Appliance for conditioning a milk-based liquid

An appliance is arranged for frothing a body of a milk-containing liquid contained in a frothing tank. Such appliance includes an air incorporation device that is arranged to dip or to be immersed in such body of liquid during frothing for incorporating air thereinto; and a steam supply arrangement having a steam outlet that is arranged to be immersed in such body of liquid during frothing for incorporating steam thereinto. The steam outlet and the air incorporation device are arranged to be at spaced apart locations in such body of liquid during frothing so as to form steam bubbles at the steam outlet separately from air bubbles that are being formed by the air incorporation device.

This application is a 371 filing of International Patent Application PCT/EP2009/061209 filed Aug. 31, 2009.

FIELD OF THE INVENTION

The present invention relates to an appliance for frothing a milk-based liquid to be used, for example, in the context of the preparation of beverages such as cappuccinos, milky coffees, café latte and macchiato, chocolate and other hot beverages.

BACKGROUND ART

Speciality beverages in which at least a portion is made up of frothed or heated milk are becoming more and more popular. The best-known beverage of this type is a coffee of the cappuccino type. It comprises a liquid portion consisting of coffee topped by a layer of frothed milk which, because of its very much lower density, floats atop the surface of the liquid.

The simplest method to froth milk consists in plunging a steam supply pipe into a cup of milk to be frothed. However, the results of such a method depend largely on the skills of the user and generally provide a poor froth quality. In fact, usually, milk ends up merely heated and not significantly frothed for lack of air incorporation into the milk during the conditioning treatment. Indeed, for achieving an acceptable frothing, the user is expected to skillfully move the steaming pipe in the milk so as to agitate it at the same time for simultaneous incorporation of air into the milk. Such systems are for instance disclosed in EP 0 791 321 and EP 1 716 796. The mere heating by vapour without frothing of milk is well known and may also be achieved automatically, for instance as disclosed in EP 1 776 904. In particular, a vapour source may be combined with a stirrer to avoid burning of milk on the steam injection duct by injecting the vapour into the milk via the stirrer enclosed in a stirring chamber in fluid communication with milk, as for instance disclosed in U.S. Pat. No. 3,182,975 and U.S. Pat. No. 4,479,908.

Milk may also be frothed by sucking fresh milk from a milk source into a device that includes an air inlet and a pressurised vapour inlet forming a Venturi arrangement, for heating and mixing such milk with vapour and air, and an outlet for the thereby frothed milk. Such devices are for example disclosed in the abovementioned EP 1 716 796, DE 10 2006 031 064 and DE 44 45 436. Such systems may also be combined with an in-line stirrer, as for example disclosed in DE 20 2007 009 537. Furthermore, in EP 1 785 074 milk is pumped from a reservoir into a mixing chamber into which a mixture or vapour and milk is injected for frothing the milk, and then the frothed milk is delivered to a delivery opening.

Another known approach involves the use of a frothing receptacle which comprises a frothing arrangement and into which milk is supplied and then frothed. The frothing may be carried out by injecting vapour and air with a Venturi arrangement into the milk contained in the receptacle, for instance as disclosed in EP 1 725 124, WO 01/26520 and WO 2006/122916. This system produces frothed milk with a relatively liquid foam having large bubbles.

A problem involved with the suction of milk in a Venturi-type vapour arrangement is the immediate condensation of vapour in the Venturi arrangement and the poor formation of vapour bubbles in the milk.

Frothing of milk in a tank can also be achieved by an impeller for mechanically agitating and driving air into the milk and a resistive heater for heating the milk at the same time, as for example disclosed in WO 2006/050900 and PCT/EP08/056349. This system produces a relatively dry firm foam with large bubbles as well.

There are frothing devices that combine a vapour supply pipe for heating and a mechanical stirrer extending therebelow for simultaneously agitating the milk in a cup held by a used under this frothing device. Such a stirrer may be connected via a drive shaft to a drive rotor that is located inside the vapour tube and that is driven by the passage of vapour in the tube. Vapour, or a mixture of vapour and air, is delivered vertically above the stirrer. For example EP 1 197 175 discloses a vapour pipe having a plurality of openings for incoming milk and air and at its outlet a stirrer for agitating the milk. DE 10 2004 063 285 discloses a similar device in which the vapour pipe is arranged for guiding only vapour and is impervious so as to avoid entry of milk into the tube for hygiene reasons. EP 1 597 992 discloses another such device of the same type. In WO 2007/095770 a lance with a Venturi arrangement is arranged to inject a steam-air mixture in a receptacle of milk. A similar arrangement is disclosed in WO 2007/144735.

A problem with such systems comes from the fact that the user has to properly hold the cup under the vapour pipe, or vice versa, so that the pipe's outlet does not emerge from the milk during the treatment which would lead to the projection of milk all over the place. This risk is even increased when such a vapour pipe cooperates with a stirrer located thereunder that can lead to the formation of a vortex in the milk nearby the stirrer and space the pipe's vapour outlet above the surface of the milk which can also lead to the projection of milk in addition to poor or non incorporation of vapour into the milk.

Moreover, the use of interdependent stirrers, vapour and/or air injection means, such as systems using a Venturi arrangement or systems having a stirrer driven by the injected vapour, have a very limited versatility. Normally, they allow only one mode of operation and require the user's skills, e.g. motion of the frothing arrangement and/or of the receptacle to adjust the frothing characteristics.

One solution to this latter problem has been proposed in EP 1 501 398 in which vapour is mixed with air in a ratio determined by the user and then injected as a mixture into a receptacle containing milk. The vapour-air ratio is adjusted with the view to heat the milk with a user-adjusted amount of froth. EP 1 776 905 discloses a similar system in which pulses of compressed air are incorporated into a flow of vapour and then injected as a mixture into a jug of milk. A further system of the same type is disclosed in U.S. Pat. No. 6,006,654 in which milk is pumped into a vapour stream and then mixed with air for frothing and then delivered at a discharge nozzle.

There is therefore still a need to provide a reliable system that is simple and reliable for producing frothed milk.

SUMMARY OF THE INVENTION

Therefore, the invention relates to an appliance for frothing a body of a milk-containing liquid contained in a frothing tank. The appliance comprises: an air incorporation device that is arranged to dip or to be immersed in said body of liquid during frothing for incorporating air thereinto; and a steam supply arrangement having a steam outlet that is arranged to be immersed in the body of liquid during frothing for incorporating steam thereinto.

In accordance with the invention, the steam outlet and the air incorporation device are arranged to be at spaced apart locations in this body of liquid during frothing so as to form steam bubbles at the steam outlet separately from air bubbles that are being formed by the air incorporation device.

The air bubbles introduced into the milk-containing liquid are thus distinct from steam bubbles formed at the steam outlet opening.

By separating the steam and the air incorporation into the milk-containing liquid, especially during the entire frothing process, the properties of the bubbles of steam and the bubbles of air are not modified before or at incorporation into the milk-containing liquid.

It has been found experimentally that the interaction of air bubbles and vapour bubbles upon incorporation into the milk-containing liquid, produces a higher froth quality than when air and vapour are mixed together before incorporation into the milk-containing liquid. Even though the physical and/or chemical phenomenons are not as yet well understood, and without being bound to any theory, it is believed that milk, in particular the conditioning of the milk proteins which is believed to influence directly the characteristics of the milk-containing foam, reacts differently when exposed to (especially substantially non-condensed) steam and air separately than when the milk-containing liquid is treated with a pre-mix of air and steam or, even worse, air and condensed steam, i.e. hot water.

The steam outlet may be associated with a valve means for controlling characteristics of the steam bubbles, in particular a bubble size thereof.

By adjusting the characteristics of the valve, in particular by regulating the flow of steam through the valve, the characteristics of the resulting frothed milk-containing liquid are influenced correspondingly, in particular the size of the pores in the frothed milk which depends on the size of the injected steam bubbles can be adjusted to reach a desired foam quality.

In particular, it has been found that injecting steam with a very fine bubble size into the milk-containing liquid, for instance by forcing the steam under pressure through a resilient valve member at the steam outlet opening, a foam quality close to the one obtained manually by the barista or highly skilled coffee sommelier, e.g. a foam with bubbles around approx. 150 micron, can be obtained, instead of a pore size at or above approx. 200 micron obtained with the known automatic milk frothers.

For instance, the valve means comprise a biased resilient member arranged to allow a release of steam through the outlet when such steam reaches a sufficient pressure in the steam supply arrangement. The resilient member is optionally associated with a user-adjustable bias means for adjusting the resilient member's bias. The valve means may include at least one of a lid, plug or cover resiliently closing the steam outlet opening.

The valve means may comprise a resilient section, in particular a tubular or conical section, for guiding such steam towards the steam outlet opening. This resilient section can be part of the steam outlet and the valve means may further comprise means for squeezing this resilient section to regulate the steam flow therethrough. Typically, the valve means further comprise a retaining surface, the resilient section being biased to urge the outlet opening against the retaining surface. In this case, a steam pressure may build up behind the valve to a level necessary to force the passage along the resilient section through the outlet opening and thus inject vapour bubbles with an adjusted size into the milk-containing liquid.

The appliance can also include a one-way valve or a non-return valve to prevent the introduction of the milk-containing liquid into the steam supply arrangement, in particular into its outlet. The valve for adjusting the bubble size may itself be a one-way valve or a non-return valve.

At delivery of such steam bubbles into this liquid, the outlet of the steam supply arrangement can have a section of up to 0.7 mm2, typically in the range of 0.1 to 0.65 mm2, in particular in the range of 0.2 to 0.55 mm2such as 0.25 to 0.45 mm2.

The air incorporation device may comprise an air supply duct arranged to dip in the milk-containing liquid, in particular associated with an air pump, and/or an agitator, such as a stirrer.

Typically, the appliance of the invention has an operative position in which the steam outlet is located at a level below or at a side of the agitator. Hence, should the agitator such as a stirrer cause the formation in the milk-containing liquid of a vortex above the agitator, the steam outlet would still deliver steam into the milk-containing liquid and not above such a vortex.

In an embodiment of the invention, the appliance has a reservoir module with a frothing tank for containing the body of milk-containing liquid. The air incorporation device and the steam supply arrangement are optionally secured to the reservoir module. In particular, the air incorporation device can comprise a stirrer secured to a bottom of the frothing tank. The steam outlet can be located at or adjacent a bottom of the frothing tank.

Normally, the frothing tank has a central vertical axis, for example extending vertically from the centre of a generally circular or polygonal bottom face, the air incorporation device having an agitator in the tank off-centred relative to this vertical axis, whereby an asymmetric agitation can be generated in the body of milk-containing liquid. Alternatively or cumulatively, it is also possible to incorporate flow-perturbation obstacles in the structure of the tank, e.g. protrusion extending into the tank's cavity, so as to generate an asymmetric agitation. Optionally, the steam outlet is off-centred relative to this vertical axis and is in particular located opposite the agitator, whereby the location of the steam bubble formation and the location of the air bubble formation are separated in the tank.

Typically, the body of milk-containing liquid has a moving upper surface during frothing in the tank. To avoid dispensing of steam above the moving surface, the steam outlet should be so positioned in the frothing tank to be below this moving surface at all time during frothing. Optionally, the position of the steam outlet can be adjusted by the user for this purpose. The steam outlet may be associated with a safety valve for allowing the exit of steam via the outlet only when the outlet is located below the moving upper surface.

The steam arrangement may have a duct that is connectable to an external steam generator, in particular a steam generator of a hot beverage machine.

In an embodiment of the invention, the steam arrangement of the appliance includes a steam generator and a steam passage for guiding steam from the steam generator to the steam outlet. The steam generator may be located at a periphery and/or a bottom part of the reservoir module and may be giving into this steam passage. Such a passage may extend over a tank upright wall and down along the upright wall inside the frothing tank to the steam outlet. This upright wall is optionally associated with a movable cover member, for example hinged to the reservoir module and/or separable therefrom. Such a movable cover member may delimit a section of the steam passage for guiding steam over said upright wall and/or may give access to a water receptacle of the steam generator, in particular an access along the upright wall outside the tank.

The appliance may include a support base which is electrically connectable, e.g. to the mains, for powering the appliance. The reservoir module can be made separable from the support base in particular for cleaning the reservoir module or dispensing therefrom frothed milk-containing liquid. The support base may include at least one of a steam generator and a drive module for actuating an agitator in the frothing tank. Likewise, the reservoir module may have a cavity, typically adjacent the frothing tank, for containing at least one of a steam generator and a drive module for actuating an agitator in the frothing tank.

To increase the flexibility and user-adaptability, the appliance can be arranged to be user-configurable for incorporating air and steam simultaneously and/or sequentially while frothing the milk-containing liquid. The appliance may also be arranged to let the user choose the velocity of incorporation of air and/or steam into the milk-containing liquid. Optionally, the air incorporation means can be associated with a resistor heater for heating the milk-containing liquid in combination or without steam heating. Hence, a user may adjust the characteristics of the desired froth, in particular the fluidity and dryness thereof, as well as the foam structure by reducing or suppressing the steam incorporation and compensating the lack of steam heating by a resistor heating of the milk-containing liquid, for example a resistor assembled to a tank for containing the liquid or to the agitator.

Another aspect of the invention relates to an appliance, in particular of the type described above, having a reservoir module. Such a module comprises: a tank with a bottom part and at least one upright wall for containing a body of a milk-containing liquid which is to be heated and/or frothed therein. The module further incorporates a steam supply arrangement comprising a water receptacle and a water heater for producing steam adjacent to said tank and a passage for guiding the produced steam into the body of milk-containing liquid within the tank, such passage extending in particular over the tank's upright wall into the tank. Optionally, the module has a movable cover member on the tank's upright wall, the movable cover member delimiting a section of the steam passage for guiding steam over said upright wall and/or giving access to the water receptacle of the water heater.

The invention also relates to an appliance, in particular as described above. This appliance has a support base and a reservoir module that is separable from the support module and that is arranged for containing a body of a milk-containing liquid which is to be heated and/or frothed therein. The support base comprises: a steam generator with a water receptacle for producing steam adjacent to the reservoir module, the steam generator being associated with a steam passage for guiding the produced steam into said reservoir module; and a drive arrangement for driving an agitator, in particular magnetically driving the agitator, located in the reservoir module. The reservoir module and/or the support base is optionally assembled to a movable cover member that delimits a section of the steam passage and/or that gives access to the water receptacle.

Such appliances may incorporate any feature or combination of features described above.

DETAILED DESCRIPTION

As shown inFIG. 1, a reservoir module1aof an appliance1according to the invention has a tank10for receiving the milk-containing liquid that is to be frothed. Reservoir module1ais arranged to be removably electrically connected to a support base (not shown) via an electric power supply connector7.

Tank10has a spout11for dispensing the liquid upon frothing. Furthermore, a closed compartment2is located under tank10that contains an electric drive arrangement5associated with a user-switch6and power supply connector7for connection to a support base (not shown). Drive arrangement5is arranged to drive a stirrer8that is located on a bottom12of tank10and that is magnetically coupled with drive arrangement5through bottom12. Compartment2is formed by a generally tubular stand3assembled to a rigid bottom4. The upper part of compartment2is delimited by tank10which is located inside and secured to stand3. Thus, tubular stand3, bottom4and tank10are three casing parts which, when assembled, form together chamber2. Further details and variations on the appliance's structure, drive means, switches, electric connector and stirrer are for instance disclosed in WO2006/050900 and in WO/2008/142154, the contents of which are hereby incorporated by way of reference.

Moreover, appliance1has an arrangement for the supply of steam comprising a pipe20with an inlet21for connection to a steam generator (not shown) and an outlet22for delivering bubbles of steam at bottom12of tank10. Pipe20is secured to tank10via a member13extending from the upper edge of tank10.

Pipe20is made of resilient material, e.g. a plastic material, and urged under elastic constraint against bottom12so that pipe20, which is compressed along its longitudinal direction, and bottom12, which forms a retaining surface against the pipe's outlet22. Longitudinally compressed pipe20and retaining surface12cooperate together to form a valve system for regulating the bubble size of steam bubbles exiting during use via an adjusted opening25formed by the periphery of steam outlet22urged against retaining surface12of tank10. Hence, outlet opening25formed between tube20and bottom12has a section which is adjusted by the valve means, i.e. tube20biased against bottom12, and the steam under pressure inside and along tube20forcing its way out via opening25.

For operating appliance1, steam inlet21is connected to a steam generator, in particular to the steam nozzle of a coffee machine via a steam connection tube (not shown), and milk or a milk-containing liquid is poured into tank10.

Off-centred stirrer8is made to rotate in the liquid by switching on drive arrangement5by pushing button6. The rotation of stirrer8leads to the formation of a vortex in the liquid contained in tank10. This vortex, the surface of which is shown schematically in dotted lines15, may extend down to and even below stirrer8at start-up. Consequently, air in the form of bubbles is mechanically incorporated into the liquid by the action of stirrer8rotating through the bottom part16of surface15of the agitated liquid.

Steam is produced by the steam generator and supplied via inlet21into pipe20and guided therealong to bottom12. At outlet22, the steam forces its way under pressure out through opening25to form very small steam bubbles in the rotating milk-containing liquid to heat the liquid and to form with the air bubbles a fine emulsion of milk-containing liquid, air and steam and condensed steam.

Moreover, at delivery of the steam bubbles into this liquid, the outlet opening25can have a section of about 0.3 or 0.4 to 0.5 or 0.6 mm2.

In accordance with the invention, as illustrated inFIG. 1, steam outlet opening25and the air incorporation stirrer8are at spaced apart locations in the milk-containing liquid contained in tank10during frothing so as to form steam bubbles at steam outlet opening25separately from air bubbles that are being formed at stirrer8.

FIGS. 2 to 4, in which the same numeric references generally designate the same or similar elements, illustrate a second embodiment of an appliance1according to the invention, which includes a steam generator27,28.

Appliance1has a reservoir module1aon a support base1b, reservoir module1abeing separable from base1b, e.g. for serving frothed liquid or cleaning. Reservoir module1aincludes a tank10having a bottom12located above a cavity2containing an electric drive arrangement5and a user-interface module6. Reservoir module1ais removably mounted on support base1bwith a connector arrangement7for transmitting electric power from the mains via an electric cable71to reservoir module1a.

Reservoir module1aof appliance1has an air incorporation device in the form of a stirrer8arranged to be immersed in a milk-containing liquid within tank10. Stirrer8has an annular shape and is positioned around a protrusion121of tank bottom12. Stirrer8is driven by drive arrangement5via a rotating magnetic transmission member51magnetically actuating stirrer8through bottom12. Furthermore, reservoir module1aincludes a steam supply arrangement having: a steam generator in the form of a water receptacle28situated adjacent to tank10and a heater27, in particular a resistive heater, located in or extending into reservoir28for heating and evaporating water contained in reservoir28; and a passage20,21,22,23,23′,23″,24for guiding steam produced by the steam generator27,28to steam outlet opening25in tank10.

In accordance with the invention, as apparent inFIGS. 3 and 4, steam outlet opening25and stirrer8are at spaced apart locations above bottom12of tank10and thus at spaced apart locations in the milk-containing liquid when tank10is filled therewith. Steam outlet25is located below and sufficiently spaced apart from air incorporator8so that the formation of steam bubbles and the formation of air bubbles in the milk-containing liquid are carried out separately therein. As explained above, entire separation of the processes of air incorporation and of steam incorporation leads to the improved froth quality.

Moreover, like in the previous embodiment of the invention shown inFIG. 1, pipe20is made of resilient material, e.g. a plastic material, and urged under elastic constraint against bottom12by means of support element13so that pipe20, which is compressed along its longitudinal direction, and bottom12, which forms a retaining surface against the pipe's outlet22, cooperate together to form a valve system. This valve system being suitable to regulate the bubble size of steam bubbles exiting during use via opening25. Indeed, outlet opening25formed between tube20and bottom12has a section which is adjusted by the valve means, i.e. tube20biased against bottom12, and the steam under pressure inside and along tube20forcing its way out via opening25.

It is possible to force pipe20through bore13aof support element13such that pipe20may be slightly displaced downwards or upwards within support element13. Hence, outlet22can be so positioned relatively to bottom12or urged thereagainst to adjust the desired bubble size of the steam bubbles exiting from outlet opening25formed between outlet22and bottom12.

Moreover, to improve the stirring effect of stirrer8and the homogenisation of the milk-containing liquid within tank10, stirrer8is located on bottom12off-centred relative to a central vertical axis10′. Steam outlet25is also off-centred relative to vertical axis10′ and located opposite stirrer8on a peripheral part of tank10.

Also shown inFIGS. 2 and 3, appliance1has a cover member14movably assembled via a hinge14′ to reservoir unit1aalong the top edge of the rear upright wall of water receptacle28. Instead of being hingedly connected to the unit's structure, cover member may also be formed as a separable, freely removable cover member on the unit. Furthermore, cover member14partly covers tank10so that the inside of tank10remains visible for the user. In a variation, such cover member may extend over the entire mouth of tank10, for instance to avoid any milk projections outside tank10during use; such a cover member may be partly or completely made of transparent or translucide material so that the user may still examine the content of tank10during use without having to uncover it.

Cover member14incorporates part of the steam supply arrangement, namely a steam collector24leading into a steam duct23formed in and along an inner protuberance23aof cover member14.

When cover member14is in its closed position, as shown inFIG. 2, its generally u-shaped peripheral front part is urged against rim12′ of the upright wall12″ of tank12and steam collector24is located immediately above water receptacle28for collecting the water evaporated therefrom by heater27and canalise this evaporated water into an inlet23′ of steam duct23. Moreover, protuberance23ais urged against support member13whereby outlet23″ is joined to inlet21of tube20via bore13aof support member13so as to allow the passage of steam from duct23into pipe20and therefrom into a bottom part of tank10via opening25.

When cover member14is turned open, as shown inFIG. 3, water receptacle28is uncovered and may be accessed by the user, for instance for refilling with water, emptying or even for cleaning or servicing.

Moreover, appliance1is fitted with a second resistor in thermal communication with tank10, e.g. in the form of a thick-film heater27′ extending under bottom12of tank10.

Second resistor27′ may be powered when a user wishes a frothed milk which has a lower water content, i.e. which is drier than when heated only by means of steam from the steam generator27,28. When steam generator27,28is not activated, i.e. when heating is only carried out by second resistor27′, the froth obtained is of the same kind as with frothers of the type disclosed in WO2006/050900 and in WO/2008/142154.

It follows that appliance1shown inFIGS. 2 to 4is arranged to be user-configurable for dry frothing or steam frothing or a combination of dry frothing and steam frothing.

Moreover, appliance1may be user-configurable so that air and steam are incorporated simultaneously or sequentially with or without dry heating via second resistor27′. It is also possible to configure appliance1so that steam heating and heating with second resistor27′ are combined in order to accelerate the frothing.

Hence, appliance1gives the user the possibility to choose between dry and steam frothing depending on the heating used, i.e. steam and/or second resistor27′, and between small and large bubble frothing depending on the positioning of tube20over tank bottom12.

In a variation, the same resistor heating may be used for dry and/or steam heating of the milk-containing liquid, for instance by allowing the user to bring this resistor in direct contact either with the tank or with the steam generator or with both at the same time.

FIGS. 5,6aand6b, in which the same numeric references generally designate the same or similar elements, illustrate a further embodiment according to the invention of an appliance1for frothing a body of a milk-containing liquid17contained in a frothing tank10.

In this embodiment, the reservoir module1adoes not include the steam generator or the drive arrangement. Steam generator26,27,28, drive arrangement5, user interface6and cover member14are incorporated into support base1bwhich has a connection (not shown) to the mains.

Reservoir module1bhas a tank10which contains an agitator, for instance in the form of an off-centred stirrer8, and a steam pipe20secured in a support element13with a bore13a, like in the previous embodiment. Moreover, reservoir module1ais separable from support base1band mountable thereon as indicated by arrow1abinFIG. 5.

Stirrer8is off-centred in tank10with respect to tank's vertical central axis10′. Drive arrangement5is incorporated in support base1band has a rotatable magnetic transmission member51protruding through support surface1b′ for magnetically actuating stirrer8through a correspondingly shaped protrusion121in tank's bottom12. Support surface1b′ and arched vertical face1b″ have a shape that matches bottom12of reservoir unit1band the reservoir's upright wall12″. Hence, transmission member51is as off-centred relatively to surface1b′ as stirrer8relatively to bottom12. The location of transmission member51relatively to vertical face1b″ acts as a discriminator means for properly positioning and correct orientation of reservoir unit1aon support base1b. Hence, inlet21of pipe20secured to tank10automatically matches outlet23″ of duct23in cover member14of support base1bwhen reservoir1ais mounted onto base1b.

Moreover, reservoir module1adoes not include any electric component and is not affected by any water or humidity infiltration. Hence, reservoir module1a, or at least tank10when the remaining components of module1aare separable therefrom, can be made of detergent-resistant material for permitting dishwasher cleaning.

FIG. 6aillustrates how appliance1ofFIG. 5operates at start-up. During operating, reservoir unit1ais in place on support base1band cover member14is in its closed position over water receptacle28and part of the mouth of tank10against rim12′.

Tank10contains a body of liquid17, such as milk, at the beginning of the frothing process shortly after stirrer8has started to rotate in liquid17. Stirrer8drives liquid body17to rotate and form a vortex in tank10. Hence, liquid body17has a generally v-shaped surface15extending down to bottom12. At this period of time, stirrer8rotates through surface15at level16and thereby incorporates air in the form of bubbles into liquid17.

Simultaneously, water is passed from water tank28into in-line heater27where it is evaporated and guided as steam via pipe26through duct23and pipe20into milk body17, thereby forming therein steam bubbles at outlet opening25and heating the milk17. Upon formation, steam bubbles and air bubbles that are formed and incorporated at different locations in tank10, may combine to form a foamy milk-containing phase in body17.

The more air and steam is incorporated into body17, the foamier body17becomes and increases in volume and viscosity.

As shown inFIG. 6b, after a certain period of frothing time, the heated milk body17is so emulsified by steam and air that its surface15′ generally forms a v-shape extending only about half-way down body of milk17. At this stage, stirrer does not anymore extend through body surface15′ and hence no further air incorporation takes place at the level of stirrer8. A limited amount of air may only be incorporated at surface15′ at a low rate by the result of agitation. In a variation, it is also possible to incorporate air and steam in a time overlap and/or sequentially.

As illustrated inFIGS. 6aand6b, steam outlet25is at all time located within milk body17underneath moving surface15,15′ so that steam is incorporated into milk body17and not thereabove. Hence projections of hot liquids are prevented.