Tea processing
TEA
Chemical
constituents of tea leaf (soluble substances)
Substance |
Percent
dry weight |
Percent total soluble solids black tea |
|
Fresh
shoot |
Black
tea |
||
1. flavanol (quality) |
|
|
|
i). EGCG (epigallocatechin gallate) |
9-13 |
1-3 |
3-8 |
ii).
ECG (epicatechin gallate) |
3-6 |
These are polyphenols which are
derivatives of gallic or catechin e.g tannins |
|
iii).
EGC (epigallocatechin) |
3-6 |
||
iv).
EC (epicatechin) |
1-3 |
||
v).
Others |
1-2 |
||
2. Flavonols and their glycosides |
3-4 |
2-3 |
6-8 |
3. Flavandiols |
2-3 |
- |
- |
4. Phenolic acids and depsides |
5 |
4 |
- |
5. Thioflavins
(if high then high quality tea) |
- |
1-2 |
3-6 |
6. Dialyzable
biflavonols and dialyzable thearubins |
- |
2-4 |
6-10 |
7. Non-dialysable materials-
thearubigins |
- |
1-2 |
3-5 |
8.
Polysaccharides |
14 |
14 |
3-4 |
9. Proteins |
15 |
15 |
0.5-1 |
10.
Caffeine |
3-4 |
3-4 |
8-11 |
11. Amino
acids and peptides |
4 |
5 |
14 |
12.
Sugars |
4 |
4 |
11 |
13. Organic acids |
0.5 |
0.5 |
1.5 |
14. Mineral substance |
5 |
5 |
10 |
15. Volatile substance |
0.01 |
0.02 |
0.05 |
Factors affecting composition of tea
The tea composition is synonymous to tea leaf
a) Genotype characteristics
The two extreme gynotype are Camellia assamica and Camellia
sinensis. There are also intermediates between the two. The chemical
composition of shoots differ: Flavanol content in assamica (30%) sinensis
(10%); the proportions of individual flavanols differ with assamica containing
high proportion of gallate esters than sinensis. Volatile fraction-
proportionately large quantities of relatively high boiling terpenoids and
aromatics substances generally considered responsible for desirable flavoring
characteristics in sinensis.
While assamica has generally large leaves, high
yields and rich in flavanols and caffeine of which are responsible for colour,
strength and briskness, sinensis are superior in certain flavor characteristics.
b) Environment
i) Cultural practice
Age: yields
and quality potentials of tea bush not fully realised till 10 years after
planting. Economic importance of tea bushes generally 40-45 years but may be
larger if maintained.
Pruning: Most important cultural practice. Kenya’s
pruning cycle 3-4 years to keep bushes at manageable heights and keeps the
plant in vegetative phase. Difference in quality can be detected according to
time elapsed since last pruning. Stimulates young shoots constitute cropped
proportions. After pruning the quality increases with time.
- The 4 types of pruning include;
a) Collar pruning (near the ground )
b) Medium pruning
c) Top pruning
d) Fringe or lung pruning
Shade (shading of tea): Has effect on the under growth.
Depending on the climate provided in form of the shades to protect from
relative high temperatures. Shade trees were removed because of reduced yields
but the effect of shade on tea quality not very clear.
Fertilizers: K fertilizers have little effect on quality ; PO4s
improves quality (vary with district. But these are not usually used because they
do not sufficiently increase yields. Nitrogenous fertilizers commonly used
increases yields considerably and studies show increase in Nitrogen rates reduces
the quality of made black tea and reduced flavanol levels. The flavour
decreases and thus low quality final made tea.
Altitude: In high altitude, tea universally regarded
as superior to low one. It is considered to have important effect particularly on
volatile fractions.
Climate: Best quality associated with slow growth of
tea bush.
c) Harvesting
i) Plucking standards
·
70-90 days for another plucking.
·
Size of harvested shoot or the plucking
standards influence quality.
·
Best tea determined by plucking only 2
terminal leaves and bud (fine plucking).
·
Course plucking- 3 or more leaves below
the apical bud.
·
Caffeine content decreases with maturity
thus high caffeine content only produced by fine plucking standards
·
Also the theaflavin value decreases as
plucking become course.
ii) Handling leaf
·
Avoid damage after plucking or
overheating leaf to maintain quality.
·
Pluckers to use basket sacks, after
weighing and collection leaves should be transported with open sacks or baskets
rather than bulking in lorry or trailers
·
This helps prevent overheating damage
which will initiate chemical reaction that will change the composition of tea
leaves.
TEA PROCESSING
The 3 products of tea processing are,
·
Black tea or orthodox fermented dried
·
Green tea-unfermented
·
Oolong tea-partially fermented
- Black tea differs from green tea in that during
fermentation stage black tea is fully fermented while green tea is not.
- The enzymes responsible for fermentation are
inactivated after maceration hence maintains the chlorophyll.
- Oolong tea is between the black and green tea.
There is potential fermentation of macerated tea leaves before drying.
Black tea Manufacturing
Withering
Prime objective is to prepare the leaf for rolling
by making it:
·
flaccid
·
permeable to juices
There is potential water loss for moisture contained
in the tea leaves.
Tea is spread thinly on tray banks or tats space 5-6
inches to allow free air access with alleyways to allow distribution and
removal of leaf. This can be in open air or building. Open air has no control
except you use hygrometric conditions of the ambient air. In closed building,
air is circulated by fans; drawing air from bulking chambers.
Under-withering is not desirable because leaf thrown
out of rollers, juice may be expressed and lost, wet leaves clog sieves of
sifting machines, produce flack tea of low specific gravity, too much withering
overtax the drier (firing) and stalk cortex removed from the rollers and tea
becomes more red and so more color than quality.
Physical changes-physical withered
Cell juice looses water first which is rapid then
plasma colloids begin to loose water more quickly. It is then followed with
water evaporation resulting from changes of plasma colloids which lose hydrolic
nature. The cell protoplasm irreversibly looses its hydrophilic properties and
leaf does not restore initial turgor. The leaf becomes soft and elastic.
Biochemical changes- chemical withered
Enzymatic
activity increases in the leaf required for fermentation.
Oxidation reactions:
·
Involves oxygen absorption by the
leaf
·
Followed by oxidative reaction within
the leaf; this confirmed by intensification of color of water extract of
withered leaf compared to fresh leaf.
Degree of the wither
This is the extent to which withering process
proceeds and may be expressed in 2 ways:
i) Weight of leaf withered as percentage of fresh
leaf weight
= withered wt/fresh leaf wt x 100.
ii) Out-turn of the black tea= wt made tea/wt
withered tea x 100. It is used to
control drying process. 40-42 %= light withered, 45-50 %=medium to hard wither.
Withering
systems
Natural withering
Air in natural circulation is used and no control of
speed, temperature or relative humidity. Open sided shade is normally used. Tea
leaves are placed on series of floors usually wire supported large mesh wires
netting covered with removable hesion cloth. It results to potential water
evaporation from the tea leaf and biochemical and physical changes. Requires
large space/area.
Controlled withering
Air temperature is controlled particularly where
heating is done. Withering is more efficient and more uniform not just within a
particular path of tea leaf but from day to day. More uniform product quality
with mc etc. can be:
i) Tunnel withering
Tea leaf is spread on layers of trays on the mobile trolleys
hession or wire netting. The leaf loading on the trays is about 1.8 kg/m2 the trolleys
are wheeled into a withering tunnel and subjected to warm air through tunnel,
typically air mass at speed 2.5 m/s and has wet bulb depression 7.5 ֯ C. It is difficult to estimate the dry
bulb temperature and this can say something about relative humidity. The warm
air should not exceed 38 ֯ C.
ii) Trough withering
There is a wire, hession, nylon netting fixed
horizontally on the upper part of the tough about 9 inches below its top. A
layer of tea leaf, 8 inches (20 cm) thick is spread on the netting resulting in
a loading of about 23.5 kg/m2. At the bottom of trough is tunnel which serves as
an air condition. Air is circulated by a fan made of steel tubes through which
steam flows and condenses as it looses heat to the surrounding air. The
condensate is re-circulated back to the boiler via steam trap (prevents escape
of live steam and allows flow of condensate).
iii) Continuous withering machine
Factors affecting withering of tea leaf
i) Extent of
tea damage- damage tends to cause premature critical changes and hence
uneven withering. There is deterioration of leaf appearance and liquor quality
if damage is significant.
ii) Type of
leaf received- course plucking results to slow withering leaves than
fine plucking. Leaves from different yields may have different sizes and
composition which may influence rate of withering (physical and chemical
changes).
iii) Moisture
content of leaf surface- surface water content determines degree of
withering or time. Its presence encourages bacterial growth which adversely
affects final tea quality.
iv) Thickness
of spread- for given withering trough, design of pan determines speed of
withering. Air flow rate depends on thickness of spread of leaf, if thick
requires higher air flow rate and hence affect degree of wither and increase
time of wither.
v) Drying
air parameters- drying capacity of air is influenced by temperature,
rainfall, air flow rate. If the air is saturated (100% relative humidity), no
evaporation of water will be effected from the leaves. The lower the relative
humidity, the higher the rate of withering. Temperature affects rh, rh decreases
with its increase and vice versa.
vi Duration
of withering- influences quality, high for long wither and v.v. usually
takes 18-20 hrs in Kenya but can be shortened to 6-8hrs with proved sufficiency
to allow chemical and physical changes to have high quality.
Rolling
The object of rolling is to macerate the leaf so
that the enzymes and their substrates get intimately mixed up. This is achieved
mechanically either by the use of an orthodox roller, the rotorvane, or
by CTC (crushing, tearing and curling) machines. Rolling ruptures the cell wall
thereby enabling the production of enzymes.
Essentially macerated and formed into particular
shapes. In the cut tear and curl system they are curled.
a) Orthodox
method
Orthodox tea- tis distinct and demand by some markets.
It uses pressure rollers and is batch process where withered leaf
(55-60 % mc) is fed to rollers of vertical metal cylinders open at both
ends. The lower end rests on larger circular table fitted with series
of ribs and battens. The fittings are basically protrusions into the
leaves.
Leaf bruised and twisted
(raised battens-can be smooth or has cutting edges set at angles) depression,
and rounded off cones. Under high pressure and severe battens, leaves tend to
disintegrate. Without pressure, action more directed towards wringing and
twisting.
The juice is expressed
from leaf and spread as thin film on their surfaces (liquids forced out of cell
without rapture) ad mix catechins and enzyme which were separated and exposed
to atmosphere. Thus enzyme oxidation of catechins begins. The green color
diminishes and brown or coppery color begins to appear.
b) Legg-cutter (machine)
It was
originally designed for tobacco cutting and combines leaf into firm cake and
cuts it into strips varying in width ¼ -1/10 inch (0.625-0.025cm). The strips
are then given short light roll in roller with a flat table and center
concentration.
Not adequate when used alone but combined with
pressure rollers therefore used as pre-treatment. Has advantage that unwithered
leaf can be processed without difficulties. Numbers of rollers required may be
about 1/3 of those required by orthodox method.
c) Lawrie Tea Processor (LTP)
Modified hammer mill with barrel containing shaft
carrying a series of knives and beaters. Shaft rotates at a high speed while
leaf blown into and discharge from machine by means of centrifugal fan.
- Advantages include:
·
low capital required,
·
low maintenance cost,
- Main disadvantage is that it can only be
operated satisfactorily
over very narrow range of low moisture content
(70-72%).
d) Rotorvane machine (RVM)
- Consists of horizontal cylinder fitted with rotor
assembly which includes a worm to drive the leaf forward.
·
Series of vanes which squeeze the leaf
against battens and indentations inside barrel of machine.
·
Gap through which leaves discharge and
adjustable to allow for pressure applied to the leaf be varied.
Comparison of various rolling methods
1) Leaf appearance- pressure roller is
favored where appearance is of prime importance. Its best grade is relatively
larger pieces of leaves, black with definite twist imparted
- RVM can produce similar style leaf but small
pieces.
- CTC grades are even and graining and generally
smaller with narrow particle size distribution.
- LTP usage results in a product somewhat similar to
CTC but rather broader particles size distribution.
2) Rate of fermentation
- Rate of reaction increases as particles of
macerated leaf reduces in size thus CTC teas are fermented about 11/2 hours,
pressure rollers 2-3 hours.
- The rate limiting step during fermentation is
diffusion of oxygen to the reaction sites within leaf particles.
- Smaller or particles that have larger surface
area: volume ratio and allow oxygen transfer faster.
- Pressure roller and to less extend RVM compress the
leaf and restricts oxygen transfer to interior of particles. This effect is minimal
in CTC or LTP rolling.
3) Liquor quality
- Liquor from CTC teas tend to be strong and
brightly coloured whereas liquors from pressure rollers are generally thinner
and less coloured but often superior in flavour due to the following reasons:
i) The more complete oxidation of volatile
precursors especially area of unsaturated fatty acid lead to high levels of
less desirable volatile substances during fermentation of CTC rolled teas.
ii) Some desirable volume substances are destroyed
by oxidation during fermentation of CTC rolled tea
iii) Where pressure rollers used the relatively
anaerobic conditions created within compressed leaf allows the glycoside enzyme
responsible for the release of the desirable volatilise: linalool and geraniol
remain active for long period resulting to high levels of them.
iv) Due to
the high moisture content of CTC rolled teas and the larger surface area of the
leaf particles, there is greater opportunity for loss of desirable volatile
substance during drying.
Roll- breaking and Green –leaf Sifting
On discharge from roller leaf mass is more or less
compressed into lumps, broken in sifting (in rectangular or sieve set on moderate
mechanical agitators.
It performs 3 essential functions:
i) Cools heat to prevent retention of any excess.
ii) Aerates tea mass.
iii) Separate leaf to portions uniform for
fermentation (feed continuously spread in sieve). These now form dhools.
FERMENTATION
Critical process that if not properly done may lead
to quality impairment. Involves a series of complex reactions of polyphenols,
enzymes (polyphenoloxidases, orthodiphenol oxidases or catechol oxidases) and
oxygen
Main chemical reaction involves enzymes naturally
present in tea leaves. Hence withering temperatures have to be low so as not to
reduce the quality of the enzymes responsible during fermentation. During
fermentation there are wide varieties of chemicals responsible for colour,
taste and flavour of the final tea. It is a critical stage with regard to tea
quality.
Fermentation has to proceed to optimum level so that
we have optimum quantities of desirable chemicals and minimize quantities of
undesirable chemicals. Once optimum levels are reached fermentation reaction
has to be quickly stopped (by drying).
Fermentation process
·
It is the process of oxidation of
leaves.
·
The mechanical aspect involves spreading
out of the leaves macerated by rolling a layer (dhools) 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.
·
The thickness of spread takes care of
oxygen access, rate of moisture loss and leaf temperature.
·
Requires good air circulation.
·
If hot climate kept humid by reducing
temperatures in controlled fermentation rooms
·
Leaf changes to coppery red shade and
looses grassy oduor and then to dark colour and pleasant aroma develops.
·
Time must be kept from rolling and be
limited for after 4 hrs there is loss of quality.
·
If leaf colour (green) is retained after
this, then it means rolling was insufficient.
·
Over-fermentation decreases quality but
increases colour (dark).
·
Daily washing of fermentation trough
surfaces to remove juices which may be sources of bacteria is required.
·
Dhools are then transferred to driers
for firing.
Fermentation products:
·
Most chemical process is oxidation of
polyphenols by atmospheric oxygen with help of oxidase.
·
Results to production of o-benzoquinone derivates from
catechins.
·
Individual phenols oxidized
successively.
·
The catechin derivatives only 2 are
important- epigallocatachin and its gallate.
·
3 variants formed: ECG, EGCG and hybrid
of EGCG and EGC.
·
The primary results of oxidation are the
o-quinones that are highly reactive and undergo wide range of different
reactions plus oxidation of other substances.
·
The rearranged pairs are bisflavanols
which are capable of further reaction with ortho-quinone already
present giving theaflavins (TF).
·
An additional oxidation not enzyme
controlled transforms theaflavins to thearubigins (TR).
Non-enzymatic oxidation Thearubigins
§
Bisflavanols are formed by condensation
of 2 molecules of 0- quinones derived from GC and are colourless.
§
TF results from oxidative condensation
of one molecule of the o-quinone derived from EGC or condensation of
bisflavanols with o-quinones.
§
They influence quality of teas-if high
then higher quality. Their formation is favoured by availability of oxygen and
sufficient polyphenol oxidase and high levels of sufficient substrates.
§
The EGC levels in fresh tea has been
found to indicate the extent of theaflavin formation on fermentation and the
ratio of GC:C is very important in determining the potential for theaflavin
formation.
§
Thearubigins are coloured phenolic
oxidation products and are a mixture of various compounds (polymeric and
non-polymeric).
.
Factors affecting the formation process
i) Oxygen
supply-should be adequate for the formation of o- quinones
and theaflavins
ii) Temperature-
maximum theaflavins are obtained at low temperatures. High temperatures
encourage oxidative polymerisation leading to reduction in theflavin and
increased polymeric thearubigins due to 2 reasons:
·
High temp results in increased activity
of peroxidase believed to mediate in conversion of theaflavins to polymeric
thearubigins,
·
it also results in reduced solubility of
oxygen in the leaf juices hence less available oxygen
iii)
Time-
should be up to just peak of theaflavins. Normally subjectively judged by
appearance and aroma of fermenting leaves:
· Green
yellow yellow/brown light/brown coppery red.
· At
low temp the time does not need to be controlled very precisely but as the temp
and reaction ratios increase, control of fermentation time is much more
important.
· It
varies from 45min t0 3 hrs depending on the temp and type of processing
equipment.
iv)
Degree of wither-the
amount of TF formed decreases with increase of the degree of wither. Possibly 2
reasons:
· activity
of polyphenol oxidase decrease with increase of the degree of wither,
· Greater
heat generated on maceration of drier leaf leading to more rapid rate of
reaction but low final levels of TF due to competing influence of temp.
v)
PH—the
initial PH of leaf juice is about 5-6 but falls during fermentation to 4.6.
Adjustment of PH to 4.5-4.8 prior to commencement of fermentation increases TF
and decreases TR thereby improving the products.
vi)
U.V Radiation-
exposure of leaf to uv accelerates the rate of reaction, increases levels of TF
and this improves blackness of final product.
Other oxidation reactions
·
The o-quinone act as oxidizing agents
are themselves reduced back to flavanols.
·
Some of the reactions are significant in
determining the composition of the volatile fractions and hence tea aroma.
·
They include oxidation of amino acids,
unsaturated fatty acids and carotenes:
i)
Amino acids-
oxidative deamination yields corresponding aldehydes with loss of CHO & NH3
.
· Some
of the aldehydes are significant flavour compounds
e.g. formation of phenyl acetaldehyde with
rose-like aroma from phenylalanine.
ii)
Unsaturated fatty acids-
oxidation yields aliphatic aldehydes and acids of lower molecular wts
· e.g.
oxidation of linoleic/linolenic acids giving rise to trans-2 hexenal, hexanoic
acid and trans-2-hexenoic acid.
iii)
Carotenes-
terpenoids, aldehydes, ketones which are important in black tea aroma are
formed on their oxidation.
N/B:
·
The other enzymatic reactions on fermentation:
·
Polyphenols and oxidative products are
powerful enzyme inhibitors and seems therefore most enzyme systems in the leaf
cease to function at or soon after maceration.
But some enzymes other than polyphenol oxidase and
peroxidase do survive e.g.relatively large amounts of methanol in
unfired leaf results to activity of pectin methyl esterase (dimethylated
pectin). Chlorophillases – injured leaf after pruning does not show any
green pigment since initial stages in the breakdown of chlorophyll are enzyme
mediated. It is likely that these enzymes also survive to some extent and
contribute to appearance of final product. Glycosidase- some volatile
substances that are significant in flavour determination such as linalool and
geraniol are present and also glycosides in the intact leaf are released
during maceration which apparently as a result of enzymatic action.
Fermentation methods
-
Floor fermentation
•
Cut rolled leaves ‘dhools’ are spread on
the floor at a layer sufficient to allow oxygen to diffuse throughout the mass
(1-2 cm thick).
•
Heat of fermentation is absorbed by the
floor and frequent air humidification done to prevent leaf surface from drying.
•
Disadvantages:
require large floor surface, unless floor is clean and smooth there is danger
of building bacteria whose role may lead to unpleasant taints in the
final product.
-
Tray fermentation
•
Leaves are spread on trays which are
then packed on mobile frames in fan room.
•
Trays may be stack in layers and each
rack gives total fermentation space of 9 m2 and leaf thickness of 4 cm.
-
Tubs-trolleys
•
Most pre-dominant in Kenya.
•
Small mobile tubs supporting bed of leaf
20-25 cm deep on a wire mesh.
•
The tubs are connected to supply of
humid air which enters the base and moves upwards through the leaf bed.
•
The leaf raked from time to time to
minimize uneven fermentation due to temp variation with the leaf bed.
-
Continuous fermentation Unit (CFU)
•
Continuous leaf flow from rollers into
fermentation unit and firing.
•
KTDA have both the tubs and CFU. The
leaf is spread on perforated belts or trays and continuous airflow through leaf
is supplied.
•
Fermentation proceeds as the leaf moves
on belts.
•
Often stacked belts with intermediate
ball breakers are used and more than one belt is used.
Disadvantages:
-
Expensive especially capital cost and
running costs.
-
Difficult to maintain compared to tubs
-
Difficult to clean.
Advantages:
-
Minimal space required
-
Provision of continuity in production
process once the leaf is fed.
FIRING/DRYING
-
The process during which the enzyme
activities are stopped
-
Moisture content is brought down from
45-50 % to 2-3 % in dried black tea.
-
It also allows development of black tea
aroma
-
Physically achieved by blowing hot air
through fermented leaves as they are conveyed in chains.
-
The drying process lasts for about 20
minutes.
-
Drying in steps till last (bottom trays)
or on fluidized beds.
-
The hot air is provided for by furnace
or steam boiler through passage tubes.
-
Induced drought is maintained by fans.
-
Dhools fed into a hopper ancillary to
which is an automatic spreads.
-
There are automatic thermometers
recording inlet and outlet temperatures.
-
Subject to temperatures of 140 oC< in
dryers. There are 3 stages:
1. Chemical changes take place during drying which
are very important for quality of the final product.
·
There is color development.
·
Dried tea particles come out of dryers
which vary widely in sizes.
a)
It subjects fermented tea to forced
blast of hot air in such manner that hottest air comes first into contact with
driest tea.
b)
Through water evaporation fermented leaf
looses coppery red colour and transforms to black tea.
c)
At commencement fermentation proceeds at
accelerated rate.
d)
Enzyme activity declines steadily and
only ceases all together when moisture content falls to about 20 %.
e)
Time taken to reach this point is
critical:
•
If drying is too rapid the outer layer
of the leaf particles or agglomeration of leaf particles will harden and
prevent diffusion of moisture within.
•
If too slow the period of high
temperature fermentation will be too long and unpleasant taste develops and tea
is said to be stewed.
·
Precise temperature time regime is less
important once enzyme activities have been completely arrested but it is
desirable that leaf reaches highest possible temperature consistent with
avoidance of burnt taste.
·
This ensures maximum possible enzyme
inactivation.
·
The typical drying temperature ranges
100-150 oC at the start and low temperatures used as the end of leaf
temperature approaches air temperature.
·
Low temp is sometimes employed with
object of retaining high proportion of flavoury volatiles.
Chemical changes during firing
·
Arrests enzymatic activities and
consequently biochemical processes.
·
Large proportions of essential oils
formed on fermentation are volatilized (75-80 %).
·
Practically every fraction of essential
oils is reduced.
·
Total acids increase.
·
Quantity of nitrogenous compounds is
reduced.
·
Methanol content reduces as it is
volatilized (formed after fermentation due to demethylation of methyl thus
decreased toxicity.
·
Heat sensitive vitamins e.g. E are lost.
·
Contents of glucose, sucrose and starch
decrease insignificantly which is very important for good quality since firing
results in specific tea aroma due to partial caramelization of soluble
carbohydrates.
·
At high temperature fermented tea leaf
undergoes certain transformation leading to typical lactic acid aroma and
contents of many compounds determining aroma increase e.g. alcohols, aldehydes,
acids.
Tea driers
Tea drying is predominantly done on fluidized beds
but 15 years ago it was done on endless chain driers.
1. Endless chain drier (ECD)
·
As stacked bed continuous fermentation
unit without ball breakers.
·
Used in series of slowly moving
perforated belts of trays placed above each other.
·
Fermented tea is fed at one end and
falls and moves to the other end where it falls to the next lower level of the
belt.
·
The process is repeated till tea is
discharged at the end of the lowest belt.
·
Heated air is fed at the bottom of
stacked belts and flows upwards through tea leaf and discharged to the
atmosphere after the top tea.
·
Counter current occurs with driest tea
leaf at the bottom belt in contact with hottest incoming air and moist tea leaf
on top belt contact air of lowest temperature.
·
The stack belt thus is like black box
giving counter current but also cross-flow.
·
Drying temperatures were in the region
of 104-110 ֯C
and some literatures indicate low temp of 82-94 ֯ C
2. Fluidized bed driers (FBD)
·
Layer of tea in the drying chambers is
fluidized by passing hot air through an upward direction.
·
There is intimate contact between air
and leaf particles resulting in efficient uniform drying.
·
2-3 drying sections each with own air
supply.
·
Fluidization due to upward air movement
through the bed into the tea particles.
·
The leaves are not stationary and do not
rest on each other but move continuously on suspension.
·
It is very important to control air velocity.
·
At very low velocity no fluidization and
beyond high limit, particles are carried away.
·
Thus minimum and maximum limits of
velocity are observed.
·
There is always entrainment of
particles. Air passes through cyclone which separates air
·
Air heating is usually done by steam but
is the same as endless chain drier
Advantages
·
Few moving parts hence low maintenance
costs
·
Produce cleaner tea with less fibre and
more grainy as fibre is entrained by air.
·
More surface area of heat and mass transfer
therefore more efficient drying.
Disadvantages
·
May be tendency to stewing in entirely stages
where thick bed not fluidized and heat transfer is poor.
·
The leaf is moved through machine by air
flow with mechanical aid consequently precise control over residence time not
possible and particles are unevenly fired. Particles taking too long may over
dry and have poor quality and vice versa under dried.
·
Very high air flows required for
fluidization may result in greater loss of volatile substance than in tradition
ECD.
-
Tea from drier is a mixture of fibre and
tea leaf with broad particle ; Size distribution to be separated into sizes. Fiber
extraction– sorting by means of PVC coated rollers.
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Friction causes the surfaces to acquire
electrostatic charges which attract fibre and attached as tea moves along belt.
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Fiber is removed from the belt.
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Grading-
by screening: series of screens of different of different sizes with one
largest and the bottom smallest aperture.
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Whole unit vibrates ( vibro screens )
separates to 4 grades:
•
BP1-Broken
pekoe 1
•
PF1-
pekoe fanning 1
•
PD1-
pekoe dust 1
•
D1-
Dust 1
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In factory for export and local
packaging industry, not for consumers.
-
Can be blend on requirement.
- Pure grade is rare in the market.
·
To minimize water vapour since tea is
hydroscopic.
·
Flavour retention very important: Use
paper sack for export.
·
4 layers Kraft paper lined with Al foil
normally used.
·
Tea sold to packers in canvas bags lined
with polythene sheet.
·
Looking at tea leaf particles as they appear
(dry tea, black or brownish.
·
Cleanliness:
(fibrous or dusty), even, twisted, curled or desired.
·
Infused leaf (after extraction) colour,
dust, brightness, is colour mixed or one type.
·
Taste of liquor:
taste, colour, body (thickness) and flavour.
Instant tea manufacture : Teas
soluble in hot and cold water; used in some countries as ice tea mixed
with other ingredients e.g. cold lemon tea mix which has sugar, citric
acid, natural lemon flavour and tricalcium PO4.
·
Processing operations include:
·
Black tea fermented and dried leaf,
oolong or green tea i.e. uses finished products of primary tea factory.
·
Choice made on:
•
Processing requirement
•
Market tea intended for
•
Commercial restriction on manufacture
Extraction:
·
Use of water solution compounds by
extension liquid (water recycled).
- Final concentration of water fairly low by 3-5 %
TS.
·
Can be batch or continuous
·
Batch-
use extension tanks and water at 80-900c. Tanks remain stationary but
extraction liquid moves from tank to another.
·
Extraction continues for some time and
tank emptied before introduction of fresh leaf.
·
Continuous more convenient e.g.
Counter-current continuous extraction
-tea leaves introduced at lower end and conveyed
upwards by rotation of screen while extracting water flows in reverse
direction.
·
Soluble solids transferred from leaves
to water.
·
The screws perforated to allow water
flow more easily through tea body.
·
Extraction temperature controlled by
series of heater on the body of the unit.
Extraction results in loss of aroma compounds
therefore need to retain.
·
Vapour which aroma compounds condensed
(dilute aroma) hence concentrated by refractionating column.
·
Aroma comes top as vapour and is
condensed.
·
Some aroma taken back to distribution
column to aid it reflux but the rest taken to storage tank.
·
Reflux ratio= 20:1 (storage: tank)
·
The distillation column has 2 fractions
(top- aroma concentrate, bottom is water with some aroma compounds which form
part of extraction liquid in the extractor.
·
The dearomatized extract from the
separator is evaporated by steam heating.
·
Vapour concentrated mixture goes to
separator (separate to liquid and vapour) and
·
Liquid is then concentrated and taken to
storage tank as aroma concentrate.
·
Part of concentration is further
concentrated.
·
Vapor fraction from separator is mixture
of vapour and aroma compounds.
·
Thus dearomatized extraction has some
aromas condensed in 2 stages.
•
Using ordinary water and condensate
forms part of extension liquid in extractor.
•
Vapour fraction from separator (those
not condensed in first condenser). It is a refraction condenser using chilled
liquid to condense aroma compounds.
- Condensate taken to storage tank and part of it
joins dilute aroma on its way to distillation column.
· Concentrate
plus aroma concentrate and part of aroma condensate are stored together.
· Corn
syrup solids usually added to these concentrates before drying to retain as
much aroma compounds as possible.
· Drying
also results in loss of aroma due to evaporation.
Drying- 2 methods
commonly used:
· Spray
drying- mostly
· Freeze
drying- more expensive. This tea is more expensive, high quality due to
superior reconstitution ability.
-instant tea powder has MC 3% but very hygroscopic
and absorbs moisture if exposed on air to unsafe levels. Ensure minimal entry
of water by always tinning.
Cream processing -
Done only when tea is subjected to temperatures below 6 0c. Thus avoided by not
cooling to these low temperatures, which turn extract into cream of opaque
brown colour which gives non uniform product. It is processed especially alone
before concentration.
Quality Questions
Biochemistry researches determine constituents that
promote good quality:
Ethyl acetate dissolving substances indicate quality
produced by efficient fermentation.
For good colour TR/TF should be fairly Ltd (at best
10-12 range) by regulating fermentation time and method since TR is 10*
Prime consideration of quality is nature of plucked
leaf, good leaf of high polyphenol content, high enzyme activity, physically
allows tissues to be worked on. Thus course plucking results in poor quality
tea because of
·
Low average polyphenols
·
Mature and more lignified leaf not
easily macerated and cells distorted giving free access to air on fermentation
and contains enzymes mingled with polyphenols
·
Rhythm of growth of tea bush affects
quality e.g. Quality improves with age after pruning up to 4 years.
·
Damaged leaf not good due to
uncontrolled fermentation.
·
Slow growth results to high quality
·
Long withering increase bacteria which
is not required Finished tea should have 3 %mc and since they are hygroscopic
will pick moisture if not controlled (9-10%) mc encourages molds giving musky
taint to the dry liquor
Healthy benefits of tea
Body protection against heart diseases, stroke,
cancer and more; due to high levels of antioxidants (polyphenols, flavonoids
and catechins) they scavenge radicals.
White tea has more antioxidants than all.
Green tea has more catechins than black.
Unfermented has more polyphenols.
Brewed (caffeinated) has more antioxidants. Tea
drinking has the following benefits: Low blood pressure, may lower the cholesterol level therefore reduce chances
of heart attack.
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