The invention relates to a milk frother and a drive mechanism for a milk frother.

Espresso based coffee drinks such as caffe latte, cappuccino and flat white have become increasingly popular and consumers want to be able to easily and reliably produce these drinks at home. Traditionally, baristas have been trained to use the steam generated by an espresso machine to heat and froth milk in a jug to produce the hot milk and froth required for these drinks. Producing the appropriate amount of hot milk and froth without burning the milk or damaging the jug requires some training or experience. Smaller espresso machines are also much harder to use to treat the milk.

Capsule machines are increasingly dominant in the home market due to their ease of use and in the known capsule machines, each capsule produces a single serving of coffee.

A number of separate milk frothers have been produced which are suitable for home or small scale commercial use. EP1656866 discloses a frother with a tank to receive a milk-based liquid food with a magnetic beater. A magnetic beater driving system produces a magnetic field which drives the beater in rotation in the tank. The system, beater and a beater positioning unit break or prevent symmetrical circulation of the liquid around a median of vertical axis of the tank. Heating units are disposed in association with the tank for heating the liquid. This is used for preparing foam from a milk-based liquid food product.

GB2454421 discloses a device for frothing milk that is provided with a rotating heating element to be inserted in the milk. The device is provided with a whisk attachment for producing froth and a paddle attachment for producing a more creamy finish.

EP1827188 discloses a cordless milk frother having a stirrer mounted on a spindle, wherein the stirrer is held and driven magnetically in the tank by the stirrer drive system.

Milk frothers on the market are typically aimed at the capsule machine market and froth enough milk per cycle to make one drink. However, traditional style espresso coffee machine can make more than one espresso at a time. Further approaches that have not been commercialised can be found in the patent literature. WO2010106372 discloses a milk frother which uses ultrasonic acoustic signals to determine the volume of froth in the milk. AU2015202341 discloses a frother that has user temperature control.

Whilst it would be desirable to have a frother capable of working with multi cup volumes of milk, the technical problems to overcome are considerable. For any given volume of milk, the speed of the whisk needs to vary, to achieve the desired drink the user would like to make. Typically, drinks are Cappuccino, approximately 50% froth 50% milk and Latte, 90% milk 10% froth. The flat white about 2/3 milk to 1/3 foam. If the whisk speed is not correct for the desired milk volume put in the frother, then the % volume of froth in the finished drink will not be correct.

The present invention therefore seeks to provide a milk frother capable of frothing multiple cups of milk

According to the invention a milk frother comprising a milk frother comprising a tank for receiving milk or a milk based liquid, drive means adapted to drive whisk means, which whisk means is adapted to rotate in the milk or milk based liquid to mix the heating milk or milk based liquid and to produce foam, the frother further comprising heating means adapted to heat the milk or milk based liquid and a temperature sensor arranged to measure the temperature of the milk, the frother being provided with a control unit to control actuation of the drive means and heating means, wherein the control unit controls the duration of the actuation of the drive means and heating means at least in part on the change of temperature of the milk detected by the temperature sensor during a pre determined time interval.

Preferred aspects can be found in the sub-claims.

The invention addresses the problem that if the whisk speed is not correct for the desired milk volume put in the frother, then the % volume of froth in the finished drink will not be correct for the desired drink. Users are unlikely to able to control exactly how much milk they put into the frother and then input that amount and what type of drink (% frother) they would like, using controls. This could lead to errors in measurement and during input, causing an incorrect % of froth in the finished drink. The concept of the invention allows the frother to detect the volume of milk placed in the frother by monitoring the temperature rise of the milk during the first stage of the heating / frothing cycle. The volume of milk determination can be based on preprogramed set of data / formulas and set the whisk speed according to the type of drink (desired % of froth) inputted by the user. Thus, producing a drink with the correct % froth every time with minimal input from the user.

The temperature of the milk is advantageously controlled by a thermal sensor mounted on the heating plate, near the element or a probe in the milk container. The thermal sensor measures the drink temperature and is used as a signal to finish the whisk and heating cycle.

An exemplary embodiment of the invention will now be described in greater detail with reference to the drawings in which:

Fig. 1 shows an exploded view of the milk frother Fig. 2 shows a graph of temperature against time Fig. 3 shows a table of motor speed values

Figure 1 shows an exploded view of the frother comprising a base 1 , a main body 2, whisk means 3 and a lid 4. The frother is cordless and the base is provided with an upstanding boss to provide the electrical connection from mains electricity to the frother body 2 such that the frother body can rotate through 360° on the base and be lifted clear when the milk or liquid is frothed without a cable be attached to the body 2.

The main body 2 comprises at its upper end a tank 5 which receives the milk or liquid to be frothed and heated. The tank has a flat bottom 14. Magnetic drive means 15 and heating means 16 are located in the main body beneath the flat bottom of the tank 5.

The whisk means 3 comprises an annular upstanding wall 7, which wall is joined by two radial arms 8 to an axial spindle 9. The spindle 9 extends above and below the radial arms 8. A whisk 10 is rotatably mounted on a rotatable drive shaft at the lower end of the spindle 9, which drive shaft is housed within the spindle 9. The upper portion of the spindle is provided with first and second annular protruding walls 11 , 12. The first, lower wall 11 indicates the maximum advised level for frothed milk. The second, upper wall 12 indicates the maximum advised level for heated milk. The height of the radial arms indicates the minimum advised milk level.

The lid is provided with a seal around its edge to seal the tank in use and has two holes 13, which allow heated air to escape hereby increasing the working volume of the tank to be larger than it otherwise would be.

In use, the whisk means holds the whisk 10 substantially concentrically with the magnetic drive means located in the main body. The whisk 10 is held above the surface of the flat bottom, typically by about 3mm, thereby avoiding that the surface of the flat bottom might be scratched by the whisk action. This permits the interior of the tank 5 to be provided with either a polished stainless steel surface or alternatively a non stick coating. This also facilitates the magnetic drive means to run on for 20-30 seconds after the heating element is switched off. This enables the milk to be heated to a higher temperature suitable for hot chocolate drinks. The heat is dissipated throughout the milk and does not burn on the bottom surface of the container.

The radial arms 8 will also interrupt the vortex that would develop due to the rotation of the whisk, which advantageously results in a better froth as well as increasing the working volume of the tank.

In the exemplary embodiment, the frother is provided with a selector and an actuator. The selector enables the user to select which drink and hence how much froth they desire. The selector could have several settings: cappuccino, latte, flat white and/or no froth. The actuator can also have two settings, hot and cold. It would be possible to combine all the settings into a single button.

As well as pre-sets for different froth levels (e.g. cappuccino and latte), it will also be possible to have pre-sets for milk drinks with additives, such as hot chocolate or other milk types.

As the volume of milk in the tank increases, a greater speed is required to spin the milk and raise it into a vortex as the cross sectional area of the tank is fixed. The vortex allows the whisk to be exposed to air and thus introduce bubbles into the milk, which forms the froth. To detect the volume of milk in the product, the device measures the rate of temperature rise (delta T) of the milk during an initial part of the frothing cycle. During this initial part of the cycle the motor turns the whisk at a first speed. The first speed is lower than the speed of rotation of the whisk during the second phase. An exemplary period for the first phase is one minute. During the first phase, the greater the amount of milk within the jug, the slower the temperature rise and the vice versa for the lesser amount of milk. This delta T can then be used to calculate an optimum whisking speed for the remainder of the cycle, dependent on the drink or (percentage of % froth selected at actuation.

In the second phase, the whisk speed does not need to be constant and could be varied to go faster or slower, to fine tune the type of froth created. For example, by varying the speed, it is possible to simulate the skill of a barista by speeding up the motor to generate more frother and slowing it down to fold froth into the milk.

For 100ml of milk an exemplary time for the second phase for a cappuccino is 1 minute.. The cycle length can be determined by the target temperature (eg 67C), the motor speed (as faster speeds introduce more air so the milk warms up more slowly) and the volume of milk (as larger volumes warm up more slowly).

The heater is controlled by a thermal sensor 18 on the base of the jug / or in the jug (probe). The probe has a trigger temperature, wherein when the detected temperature of the milk reaches this temperature, the cycle is ended. An exemplary trigger temperature is 67°. It would be possible for this temperature is be a programmable range of temperatures or selected pre-programmed temperatures so that a user could control the final temperature. This may be desirable where the frother is used to make hot chocolate, for example, as this drink usually as a higher temperature.

In a further embodiment the value of delta T could also be used to calculate a heating cycle where the power of the elements could be variable. For example, more power could be provided at the beginning of the cycle and then throttled back towards the end of the cycle, to fine tune the type of froth produced.

Figure 2 shows heat up time for milk sensed by the frother for the first phase 60 seconds of the cycle from various start temperatures of the milk. (1 degree to 20 degrees.) For 100ml the delta T is 35 degrees and for 540ml the delta T is 11 degrees. Therefore, there is a variation of 25 degrees between the smallest and largest volumes for a two cup frother, i.e. between the volume for a one cup cappuccino and a two cup latte. This temperature change can be easily and reliably detected using low cost thermal sensors.

In a preferred embodiment for a 2 cup frother, the geometry of the frother jug and the element wattage is as follows. The shape of the jug/heating plate has inner diameter of 89.4mm and depth of 126.4mm. The heating plate is provided with a 500w element and the motor has a voltage of 5V. In this embodiment, two temperature measurements are made: the first at 10s and the second at 60 seconds. The first time interval of 10s is chosen so that the milk can be mixed prior to the temperature being measured so that any local effects due to residual heat from a previous use or use of a mix of milk temperatures when for example pat of the milk comes from a different bottle taken from a refrigerator and part was at room temperature.

The volume of milk can then be calculated by the formula V = a delta T ^b where a and b are constants determined by the geometry of the frother. For the above frother an exemplary value for a is 11762 and for b is -1.275.

In a second approach, motor speed is calculated based on the pre-set selected and the volume of the milk, will be different if any of the following factors are changed:

In this embodiment the shape of the jug/heating plate has an inner diameter of 89.4mm and depth of 126.4mm. The heating plate is provided with a 500w element.

The motor voltage is calculated according to the formula

M = aV ² - bV + C

Exemplary values for a, b and c can be found in the table in figure 3. The motor is tuned off when the measured temperature reaches 67 C.

The design of the whisk and it holder / mounting method can also influence these values.

The motor can be allowed to run for a set time, for example 20 s, after the element is switched off to avoid stationary milk burning on the jug. If the calculated volume of milk is below a lower limit or above an upper limit, the frother element and motor are turned off.

Although a 2 cup version is specifically described here, the concept can be scaled up to larger capacity devices for large families or commercial purposes.

The max and minimum levels for the milk could be either marked on the frother jug, or the frother could be programmed to end the cycle if it detects too little or too much milk. This is a useful safety feature to prevent froth spilling out or milk burning onto the element.

The desired % of froth could be input using pre-set controls, programmable controls or an infinite control from 100% froth to 0% froth. The control could be touch screen, capacitive, push button, sliders, Bluetooth connected app, dials or voice activated. The controls could be on the frother jug, the handle, the power base or remote device.