GB2035039A - Apparatus and method for tea fermentation - Google Patents

Abstract

Apparatus for continuous fermentation of tea under temperature controlled conditions, comprises at least one conveyor (1,2,3) having a perforate support (20) for supporting a layer of macerated tea leaves, and cooling means (6-12) for passing a stream of oxygen-containing cooling gas through the perforate support and the layer of tea so as to control the temperature of the layer. The perforate support may be a fabric or perforate metal conveyor belt. The cooling gas is preferably refrigerated air, optionally enriched with pure oxygen. <IMAGE>

Description

SPECIFICATION Apparatus and method for tea fermentation The present invention relates to the continuous so-called "fermentation" of macerated tea leaves and provides an apparatus for controlling the temperature of fermentation, which is particularly suitable for use in regions of high ambient temperature during the tea cropping season e.g.

Central Africa.

The production of black tea generally involves three stages. In the first stage the green tea leaf is macerated by cutting and/or rolling in a processing machine so as to disrupt the leaf cells and release some of the juices. This mass of cut tea is called the dhool. In a second stage the dhool is allowed to "ferment" by allowing enzymes present in the juices to oxidise certain of the substrate compounds present, so as to produce the coloured tasting compounds characteristic of the tea beverage.

In a third stage the dhool is dried or fired to obtain black tea.

During the fermentation stage the exothermic enzymic oxidation of leaf substrate leads to a rise in temperature of the dhool. However, it is well known that the temperature rise has a detrimental affect on the final quality of the tea.

It has been proposed in British Patent Specification 1 478 1 39 to provide a process for the continuous fermentation of tea leaf which involves controlling the temperature of fermentation to between 20 and 30"C whilst simultaneously agitating the tea leaf. The specification also describes an apparatus for carrying out the process on conveyor belts and which provides for aerating the fermenting dhool at intervals in a ball breaker fed with conditioned air. However, at high ambient temperatures, such as are commonly encountered in tea growing regions during the cropping season, the cooling is often insufficient to provide adequate temperature control.

The present invention provides an apparatus for continuous fermentation of tea under temperature controlled conditions, which comprises at least one conveyor having a perforate support for supporting a layer of macerated tea leaves, and cooling means for passing a stream of oxygen-containing cooling gas through the perforate support and the layer of tea so as to control the temperature of the layer.

The invention also provides a method for the continuous fermentation of tea under temperature controlled conditions, which comprises providing at least one conveyor having a movable perforate support a layer of macerated tea leaves, and passing a stream of oxygencontaining cooling gas through the perforate support and the layer of tea so as to control the temperature of the layer.

The perforate support allows the cooling gas to pass freely through and is advantageously formed of perforated fabric, open-weave fabric, perforated metal sheet, perforated plastics or rubber film, or metal gauze. Generally the perforate support will be a movable conveyor belt, but may also be an endless chain of interlinked trays.

The rate at which the cooling gas passes through the layer of dhool is sufficient to cool the dhool, and preferably such as to prevent any compaction of the layer and to maintain a uniform temperature throughout the layer. However, the dhool remains in a layer on the support.

Dhool leaves the processing machine at a relatively high temperature e.g. 35 to 40"C.

Preferably, three conveyors arranged in series are used. A preliminary cooling may be achieved on the first conveyor by means of air at ambient temperature. The layer of tea is preferably relatively thin so that cooling substantially to wet bulb ambient temperature e.g. 21 to 27"C may be achieved in a short time e.g. 2 to 3 minutes.

The tea then passes onto the second conveyor, which preferably runs at a slower speed than the first conveyor so as to build up a deeper layer of tea. In order to cool the tea still further, air cooled below ambient temperature by refrigeration means, may be blown through the perforate support to reduce the temperature of the tea to e.g. 1 2-20 C.

On the third conveyor, the tea is maintained at the desired optimum temperature e.g.

1 2-20 C so as to complete the fermentation process. The third conveyor is preferably run still slower than the second conveyor, so that most of the fermentation occurs at the optimum temperature.

The conveyors may be provided with variable speed drives e.g. hydraulic or electric motors, and the first, second and third conveyors are preferably operated at relative speeds of 14-5:5-1:1 respectively, more preferably substantially 9:3:1. The thickness of the respective layers will be in the inverse ratio of the conveyor speeds.

A spreader may be provided at least on the first conveyor for ensuring a layer of uniform depth, so as to prevent temperature gradients.

The creation of a uniform temperature within the tea may also be assisted by a homogenising and aerating device such as a ball breaker or vibrating sieve before the second and third conveyors. Hot spots and non-uniform fermentation may thus be avoided.

The cooling gas is preferably air, possibly enriched with oxygen to ensure an adequate supply of oxygen to the leaf during fermentation. The air may be ambient air flown by a fan through ducts provided below the perforate support. If necessary, refrigeration means e.g. a cooled heat exchanger matrix may be located in the duct.

The apparatus ensures a continuous cooling of the dhool and thus provides good uniformity of temperature throughout substantially all the fermentation time, even at very high ambient temperatures. e.g. 30 to 35"C.

An embodiment of the invention will now be described by way of example with reference to the accompanying drawing, which is a purely schematic representation including a fermentation apparatus.

The drawing shows a conventional processing machine 4 having a rotary drum 5 (e.g. a Lawrie Tea Processing Machine) for disrupting the tea leaf and feeding it to the fermentation apparatus.

The fermentation apparatus shown comprises three belt-conveyor assemblies 1, 2 and 3. Each conveyor assembly comprises a front and rear drive wheel 19, 1 3 respectively, driven by a variable speed motor (not shown) and driving a perforated conveyor belt 20. The belt 20 defines with walls 21 a tray for containing a layer 22 of macerated tea leaf.

Ducts 8, 9 and 10 are provided for blowing cooling air through the respective perforated conveyor belts to cool the layer of tea thereon and maintain a substantially uniform temperature throughout the layer. Air at ambient temperature is fed into the ducts via an inlet duct 7 and fan 6. The ambient air is fed directly through duct 8 to achieve a preliminary cooling of the tea in the first conveyor assembly 1. However, the air in ducts 9 and 10 is cooled by passage through heat exchanger matrices 11 and 1 2 of a refrigeration unit (not shown).

Macerated tea (dhool) is fed onto the belt 20 of the first conveyor assembly 1 from the processing machine 4 via a duct 23. A rotary spreader 1 6 spreads the tea into a uniform layer 22. An exhaust air duct 1 7 is provided for removal of the cooling air after passage through the perforated belt 20. The use of ducts 23 and 1 7 helps ensure that the tea is deposited on the upstream end of the belt 20 and not blown away by the cooling air.

At the downstream end of conveyor assemblies 1 and 2 are provided rotary ball breakers 14 and 1 5 respectively. The ball breakers break up any agglomerations of tea leaves so that a homogenous layer is deposited onto the succeeding conveyor belt. In this way localised high temperature regions within the layer of tea are avoided.

Operation is as follows: Green tea leaves 1 8 are fed into the processing machine 18 and macerated tea is deposited onto the perforated belt 20 of the first conveyor assembly. At this stage the tea is at a temperature of around 38"C due partly to heat generated mechanically in the processing machine 18 and partly to the onset of enzymic "fermentation" of the leaves. The blast of ambient air through the belt 20 reduces the temperature of the tea to wet bulb ambient temperature e.g. around 24"C in two to three minutes.

After this initial cooling, the tea passes onto the second conveyor assembly where it is cooled still further to below wet bulb ambient temperature e.g. to around 16"C. On the third conveyor assembly the tea is held at the desired optimum fermentation temperature e.g. 12"-20"C.

The fermentation time spent by the tea on the conveyor assemblies may be from 1 5 mins to 3 hours. Typically most of this time is spent on the third conveyor assembly. Thus, the speeds of assemblies 1 and 2 are typically 9 and 3 times respectively the speed of the third conveyor assembly. Consequently, the thickness of layers 16, 24 and 25 are in the ratio 1:3:9.

After leaving the third conveyor assembly the fermented tea is passed to a drier.

Examples of the invention will now be described: Example I Indian hybrid seedling tea which had been withered to a moisture content of 70%, was macerated by a Lawrie Tea Processing (LTP) unit. The comminuted tea (the dhool) was allowed to ferment in three different systems, two simulating acceptable commercial batch processes and the third according to the current invention.

The dhool was spread to a depth of 5cm for a tray fermentation, 12.5cm for trough fermentation with a forced ambient air current, and 2.5, 7.5, and 22.5cm in stages 1, 2, and 3. of a continuous fermentation unit according to the invention.

6 temperature recording were made at various depths at intervals throughout the course of fermentation.

Mean Ambient Temperature = 1 9.2 C.

Mean Temperature of leaf from L.T.P. = 32.9"C.

A Tray Fermentation System (Comparative) Fermentation Temperature Temperature Standard Time Range Mean Deviation Min. "C "C "C 5 32.0-34.5 33.25 0.85 10 32.5-35.5 34.25 0.96 15 31.5-36.0 34.66 1.52 30 31.0-35.0 34.08 1.43 45 30.0-32.5 32.00 0.91 60 28.0-32.0 30.42 1.55 75 25.0-29.0 26.75 1.52 90 23.0-26.0 24.08 1.10 105 21.5-26.0 22.92 1.07 120 21.0-25.5 23.17 1.46 Mean Temp. "C throughout fermentation 29.56 + 4.60 B Trough Fermentation System (comparative) Fermentation Temperature Temperature Standard Time Range Mean Deviation Min. "C "C "C 5 25.0-34.0 28.83 3.24 10 22.5-28.5 25.42 2.21 15 19.5-25.0 23.00 1.41 30 18.5-22.0 19.75 1.21 45 18.0-20.5 18.83 0.69 60 18.0-19.5 18.58 0.55 75 17.0-18.5 17.66 0.49 90 17.5-19.0 18.08 0.55 105 17.0-18.5 17.58 0.54 120 17.5-18.0 17.42 0.46 Mean Temp. C throughout fermentation 21.29 + 3.76 C. Continuous Fermentation Unit (not using refrigeration facility) Fermentation Temperature Temperature Standard Time Range Mean Deviation Min. C "C "C 5 18.21 20.00 1.04 10 16.5-18.0 17.08 0.57 15 17.0-18.5 17.83 0.64 30 17.0-19.0 18.08 0.80 45 16.0-18.5 18.00 0.91 60 17.5-20.0 18.42 0.90 75 16.0-19.0 18.17 1.11 90 17.0-18.5 18.25 0.56 105 17.0-19.0 18.33 0.85 120 17.0-18.0 17.33 0.39 Mean Temp. C throughout fermentation 18.5#0.86 The continuous fermentation unit thus provided a temperature which was generally lower and much more constant than the other two methods.

EXAMPLE II Performance of the unit in controlling fermentation temperature during three typical occasions during the hot season, when ambient air temperatures are high, was monitored. Ambient air temp. C 22.5 24.4 26.3 Temp. of leaf ex L.T.P. C 34.6 35.8 37.2 Mean temp. during tray fermentation C 30.17#4.38 32.42#4.85 33.08#4.74 Mean temp. during trough fermentation C 24.2#3.61 26.17#3.65 28.20#4.10 Mean temp using continuous unit C 21.83#0.93 23.48#1.23 25.73#1.02 Mean temp. using continuous unit with refrigeration system set to operate at 17 C 20 C 20 C 19.02#0.79 20.85#0.95 21.14#0.93 Teas made from the different fermentation systems were analysed for quality parameters and evaluated as under.

Mean Fermentation Optimum Chemical Analysis Evaluation Temp. Time TF TR TC P Kg-1 Tray 30.25#4.57 60 min. 11.2 17.8 3.97 88 Trough 23.48#3.41 75 min. 14.9 16.4 3.71 95 Continuous Fermenter 19.21#0.83 105 min. 17.2 15.2 3.64 102 Set to operate at 17 C The tea produced in the continuous fermenter was thus of higher quality than in the other two methods.

Claims (20)

1. An apparatus for continuous fermentation of tea under temperature controlled conditions, which comprises at least one conveyor having a perforate support for supporting a layer of macerated tea leaves, and cooling means for passing a stream of oxygen-containing cooling gas through the perforate support and the layer of tea so as to control the temperature of the layer.

2. An apparatus according to claim 1, wherein the conveyor is a continous belt-conveyor and the perforate support is a fabric or metal conveyor belt.

3. An apparatus according to claim 1, wherein the perforate support is a series of interlinked perforate trays.

4. An apparatus according to any preceding claim, wherein the conveyor is provided with a variable speed drive.

5. An apparatus according to any preceding claim, which further comprises a spreader for forming a layer of tea of uniform thickness.

6. An apparatus according to any preceding claim, wherein the cooling means comprises a blower and ducts for directing a stream of air, at or below ambient temperature, through the perforate support.

7. An apparatus according to any preceding claim, which comprises three conveyors arranged in series, the tea being conveyed by each conveyor in turn.

8. An apparatus according to claim 7, wherein the cooling means further comprises refrigeration means for cooling the air directed to the second and/or third conveyors.

9. A method for the continuous fermentation of tea under temperature controlled conditions, which comprises providing on at least one conveyor having a movable perforate support a layer of macerated tea leaves, and passing a stream of oxygen-containing cooling gas through the perforate support and the layer of tea so as to control the temperature of the layer.

10. A method according to claim 9, wherein the cooling gas is air, optionally enriched with oxygen.

11. A method according to claim 9 or 10, wherein there are three conveyors arranged in series and tea is conveyed by each conveyor in turn.

1 2. A method according to any of claims 9 to 11, wherein on the first conveyor the tea is cooled down to 21 to 27"C.

1 3. A method according to claim 12, wherein the cooling is achieved in 2 to 3 minutes.

14. A method according to any of claims 11 to 13, wherein on the second conveyor the tea is cooled down to 1 2-20 C.

1 5. A method according to claim 14, wherein the tea is maintained at a temperature of 1 2-20 C on the third conveyor.

1 6. A method according to any of claims 11 to 15, wherein the speeds of the first, second and third conveyors are in the ratio 14-5:5-1:1.

1 7. A method according to any of claims 11 to 1 6 carried out in from 1 5 mins to 3 hours.

1 8. A method for the continous fermentation of tea substantially as described with reference to the drawing.

1 9. Black leaf tea wherever produced from tea subjected to the method of any preceding method claim.

20. An apparatus for continous fermentation of tea substantially as described with reference to and as shown in the drawing.