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.

 

 

 

 

 

 

 

SORTING AND GRADING

-        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.

-        Friction causes the surfaces to acquire electrostatic charges which attract fibre and attached as tea moves along belt.

-        Fiber is removed from the belt.

-        Grading- by screening: series of screens of different of different sizes with one largest and the bottom smallest aperture.

-        Whole unit vibrates ( vibro screens ) separates to 4 grades:

•         BP1-Broken pekoe 1

•         PF1- pekoe fanning 1

•         PD1- pekoe dust 1

•         D1- Dust 1

Packaging

-        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.

Tea Tasting

- Quality control involves:

·        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:

Selection of raw materials

·      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.

 

Aroma stripping

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.

 

Concentration

·      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.

 

Packaging

-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.