Beverage dispensers

Abstract

A system for dispensing a beverage infuses the beverage with compressed air using an infusion-acceleration arrangement. The general principle is to increase the liquid-gas interfacial surface area to

Description

BACKGROUND OF THE INVENTION This invention relates to dispensers for gas-infused beverages. Nitrogenated beverages are increasingly popular. Nitrogenated beverages typically create a drink that tastes ‘creamy’, with an improved mouth feel, and have an appealing head or crema. Examples of nitrogenated drinks include “nitro” cold-brew coffee. These beverages are typically prepared by infusing nitrogen into a preprepared liquid beverage in a pressurised environment. Typical methods of creating nitrogenated beverages are costly and complex, usually requiring the use of gas cartridges to nitrogenate the beverage prior to dispensing. Typical systems for preparing nitrogenated beverages also require batch preparation prior to dispensing, making them unsuitable for producing either single or a low number of servings. Owing to the need for specialised equipment to infuse the liquid beverage with the gas and the cost and complexity of the process, nitrogenated beverages are typically only available in professional settings such as bars or restaurants. Although there have been some proposals for devices which are more suited to a casual domestic setting, these come with significant drawbacks. SUMMARY OF THE INVENTION From a first aspect, the invention provides a beverage dispensing system comprising: an infusion-acceleration arrangement arranged to infuse a pressurised gas into a liquid beverage; a holding chamber arranged to hold gas-infused liquid beverage; and a gas delivery system comprising an ambient air inlet and a compressor, wherein the gas delivery system is arranged to draw air into the gas delivery system through the ambient-air inlet and compress the air using the compressor; wherein the beverage dispensing system is arranged to carry out: an infusion phase comprising: producing a gas-infused liquid beverage by using the infusionacceleration arrangement to infuse the liquid beverage with the pressurised air supplied from the gas delivery system and storing the gas-infused liquid beverage in the holding chamber; and a dispensing phase comprising dispensing the gas-infused liquid beverage from the holding chamber. From a further aspect the invention provides a dispensing system comprising: an infusion-acceleration arrangement arranged to infuse a pressurised gas into a liquid beverage and a holding chamber arranged to hold gas-infused liquid beverage; wherein the beverage dispensing system is arranged to carry out: an infusion phase comprising: producing a gas-infused liquid beverage by using the infusionacceleration arrangement to infuse the liquid beverage with the gas and storing the gas-infused liquid beverage in the holding chamber; and a dispensing phase comprising dispensing the gas-infused liquid beverage from the holding chamber. From a further aspect the invention provides a method of dispensing a beverage using a beverage dispensing system comprising: an infusion-acceleration arrangement; a holding chamber; and a gas delivery system comprising an ambient air inlet and a compressor, the method comprising: drawing air into the gas delivery system through the ambient air inlet and compressing the air using the compressor; an infusion phase comprising: producing a gas-infused liquid beverage using the infusion-acceleration arrangement to infuse a liquid beverage with pressurised air supplied from the gas delivery system, and storing the gas-infused liquid beverage in the holding chamber; and a dispensing phase comprising dispensing the gas-infused liquid beverage from the holding chamber. From a further aspect the invention provides a method of dispensing a beverage using a beverage dispensing system comprising: an infusion-acceleration arrangement; and a holding chamber; the method comprising: an infusion phase comprising: producing a gas-infused liquid beverage using the infusion-acceleration arrangement to infuse a liquid beverage with a pressurised gas and storing the gas-infused liquid beverage in the holding chamber; and a dispensing phase comprising dispensing the gas-infused liquid beverage from the holding chamber. Thus it will be seen that, in accordance with at least some embodiments of the invention, a gas-infused liquid beverage is produced using an infusion-acceleration arrangement and stored in a holding chamber before, preferably immediately thereafter, dispensing the gas-infused liquid beverage. The Applicant has found that this is a convenient and advantageous way to create and dispense a gas-infused liquid beverage, without requiring manual intervention from the user. In accordance with embodiments of the invention, the pressurised gas is compressed ambient air. Whilst pure nitrogen (N2) has previously been used to prepare nitrogenated beverages, the applicant has appreciated that a drink of comparable quality can be created using compressed ambient air. Using ambient air rather than nitrogen advantageously reduces the cost and complexity of preparation of a nitrogenated beverage as inter alia it obviates the need to provide a source of pressurised nitrogen such as a gas canister which must be regularly replaced. In a set of embodiments, the beverage is capable of forming a foam. Thus, the beverage may form a foam when dispensed from the beverage dispensing system. The beverage may comprise one or more lipids, oils, proteins, polypeptides or emulsifiers such that it is capable of forming a foam. The presence of these substances in a liquid enables the formation of persistent bubbles when the liquid is infused with gas. This creates a foam when the gas-infused beverage is dispensed. Examples of suitable beverages for this purpose include cold-brew coffee, juices, fruitbased infusions, and cocktails. When these beverages are infused with pressurised air, it typically creates a drink that tastes creamy, with an improved mouth-feel and appealing head or crema. In a set of embodiments the beverage dispensing system comprises an infusion chamber which contains the liquid beverage whilst it is being infused with the pressurised gas during the infusion phase. The infusion-acceleration arrangement may therefore be provided within or provided at least partially by the infusion chamber in order to achieve this. In a set of embodiments, the holding chamber comprises the infusion chamber - i.e. the holding chamber is arranged to contain the liquid beverage whilst it is being infused with the pressurised gas, and arranged to store the gas-infused liquid beverage , before dispensing the gas-infused liquid beverage from the holding chamber. This may reduce the complexity of the beverage dispensing system. It is also envisaged however that in other embodiments, the holding chamber could be provided separately to the infusion chamber, and arranged to receive the gas-infused liquid beverage from the infusion chamber during the infusion phase, before dispensing the gas-infused liquid beverage from the holding chamber in the dispensing phase. In the embodiments described below the chamber in which the liquid beverage is infused with the pressurised gas is referred to as the infusion chamber. However, it should be understood that as explained above, this chamber could be provided by the holding chamber. In accordance with embodiments of the invention the beverage dispensing system comprises a gas delivery system. The gas delivery system may be connected to the infusion chamber by a gas inlet valve, which could restrict the direction of the flow of gas so that it can only flow from the gas delivery system to the infusion chamber. The compressor may comprise an electric compressor or a manual pump. The pressure of the pressurised air supplied by the gas delivery system, e.g. to the infusion chamber, may be between 3 bar and 6bar, e.g. between 4 bar and 5bar. The gas delivery system may further comprise a filter for filtering particulate matter out of the ambient air. In a set of embodiments, the infusion chamber and / or the holding chamber are / is dimensioned for storing up to four servings of beverage. In such embodiments, the interior volume of the or each chamber may be between 150 cm3 and 1000 cm3. The or each chamber may be designed to store between 100ml and 750ml of liquid beverage. In some embodiments the infusion chamber and / or holding chamber are / is dimensioned for storing a single-service of the liquid beverage. In these embodiments, the interior volume of the or each chamber may be between 150 cm3 and 250 cm3, and the or each chamber may be designed to store between 100ml and 250ml of liquid beverage. The infusion chamber may be only part-filled with the liquid beverage, e.g. up to half, or two-thirds of the volume of the infusion chamber may be filled with liquid beverage prior to the infusion phase. The Applicant has appreciated that when using compressed air instead of nitrogen to nitrogenate the beverage, a quicker dispense after the beverage has been infused is desirable for producing a high-quality beverage. Leaving the beverage pressurised for long periods of time to absorb the gas is undesirable, because the oxygen in the air will degrade the quality of the beverage via oxidation. By providing a beverage dispensing system suitable for dispensing a small number of servings of liquid beverage, e.g. a single-serving, this problem is avoided. In a set of embodiments, the start of the dispensing phase is controlled by the beverage dispensing system - e.g. with a microcontroller - such that it occurs soon or immediately after the infusion phase has completed for the reasons set out above. The dispensing phase may be controlled such that the dispensing phase begins after a predetermined time, e.g. of less than 20 seconds after the infusion phase has finished e.g. less than 10 seconds thereafter. The infusion phase may have a duration of between 10 and 60 seconds - e.g. between 20 and 40 seconds. Thus the entire infusion and dispensing operation may take less than two minutes - e.g. only of the order of a minute. Alternatively or additionally, the dispensing phase may be controlled such that it begins after confirmation that a receptacle is in place to receive the gas-infused liquid beverage. The beverage, which could be a pre-prepared beverage, may be poured into the beverage dispensing system, e.g. into the infusion chamber, before the infusion phase is carried out. The infusion chamber may comprise a beverage inlet for receiving the liquid beverage. The beverage inlet may be sealed during the infusion and dispensing phases, e.g. the beverage inlet may be sealed by a compartment closure, e.g. a removable lid. In a set of embodiments, the infusion chamber is removable from the beverage dispensing system - e.g. as part of an infusion cartridge. Providing a removable infusion chamber may advantageously enable a user to cold-brew a beverage, e.g. coffee, overnight, by cooling the removable brewing chamber, e.g. by placing the removable infusion chamber in a refrigerator, without cooling the other components of the dispenser. It may also increase the convenience of filling and / or cleaning the infusion chamber. The infusion chamber may contain solid particles for infusing a flavour into the liquid beverage prior to the (gas) infusion phase. The solid particles may be contained within an enclosure comprising an integrated filter, to prevent particles from moving freely within the infusion chamber (e.g. a strainer basket), or may be allowed to move freely within the infusion chamber during the infusion phase, before being filtered out of the gas-infused liquid beverage in the dispensing phase. The infusion-acceleration arrangement accelerates the rate of infusion of the pressurised gas into the liquid beverage compared to the rate of infusion which would occur if the pressurised gas were left to infuse passively into the body of liquid under the same pressure. The infusion-acceleration arrangement may achieve such acceleration by increasing the aggregate surface area of interface between the pressurised gas and the liquid beverage. Infusion of the gas into the liquid beverage should be understood to comprise dissolving the pressurised gas into the liquid beverage, in other words, the pressurised gas is absorbed into the liquid beverage. As outlined above, when using compressed air instead of nitrogen to nitrogenate the beverage, it is desirable to avoid oxidation of the liquid beverage and as short an infusion-time as possible is therefore desirable for producing a high-quality beverage. In a set of embodiments, producing a gas-infused liquid beverage using an infusionacceleration arrangement does not require manual intervention from the user of the beverage dispensing system. This may advantageously reduce the amount of preparation required by the user for producing a nitrogenated drink, especially in comparison to prior-art proposals which require manual shaking to adequately infuse gas into the liquid beverage. It may further improve the consistency of the nitrogenated drink, rendering the quality of the nitrogenated drink independent of the user preparing it. In a set of embodiments, the infusion-acceleration arrangement is provided by a gasdispersion plate within the infusion chamber, the gas-dispersion plate comprising a plurality of through-going holes. In some embodiments, the diameter of each of the through-going holes is between 5pm and 500pm, e.g. between 10pm and 50pm. In such embodiments, the infusion phase will typically comprise the pressurised gas being forced through the plurality of though-going holes of the gas dispersion plate into the liquid beverage. The small diameter of the through-going holes creates small gas bubbles in the liquid beverage. By creating small bubbles, the high aggregate surface area of interface between the liquid beverage and the gas can advantageously cause the gas to infuse into the liquid beverage quickly. Advantageously, the diameter of each of the through-going holes may be such that the gas dispersion plate filters solid particles out of the gas-infused liquid beverage when the gas-infused liquid beverage is dispensed in the dispensing phase. In an alternative set of embodiments, the infusion-acceleration arrangement comprises a spray-nozzle which sprays the liquid beverage onto a surface inside the infusion chamber. The surface inside the infusion chamber may comprise an internal wall of the infusion chamber, or a protrusion therefrom but in some embodiments the surface comprises a separate member such as a plate - which could for example be flat or dished and which could be supported within the infusion chamber by any suitable support means. In such embodiments, the infusion phase may comprise supplying pressurised gas to the infusion chamber to pressurise the infusion chamber. The pressurised gas may be supplied to a space in the infusion chamber above the liquid line. The liquid beverage may be supplied to the spray-nozzle using a pump e.g. to draw liquid beverage from below the liquid line inside the infusion chamber. Spraying the liquid beverage onto a surface in the infusion chamber has the effect of breaking up the jet of liquid, producing droplets of the liquid beverage. This arrangement may thus advantageously improve the rate of gas absorption into the liquid by increasing the surface area of the gas-liquid interface. In an alternative set of embodiments, the infusion acceleration arrangement comprises a tangential air-inlet to the infusion chamber. In such embodiments, the infusion phase comprises supplying pressurised air to the infusion chamber through the tangential air-inlet such that a swirling motion of the liquid beverage is encouraged. Encouraging a swirling motion of the liquid beverage promotes mixing, and helps to entrain bubbles of gas into the liquid flow. This arrangement may thus advantageously improve the rate of gas absorption into the liquid. The tangential air-inlet may be arranged to supply pressurised air from the gas delivery system into space in the infusion chamber above the liquid line, or alternatively directly into the liquid beverage contained within the infusion chamber. A tangential flow of gas entering the liquid beverage in the infusion chamber at a suitably high pressure -e.g. greater than 1 bar - encourages a swirling motion of the liquid within the infusion chamber. In some embodiments comprising a tangential air-inlet, the infusion chamber is conically shaped. In such embodiments, the infusion phase may comprise supplying pressurised air through the tangential air-inlet at a position in the infusion chamber which is above, or close to the top of, the liquid line of the liquid beverage, such that a swirling motion of the liquid beverage is encouraged in a first direction, and a downward-travelling cyclone which swirls in the first direction is created in the infusion chamber. Due to the conical shape of the infusion chamber, when the liquid beverage in this downward-travelling cyclone hits the base of the conically-shaped infusion chamber, an upward-travelling cyclone which swirls in the opposite direction to the first direction is created in the centre of the infusion chamber. The creation of such a reverse-flow cyclone promotes mixing and can thus advantageously improve the rate of gas absorption into the liquid. In addition to the embodiments described above, other infusion-acceleration arrangements are envisaged - for example, mechanical agitation of the infusion chamber itself such as by spinning, ultrasonic vibration, or shaking. Alternatively or additionally, infusion-acceleration could be provided by a mechanical component which stirs or swirls the liquid in the infusion chamber. Each of these methods promotes mixing, thereby mass transfer, and thereby increasing mass transfer and increasing the rate of gas infusion. It is also recognised that any combination of such techniques could be employed. The holding or infusion chamber may comprise a beverage outlet for dispensing the gas-infused liquid beverage therefrom, wherein the beverage outlet is sealed during the infusion phase and opened in the dispensing phase. In embodiments where the infusion chamber is configured to be removable from the beverage dispensing system, the beverage outlet may comprise a dry-disconnect valve. The dry-disconnect valve may be arranged to allow the gas-infused liquid beverage to be dispensed from the infusion chamber during the dispensing phase, and arranged to form a dry-seal when the infusion chamber is removed from the beverage dispensing system. The drydisconnect valve may comprise a resiliently-sprung piston which is biased towards a sealed position but can be displaced to allow fluid flow upon the application of upward pressure. The dry-disconnect valve may be opened when the infusion chamber is reconnected to the beverage dispensing system. Having a dry-disconnect valve at the beverage outlet of the infusion chamber may advantageously enable a user to remove the infusion chamber from the rest of the beverage dispenser whilst minimising leakage. The dispensing phase may end when the liquid beverage has been or is close to being emptied from the holding or infusion chamber. Controlling the behaviour of the beverage dispensing system in this manner may advantageously prevent spluttering of pressurised gas by ensuring that dispensing stops when the holding chamber is emptied. Therefore the beverage dispensing system may comprise means for detecting when the holding chamber has been or is close to being emptied. In a set of embodiments the beverage dispensing system comprises a flow sensor for sensing an interruption of flow of the gas-infused beverage from the holding chamber during the dispensing phase, and the dispensing phase may be ended when the flow sensor senses that there is an interruption to gas-infused beverage flowing out of the holding chamber. The flow sensor could, for example, comprise a conductivity sensor. The outlet could be closed to end the dispensing phase by a suitable valve - e.g. a solenoid-controlled valve. In other embodiments the beverage dispensing system comprises a level sensor, e.g. a float, for determining the level of the liquid in the holding or infusion chamber, and the dispensing phase is ended when the level sensor senses that the level of the liquid in the holding or infusion chamber has reduced to below a predetermined threshold. Such a sensor could be electrical and used to operate an electrical valve to end dispensing or a flow cut-off valve could be mechanically coupled to a mechanical sensor such that it closes in response to the predetermined threshold level being reached. In a set of embodiments the beverage dispensing system comprises a flow restrictor disposed at or upstream of a dispensing outlet of the beverage dispensing system. The flow-restrictor may advantageously control the flow of the beverage in such a way that a desirable crema or foam is produced on the beverage when it is dispensed from the dispensing outlet. The flow restrictor could comprise a flow-restricting obstacle arranged to create a small gap between a wall of an outlet conduit and a surface of the flow-restricting obstacle. The width of the gap may be 10-200 pm, e.g. 50-100 pm. The beverage dispensing system may further comprise an integrated cleaning system. The integrated cleaning system may comprise a water source and a water pump, and may be connected to the infusion and / or holding chamber by a water inlet valve. The water source may comprise a reservoir contained within a countertop beverage dispenser, but can alternatively comprise a connection to a mains water supply. The integrated cleaning system may use the water pump to pump water from the water source through the water inlet valve to the infusion chamber and / or the holding chamber. Supplying water to the or each chamber enables it to be flushed, removing residue left by the liquid beverage after use of the beverage dispensing system. This may advantageously obviate the need for the user to disassemble the beverage dispensing system to clean it. In a set of embodiments, the beverage dispensing system comprises a countertop beverage dispenser - e.g. its outer dimensions might all be less than 50cm and / or the total volume of space occupied by the countertop beverage dispenser does not exceed 0.05m3, e.g. does not exceed 0.03 m3, advantageously making it suitable for use at home, by the mass-market, rather than requiring a professional setting for preparation. Features of any aspect or embodiment described herein may, wherever appropriate, be applied to any other aspect or embodiment described herein. Where reference is made to different embodiments or sets of embodiments, it should be understood that these are not necessarily distinct but may overlap. BRIEF DESCRIPTION OF THE DRAWINGS Certain preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a perspective view of the exterior of a beverage dispenser in accordance with the invention; Figure 2 is a high-level system diagram of the beverage dispenser of Figure 1; Figure 3 is a system diagram of the gas delivery system; Figure 4 is a more detailed view of the infusion system; Figure 5 is a perspective view of the gas dispersion plate of the infusion system shown in Figure 4; Figure 6 is a more detailed diagram of the dispensing system; Figure 7 is a more detailed view of the flow restrictor in the dispensing system shown in Figure 6; Figure 8 is a flow-chart showing the operation of the beverage dispenser in accordance with the first embodiment of the invention; Figure 9 is a schematic diagram of the infusion system of a second embodiment of the invention; Figure 10 is a schematic diagram of the infusion system of a third embodiment of the invention; and Figure 11 is a schematic diagram of the infusion system of a fourth embodiment of the invention. DETAILED DESCRIPTION Figure 1 is a perspective view of the exterior of a beverage dispenser 100 in accordance with the invention. In the examples described in detail herein the beverage dispenser 100 is a countertop appliance, and is designed such that it is particularly suitable for dispensing “Nitro-style” cold-brew coffee. However, it is anticipated that the countertop beverage dispenser 100 could equally be used for dispensing other nitro-style beverages, for example, juices, fruit-based infusions, or cocktails. Nitrogenated beverages (i.e. beverages infused with nitrogen) typically create a drink that tastes creamy, with an improved mouth-feel and appealing head or crema. The countertop beverage dispenser 100 comprises an infusion cartridge 120, a dispensing outlet 150, and a beverage receptacle platform 160. The infusion cartridge 120 is removable from the rest of the beverage dispenser 100 so that it can be filled with a liquid beverage. This enables a user to fill it with a liquid and a solid component to infuse the liquid with, and also allows the user to cool the liquid by, for example, placing the infusion cartridge 120 in a refrigerator overnight. The infusion cartridge 120 may have an integrated strainer basket (not shown) for holding the solid component to infuse the liquid with. Where the beverage dispenser is used for dispensing nitro-style cold-brew coffee, the infusion cartridge can be filled with water, with coarse-ground coffee added to the strainer basket, and placed in the refrigerator overnight to brew. During operation the beverage dispenser 100 infuses the liquid beverage (e.g. the cold-brew coffee) with a gas containing nitrogen during an infusion phase, and promptly dispenses the liquid beverage from the dispensing outlet 150 to a receptacle placed on the platform 160 during a dispensing phase. The dispensed nitro-style liquid beverage does not require any further mixing from the user prior to consumption - i.e. is ready to be consumed when dispensed. The structure and operation of the beverage dispenser are described in more detail below. Figure 2 is a high-level system diagram of the beverage dispenser 100 shown in Figure 1. The beverage dispenser 100 comprises a gas delivery system 210, an infusion system provided by the infusion cartridge 120, a water delivery system 230 and a dispensing system 240. The infusion cartridge 120 is connected to the gas delivery system 210 by a one-way gas inlet valve 212. The gas inlet valve 212 restricts the direction of the flow of gas so that it can only flow from the gas delivery system 210 to the infusion cartridge 120. Figure 3 shows the gas delivery system 210 in more detail. It comprises a gas filter 214, a compressor 216, and an ambient-air inlet 218. The gas delivery system 210 draws air into the gas delivery system 210 through the ambient-air inlet 218, compresses the air using the compressor 216, and delivers the compressed air to the infusion cartridge 120 through the gas filter 214 and gas inlet valve 212. The advantage of a gas delivery system 210 as shown in Figure 3 is that gas cannisters are not required for operating the beverage dispenser, thus reducing expense and inconvenience for the user. Whilst nitrogen is traditionally used to create nitro-style drinks, the applicant has appreciated that a drink of comparable quality can be created using compressed ambient air (which of course contains a large percentage of nitrogen). Referring again to Figure 2, the infusion cartridge 120 comprises a compartment closure 222, an infusion chamber 224, an infusion-acceleration arrangement 226 and a beverage outlet 228. The compartment closure 222 comprises an opening which can be opened to fill the infusion chamber 224, and sealed during the infusion and dispensing phases - this could straightforwardly be provided by a lid, for example. Figure 4 is a more detailed diagram of the infusion cartridge 120 showing the infusion chamber 224 and a gas dispersion plate 226 mounted across the lower part of the infusion chamber 224 to provide the infusion acceleration arrangement. The top of the infusion chamber 224 is sealed by the closure member 222 in the form of a removable lid. Although not shown a filter or basket could be provided to retain solids such as coffee grounds used to infuse the beverage. The base of the infusion chamber 224 comprises the gas inlet valve 212 and a drydisconnect valve 227 which comprises a resiliently-sprung piston which is biased towards a sealed position but can be displaced to allow fluid flow upon the application of upward pressure. A more detailed perspective view of the gas dispersion plate 226 is provided in Figure 5. From this it can be seen that the gas dispersion plate 226 comprises a perforated, flat, circular disc 202 (e.g. of a metal with suitable anticorrosion properties or coating) with a plurality of through-going holes 204 arranged here in a series of concentric annular rings, although the particular pattern is not essential. The diameter of each of the through-going holes could be between 5pm and 500|jm. Advantageously the diameter of each of the through-going holes 204 may be designed such that the gas dispersion plate 226 also filters any solid particles from the liquid beverage when the liquid beverage is dispensed from the infusion chamber 224. Referring back to Figure 4, the dry-disconnect valve 227 engages in use with the open mouth 228 of a conduit which forms part of the dispensing system 240 such that when the infusion cartridge 120 is installed in the apparatus 100 the dry-disconnect valve 227 is opened by the end of the conduit 228 compressing the integral spring in the valve 227, allowing the beverage to be dispensed from the infusion chamber 224. When the infusion cartridge 120 is removed however, the dry-disconnect valve 227 is disengaged from the end of the conduit 228 such that the spring automatically closes the valve 227 forming a dry seal that allows the infusion cartridge 120 to be removed whilst minimising leakage. As described above with reference to Figure 1, this enables a user to remove the infusion cartridge 120 from the rest of the beverage dispenser to create their initial liquid beverage by brewing it and / or storing it in the refrigerator. Referring again to Figure 2, the dispensing system 240 comprises a solenoid flow cutoff valve 242, a flow restrictor 244 and a dispensing outlet 246. These are shown in more detail in Figure 6. The dispensing system 240 includes a conduit 241, at the upper end of which is the open mouth 228 which provides a beverage inlet and engages the dry-disconnect valve 227; the solenoid cut-off valve 242; a conductivity sensor 243; the flow restrictor 244; and the dispensing outlet 246. The flow restrictor 244 is positioned at the outlet 246 at the downstream end of the conduit 241. The conductivity sensor 243 is mounted in the conduit 241 at an intermediate position between the beverage inlet 228 and the solenoid cut-off valve 842. The conductivity sensor 243 senses the flow of gas-infused liquid beverage in the conduit 241, and can therefore detect when the stream of gas-infused liquid beverage through the conduit 241 has stopped, or is about to stop since the flow will be interrupted by pockets of air which will be sensed by the conductivity sensor as having a different conductivity to the liquid. The solenoid cut-off valve 242 is communicatively coupled (via a microprocessor - not shown) to the conductivity sensor 243 such that it can be closed in response to a cutoff signal received from the conductivity sensor 243. When the solenoid cut-off valve 242 is closed, liquid beverage cannot flow through the conduit 241 i.e. no liquid can be dispensed by the dispensing system 240. In alternative embodiments, a flow cut-off valve could be coupled to a mechanical sensor such as a float valve, the float valve being located in the infusion chamber. In these alternative embodiments, the flow cut-off valve can be closed in response to the float reaching a certain position within the infusion chamber which is indicative of predetermined a volume of liquid remaining in the infusion chamber. Figure 7 shows the flow restrictor 244 in more detail. The flow restrictor 244 comprises a flow-restricting obstacle 245 disposed at the end of the conduit 241 in a flared section thereof which forms the dispensing outlet 246. The shape of the flow restrictor interacts with the flared shape of the conduit 241 to form a tight and narrowing annular gap 247 between the walls of the conduit 241 and the flow-restricting obstacle 245. The width of the annular gap is ideally 10-200 pm at its narrowest point. The annular gap 247 has the effect of controlling the flow of the beverage in such a way that a desirable crema or foam is produced on the beverage when it is dispensed from the dispensing outlet 246. Referring again to Figure 2, the water delivery system 230 comprises a water source 234 and a water pump 236. The water source 234 can be a reservoir contained within the beverage dispenser 100, but can alternatively comprise a connection to a mains water supply. The water delivery system 230 is connected to the infusion cartridge 120 via a water inlet valve 232 provided in the base of the infusion cartridge . The water inlet valve 232 allows water to flow from the water delivery system 230 to the infusion cartridge 120 but maintains pressure in the chamber 224 during infusion. The water delivery system 230 is used to clean the infusion cartridge 120 by using the water pump 236 to pump water from the water source 234 through the water inlet valve 232 to the infusion system 220, removing residue left by the liquid beverage after use. Operation of the beverage dispenser 100 will now be described with reference to the flow chart of Figure 8. In a filling phase 302, the user fills the infusion chamber 224 of the infusion cartridge 120 with a beverage of their choice by removing the lid 222. This can comprise removing the infusion cartridge 120 from the rest of the beverage dispenser -disengaging the dry-disconnect valve 227. The user then replaces the infusion cartridge 120 into the rest of the beverage dispenser 100 and replaces the infusion chamber lid 222. The user then presses a button (not shown) to initiate the infusion phase 304. This begins with the compressor 216 being operated to draw in air through the inlet 218, compress it and pass it through the filter 214 into the inlet valve 212 which is opened. The compressed gas then passes into the lower part of the infusion chamber 224. Where some liquid beverage is present below the gas dispersion plate 226 after the infusion chamber 224 has been filled in the filling phase 302, supplying the compressed gas to the lower part of the infusion chamber forces some or all of this liquid beverage through the holes 204 of the gas dispersion plate 226 into the area of the infusion chamber 224 above the gas dispersion plate 226. Along with some or all of this liquid beverage, the compressed gas is forced through the holes 204 of the gas dispersion plate 226 up into the rest of the liquid beverage above the gas dispersion plate 226. The small diameter of the through-going holes 204 in the gas diffusion plate 226 forces small gas bubbles into the liquid beverage. By creating smaller bubbles, the increased aggregate surface area of interface between the liquid beverage and the gas causes the gas to infuse into the liquid beverage more quickly compared to the rate of infusion which would occur if the compressed gas and liquid beverage were left to infuse passively under pressure. Infusion of the gas into the liquid beverage should be understood to involve dissolving the gas into the liquid beverage, in other words, the pressurised gas being absorbed into the liquid beverage. Throughout the infusion phase 304, the solenoid shutoff valve 242 is closed, preventing the gas-infused liquid from leaving the infusion chamber 224. The gas-infused beverage is stored in the infusion chamber 224 until the end of the infusion phase 304. After the infusion phase 304 has been completed - e.g. after a predetermined time controlled by the microcontroller (not shown) and optionally after confirming that a receptacle has been placed on the receptacle platform 160 - the beverage dispenser 100 automatically initiates a dispensing phase 306 in which the gas-infused liquid beverage is dispensed from the infusion chamber 224. The applicant has appreciated that when using compressed air instead of nitrogen, a shorter infusion time and a quicker dispense after the beverage has been infused is desirable for producing a high-quality “nitro-style” beverage. Leaving the beverage pressurised for long periods of time to absorb the gas is undesirable, because the oxygen in the air will degrade the quality of the beverage via oxidation. During the dispensing phase 306, the cut-off valve 242 is opened, allowing the gas-infused liquid beverage to flow under pressure to the dispensing outlet 246, through the flow restrictor 244. The gas-infused liquid beverage is forced through the gas dispersion plate 226 toward the dispensing outlet 246 by the pressurised gas in the infusion chamber 224. The flow restrictor 244 restricts the flow of beverage in a controlled way such that the flow produces a desirable crema or foam on the dispensed beverage as it exits into a receptacle placed on the platform 160. During the dispense phase 306, the conductivity sensor 243 provides a signal indicating that liquid is flowing. Once the chamber 224 is nearly empty such that flow starts to be interrupted by pockets of air, the conductivity sensor 243 detects this and the microcontroller initiates the end-of-dispense phase 308. In the end-of-dispense phase 308, the cut-off valve 242 is closed. Controlling the behaviour of the dispensing system 240 in this manner advantageously prevents spluttering of pressurised gas by ensuring that dispensing stops before the dispensing system 240 is fully emptied. The end-of-dispense phase 308 also includes subsequently operating the water pump to flush the chamber 224 with water. The user could, for example, be directed to place a suitable cleaning receptacle on the platform 160 at this point and the valve 242 reopened. Optionally the compressor 216 could be operated to drive the water through the system. Figure 9 is a schematic diagram of the infusion cartridge 420 of a second embodiment of the invention. The features of this embodiment can be the same or similar to those of the first embodiment and thus are not described in detail here. The infusion cartridge 420 comprises an infusion chamber 424, a pump 470, a spray-nozzle 472 and an impaction plate 474. The impaction plate 474 may be supported within the infusion chamber 424 by a strut or any other suitable support means (omitted for clarity). The infusion cartridge 420 further comprises a gas inlet vale 412 for introducing compressed air into the infusion chamber 424, although in this embodiment it is provided on the side of the chamber rather than the base. A beverage outlet (not shown) is also provided for dispensing the gas-infused liquid beverage from the infusion chamber 424. This could comprise a dry-disconnect valve like that described with respect to the first embodiment, or any other suitable arrangement. Prior to the infusion phase, a user will add their beverage of choice into the infusion chamber 424 in a similar manner to that described in the first embodiment. During the infusion phase, compressed gas is supplied to the space in the infusion chamber 424 above the liquid via the gas inlet valve 412. Once the infusion chamber 424 has been pressurised, the pump 474 draws liquid from the bottom of the chamber and delivers liquid to a small orifice in the spray nozzle 472, which therefore sprays a jet of the liquid onto the impaction plate 476 which breaks up the jet of liquid, producing small droplets of liquid. This arrangement improves the rate of gas infusion into the liquid beverage by increasing the surface area of the gas-liquid interface. This can be continued until a sufficient degree of infusion has bene achieved. Thereafter dispensing and optional flushing can be carried out in the same manner as in the first embodiment. Figure 10 is a schematic diagram of the infusion system 520 of a third embodiment of the invention. The infusion system 520 comprises a conically-shaped infusion chamber 524, a pump 570 disposed at the end of a central conduit 576 which extends at right angles into the centre of the chamber 524, an air-inlet valve 512, a tangential air-inlet conduit 580, and a beverage outlet 528. During the infusion phase, compressed air is supplied through the tangential air-inlet conduit 580 from the air delivery system (not shown but which can be as shown in Fig. 3), through the air-inlet valve 512. The tangential compressed air jet sets up a cyclone at the top of the chamber 524 which induces a swirling motion the liquid beverage as it is entrained by the air. The conical shape of the infusion chamber 524 encourages a downward-travelling cyclone comprising air and liquid. When this downward-travelling cyclone hits the base of the conically-shaped infusion chamber 524, it causes a reversal of the flow direction - i.e. an upward-travelling cyclone which swirls in the opposite direction the centre of the infusion chamber 524. The creation of the reverse-flow cyclone generates strong shear forces which promotes rapid infusion of the gas into the liquid. The pump 570 operates to draw the gas-infused beverage through the central conduit 576 to enable recirculation of the gas-infused beverage, and thus create a continual cyclone. Thereafter, the gas-infused beverage is dispensed to the dispensing system from the base of the infusion chamber 524, via the beverage outlet 528. Figures 11A and 11B show schematically the infusion system 620 of a fourth embodiment of the invention. The infusion system 620 comprises an infusion chamber 624 and wedge-shaped air inlet plenum 626 comprising a tangential air inlet 680 at an upper part thereof and a tangential air outlet 678 at a lower part thereof. During the infusion phase, compressed air is supplied from the gas delivery system into the plenum 626 through the tangential air inlet 612 such that it is delivered via the tangential air outlet 678 into the liquid beverage, below the liquid line. Such a tangential flow of gas entering the liquid in the infusion chamber 624 at a suitably high pressure (ideally greater than 1 bar) encourages a swirling motion of the liquid within the infusion chamber 624 - thereby promoting mixing and helping to entrain bubbles of gas into the liquid flow. This leads to increased gas absorption rates. Whilst several alternatives have been described for providing an infusion-acceleration arrangement, there are many other possibilities: for example, mechanical agitation of the infusion chamber itself such as by spinning, ultrasonic vibration, or shaking. Alternatively or additionally, infusion-acceleration could be provided by a mechanical component which stirs or swirls the liquid in the infusion chamber. Each of these methods promotes mixing, thereby mass transfer, and thereby increasing mass transfer and increasing the rate of gas infusion. It is also recognised that any combination of such techniques could be employed. Although in the examples described, the infusion-acceleration arrangement is within the infusion chamber itself, it is equally envisaged that the infusion-acceleration arrangement could be external to and coupled to the infusion chamber- i.e. configured to mix the compressed gas and liquid beverage separately, before storing the resulting gas-infused beverage in a chamber. It will also be appreciated that whilst some of the embodiments include a removable infusion cartridge, this is not essential and the infusion system could instead be provided as an integral part of the dispenser. More generally it will be appreciated by those skilled in the art that the invention has been illustrated by describing one or more specific embodiments thereof, but is not limited to these embodiments; many variations and modifications are possible, within the general principle of using compressed air to rapidly infuse into a beverage, with a suitable acceleration arrangement, immediately before automatically dispensing it. It will be appreciated by those skilled in the art that the invention has been illustrated by describing one or more specific embodiments thereof, but is not limited to these embodiments; many variations and modifications are possible, within the scope of the accompanying claims.

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