Device for producing milk foam
A device for producing milk foam uses Couette flow and a high shear stress that is accordingly applied to a milk-air mixture between two concentrically arranged cylinders. 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 between the two cylinders and expands. The parameters of the device that mainly influence the foaming effect are the width of the gap and the relative rotation speed of the cylinders.
1. A device for producing milk foam, the device comprising:
an outer cylinder;
an inner cylinder arranged concentrically within the outer cylinder;
a fluid inlet and a fluid outlet;
a reservoir leading into the fluid inlet, the reservoir containing milk, the device configured to provide the milk and air to the fluid inlet,
the outer cylinder and the inner cylinder are rotatable with respect to each other,
a gap is formed between the outer cylinder and the inner cylinder, and the gap connects the fluid inlet with the fluid outlet; and
a motor for rotating the inner cylinder with respect to the outer cylinder, the motor comprising a shaft provided with a head part that comprises at least one first magnet, the inner cylinder comprising 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 shaft onto the inner cylinder.
2. The device according to claim 1 , wherein a width of the gap in a radius direction of the inner cylinder and the outer cylinder is in a range of 0.3 to 0.5 mm.
3. The device according to claim 1 , wherein the fluid outlet has a diameter that is greater than a width of the gap.
4. The device according to claim 1 , wherein
the inner cylinder comprises a first part having a greater 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.
5. The device according to claim 4 , wherein the chamber has a width in a radius direction of the inner cylinder and the outer cylinder in a range of 2 to 10 mm.
6. The device according to claim 1 , wherein the motor is adapted to rotate the inner cylinder at a rotation speed in a range of 4000 to 8000 rpm with respect to the outer cylinder.
7. The device according to claim 1 comprising a water-impermeable separation element arranged between the motor and the inner cylinder.
8. The device according to claim 1 , wherein the inner cylinder is a rotor, and the outer cylinder is a stator.
9. The device according to claim 1 , wherein a diameter of the outer cylinder is in a range of 25 to 35 mm.
10. The device according to claim 1 comprising a heater for heating at least one of the milk or the air flowing from the fluid inlet to the fluid outlet.
11. The device according to claim 1 , wherein
the outer cylinder has an inner wall;
the inner cylinder has an outer wall; and
the inner wall and the outer wall define two parallel boundary plates, relative movement of the inner cylinder against the outer cylinder causing a Couette flow of the mixture in a space between the two parallel boundary plates.
12. The device according to claim 1 comprising a single inlet into the gap between the inner cylinder and the outer cylinder.