is subsequently harvested regularly, with

pruning every 3–5 years, thus maintaining a height at

which the leaves and buds can be plucked easily.

During this time, the tea is treated with regular appli-

cations of fertilizers, particularly nitrogen.

Harvesting

0008 Harvesting (plucking) of tea involves manual or

mechanical removal of the terminal young tender

portions of peripheral shoots. In most countries, the

recommended plucking standard is two leaves and a

bud. However, under commercial production, it is

difficult to select leaves of one plucking standard

exclusively, and it is usual to find varying proportions

of the more mature leaves in the harvested crop. The

quality of the final product deteriorates with increase

in mature leaf content.

0009 After plucking, depending on climatic conditions,

new shoots take between 40 and 100 days from bud

break to develop into pluckable shoots. In general,

the colder or drier the production area, the slower is

the shoot growth. The intervals between plucking

rounds vary accordingly. The appropriate plucking

interval is determined by optimizing between the

extent of new shoot development and the level of

overgrowth of shoots that have been left after the

previous plucking. Hand plucking is the preferred

method of harvesting tea as it enables the selection

of the quality requirements of two leaves and a bud.

Compared with mechanical harvesting, however, hand

plucking is slow. Mechanical harvesting is carried out

in some countries where labor is either expensive or in

short supply. This can be carried out using modified

hedge trimmers, motorized machine pluckers oper-

ated by one or two people, or a self-propelled ma-

chine capable of negotiating the lines of tea bushes.

Black Tea Manufacture

Withering

0010The changes that occur in the green leaf from the time

it is detached from the plant to the time of maceration

or rolling are collectively known as ’withering.’ These

changes can be categorized into physical and

chemical processes and are thought to be important

in the manufacture of tea.

0011The main physical process that occurs during

withering is moisture loss, leading to changes in cell

membrane permeability. These changes are indispens-

able in orthodox black tea manufacture as they pre-

condition the leaf for maceration or rolling. For teas

made by alternate methods of maceration, such as

those using the Legg-cut, the Laurie Tea Processor

(LTP), or Crush Tear and Curl (CTC) (see section on

Maceration), physical withering may not be obliga-

tory. During physical withering, the stomata on the

lower surface of the leaf begin to shut gradually, but

continue to influence the rate at which water is lost.

Air temperature, atmospheric vapor pressure, and air

velocity and direction all affect the rate and degree of

physical wither. The many biochemical changes oc-

curring during withering are referred to as ’chemical

wither.’

0012Several methods of withering have been developed.

Traditionally, withering was carried out on banks of

trays in thin layers so that the largest possible leaf

surface would be exposed to air. Alternatively

withering facilities consisted of open lofts built some

distance from the factory, or on the upper, open

storeys. These methods have largely become obsolete,

as they were inefficient, time-consuming, and incap-

able of coping with increased crop production.

0013In most modern factories, withering is now carried

out in troughs that can hold between 2000 and

3000 kg of green leaf, loaded to a depth of 25–

40 cm. The troughs have dual-direction fans that

either drive air into the withering troughs or draw

air through them. Under low humidity, physical

withering can be achieved using ambient air, even if

the troughs are slightly overloaded. However, at high

TEA/Processing 5753

humidity and/or when the troughs are overloaded

because of a greater quantity of crop, stream-heated

air is necessary to assist physical withering. Excessive

use of heat during the withering process impairs black

tea quality. Chemical withering is time-dependent. It

requires a minimum of 6 h from the time the leaf is

detached from the plant during plucking. Chemical

withering beyond 20 h impairs plain black tea quality.

0014 Despite these improvements, withering still takes

up considerable factory space, and newer methods

continue to be developed. One such method involves

storing the leaf in a holding tank with minimal mois-

ture loss for about 6 h to achieve chemical wither. The

leaf is then spread on withering troughs, or a moving

belt witherer, and the moisture is rapidly reduced

by the use of warm air. This process is known as

’two-stage withering.’ Other methods of withering

used in black tea manufacture include the use of

drum withering, tunnel withering, Tocklai continu-

ous withering machines, Russian withering machines,

and automatic continuous operating installations.

Recent studies indicate that in the unorthodox

methods of producing plain black tea, physical

withering is not essential to produce high-quality

black tea. Some factories are therefore now process-

ing plain black tea after achieving chemical withering

but without physical withering.

0015 Factors that affect withering include the leaf type,

leaf conditions, standard of plucking, thickness of

spread, length of wither, drying capacity of air, and

temperature of air used in drying.

Maceration and/or Rolling

0016 Black teas are often referred to as ’orthodox’ or

’unorthodox’. These descriptions are derived from

the method of leaf distortion employed during

manufacture.

0017 In orthodox manufacture, leaves that have

achieved physical withering are subjected to rolling.

During conventional rolling, the leaf is damaged in

such a way that it becomes twisted, and the semi-

permeable membrane of the leaf is distorted, allowing

the cell juices to be expelled to cover the leaf surface.

This allows the juices to mix with the cellular

enzymes in the presence of oxygen, and the chemical

reactions necessary for fermentation will commence.

The orthodox method of manufacture is particularly

desirable in the processing of flavory black teas, but

this method is slow and requires a large amount of

factory space to process high volumes of leaves.

0018 Although some factories still make tea using the

orthodox (rolling) system, other methods of macer-

ation have been developed and introduced into many

factories. Such methods include Legg-cut, CTC,

Rotorvane, LTP, Triturator, and other miscellaneous

methods. These methods are particularly desirable for

processing plain to medium-quality black teas. They

are generally faster than the orthodox methods and

can handle high volumes of leaves more easily.

0019The CTC method is widely employed and consists

of two engraved metal rollers close together running

in opposite directions. The machines work like a

mangle, with one roller rotating approximately

70 rpm and the other 700 rpm. The leaf is cut, torn,

and rolled in the small gap between the serrated sur-

faces of the rollers. Using this method, leaf distortion

is much greater than with the most efficient orthodox

rollers, fermentation is faster, and liquoring proper-

ties are improved.

0020Another popular method of maceration involves

use of the Rotorvane. The rotor, consisting of seg-

ments, revolves in a cylinder of 15.20 or 37.5 cm

diameter and is fitted with vanes propelling the leaf

towards the discharge against resistors projecting

from the casing. Maceration is achieved by rubbing

and shearing inside the cylinder, and fermentation

occurs simultaneously.

0021The LTP is another successful machine for macer-

ation. This machine resembles a hammer mill and

employs a centrifugal fan to induce and discharge

leaves.

0022Most modern factories use a Rotorvane plus three

CTC machines in series. However, there are factories

that still carry out orthodox manufacture, and a

few use the LTP. In general, orthodox teas have a

superior aroma to CTC or LTP teas. However, CTC

and LTP teas have higher levels of theaflavins and

thearubigins and therefore have more color and are

brighter and brisker than orthodox teas. (See Phenolic

Compounds.)

Fermentation

0023In black tea processing, fermentation is defined as the

chemical transformations occurring as a result of

breakup of the cell membrane due to maceration.

0024Factories manufacturing tea by orthodox methods

generally ferment on tables or trays. In factories that

employ CTC, LTP, and Rotorvane systems, fermenta-

tion is carried out in batches using troughs or trolleys,

or continuously on moving machines. In the case

of batch fermentation, the troughs or trolleys are

connected to an air supply by a duct, which can be

humidified if necessary to reduce the temperature

of the fermenting tea (dhool). A trolley can contain

110–130 kg of macerated leaf. The advantage of a

trolley system is that the temperature of the dhool

can be controlled more precisely than other systems.

However, it has the disadvantage that the timing

5754 TEA/Processing

of fermentation duration is manual and is subject to

error.

0025 A number of systems have now been developed, in

which fermenting is carried out on a continuous fer-

menting machine. In most of these systems, the fer-

menting tea moves on a perforated belt through which

air is passed. The speed of the belt determines the

throughput and fermentation time. In other continu-

ous methods, the dhool is fed into an open-topped,

semicircular tank with two rows of rotating paddles

that push the dhool forward in a mechanism similar

to that of a screw. The speed of the rotation governs

the rate of throughput and duration of fermentation.

The temperature of the dhool is normally controlled

by external fans. The paddles that continuously turn

expose the dhool to air. Although, in most continuous

fermenting systems, temperature control can be diffi-

cult, the timing of fermentation duration is usually

very precise.

0026 The liquor characteristics of the black tea can be

determined by controlling the temperature and lime

of fermentation. Generally, the lower the fermenta-

tion temperature, the better the black tea. Usually, the

ambient air blown through fermenting dhool supplies

adequate oxygen. Fermenting under enriched oxygen

atmosphere therefore does not improve the liquor

quality provided that the fermentation duration is

timed correctly.

Firing (Drying)

0027 Firing of tea is the process that reduces the moisture

content of fermented tea from about 60% to below

4% and renders the product in a form suitable

for storing. Firing terminates fermentation by de-

activating the enzymes by subjecting the dhool to

high temperatures. Inlet temperatures in the driers

usually range from 140 to 98



C with outlet tem-

peratures ranging from 45 to 82



C. During firing,

considerable amounts of volatile aromatic com-

pounds are lost.

0028 Firing can be carried out using conventional driers

in which the dhool is fed on to a perforated moving

belt and is discharged off after the tea is dry. In most

modern factories, fluidized bed driers are used. In

these driers, hot air is blown into the drier, and this

moves the dhool by the process of fluidization. Gen-

erally, fluidized bed driers have a higher throughput

than conventional driers.

Grading and Sorting

0029 Prior to grading, stalks are removed by the use of

electrostatic separators. The process is effective

because of the higher moisture content of stalks as

compared with leaves after emergence from the drier.

0030Grading of leaves is generally carried out using

mechanically oscillated sieves fitted with meshes of

many different sizes. In some machines, the sieves are

arranged in banks of diminishing mesh size, such that

the outfall of the upper sieve falls on to the lower.

0031The products of the various siftings constitute the

different grades (Table 1). The grade specification

is entirely artificial. However, they are generally

recognizable by their appearance; for instance, bro-

ken orange pekoe contains a high proportion of buds,

orange pekoe is characterized by the abundance of

twisted tender stalk, and pekoe and souchong grades

tend to be more compact and dense. In recent years,

the pattern of grading has been altering in the direc-

tion of smaller teas.

0032The sieve standards adopted for regulating grades

differ in the various countries and even in different

districts in the same country. However, there are

efforts to standardize the sieve sizes.

0033Winnowing in one form or another is routinely

employed and, according to the size and density of

the particles, separates fannings and dust, carrying

away the fibrous residue that is of no commercial

value as a grade.

Packaging and Storage

0034Tea is hygroscopic, and if not adequately packed and

stored, it can readily absorb considerable amounts of

moisture, leading to a deterioration in quality.

0035Most tea is transported in bulk, and the common

forms of packaging employed are either tea chests or

multiwalled paper sacks. Tea chests are normally

made out of plywood, lined on the inside with

aluminum foil, which acts as a moisture barrier.

tbl0001Table 1 Black tea grades

Leaf grades

TGFOP Tippy golden flowery orange pekoe

TFOP Tippy flowery orange pekoe

FOP Golden flowery orange pekoe

OP Orange pekoe

FP Flowery pekoe

Broken grades

TGFBOP Tippy golden flowery broken orange pekoe

TGBOP Tippy golden broken orange pekoe

GFBOP Golden flowery broken orange pekoe

TBOP Tippy broken orange pekoe

GBOP Golden orange

FBOP Flowery orange pekoe

BOP Broken orange pekoe

BP Broken pekoe

BPS Broken pekoe souchong

PS Pekoe souchong

S Souchong

BM Broken mixed

BT Broken tea

TEA/Processing 5755

Multiwalled paper sacks have flat hexagonal ends to

facilitate stacking and consist of a minimum of two

plies of Kraft paper with an additional layer of alumi-

num foil on the inside of the sack to prevent moisture

migration. Materials used for the construction of the

sacks should be free from taint or odor. Both sacks

and tea chests are designed to hold 40–60 kg of manu-

factured tea depending on grade.

0036 Retail packaging of tea is wide and varied, but the

common consideration is the requirement to keep out

moisture. Tea should be stored in conditions that

minimize the absorption of moisture, preferably at a

low humidity. If teas are stored for long periods,

however, moisture absorption and subsequent loss

of quality are inevitable. (See Storage Stability: Par-

ameters Affecting Storage Stability.)

Green Tea Manufacture

0037 Green tea is manufactured from fresh leaves that have

not been fermented. There are various methods of

green tea manufacture, but they all depend on stop-

ping enzyme activity in the green leaf. Withering is

not mandatory in green tea manufacture.

0038 In China, green tea manufacture begins by roasting

the leaf in a hot iron pan for a few minutes, followed

by hand rolling on a table. The leaf is then subjected

to two or more further roastings and rollings.

0039 In Japan, the leaf is steamed for 15–20 s in a revolv-

ing cylinder provided with an agitator. The steamed

material is cooled by a fan or by air on a belt conveyor

and then subjected to primary heating and rolling. The

leaf may undergo further heating and drying before

passing through secondary (final) rollers. The green

tea is then dried to a moisture content of about 3–4%.

Partially Fermented Tea Manufacture

0040 Oolong tea is manufactured in a similar manner to

green tea, but with the following variations. The fresh

leaf is withered at room temperature for approxi-

mately 16 h, or at 40



C for 2 h followed by another

4 h at room temperature. In both cases, during the last

4 h, the leaf is rolled by hand for a period of 30 min

every hour. This is followed by roasting (parching or

pan frying) at about 160



C for about 20 min. The tea

is rolled further and then fired.

0042 The manufacturing process for pouchong tea

is slightly different than for oolong. The leaf is

withered in the sun (solar withering) for 15 min,

during which time it is turned over once. This is

followed by indoor withering for 3 h, during which

time the leaf is turned over three times. The leaf is

then pan-fried at 160



C for 20 min, rolled by hand

for about 20 min, and dried at 80–85



C for 40 min.

Low-grade pouchong tea is often scented by blending

with jasmine flowers to enhance the flavor (jasmine

tea). Other partially fermented teas such as teek-

wang-yin and pan-fried longjing are also manufac-

tured in a similar manner.

Other Tea Products

0043Earl Grey is flavored with the peel oil of bergamot, a

citrus fruit, which is added by spraying on to black tea

before final packaging. Jasmine flowers are some-

times added to manufactured black tea in the country

of origin, and these impart a characteristic floral note.

Lapsang souchong tea is a Chinese black tea flavored

with natural smoke.

0044Instant teas are prepared in the producer countries

by infusing the undried leaf and then evaporating

the liquor by either freeze-drying, spray-drying, or

vacuum-drying. All these drying procedures avoid

using excessive heat and therefore reduce the loss of

flavor components. Instant tea is also produced in the

USA and UK, but details of the processes are secret or

protected by patents.

0045In recent years, decaffeinated teas have appeared

on the market. The tea is decaffeinated with methyl-

ene chloride or other chlorinated solvents and super-

critical carbon dioxide. (See Coffee: Decaffeination.)

See also: Coffee: Decaffeination; Phenolic Compounds;

Storage Stability: Parameters Affecting Storage Stability