WO2014096183A1

WO2014096183A1 - Device for producing milk foam

Info

Publication number: WO2014096183A1
Application number: PCT/EP2013/077362
Authority: WO
Inventors: Youcef Ait Bouziad; Alfred Yoakim; Alexa Perrin
Original assignee: Nestec S.A.
Priority date: 2012-12-21
Filing date: 2013-12-19
Publication date: 2014-06-26
Also published as: US20180303279A1; US10028613B2; RU2015129844A; AU2013366725A1; JP2016504102A; RU2617611C2; BR112015014474A2; MX2015008112A; CA2894392A1; CA2894392C; AU2013366725B2; PT2934254T; ES2707053T3; TR201901632T4; US10561270B2; EP2934254B1; CN104883943B; IL239230A0; KR20150096515A; US20160000257A1

Abstract

The present invention provides a device 1 for producing milk foam, which makes use of Couette flow and a high shear stress that is accordingly applied to a milk-air mixture between two concentrically arranged cylinders 2, 3. The cylinders are rotated relatively to another. The high shear stress leads to an emulsion of the milk and the air, which is the basis for a foaming effect, once the emulsion flows out of a gap 6 between the two cylinders 2, 3 and expands. The parameters of the device 1 that mainly influence the foaming effect are the width of the gap 6 and the relative rotation speed of the cylinders.

Description

Device for producing milk foam

The present invention is directed to a device for foaming a fluid, preferably for producing milk foam. In particular the device is designed in a way that it can be positioned to produce foam in-line of a fluid flow path. The device of the present invention uses high shear energy experienced by the fluid in the device as foaming energy for producing the foam.

In the state of the art devices for producing milk foam are known, wherein the foaming of the milk is typically carried out in batch processes. For example, a device is known that has a reservoir, into which milk can be filled. A rotating part in the reservoir, for example a whisk provided at the bottom surface of the reservoir, causes the foaming of the milk. Such a device for producing milk foam, however, can produce only a predetermined amount of milk foam at the same time, i.e. in one batch process. Afterwards, the device needs to be emptied, preferably cleaned, and refilled with milk before the next batch process can be started. Furthermore, such a device cannot be implemented in-line of a fluid flow path, and thus e.g. in a device for producing or providing milk or other beverages .

Further, the state of the art includes devices that inject hot steam into milk that is filled into a reservoir, in order to cause foaming. However, such devices cannot be used in-line of a fluid flow path of e.g. a beverage producing device. For the above-mentioned state of the art devices, the factors that influence the foaming of the milk are, for example, the geometry of the rotating part, like the whisk, or the temperature and/or pressure of the steam that is injected into the milk. These factors are difficult to understand, and are not easy to control accurately without building more complicated devices. Therefore, the milk foam of many simple state of the art foaming devices is often produced unreliably, i.e. the properties of the foam like volume, foaming level, foam stability etc. differ from one batch process to another.

The goal of the present invention is to overcome the above-mentioned disadvantages, and to improve the state of the art. In particular, the present invention has an object to provide a device for foaming a fluid, preferably for producing milk foam, which can be implemented in-line of a fluid flow path. Further, the present invention desires to provide a device, with which the foam can be produced more reliably. Therefore, the factors influencing the foaming effect should be better understandable and easier to control accurately. Finally, an object of the present invention is to provide a simple and robust device that has a long life time.

In the following the description of the present invention is given in view of foaming of milk. However, the invention is not limited to milk as a fluid, but can also be applied to other fluids, e.g. chocolate or coffee. Consequently, other foams than milk foam can be achieved by the present invention as well.

For foaming of milk, in general both milk and air have to be provided in some manner as shown in fig. 1. Optionally, the milk and/or the air can be heated. Further, some kind of foaming energy has to be provided to the milk and the air, in order to produce the milk foam. The present invention bases on the idea that the foaming energy is provided as a high shear energy. The high shear energy is achieved by designing the device such that a milk-air mixture is passed at least partly by Couette flow through the device.

Couette flow refers to a laminar flow of a viscous fluid in a space between two parallel plates. The basic principle of Couette flow is shown in fig. 2. In fig. 2 a movable two-dimensional boundary plate moves with a certain velocity u in respect to a stationary two- dimensional boundary plate. In between the two boundary plates is present a fluid. The movement of the movable boundary plate causes the fluid to move. Two boundary conditions define the movement of the fluid. Directly at the stationary boundary plate, the fluid does not move at all, due to friction forces at the stationary boundary plate. Therefore, the velocity u is zero. Directly at the movable boundary plate, friction causes the fluid to move with the velocity u of the movable boundary plate. In a simple model, the velocity u of the fluid increases linearly in a direction y measured from the stationary boundary plate. Thereby, a shear stress τ is caused in the fluid, which depends on the distance between the two boundary plates, the viscosity of the fluid, and the absolute velocity of the moving boundary plate. The shear stress in the fluid results in a shear energy, which can be used as foaming energy. Details will be described by the present invention. The present invention realizes the above-mentioned principle with a device according to the attached independent claims. The independent claims solve the above-mentioned problems of the state of the art. Further advantages of the present invention are developed in the attached dependent claims.

In particular, the present invention is directed to a device for producing milk foam comprising an outer cylinder, an inner cylinder arranged concentrically within the outer cylinder, a fluid inlet and a fluid outlet, wherein the outer cylinder and the inner cylinder are rotatable with respect to each other, wherein a gap is formed between the outer cylinder and the inner cylinder, and wherein the gap connects the fluid inlet with the fluid outlet.

The two boundary plates shown in fig.2 are respectively realized by the outer wall of the inner cylinder and the inner wall of the outer cylinder. The movement of the movable boundary plate against the stationary boundary plate in fig. 2 is caused by the relative movement of the inner cylinder against the outer cylinder. The cylinders have a common rotation axis. The distance between the boundary plates in fig. 2 is defined by the width of the gap, through which a fluid, preferably milk and air, flows from the fluid inlet to the fluid outlet. That means milk and air are preferably provided to the fluid inlet of the device. Consequently, following the principle described in respect to fig. 2, the milk-air mixture experiences a high shear stress in the gap.

The high shear stress leads to an emulsion of the air and the milk. As soon as the emulsified milk-air mixture leaves the gap it expands. Due to the expansion a foaming effect is achieved, since the size of air bubbles within the milk increases abruptly. Therefore, generally speaking a high shear energy contained in the milk-air mixture is used to provide the foaming energy, which is necessary to produce the milk foam.

The device is designed such that it can be positioned in^¬ line with a flow path of the milk, since the milk can flow into the fluid inlet, through the gap, and out of the fluid outlet. The device 1 is therefore suited to continuously receive milk from a reservoir and convert it into milk foam. Factors that influence the milk foam properties are e.g. the width of the gap, the tangential speed of the cylinders with respect to each other, the cylinder surface and the time during which the milk is exposed to the shear stress, i.e. the time spent in the device. Such parameters are easy to understand and can be controlled precisely. Moreover, the device can be designed rather simple, but can still produce very reliable results.

Preferably, a width of the gap in the radius direction of the inner cylinder and the outer cylinder is in a range of 0.2 to 1.0 mm, even more preferably in a range of 0.3 to 0.6 mm .

As has been stated above, the shear stress experienced by the milk-air mixture in the device depends largely on the width of the gap that is formed between the two cylinder walls. Here the gap diameter is chosen such that a shear stress is achieved, which yields an optimal foaming effect of the milk. That means, for instance, that the foaming effect leads to milk foam having optimal properties. When referring to properties of the foam, the present invention understands e.g. a desired volume, good foam stability and/or a sufficient foaming level. Stability is defined by the amount of time the milk foam is stable, i.e. substantially keeps its volume. The foaming level is defined by a ratio of the volume of the milk fed into the fluid inlet to the volume of the milk foam dispensed out of the fluid outlet.

Preferably, the fluid outlet has a diameter that is larger than the width of the gap, preferably in a range of 2 to 10 mm. The broadening of the fluid outlet compared to the width of the gap leads to the expansion of the milk-air emulsion produced by the high shear stress resulting from the relative rotation of the two cylinders. The expansion increases the size of air bubbles within the milk-air mixture, and thus causes a foaming effect. The preferred diameter of the fluid outlet causes an expansion that achieves milk foam with optimal properties.

Preferably, the inner cylinder comprises a first part having a larger diameter and a second part having a smaller diameter, the gap is formed between the first part of the inner cylinder and the outer cylinder, and a chamber is formed between the second part of the inner cylinder and the outer cylinder.

The offset between the two cylinders creates a first zone (in this case the gap) between the two cylinders, where the milk-air mixture experiences higher shear stress, and a second zone (in this case the chamber) between the two cylinders, where the milk-air mixture experiences lower shear stress. The high shear zone is used for emulsification of the milk-air mixture, whereas the low shear zone is used for expansion of the emulsification . Thereby, the foaming effect can be generated between the two concentric cylinders. If further the fluid outlet is even lager in diameter than the chamber between the two cylinders, a further expansion and foaming effect can occur. The device could even be designed such or means could be provided in the device, so that the chamber diameter is adjustable. Thus, the foaming effect and the properties of the milk foam could be controlled and adj usted . Preferably, the chamber has a width in the radius direction of the inner cylinder and the outer cylinder in a range of 2 to 10 mm.

With the preferred values for the chamber width, the optimal foam properties can be achieved within the device.

Preferably, the device further comprises a motor for rotating the inner cylinder with respect to the outer cylinder .

Preferably, the motor is adapted to rotate the inner cylinder with a rotation speed in a range of 4000 to 8000 rpm with respect to the outer cylinder. The shear stress between the two cylinders depends on the movement velocity, i.e. the relative rotation speed of the two cylinders. The preferred values for the rotation speed have been found to yield the best foaming effect. That means the generated milk foam has the optimal properties. The relative rotation speed can be achieved by fixing the outer cylinder and rotating the inner cylinder with the above-given speed values, or vice versa. The relative rotation speed can also be achieved by rotating the two cylinders in opposite directions to another.

Preferably, the motor comprises a shaft provided with a head part that comprises at least one first magnet, the inner cylinder comprises at least one second magnet, and the at least one first magnet and the at least one second magnet are adapted to contactlessly transfer a rotation of the motor shaft onto the inner cylinder.

Due to the fact that the inner cylinder is driven magnetically, the shaft does not need to be mechanically connected to or inserted into the inner cylinder. Therefore, less friction will occur in the device, since no friction between the shaft and a guiding to or into the inner cylinder exists for rotating of the device. Less friction results in less energy consumption and a longer life time of the device.

Preferably, the device further comprises a water- impermeable separation element arranged between the motor and the inner cylinder.

The water-impermeable separation of inner cylinder and the motor is possible due to the magnetic coupling that transfers the rotation of the shaft onto the inner cylinder. The risk that milk enters from the gap into the part of the device, in which the motor is housed, is more or less eliminated. The motor thus requires less cleaning, since it is not contaminated with milk, and has also a longer expected life time. Preferably, the separation element is made of metal or plastic. The separation element can be part of a housing of the device.

Preferably, the inner cylinder is a rotor and the outer cylinder is a stator.

The inner cylinder is thus preferably rotated against a fixed outer cylinder. The shear stress in the gap depends also on which of the two cylinders is rotated. Best results for foaming the milk have been found with only the inner cylinder rotating. Then the resulting milk foam has the best properties.

Preferably, a diameter of the outer cylinder is in a range of 25 to 35 mm, preferably about 30 mm.

The total volume of the gap defined between the two cylinders, which depends not only on the width of the gap but also on the absolute diameters of the two cylinders, yields the desired amount of milk foam per time, when the device is operated.

Preferably, the device further comprises a heater for heating fluid flowing from the fluid inlet to the fluid outlet.

Heating of the milk and/or air provided via the fluid inlet into the gap can enhance the foaming effect due to additional available energy and/or protein denaturation . Further, hot milk foam is typically desired for preparing beverages like cappuccino or the like. The heater is preferably integrated within one of the two cylinders. For, example, the heater can be housed inside the inner cylinder. If the inner cylinder is further made of a heat conducting material, preferably a metal the heat can be well transferred to the outer surface of the inner cylinder. The outer surface defines the gap, and can thus efficiently heat up the milk-air mixture flowing through the gap. Preferably, the heater heats the cylinder surface along the complete length (i.e. the height) of the inner cylinder .

In an embodiment, the outer cylinder has an inner wall and the inner cylinder has an outer wall, the inner wall and the outer wall realizing two parallel boundary plates, the relative movement of the inner cylinder against the outer cylinder causing a Couette flow of a fluid in a space between the two parallel plates.

The invention also relates to a method for producing milk foam in a device as described above. The method comprises rotating with respect to each other the inner cylinder and the outer cylinder.

A further aspect of the invention relates to a use of milk as a fluid to be frothed by a device as described above.

In the following the present invention is described in more detail with reference to the attached figures.

Figure 1 shows schematically a basic principle for producing milk foam. Figures 2a and 2b show schematically a basic principle of Couette flow for generating shear stress.

Figure 3 shows a schematic of a device for producing milk foam according to the present invention. Figure 4 shows a cross section of a device for producing milk foam according to the present invention. Figure 5 shows a perspective view of a device for producing milk foam according to the present invention.

Figure 6 shows a schematic of a device for producing milk foam according to the present invention.

Figure 3 shows schematically a device 1 for producing milk foam according to the present invention. The device 1 comprises an outer cylinder 2, which is at least partly hollow, wherein the hollow defines an inner diameter o of the outer cylinder 2. Inside of the outer cylinder 2, i.e. inside the hollow, is arranged an inner cylinder 3 concentrically with the outer cylinder 2. The inner cylinder 3 has an outer diameter i. The outer cylinder 2 and the inner cylinder 3 are rotatable against each other around a common rotation axis. To this end, preferably the inner cylinder 3 is rotatable around the rotation axis, which is indicated in Fig. 3 by the broken line, i.e. the inner cylinder 3 is a rotator. The outer cylinder 2 is preferably fixed, i.e. the outer cylinder 2 is preferably a stator. The outer cylinder can e.g. be held fixed in a beverage producing device, which includes the device for producing milk foam of the present invention. Alternatively, the outer cylinder 2 can be a housing of the device 1 of the present invention, which can be held fixed by its own weight or the weight of the device 1 when e.g. standing on a surface. However, other solutions are possible, for example, that the outer cylinder 2 is rotated against a fixed inner cylinder 3 or that both cylinders 2 and 3 are rotated against each other. The outer diameter i of the inner cylinder 3 is smaller than the inner diameter o of the outer cylinder 2, so that a gap 6 is formed between the cylinders, which has a width w that is defined by the difference of the respective cylinder diameters (i.e. w=o-i/2) . The gap 6 connects a fluid inlet 4 of the device 1 with a fluid outlet 5 of the device 1 in a way that a fluid, e.g. milk provided together with air, can pass through the device 1. That means in use of the device 1, preferably milk and air are entered into the fluid inlet 4, the mixture then flows through the gap 6 along the extension direction of the two cylinders 2 and 3 (i.e. the height of the cylinders), and finally exits the device 1 through the fluid outlet 5.

Since the inner cylinder 3 is rotated with respect to the outer cylinder 2, while the milk-air mixture flows through the gap 6, the mixture experiences a high shear stress according to the principles of the Couette flow explained above. The high shear stress causes an emulsion of the milk and the air. After the emulsion flows out from the gap 6 and of the device 1 through the fluid outlet 5, the emulsion expands and is thereby foamed, because air bubbles within the milk expand abruptly. Preferably, the width w of the gap 6, as measured in the direction of the radius of the inner cylinder 3 and the outer cylinder 2, respectively, is in a range of 0.1 to 1 mm, more preferably 0.2 to 0.6 mm, most preferably 0.3 to 0.5 mm. With such a gap the best foam properties for milk foam are achieved.

The inner cylinder 3 can be solid or hollow. The inner cylinder 3 can include a heater 15, which is adapted to heat the milk and the air flowing through the gap 6. Alternatively, the heater can be arranged in the outer cylinder and thus heat the milk and the air flowing through the gap 6 from the outside. The heater 15 can be provided with electricity or it can include parts that move due to the rotation of the rotating cylinder, and are designed to convert the movement into heat.

Fig. 4 shows the device 1 in a cross-sectional view. The inner cylinder 3 is located within the outer cylinder 2, and the gap 6 is formed between the cylinders 2 and 3, through which the milk-air mixture can flow and exit at the fluid outlet 5. The fluid outlet 5 preferably has an inner diameter d that is substantially larger than the width w of the gap 6. Preferably, the diameter d is in a range of 1 to 20 mm, more preferably 2 to 10 mm, most preferably 5 to 10 mm. A ratio of the width of the gap 6 to the diameter d of the fluid outlet is preferably between 1:1 and 1:200, more preferably between 1:3 and 1:50, most preferably between 1:5 and 1:30. Due to the expansion of the milk-air mixture flowing out from the gap 6 into the fluid outlet 5, air bubbles within the milk increase in size, whereby a foaming of the milk is caused. Thus, the device 1 can provide milk from at its fluid outlet 5.

Fig. 4 also shows that the device 1 comprises a motor 8, which is adapted to cause the rotation of the inner cylinder 3 with respect to the outer cylinder 2. The motor 8 is preferably provided separately to the two cylinders 2 und 3 e.g. in a separate chamber of the device 1 or at least with a separation 13, e.g. a plate, between the motor 8 and the cylinders 2 and 3. The separation 13 is preferably water impermeable, so that milk cannot enter the part of the device 1, in which the motor 8 is located. The motor 8 has preferably a shaft 9, which is rotated. The shaft 9 is preferably provided with a head part 10, which is wider than the shaft 9 and includes at least one first magnet 11. The separation 13 between the motor 8 and the inner cylinder 3 preferably comprises a protruding portion 13a, in which a recess 13b for receiving the head part 10 of the motor 8 is formed. The protruding portion 13a of the separation 13 is preferably received by a recess on the top surface of the inner cylinder 3. The inner cylinder 3 is preferably provided with at least one second magnet 12 arranged near its top surface, which is configured and positioned such that it interacts magnetically with the at least one first magnet 11 arranged in the head part 10 of the motor 8.

When the shaft 9 is rotated by the motor 8, the at least one first magnet 11 is also rotated and transfers its rotation through the magnetic coupling onto the at least one second magnet 12 of the inner cylinder 3. Thus, the rotation of the inner cylinder 3 with respect to the outer cylinder 2 is caused.

Preferably, the relative rotation speed of the two cylinders 2, 3 against each other is in a range of 1000 to 15000 rpm, more preferably 2000 and 10000 rpm, most preferably 4000 and 8000 rpm. With the preferred rotation speed the best emulsification of the air-milk mixture in the gap 6 is achieved, and the best foaming properties of the milk foam are realized after expansion. Due to the contactless transfer of the rotation of the motor shaft 9 to a rotation of preferably the inner cylinder 3, the part of the device 1, which includes the motor 8, can be separated from the part of the device 1, which includes the two cylinders 2, 3. This separation avoids that milk enters the part of the device 1, which includes the motor 8. The milk could harm the electrical or mechanical parts of the motor 8. Thus, by decoupling the two parts of the device 1, a longer life time is achieved. Further, the parts of the motor 8 are not contaminated with milk, and thus need not necessarily be cleaned very often.

Figure 5 shows the device 1 in a perspective view. The device 1 comprises two parts, an upper part la that includes the motor 8 and a lower part lb that includes the two cylinders 2 and 3. The two parts la and lb of the device 1 are preferably coupled together by two flanges 14a and 14b, and are fixed with screws 15 as shown in figure 5. An upper flange 14a can be designed to hold the motor 8 of the device 1, and a lower flange 14b can be designed to hold the cylinders 2 and 3 of the device 1. The upper flange 14a can also be produced integrally with a housing of the motor 8, and the lower flange 14b can be produced integrally with the outer cylinder 2. At least the lower flange 14b acts as the separation 13 of motor 8 and cylinders, as can be gathered from fig. 4. The fluid inlet 4 is preferably arranged on an outer side surface of the outer cylinder 2, and can be produced integrally with the outer cylinder 2. The same is true for the fluid outlet 5, which is preferably arranged on an outer bottom surface of the outer cylinder 2, and is preferably produced integrally with the outer cylinder 2.

Both the fluid inlet 4 and the fluid outlet 5 are preferably designed to attach a fluid line, like a tube. The device 1 can then e.g. be integrated into a beverage producing device having e.g. pumps to provide milk and air to the fluid inlet 4. Alternatively, the device 1 itself can be provided with a mechanism so that a relative rotation of the cylinders 2 and 3 causes milk and air to be sucked into the fluid inlet 4, e.g. via a tube leading into an external milk reservoir. Through the fluid outlet 5 the milk foam can be provided directly, or can be provided via a tube to a dispenser. The device 1 of the present invention can thus provide milk foam in-line of a flow path of the milk, and can e.g. be part of a beverage producing device, e.g. a coffee machine. Figure 6 shows a cross section of an alternative embodiment of the device 1 of the present invention. The inner cylinder 3 comprises a first part 3a and a second part 3b. The first part 3a has diameter i that is larger than a diameter a of the second part 3b. In particular, a diameter i of the first part 3a is preferably chosen such that a gap 6 of a width w in a range of 0.1 to 1 mm, more preferably 0.2 to 0.6 mm, most preferably 0.3 to 0.5 mm is formed between a first part 3a and the outer cylinder 2. The diameter s of the second part 3b of the inner cylinder 3 is preferably designed such that a chamber 7 is formed between the second part 3b and the outer cylinder 2, wherein the chamber 7 has preferably a width measured in the radius direction of the inner cylinder 3 that is larger than the width w of the gap 6, preferably in a range of 1 to 20 mm, more preferably 2 to 10 mm. The diameter o of the outer cylinder 2 is preferably 20 to 40 mm, more preferably 30 mm as in the first embodiment of the present invention. Therefore, the diameter s of the second part 3b is most preferably in a range of 20 to 28 mm, and the diameter i of the first part 3a is most preferably in a range of 27.5 to 27.7 mm as in the first embodiment . Milk and air provided to the fluid inlet 4 for flowing through the gap 6 are emulsified due to the high shear forces experienced in the narrow gap 6 between the inner cylinder 3 and the outer cylinder 2, when the inner cylinder 3 is rotated with respect to the outer cylinder 2. When the emulsified milk-air mixture flows out of the gap 6 into the chamber 7, it is expanded and consequently foamed. Due to the expansion, the foam is pressed out of the fluid outlet 5. If the fluid outlet 5 is even larger in diameter than the chamber 7, the milk is foamed even more .

Preferably, both embodiments of the present invention are designed such that a shear stress value for the milk in the gap 6 that is in a range from 20 to 80 Pa, more preferably, 30 to 60 Pa, most preferably of 40 to 50 Pa is achieved, when assuming Newton's law of viscosity. The results in terms of milk foam quality depend not only on the instantaneous shear stress, but on the time during which this shear stress is applied. A typical value is 0.2s at a shear stress of 40 to 50 Pa, but good reulsts are also obtained with longer times. The preferred shear stress values can be achieved by a gap having a width w of preferably 0.1 to 1 mm, more preferably 0.2 to 0.6 mm, most preferably 0.3 to 0.5 mm, a rotation speed of preferably 1000 and 15000 rpm, more preferably 2000 and 10000 rpm, most preferably 4000 and 8000 rpm, an inner diameter o of the outer cylinder 2 of preferably 20 to 40 mm, more preferably 30, and by using milk that has at ambient temperature (20 °C) . Milk at ambient temperature is assumed to have a viscosity of μ = 2 mPa-s and is assumed to be a Newtonian fluid. In summary, the present invention provides a device 1 for producing milk foam, which makes use of the Couette flow principle, and uses a high shear stress that is applied to a milk-air mixture between two cylinders 2, 3 rotating against each other. The shear stress leads to an emulsion of the milk and the air, which again leads to a foaming effect, once the emulsion flows out of a gap 6 between the two cylinders 2, 3 and expands. The parameters of the device 1, i.e. the key factors for the foam quality are the size of the gap 6, the size of the chamber 7 (for the second alternative) and the relative rotation speed of the inner cylinder 3 with respect to the outer cylinder 2. These factors are much easier to understand and control then, for example, the geometry of whisks in some state of the art devices, or parameters of state of the art devices that use hot steam injection. Thus, milk foam can be produced in a well controlled way with a relatively simple device. The foam can be in-line of a flow path of milk.

Claims

1. Device (1) for producing milk foam comprising

an outer cylinder (2),

an inner cylinder (3) arranged concentrically within the outer cylinder (2),

a fluid inlet (4) and a fluid outlet (5),

wherein the outer cylinder (2) and the inner cylinder (3) are rotatable with respect to each other,

wherein a gap (6) is formed between the outer

cylinder (2) and the inner cylinder (3), and

wherein the gap (6) connects the fluid inlet (4) with the fluid outlet (5) .

2. Device (1) for producing milk foam according to claim 1, wherein

a width (w) of the gap (6) in the radius direction of the inner cylinder (3) and the outer cylinder (2) is in a range of 0.3 to 0.5 mm.

3. Device (1) for producing milk foam according to claim 1 or 2, wherein

the fluid outlet (5) has a diameter (d) that is larger than the width (w) of the gap (6), preferably in a range of 2 to 10 mm.

4. Device (1) for producing milk foam according to one of the claims 1 to 2, wherein

the inner cylinder (3) comprises a first part (3a) having a larger diameter (i) and a second part (3b) having a smaller diameter (s) ,

the gap (6) is formed between the first part (3a) of the inner cylinder (3) and the outer cylinder (2), and a chamber (7) is formed between the second part (3b) of the inner cylinder (3) and the outer cylinder (2) .

5. Device (1) for producing milk foam according to claim 4, wherein

the chamber (7) has a width (a) in the radius direction of the inner cylinder (3) and the outer cylinder (3) in a range of 2 to 10 mm.

6. Device (1) for producing milk foam according to one of the claims 1 to 5 further comprising

a motor (8) for rotating the inner cylinder (3) with respect to the outer cylinder (2) .

7. Device (1) for producing milk foam according to claim 6, wherein

the motor (8) is adapted to rotate the inner cylinder (3) with a rotation speed in a range of 4000 to 8000 rpm with respect to the outer cylinder (2) .

8. Device (1) for producing milk foam according to claim 6 or 7, wherein

the motor (8) comprises a shaft (9) provided with a head part (10) that comprises at least one first magnet (11),

the inner cylinder (3) comprises at least one second magnet ( 12 ) , and

the at least one first magnet (11) and the at least one second magnet (12) are adapted to contactlessly transfer a rotation of the motor shaft (9) onto the inner cylinder (3) .

9. Device (1) for producing milk foam according to one of the claims 6 to 8 further comprising a water- impermeable separation element (13) arranged between the motor (8) and the inner cylinder (3) .

10. Device (1) for producing milk foam according to one of the claims 1 to 9, wherein

the inner cylinder (3) is a rotor and the outer cylinder (2) is a stator.

11. Device (1) for producing milk foam according to one of the claims 1 to 10, wherein

a diameter (o) of the outer cylinder is in a range of 25 to 35 mm, preferably about 30 mm.

12. Device (1) for producing milk foam according to one of the claims 1 to 11 further comprising

a heater (14) for heating liquid flowing from the fluid inlet (4) to the fluid outlet (5) .

13. Device (1) for producing milk foam according to one of the claims 1 to 12, wherein

the outer cylinder (2) has an inner wall and

the inner cylinder (3) has an outer wall,

the inner wall and the outer wall realizing two parallel boundary plates, the relative movement of the inner cylinder (3) against the outer cylinder (2) causing a Couette flow of a fluid in a space between the two parallel plates.

14. Method for producing milk foam in a device (1) according to any one of claims 1 to 13, said method comprising rotating with respect to each other the inner cylinder (3) and the outer cylinder (2) .

15. Use of milk as a fluid to be frothed by a device according to any one of claim 1 to 13.