Group head for espresso machine
Patent Information
- Authority / Receiving Office
- RU · RU
- Patent Type
- Patents
- Current Assignee / Owner
- BREVILLE HLDG PTY LTD
- Filing Date
- 2022-03-24
- Publication Date
- 2026-07-06
Smart Images
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Abstract
Description
FIELD OF TECHNOLOGY
[0001] The present invention relates to espresso machines. In particular, the present invention relates to controlling the temperature of brewing water passing through ground coffee. BACKGROUND OF THE INVENTION
[0002] Espresso coffee is made by forcing hot water under pressure through compressed ground coffee. The ground coffee is typically placed in a filter fitted with a holder (known as a portafilter), which is detachably connected to the espresso machine at the "group head" (GH). To enhance the flavor and aroma of the ground coffee, the ground coffee is compacted or pressed in the portafilter to form a disc known as a "tablet."
[0003] To achieve optimal espresso extraction, this espresso machine first heats water and then forces it through a pressurized pod. The optimal "brew water" temperature (as it's called) for achieving the desired espresso quality will vary slightly depending on various factors, such as the coffee beans, grind (i.e., how finely ground the roasted beans are), and user preference. However, as a general rule, the optimal temperature will be between 90°C and 95°C.
[0004] Espresso machines typically heat the brewing water using a main heater in the boiler and / or a thermoblock upstream of the portafilter. The thermoblock is a metal block (usually cast aluminum) with an internal flow channel and a resistive heating element. Water flowing through the thermoblock is galvanically heated by the internal surfaces of the flow channel. Unfortunately, these heating mechanisms can be slow to heat the brewing water to the desired temperature due to their physical mass and thermal properties. During this time, the brewing water temperature cannot be controlled during the short period of coffee extraction. As described above, extracting coffee with water that is not at the correct temperature negatively impacts the espresso.
[0005] Thermoblock heaters have manufacturing and material-related issues. Cast aluminum thermal blocks require a beverage-grade surface coating to resist corrosion. Cast aluminum corrodes when exposed directly to water and heat, so it must be coated (such as Teflon or another coating) to prevent oxidation. Aluminum has a relatively high specific heat capacity, requiring more energy to heat (compared to, for example, stainless steel or brass), and therefore takes longer to reach a certain temperature. Aluminum also has relatively high thermal conductivity, so it dissipates heat faster than stainless steel. Cast parts are porous, which can make manufacturing less repeatable and reliable.Thermal block castings also tend to be relatively large and heavy, which defeats the purpose of creating a compact overall design.
[0006] Water heated by the main heater becomes a "heat transfer medium," transferring heat to adjacent components and the surrounding environment throughout the machine. This conductive and radiant heating can be uneven, causing inconsistencies between the group head temperature and the water flow. Despite this, the main heater in the boiler is expected to warm the machine over time and reduce temperature loss in the brewing water as it flows through the various tubes, fittings, valves, and connectors. While this may help limit temperature loss, the brewing water temperature is still not properly controlled.
[0007] The temperature profile along the water flow path will also change. Modern heaters, such as thermoblocks and boilers, can be characterized as flow-through systems. Water flows through the boiler and thermoblock at a relatively high flow rate, resulting in uneven heating. Water closer to the inner surfaces of the thermoblock typically has a higher temperature than water closer to the center of the flow path. Such discrepancies between the actual and optimal water temperatures for brewing can significantly reduce the quality of espresso extraction. SUMMARY OF THE INVENTION
[0008] The object of the present invention is to eliminate one or more of the above-mentioned disadvantages, or at least to provide a useful alternative to the above-mentioned approaches.
[0009] In one aspect, the present invention provides a group head for an espresso machine, comprising: a group head bracket for removably engaging a portafilter that holds a ground coffee tablet in a filter basket for dispensing espresso coffee; a channel for receiving a flow of water to the portafilter; and a heater installed in the group head for heating the flow of water entering the portafilter.
[0010] Preferably, the group head further comprises a flow diffuser adjacent to the heater to define a space for distributing the water flow from the channel over the surface of the heater to improve conductive heating.
[0011] Preferably, the diffuser has a plurality of channels for directing the flow of water over the surface of the heater, wherein the channels have channel walls that abut the surface of the heater for heat conduction into the diffuser.
[0012] Preferably, each of the channels is in fluid communication with a channel for receiving a water flow, and each of the channels has at least one outlet end for fluid communication with a portafilter.
[0013] Preferably, the group head further comprises a mesh located between the outlet ends of the channels and the portafilter, wherein the outlet ends are located such that the flow of water through the mesh to the portafilter has the desired distribution through the ground coffee tablet.
[0014] The diffuser is preferably made of corrosion-resistant material with high specific heat capacity.
[0015] The heater is preferably installed in the group head bracket for heat conduction from the heater to the group head bracket.
[0016] Preferably, the detachable engagement between the group head bracket and the portafilter is configured to conduct heat from the group head bracket to the portafilter.
[0017] Preferably, the heater is a resistive heater with a conductive path made of an electrically resistive material.
[0018] Preferably, the resistive heater is adjustable to heat the water flow to a predetermined temperature.
[0019] The preferred set temperature is the temperature selected by the user.
[0020] Preferably, the group head further comprises a temperature sensor for feedback control of the resistive heater.
[0021] Preferably, the resistive heater is a thick film heater with an electrical resistive material deposited in the form of a thick film on a substrate.
[0022] Preferably, the thick film heater surrounds at least one section of the channel.
[0023] Preferably, the thick film heater is a disk arranged such that during use, the disk extends in a plane substantially parallel to the top surface of the tablet.
[0024] Preferably, the channel has an outlet end in the center of the disc, and the diffuser channels are configured to radially distribute the water flow over the surface of the disc.
[0025] Preferably, the thick film heater is configured to heat the water flow through the group head to a temperature of from 89°C to 96°C.
[0026] Preferably, the thick film heater is configured to heat a stream of water withdrawn from a reservoir inside the espresso machine to 89-96°C in less than 10 seconds from the moment the thick film heater is activated.
[0027] Preferably, the group head comprises a chamber defined by the inner walls of the group head, wherein the chamber has an opening for receiving the portafilter, and wherein the heater is mounted on the chamber.
[0028] In a further aspect, the present invention provides an espresso machine comprising a group head as described above with respect to the first aspect and associated preferred features.
[0029] In another aspect, the present invention provides a method for producing espresso coffee, comprising the following steps: placing a ground coffee tablet inside a portafilter filter basket; attaching the portafilter to a group head of an espresso machine, wherein the group head has a heater and a channel for flowing water to the portafilter; providing a flow of water through the channel to the ground coffee tablet; and heating the flow of water through the group head using a heater.
[0030] Preferably, the heater is a resistive heater having a conductive path made of an electrically resistive material.
[0031] Preferably, the method further includes the step of regulating the electrical power of the resistive heater to heat the water flow from the group head to a predetermined temperature. In some embodiments, the predetermined temperature is selected by the user.
[0032] Preferably, the method further includes the step of providing a temperature sensor for feedback control of the resistive heater by a control unit inside the espresso machine.
[0033] Preferably, the heater is configured to surround at least one section of the channel.
[0034] Preferably, the resistive heater is a thick film heater with an electrical resistive material deposited in the form of a thick film on a substrate.
[0035] Preferably, the thick-film heater is in the form of a disk extending in a plane substantially parallel to the upper surface of the tablet.
[0036] Preferably, the method further includes providing the head of the group with a flow diffuser adjacent to the heater to define a space for distributing the flow of water from the channel over the surface of the heater to improve conductive heating.
[0037] Preferably, the diffuser has a plurality of channels for directing the flow of water over the surface of the heater, wherein the channels have channel walls that abut the surface of the heater for heat conduction into the diffuser.
[0038] Preferably, each of the channels is in fluid communication with a channel for receiving a water flow, and each of the channels has at least one outlet end for fluid communication with a portafilter.
[0039] Preferably, the method further includes the step of providing a mesh between the outlet ends of the channels and the portafilter and arranging the outlet ends so that the flow of water through the mesh into the portafilter has the desired distribution through the ground coffee tablet.
[0040] Preferably, the method further includes the step of providing a group head bracket for mounting the group head on the espresso machine, wherein the heater is mounted in the group head bracket for heat conduction from the heater to the group head bracket.
[0041] Preferably, the detachable engagement between the group head bracket and the portafilter is configured to conduct heat from the group head bracket to the portafilter.
[0042] Preferably, the method further includes the step of adjusting the thick film heater to heat the water flowing from the group head to a temperature of 89-96°C.
[0043] Preferably, the method further includes the step of adjusting the thick film heater to heat the water flowing through the group head to at least 89°C in less than 10 seconds from the moment the thick film heater is activated.
[0044] Preferably, the diffuser is made of corrosion-resistant material with high specific heat capacity. BRIEF DESCRIPTION OF GRAPHIC MATERIALS
[0045] The present invention will now be described by way of example only with reference to the following illustrative embodiments and the accompanying drawings, in which:
[0046] Fig. 1 shows a schematic diagram of an espresso machine according to the present invention;
[0047] Fig. 2 is a perspective view of the head of the group in accordance with the present invention;
[0048] Fig. 3 is a cross-sectional view of a portafilter releasably engaged with a group head in accordance with the present invention;
[0049] Fig. 4 is a cross-sectional view of the head of the group shown in Fig. 3, in isolation;
[0050] Fig. 5 is a schematic cross-sectional view of the group head engaged with the portafilter, showing the flow of brewing water and thermal conductivity through the group head to the portafilter;
[0051] Fig. 6 is a perspective view of the water channel and the heater shown in Fig. 4, in isolation;
[0052] Fig. 7 is a perspective view of the inner bracket of the group head and the channel surrounded by the heater, separately;
[0053] Fig. 8 is a perspective view of the inner bracket with the channel and the heater together with the end bracket and the temperature sensor for operating the heater;
[0054] Fig. 9 is an expanded perspective view of the components showing the inner bracket of the group head together with the end bracket for the heater and the temperature sensor, with the diffuser, dispersion mesh and central fastening screw located under the inner bracket;
[0055] Fig. 10 is an expanded perspective view showing the components of the group head;
[0056] Fig. 11 is a sectional view of an espresso machine with a portafilter releasably engaged with a group head in accordance with the present invention;
[0057] Fig. 12 is a partial perspective view in section of the espresso machine shown in Fig. 11;
[0058] Fig. 13 shows a graph of the temperature and pressure of the brewing water at the inlet and outlet of the group head during the preparation of espresso coffee; and
[0059] Fig. 14 shows a block diagram of the operation of an espresso machine with a group head in accordance with the present invention. DESCRIPTION OF EMBODIMENTS
[0060] With reference to the figures, Fig. 1 schematically shows the main components of the espresso machine 10 according to the present invention. In the case of a stand-alone machine 10, water 12 is drawn from a water tank 14 by a flow pump 18 to increase the water pressure to about 9 bar and a maximum of 15 bar. A flow meter 72 at the outlet of the tank sends a flow rate feedback signal to a controller 86.
[0061] Pump 18 forces water through heater 70 to raise the water temperature to approximately 120°C. The heated water flows to solenoid valve 84. The user selects (via the user interface) espresso extraction, hot water dispensing (e.g., for a less intense black coffee, etc.), or steam dispensing for frothing milk. When machine 10 is not operating, valve 84 is displaced to direct the flow into drip tray 88 to remove any residual water under pressure. Similarly, overpressure relief valve 80 releases air into drip tray 88 if the water pressure exceeds the safe maximum value.
[0062] For the sake of clarity, only the espresso extraction flow line 83 is shown. When the extraction flow line 83 is opened by the solenoid valve 84, heated water under a pressure of approximately 9 bar enters the group head 20. The water temperature is no longer equal to the outlet temperature of the flow through the heater 70. Heat transfer to the channels, valve connectors, and fittings reduces the temperature, making it difficult to actively control the flow through the heater 70 with feedback. Some espresso machines use a thermoblock heater upstream of the group head to improve temperature control of the brewing water through the portafilter 24. However, as discussed above, thermoblock heaters have a relatively large heat capacity, making precise control of the brewing water temperature difficult.Additionally, even if the brew water temperature leaving the thermoblock heater is well controlled, there are still losses along the flow path to the group head and through the group head itself.
[0063] To address this issue, the applicant utilizes a group head heater 32 integrated into the group head 20. As described in more detail below, the group head heater 32 provides greater control over the temperature of the brewing water entering the portafilter 24. Brewing water below the required temperature is quickly heated to eliminate any significant fluctuations throughout the extraction process. Maintaining the required brewing water temperature (e.g., 93°C) significantly improves the quality of the extracted espresso 30. The temperature sensor 60 can transmit output data to the processor 86 for closed-loop control during the extraction process and for protective shutdown in the event that the maximum temperature (e.g., about 200-220°C) is reached.
[0064] After espresso extraction, the flow of brewing water into portafilter 30 is shut off. Optionally, backflush valve 85 is used to rinse the group head and drain water into drip tray 88. The water pressure and temperature upstream of the group head may drop, causing a small amount of extracted coffee to be drawn back from the portafilter. Backflush of the drip tray removes any residue and prepares the group head for the next extraction.
[0065] The complete group head 20 is shown in Figs. 2, 4 and 10. Fig. 5 is a schematic cross-sectional view of the portafilter engaged with the group head assembly, showing heat conduction from the heater 32 GG to the group head 20, the portafilter 24 and the flow of brewing water to the tablet 22. Figs. 6-9 show specific subassemblies and components in isolation for clarity, and Fig. 3 shows the group head 20 removably engaged with the portafilter.
[0066] The group head 20 is mounted on the body of the espresso machine 10 via a group head bracket 36 and includes a chamber defined by the inner walls of the group head, the chamber having an opening for receiving a portafilter 30. The group head bracket 36 is an assembly of an inner bracket 44 and an outer bracket 42 (see Fig. 4). The inner bracket 44 mounts a heater 32 GG on the chamber and holds the heater 32 GG close to the upstream side of the diffuser 38. The heater 32 GG is configured as a disk with a central opening surrounding a channel 36 for receiving brewing water from the solenoid valve 84. As best shown in Figs. 4 and 5, the heater 32 GG and the diffuser 38 define a space 134 that extends horizontally through the downward facing surface 136 of the heater 32 GG.The flow 132 of water for brewing through the channel 34 enters the horizontal space 134 through the outlet ends 150 of the channel, formed in the central screw 58, screwed into the lower end of the channel 34 (further described below). The flow 132 of water for brewing is directed along the surface 136 of the heater 32 GG by means of the diffuser 38. The space 134 has such dimensions that all the water in the flow 132 is retained close to the surface 136 of the heater and is quickly heated.
[0067] As best shown in Fig. 5, the group head 20 and the diffuser 38 are also configured to be conductively heated by the heater 32 GG. To facilitate this, the bracket 36 of the group head and the diffuser 38 have high thermal conductivity and are preferably made of materials with high specific heat capacity and corrosion resistance, such as stainless steel. Thermal conductivity 144 through the bracket 36 of the group head, in turn, will heat the diffuser 38, which also has high specific heat capacity and corrosion resistance (for example, made of stainless steel). Heating the diffuser 38 promotes thermal conductivity into the flow 132 of brewing water through the space 134.
[0068] The diffuser 38 comprises channels 138 defined by channel walls 140 that abut against the surface 136 of the heater 32 GG. This contact with the heater 32 GG further promotes thermal conductivity into the diffuser 38. Each of the channels 138 has at least one outlet end 142 of the diffuser for fluid communication with the dispersion mesh 40. The number and arrangement of the outlet ends 142 of the diffuser are such that the flow of brewing water through the dispersion mesh 40 has the required distribution. Typically, the outlet ends 142 of the diffuser are configured to have a relatively uniform flow rate and temperature, which in turn ensures a uniform distribution of the flow through the dispersion mesh 40 onto the tablet 22 for better coffee extraction using uniformly and accurately heated water.
[0069] As shown in Fig. 4 and 10, the central screw 58 is screwed into the end of the channel 34 to hold the dispersion mesh 40 and the diffuser 38 in place. The rod of the central screw 58 is hollow with an axial hole leading to two holes on the side of the rod, which form the outlet end 150 of the channel 34. The brewing water from the electromagnetic valve 84 enters the inlet end 148 of the channel and exits from the outlet ends 150 into the space 134 between the heater 32 GG and the diffuser 38. The water is radially diffused, filling the space 134, which is sealed around the periphery of the diffuser 38 by the seal 46 GG. The volume of this space is relatively small compared to the area of the lower surface 136, which helps to enhance the heat conduction into the water. Since the space is narrow, the temperature of the water is relatively uniform. The formation of the heater 32 in such a way that it surrounds a portion of the channel 34 also facilitates rapid heat conduction into the water.From the initial activation of pump 18 to the supply of water from group head 20 to coffee tablet 22 at a temperature of 89°C to 96°C, less than 10 seconds pass.
[0070] As described above, the flow of brewing water from each outlet end 142 of the diffuser to the dispersion mesh 40 is relatively uniform (in terms of temperature and flow rate). The water spreads through the dispersion mesh 40 with holes before uniformly passing to the upper surface of the tablet 22 (see Fig. 3). The tablet 22 is held in the filter basket 64 of the portafilter 24. The upper part of the filter basket 64 also fits tightly with the seal 46 GG to prevent leakage of brewing water and ensure infusion through the tablet 22 while maintaining the correct water pressure (about 9 bar).
[0071] The extracted espresso coffee flows from the filter basket 64 into the spout 26, from where it flows into a coffee cup placed on the drip tray 88. The control unit 86 can determine the volume of the portion in accordance with the cup 28, or the user can manually control the volume of the portion by turning off the pump 18 and / or the electromagnetic valve 84 through the user interface 130 (see Fig. 11).
[0072] After extracting a portion of espresso, the portafilter 24 is disconnected from the group head 20 to remove the wet puff 22. The portafilter 24 is removably connected to the group head 20 via a bayonet fitting. Diametrically opposed projections (not shown) on each side of the portafilter 24 slide upward through recesses on a radially inner surface of an insert 50 inside the outer bracket 42 of the group head 20 (see Fig. 10). The insert 50 retains a pair of guides 48, each of which defines a surface for sliding engagement with one of the projections of the portafilter 24 via removable guides 48, which allows the group head 20 to be easily adapted for replacement without disassembling the group head. The insert holds the guides 48 in the correct position between the outer bracket 42 and the inserted portafilter 24. Optionally, the insert 50 can be an integral part of the outer bracket 42.
[0073] The guides 48 are designed in such a way that the sliding movement of the projections into the engaged position also pushes the portafilter 24 upward, so that the upper part of the filter basket 64 is pressed into sealing engagement with the seal 46 GG.
[0074] The heater 32GG is best shown in Fig. 5. The heater 32GG is a disk mounted in the group head 20. Optionally, the heater 32GG and the inner bracket 44 can be formed as an integral component. The disk is a thick-film heater in which conductive paths made of electrically resistive material 66 are applied to a substrate 68.
[0075] Voltage is supplied to the conductive paths 66 made of electrically resistive material through electrical terminals 56 held in the connector bracket 54 (see Fig. 7). For the purposes of feedback control, the heater is provided with a thermistor 60 with a negative temperature coefficient (NTC), connected to the control unit 86 through conductors 62. A thermal fuse 52 is also provided on the bracket 54 as a safety device for protection against overheating.
[0076] Next, the operation of the espresso machine 10 will be described with specific reference to the block diagram shown in Fig. 14. However, as a preliminary step, the user adds a tablet 22 of ground coffee to the portafilter 24 and attaches the portafilter to the group head 20 using the bayonet connection described above. A coffee cup is placed on the drip tray 88 under the spout 26 (see Fig. 1).
[0077] As shown in Fig. 14, the operation of the espresso machine includes initialization 102 of the subsystem before the espresso extraction operation 104. Initialization 102 of the subsystem begins with a user-initiated power-on 106 via interface 130 (see Fig. 11). In some examples, other input data is selected via interface 130, such as a single or double portion and / or a preferred water temperature for brewing.
[0078] When the power supply 106 is turned on, the control unit 86 performs a diagnostic check 108 of the inputs and outputs associated with the sensors, pumps, and heaters. If the diagnostic check 108 reveals an error, the control unit 86 records an error condition 114 and reports it. If the control unit 86 determines 110 that the diagnostic check is successful, the heater 32 GG is activated for a specified period 122 (for example, from 8 seconds to 12 seconds). After the specified period of time, the change in the temperature of the heater 32 GG is measured. If the temperature change is less than a specified value (for example, a change of 30°C), the control unit 86 records an error condition 114 and reports it. If the increase in temperature of the heater 32 GG corresponds to or exceeds the set value, the initialization 102 of the subsystem is completed and the espresso extraction process 104 can begin.
[0079] The heater 32GG and the group head 20 are kept warm at step 118 by feedback control set to a predetermined temperature. To begin the extraction process, the user opens the outlet valve 84 to initiate the flow of brewing water at step 124 via the user interface 130. At this stage, the control unit 86 can increase the power of the heater 32GG for a short period of time to compensate for the drop in temperature of the brewing water at the beginning of the extraction process. Similarly, if the NTC thermistor 60 on the heater 32GG indicates a drop in temperature during the extraction process, the heater power is increased to compensate. Similarly, the feedback from the thermistor 60 is used to maintain the maximum temperature of the heater 32GG and, consequently, the brewing water.For example, if the water temperature is higher than the brewing temperature selected by the user (via the user interface at step 120), the heater power is reduced.
[0080] After control unit 86 determines 126 that the required serving volume has been dispensed, the flow of brewing water stops. The pump turns off, and the outlet valve from reservoir 14 closes. At this stage, the machine returns to maintaining the heater and group head temperature at stage 118.
[0081] When no more espresso shots are required, the espresso machine is turned off 128 by the user via the interface 130 and / or it can be automatically turned off after a set period of inactivity.
[0082] Fig. 13 is a graph showing the improved control of the brewing water temperature achieved by incorporating the heater 32GG into the group head 20. The incoming water temperature 90 (i.e., the temperature of the water at the inlet end 148 of the channel) and the extraction water temperature 92 (i.e., the temperature of the water passing through the dispersion mesh 40) are displayed on the graph for all phases of coffee extraction, namely, preheating 94, pre-infusion 96 and extraction 98.
[0083] The water temperature graphs demonstrate the functionality of the group head. Incoming water 12 (see Fig. 1) is heated by water heater 70 and then supplied to group head 20, which uses temperature feedback to control the power of heater 32 GG. The temperature at the entrance to group head 90 initially increases rapidly during the preheating phase 94 and exceeds the ideal temperature of 100. However, heat is dissipated in the structure of the group head, channels, valves and connectors, resulting in the temperature of the brewing water falling below the ideal temperature of 100°C. Group head heater 32 quickly increases the temperature of the brewing water in response to feedback from NTC thermistor 60. The feedback algorithmic control via the control unit 86 effectively smooths out temperature variations in the inlet temperature 90 and maintains the outlet temperature 92 of the GG at or close to the ideal temperature of 100.Initially, the 92°C GG outlet temperature fluctuates slightly as it reaches the ideal temperature of 100°C, but then remains exactly at the ideal temperature of 100°C during the 98°C extraction phase. Conversely, the 90°C GG inlet temperature continues to fluctuate significantly.
[0084] Controlling this aspect of the extraction process directly impacts the quality of the espresso 30 dispensed into your cup.
[0085] In some embodiments, espresso machine 10 includes only a 32-barrel heater. This provides a compact and less expensive machine for users who do not require a milk frothing function for their coffee with milk. These embodiments, with a single 32-barrel heater, require minimal countertop space and provide high-quality single or double espresso in a very short time.
[0086] The present invention is described herein by way of example only. Many variations and modifications will be apparent to those skilled in the art without departing from the spirit and scope of the broad concept of the invention.
Claims
1. A group head for an espresso machine, comprising: a group head bracket for releasably engaging a portafilter that holds a ground coffee tablet in a filter basket for dispensing espresso coffee; a channel for receiving a flow of water to the portafilter; and a heater installed in the head of the group to heat the water flow entering the portafilter, wherein the heater is a resistive thick-film heater with a conductive path made of an electrically resistive material, and wherein the thick-film heater surrounds at least one section of the channel.
2. The head of the group according to claim 1, further comprising a flow diffuser adjacent to the heater for defining a space for distributing the flow of water from the channel over the surface of the heater to improve conductive heating.
3. The head of the group according to claim 2, in which the diffuser has a plurality of channels for directing the flow of water along the surface of the heater, and the channels have channel walls that abut against the surface of the heater for heat conduction into the diffuser.
4. The head of the group according to claim 3, in which each of the channels is in fluid communication with a channel for receiving a flow of water, and each of the channels has at least one outlet end for fluid communication with a portafilter.
5. The head of the group according to claim 4, further comprising a mesh located between the outlet ends of the channels and the portafilter, wherein the outlet ends are located such that the flow of water through the mesh to the portafilter has the required distribution through the tablet of ground coffee.
6. A group head according to any one of paragraphs 2-5, in which the diffuser is made of stainless steel.
7. The group head according to claim 1, in which the heater is installed in the group head bracket for heat conduction from the heater to the group head bracket.
8. The group head according to claim 7, in which the releasable engagement between the group head bracket and the portafilter is configured to allow heat conduction from the group head bracket to the portafilter.
9. The head of the group according to claim 1, in which the resistive heater is configured to be adjustable for heating the water flow to a predetermined temperature.
10. The head of the group according to claim 9, in which the set temperature is a user-selected temperature.
11. A group head according to any one of paragraphs 1-10, further comprising a temperature sensor for feedback control of the resistive heater.
12. A head of a group according to any one of paragraphs 1-11, in which the thick-film heater comprises an electrically resistive material applied in the form of a thick film onto a substrate.
13. The head of the group according to claim 12, in which the thick-film heater is a disk arranged in such a way that during use the disk passes in a plane parallel to the upper surface of the tablet.
14. The head of the group according to claim 13, in which the channel has an outlet end in the center of the disk, and the diffuser channels are designed to allow radial distribution of the water flow over the surface of the disk.
15. A group head according to any one of paragraphs 12-14, in which the thick-film heater is designed with the possibility of heating the water flow through the group head to a temperature of from 89 °C to 96 °C.
16. A group head according to any one of claims 12-14, in which the thick-film heater is configured to heat a stream of water taken from a reservoir inside the espresso machine to 89-96 °C in less than 10 seconds from the moment the thick-film heater is activated.
17. A group head according to any one of claims 1-16, wherein the group head comprises a chamber defined by the inner walls of the group head, wherein the chamber has an opening for receiving a portafilter, and wherein the heater is mounted on the chamber.
18. An espresso machine comprising a group head according to any one of claims 1-17.
19. A method for producing espresso coffee, comprising the following steps: placing a tablet of ground coffee inside the portafilter filter basket; attaching a portafilter to the group head of an espresso machine, the group head having a heater and a channel for the flow of water to the portafilter; providing a flow of water through the channel to the ground coffee tablet; and heating the water flow through the group head by means of a heater, wherein the heater is a resistive thick-film heater with a conductive path made of an electrically resistive material, and wherein the thick-film heater surrounds at least one section of the channel.
20. The method according to claim 19, further comprising the step of regulating the electrical power of the resistive heater for heating the water flow from the group head to a predetermined temperature.
21. The method according to claim 20, wherein the set temperature is selected by the user.
22. The method according to any one of paragraphs 19-21, further comprising the step of providing a temperature sensor for feedback control of the resistive heater by a control unit within the espresso machine.
23. The method according to any one of paragraphs 19-22, in which the thick-film heater comprises an electrically resistive material applied as a thick film to a substrate.
24. The method according to claim 23, wherein the thick-film heater is made in the form of a disk extending in a plane parallel to the upper surface of the tablet.
25. The method according to any one of paragraphs 19-24, further comprising the step of providing the head of the group with a flow diffuser adjacent to the heater to define a space for distributing the flow of water from the channel over the surface of the heater to enhance conductive heating.
26. The method of claim 25, wherein the diffuser has a plurality of channels for directing the flow of water over the surface of the heater, and the channels have channel walls that abut the surface of the heater for heat conduction into the diffuser.
27. The method of claim 26, wherein each of the channels is in fluid communication with a channel for receiving a flow of water, and each of the channels has at least one outlet end for fluid communication with a portafilter.
28. The method of claim 27, further comprising the steps of providing a mesh between the outlet ends of the channels and the portafilter and arranging the outlet ends such that the flow of water entering through the mesh into the portafilter has the desired distribution through the ground coffee tablet.
29. The method according to any one of paragraphs 25-28, wherein the diffuser is made of stainless steel.
30. The method according to any one of paragraphs 19-29, further comprising the step of providing a group head bracket for mounting the group head on the espresso machine, wherein the heater is mounted in the group head bracket for conducting heat from the heater to the group head bracket.
31. The method of claim 30, wherein the releasable engagement between the group head bracket and the portafilter is configured to conduct heat from the group head bracket to the portafilter.
32. The method according to any one of paragraphs 19-31, further comprising the step of adjusting the thick-film heater to heat the water flowing from the group head to a temperature of 89-96 °C.
33. The method of claim 32, further comprising the step of adjusting the thick film heater to heat water flowing through the group head to at least 89 °C in less than 10 seconds from the activation of the thick film heater.