TECHNICAL FIELD The present invention relates to a method for the continuous production of coffee extract, in which the roasted beans are ground and mixed with water to form a suspension which is continuously extracted and cooled.

BACKGROUND ART

The object of the majority of prior art methods for producing a coffee extract is to evaporate a coffee solution to form a powder which, when it is to be consumed, is intended to be dissolved in hot water. Normally, these production processes proceed batchwise in columns or alternatively, as described in Patent Specification WO 82/03156, in a continuous process. In the intermittent - or batchwise - process, the roasted and ground coffee beans are packed into columns. In that hot water is allowed to pass through the packed column, the flavour and aroma substances which are inherent in the ground beans are leached out to form a liquid coffee extract. However, this batchwise extraction is both difficult to control, time-consuming and, at the same time, circumstantial to carry into effect. The extracted coffee solution is evaporated and thereafter dried to a powder by treatment with hot air, or by means of freeze-drying.

The coffee aroma - which in actual fact is linked to an emulsion of coffee oil in water - consists of volatile substances. A proportion of these aromatic substances disappears more or less during the process from the roasted coffee bean to the finished, concentrated coffee powder. The roasted coffee beans display a relatively slight exposure surface area, for which reason but a slight amount of aromatic substances become volatilized prior to the grinding operation. The purpose of the roasting per se is to achieve a specific aroma precisely for those beans being roasted. Thereafter, the beans are ground, which implies a heating of the coffee which, at the same time, is comminuted or finely divided and thereby displays a large exposure surface area. This means that a not inconsiderable amount of aromatic substances is volatilized already during the grinding operation.

The largest single effect on the aroma and flavour is, however, exercised by the evaporation and drying in which hot air is allowed to act on a finely divided coffee extract. In this process step, much of the specific aromatic substances which impart to a good coffee the desired aroma and flavour are volatilized. Even with expensive freeze-concentration and freeze- drying the aroma losses are considerable.

OBJECTS OF THE INVENTION

It is, therefore, desirable in the Art to be able to produce a coffee extract which has retained most of the pristine aromatic substances which were found in the coffee beans and which are developed during the roasting operation. The major object is to realise a high quality coffee extract which, however, does not preclude favourable quantitative yield in respect of the finished product.

SOLUTION

This and other objects have been attained according to the present invention in that the method of the type disclosed by way of introduction has been given the characterizing features that the roasted coffee beans are positively fed into water, whereafter they are ground to a coffee suspension which is fed into at least one extraction apparatus for continuous extraction, and that the coffee suspension and coffee extract are treated in a closed system until the finished coffee extract is cooled.

Preferred embodiments of the present invention have further been given the characterizing features as set forth in the appended subclaims.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

One preferred embodiment of the present invention will now be described in greater detail hereinbelow, with particular reference to the accompanying Drawings, in which:

Fig. 1 shows a functional diagram relating to the method according to the present invention;

Fig. 2 shows a part of the functional diagram of Fig. 1;

Fig. 3 shows the principle of a colloid mill; and Fig. 4 shows the principle of a decanter centrifuge.

DESCRIPTION OF PREFERRED EMBODIMENT

Fig. 1 shows, in a functional diagram, the different components and process steps which are included in the method according to the present invention. The roasted coffee beans are buffer stored in a buffer tank 1 or, alternatively, are fed direct from the roasting step to the extraction process. In order to maintain pristine quality right from the outset, the ready-roasted coffee beans should be stored for as short a time as possible and they should, hence, be processed on the same day as the roasting takes place. A simple principle of feeding from the buffer tank 1 is shown in Fig. 1, where a supply conduit 2 from the buffer tank 1 discharges in a container or hopper 3. When the hopper 3 has been filled to the desired level, the beans stop in the supply conduit 2 until the level in the hopper 3 falls once again. The hopper 3 may be closed so as to reduce exposure of the coffee beans to the ambient air. The hopper 3 discharges in a tube 4 in which is disposed a screw 5, with an electric motor 6. Since the roasted coffee beans are so light as to float in water, the beans must be positively fed by means of the screw 5. By varying the quantity of coffee beans that are metered in, it is possible to obtain coffee extracts of different concentrations. Water at between 10 and 100°C, preferably at approximately 75°C, is fed into a vessel 7 with a level meter 8 which regulates such that the system always contains the requisite quantity of water. Given that the system is always filled with water, air is prevented from gaining access, which reduces the oxidation of the aromatic substances of the coffee, at the same time as volatile substances are prevented from escaping. The temperature of the water, approximately 75°C, has been selected so that the water will be able to draw out most of the aroma and flavour substances from the ground coffee beans without any negative effect on these substances. The vessel 7 is in open communication with a tube 9 to which the tube 4 also discharges. As a result of the open communication between the tubes 9 and 4, the vessel 7 and the tube 4 will function as a communicating vessel so that the liquid level in the vessel 7 and the liquid level in the tube 4 will lie flush with one another. The tube 9 also forms the inlet to a colloid mill 10.

The colloid mill 10 is of the conventional type whose operating principle is illustrated in Fig. 3. Basically, the colloid mill 10 consists of a stator 11 and a rotor 12. The rotor is caused to rotate by means of an electric

motor 13. In the interspace 14 between the stator 11 and the rotor 12, there are two toothed surfaces 15 and 16, of which the one, 16, rotates at high speed. The coffee beans mixed with water are exposed in the mill 10 to high shearing and frictional forces, but also to powerful turbulence which contributes in the fine grinding of the coffee beans and their mixing into a homogeneous coffee suspension. The grinding degree of the mill 10 is adjusted by axial displacement of the stator 11 or the rotor 12.

The coffee beans being surrounding by water throughout the entire grinding process, this water will act as a coolant during the grinding operation so that the finely ground coffee, with its large exposure surface area, is immediately cooled down, which reduces the escape of and influence on the volatile aromatic substances.

The mixed coffee suspension departs from the mill 10 through the outlet 17. The outlet 17 is in open communication with the tube 9. The colloid mill 10 has such large pumping output effect as to require an arrangement as illustrated in Figs. 1 and 2 in which the tubes 4, 9 and 17, as well as the mill 10, together form a "feed-bleed" type circulation cycle 18.

An output governor pump 19 of the gear wheel or eccentric screw pump type conveys further a given quantity of coffee suspension through the conduit 20. The remaining quantity of coffee suspension is circulated in the circulation cycle 18. The conduit 20 affords a certain hold time and forms an inlet to a continuous extraction apparatus which, in the preferred embodiment, consists of a decanter centrifuge 21.

The function of a decanter centrifuge 21 is shown in Fig. 4. In a partly conical and partly cylindrical rotor 22, there is disposed a screw conveyor 23. The screw conveyor 23 rotates in the same direction as the rotor 22 but generally at lower speed. The product, in this case the coffee suspension, is fed into the rotor 22 via the inlet 20. The liquid-mixed product, the coffee suspension, is caused to sediment in the rotor 22 so that the solid matter is accumulated in the conical end 24 of the rotor 22 where the solid matter departs from the decanter centrifuge through the outlet 25. The liquid is separated under pressure by means of index discs 26 and departs from the centrifuge 21 via the outlet conduit 27.

The extraction of coffee in a decanter centrifuge 21 is wholly continuous. The process may be carried out in a centrifuge 21 which, with specific arrangements, may be rendered as good as closed so as to prevent

the access of air and the loss of aromatic substances. The stay time in the process is further extremely limited and, thus, the coffee extract will be processed in the shortest possible time. The liquid coffee extract that departs from the centrifuge 21 via the outlet conduit 27 is an extremely high-quality coffee extract which, to the least possible degree, has been exposed to heat or air which may volatilize and oxidise the aromatic substances.

The finished coffee extract is conveyed further, possibly via a filter 28 of conventional type, or alternatively a centrifuge. Depending upon the desired coffee type, with differing quantities of suspended matter, the finished coffee extract may, as a further alternative, be led directly to a cooler 29 by means of a conventional centrifugal pump 30. For, for example, the production of coffee extract of expresso coffee type, no filter 28 is required, while such a filter may be desirable in the production of extracts for clearer, brewed coffee types. The cooler 29 may, as in Fig. 1, consist of a plate heat exchanger with cold water as the coolant medium. In the plate heat exchanger, the coffee extract is forced to circulate between the plates and, on the opposite side of the plates, cold water is circulated so that a continuous cooling process is obtained. The cooled, finished coffee extract is conveyed further to some form of storage or buffer tank 31. The now cooled extract is not as sensitive to the action of air, and in addition the absence of heat renders the aromatic substances less likely to volatilize. The finished extract may now, possibly after a certain shelf-life extending heat treatment, be packed either in large aseptic containers for further transport to packaging in consumer distribution packages, or, after formulation, i.e. dilution and the addition of ingredients such as sugar, milk or flavourings, be packed direct into such consumer packages.

The coffee extract that is the result of the above-described method maintains a high standard of quality in that the entire extraction of the coffee extract has taken place in a closed system 32. The closed system 32, where the supply of air and loss of aroma have been reduced to a minimum, encompasses the whole of the above-described process from infeed of the roasted coffee beans in hot water, and until the finished coffee extract has been completely cooled.

In order further to increase the quantitative yield gained from the process, it is possible, as shown in Fig. 1, to employ an additional decanter centrifuge 33. Water, at a high temperature, is added via the water conduit 34 to the almost solid matter that departs from the first decanter centrifuge 21 via the outlet 25. In such instance, there will be obtained a new coffee suspension of considerably poorer quality than that which had been processed in the first centrifuge 21. The new suspension is collected possibly in a container 35 in order thereafter to be pumped by means of a centrifugal pump 36 in the conduit 37 which forms an inlet to the second centrifuge 33. The function of the second centrifuge 33 basically corresponds with that of the first centrifuge 21, as described above. The solid matter that departs from the centrifuge 33 via the outlet 38 now contains very little of the soluble flavour and aroma substances of the coffee, and this matter is conveyed off for possible compost deposition. The extracted liquid departs from the centrifuge 33 via the outlet 39. This liquid consists of an extract of considerably poorer quality than the extract which is attained by means of the first centrifuge 21. The liquid from the centrifuge however contains certain soluble flavour and aroma substances which may contribute to an increase in the quantitative yield gained from the process as a whole. This liquid is recycled via the conduit 40 back to the vessel 7 where it will constitute a part of the mixing medium for the newly supplied coffee beans. The second decanter centrifuge 33 with its conduits 37 and 40 is not encompassed in the closed system 32, given that the result of the continuous extraction in this centrifuge 33 is an extract of poorer quality. As will have been apparent from the foregoing description, the present invention realises a method of continuously producing a coffee extract of high quality which retains its pristine aroma and flavour substances in that the coffee, throughout the closed process is, as far as possible, not exposed to unnecessary thermal or air effects, at the same time as aroma losses are minimised. Moreover, the concentration of the coffee extract may be varied in that different quantities of coffee beans are metered into the system. This affords the possibility of producing a concentrated extract without any extra process step such as, for example, evaporation.

The present invention should not be considered as restricted to that described above and shown on the Drawings, many modifications being

conceivable without departing from the spirit and scope of the appended Claims.