Field of the Invention

The invention relates to electrical contact grill apparatus comprising at least two heated plates for contacting the food.

Background to the Invention

Contact grills heat by conduction of thermal energy from the grill plates to the food product. This heat conduction from the plate to the food increases as the contact area with the food product increases , and increases as the temperature difference between the heated plates and the food product increases.

Contact grills are well known and established in their performance. The contact heating plates are generally metal or glass plates , and usually horizontal in orientation. The food product to be grilled is placed between the two plates. The upper plate is usually freely moveable, typically on a hinge, and the weight of the upper plate assembly constitutes the mechanical load applied to the food product. To reduce the mechanical load on the food the weight of the upper plate assembly is often limited or reduced by a counterbalance mechanism and/or additional spring to prevent excessive crushing of the product. The temperature of the plates is usually controlled by a simple thermostat and is independent of the position of the plate s. Generally the upper plate can close to the point where it touches the bottom plate. Frozen product can be defrosted and cooked by these traditional grills however the process is slow and the temperature of the plate is typically constant. This can lead to overheating of the outside of the food product with the likelihood of excessive burning or overcooking of the outside.

Summary of the Invention

The present invention provides a contact grill apparatus for heating a food product, the apparatus comprising two plates, one of which is movable relative to the other between an open position and a closed position, and heating means for heating at least one of the plates. The apparatus may further comprise biasing means, such as a spring, arranged to bias said one of the plates towards the closed position. The apparatus may further comprise sensing means arranged to sense when said one of the plates reaches the closed position. The apparatus may further comprise control means arranged to control the heating means so as to heat said at least one of the plates either before or after the closed position is reached, or both.

The position of any moveable plate may be monitored, for example by means of a limit switch, a linear or rotary transducer, or a stress sensor, and input to the control means which may be electronic and may include a microprocessor.

The biasing means may be a spring. The apparatus may further comprise an actuator arranged to apply a mechanical load to the biasing means , such that the biasing means transmits the load to said one of the plates as a biasing force. The mechanical load may be applied by a solenoid or other electromechanical actuator , or by an electric motor, or the actuator may be manually actuated, for example comprising a lever .

The plate or plates may be heated by electric resistive heating elements, or by an electric induction heating method, or by a gas flame from an LPG gas burner system.

The temperature of one or more of the plates may be measured by a temperature measuring device, the output of which may be input to the control means. The control means may be arranged to adjust the temperature of the heated plate or plates in relation to the position of those heated plates.

The heating cycle time of the heated plate (s) may be controlled by a timer, which may be an electronic timer or an electromechanical timer. The timer may be an electronic timing circuit or a software program within the control means which may be a microprocessor based electronic control system. The electronic circuit or program may make use of a time, temperature and plate position in an algorithm arranged to suit the food product. The plates may be arranged, in use, in a horizontal orientation, or in a substantially vertical orientation to assist discharge of the food product.

Opening and closing of the plate(s) may be achieved by a manual mechanism such as a pivot lever or mechanism, may be semi-automatic with manual closing and mechanically assisted opening, or may be fully automatic using an electric actuator controlled by the electronic control system or other control means.

The closed position of the moveable plate(s) and the corresponding maximum compression of the food may be controlled by a compression limiting device. The compression limiting device may be adjustable to accommodate various food products.

Removal of the food product from the apparatus may be done by hand. The heated plates may be substantially vertical and an additional mechanical positioner may be fitted to locate the food product in the correct position for grilling. The mechanical positioner may be movable into and out of position, for example by a manual lever arrangement, such that the food product may descend under the force of gravity and discharge from the apparatus. The mechanical positioner may be movable into and out of position by an electromechanical device , or by another type of actuator. The electromechanical device may be a solenoid, a rotary or linear electromechanical actuator or an electric motor.

Apparatus according to the invention may therefore have one or more moveable heated plate(s) which close onto the food product applying a controlled mechanical compression to the food product. The position of the moveable plate(s) may be monitored and the temperature of the plates may then be controlled in relation to the position of the moveable heated plates(s). In this way different temperatures may be applied to the food product as grilling proceeds.

This allows a range of different grilling methods to be used to suit different food products. These methods include as examples, defrosting and grilling of frozen product, warming or grilling or cooking of food product within its packaging, and just defrosting frozen food product. Frozen food product is considered below.

Frozen food product displays many of the characteristics of frozen water. The frozen product, typically at - 18 Centigrade is mechanically solid and cannot be easily compressed. With some embodiments of the invention, when frozen food product is placed between the plates and the moveable heated plate(s) close, then the heated plate ' s full movement will be restricted by the solid nature of the frozen product. As the control system may monitor the position of the moveable plate(s) it may then be able to detect the presence of a frozen product. The temperature of the plate(s) may then be adjusted to be lower during defrosting so that the surface of the frozen food product is not overheated as it defrosts.

As the frozen food product is heated the outside layers may defrost first and become soft allowing the moveable plate to move so the gap between the plates reduces. Certain food products are round or circular in shape. So as these products defrost the contact area increases in a non-linear manner and the temperature of the plates may be adjusted accordingly to speed defrosting. With some embodiments of the invention a compression limiting device is fitted that limits the final compression of the food product and prevents it being damaged or over compressed by the movement of the heated movable plates(s). This position may be set such that frozen food product can only be compressed to this point if it is largely defrosted.

When the moveable heated plate(s) reaches the compression limit position for a given food product then the cooking temperature may be changed automatically by the control system to the correct grilling temperature for cooking of the product. This temperature may be higher than the defrosting temperature and may be chosen to speed up the process of grilling or to achieve a particular quality of grilling within the food product. This temperature may be programmed in the control system, may be changed by the operator, and may be determined by test or experience.

The grilling or cooking time may also be programmable by the operator so that it is suitable for the particular food product and once programmed both time and temperature are consistent for subsequent cooking cycles.

Where the food product contains a number of loose food components , as is common with sandwiches and other filled bread products, the food product may be packaged in heat resistant packaging such that the food components are contained therein. The packaging may remain in place before and after the grilling process. Similarly delicate food products such as brownies may be warmed in heat resistant packaging. The temperature of the plates may be adjusted to suit the maximum temperature limits of the packaging and the temperature and time programmed to suit the food product.

Operation of apparatus according to the invention depends upon the orientation of the heated plate(s). As a result the apparatus may be manual, semi-automatic or fully automatic. When the plates are in the horizontal orientation, typical of traditional contact grills, the food product is manually inserted between the plates and removed manually, typically with the use of tongs. This constitutes manual operation.

When the plates are arranged in a close to vertical orientation, the food product may be dropped vertically into the space between the plates. A mechanical positioner may be arranged at the bottom of the slot formed between the plates, so that the food product is located in the correct position for grilling. The opening and closing of the heated plates may be automatic and the positioner may be arranged so that it rises and falls as the plates open and close. When the grill cycle is complete and the plates open then the food product can be removed by lifting it vertically out of the slot. This constitutes semi-automatic operation.

Similarly when the plates are arranged in a close to vertical orientation and the mechanical positioner is fitted with an actuation device, which is operated by the control system, the mechanical positioner may be moved out of position when the grilling cycle is completed. When the mechanical positioner moves out of position any food product previously located by the mechanical positioner may be free to descend under the force of gravity as the heating plates open. In this way the food product may be lowered or dropped into the slot between the plates before the start of the grill cycle; at the end of the cycle, as the mechanical positioner is removed and the plates open, the food product is free to fall under its own weight and discharge automatically by falling under the force of gravity. This constitutes automatic operation.

Further objects and advantages of the present invention will be apparent from the following description and claims and the accompanying drawings which, by way of illustration, schematically show preferred embodiments of the present invention and the principles thereof and what now are considered to be the best modes contemplated for applying these principles. The apparatus may embody the same or equivalent principles and may include, in any workable combination, any one or more features of the embodiments shown. Structural changes may be made to the embodiments shown as desired by those skilled in the art without departing from the present invention and the scope of the appended claims. In the drawings:

FIGURE 1 is a sectional view through of a grill apparatus with vertically orientated plates suitable for automatic operation;

FIGURE 2 is a sectional view through of a grill apparatus with vertically orientated plates suitable for semi - automatic operation; FIGURE 3 is a sectional view through of a grill apparatus with horizontally orientated plates suitable for manual operation ;

FIGURE 4 is a flowchart illustrating a heating process used in operation of the apparatus of Figure 1 ; and

FIGURE 5 is a graph showing the temperature of the heating plates of the apparatus of Figure 1 as a function of time during operation .

Referring to Figure 1 the grilling apparatus may comprise a body 1 having a housing of stainless steel or the like. The front of the body may have an aperture in which may be located a glass plate 25 or similar thermoformed non-permeable material to prevent ingress of dirt and protect access to electrical components. Within the body 1 , to the rear of the glass plate 25 and in close proximity to it, an electrical induction coil 22 may be located. In turn to the rear of this a power circuit board 26 may be mounted and connected to the induction coil 22.

In front of the glass plate 25 a metal heating plate 23 may be located to receive the heating effect of the induction circuit so that it can heat and compress the food product. It may be secured in position by four stainless brackets 24 attached, for example, to the front of the body 1. The heating plate 23 may be formed of a relatively thin sheet metal material, for example aluminium or steel of a thickness from 2mm to 6mm, and more preferably from 3mm to 5mm. The plate may have a flat main contact area 23A and may have reinforcing regions 23B which are bent out of the plane of the main contact area 23 A to provide stiffening of the plate 23. The reinforcing regions may be bent at 90° to the main contact region 23 A as shown in Figure 1 , or may be bent at a smaller angle, such as 45° so as to form a guide region to guide the food product between the plates.

At the rear of the heating plate 23 a fixing 20 may locate a temperature detecting sensor 21 to the surface of the plate. The sensor 21 is connected electrically to a controller which can be in any suitable form and in this example may comprise a control PCB (printed circuit board) assembly 3 within the body 1. The PCB 3 or other controller typically includes a processor, memory for storing settings or programs for the grill, and a clock or timer which the controller can use to time any heating programs that it is running. It may also be connected to a user input 36 such as a touch screen or a set of input buttons which may allow a user to select one of a number of modes or programs for the grill to operate in, or one or more parameters of the cooking cycle as will be described in more detail below . The moveable heating plate assembly may consist of a housing or box 15 of stainless steel or the like. At the front of the box there may be an aperture in which is located a glass plate 16 or similar thermoformed non -permeable material to prevent ingress of dirt and protect access to electrical components. Within the box 15, for example to the rear of the glass plate 16 and in close proximity to it, a second copper electrical induction coil 17 may be located.

In front of the glass plate 16 a metal heating plate 18 may be located to receive the heating effect of the induction circuit so that it can heat and compress the food product. The ferrous metal plate 18 may be secured in position, for example by four stainless brackets 19 attached to the front of the box 15. The heating plate 18 may have the same structure and dimensions as the heating plate 23.

The optimum thickness of the plates 18, 23 is determined by a number of factors, which in some cases are conflicting. One objective is to have the lowest possible mass to achieve the fastest heating speed, but it is also preferable to have an even surface temperature to get even browning (cooking) of the product. The material of the plate is preferably of a consistent good quality grade of steel for induction heating to work effectively. With resistive heating it can be steel, aluminium or even brass or bronze.

The thickness of the plate defines (for a given surface area) its mass which in turn determines its thermal capacity. The thermal capacity (for a given power input) determines how fast one can heat up the plate and the speed of response to achieve temperature changes. So theoretically for fast heating one wants the thinnest plate possible. However the thickness determines the cross sectional area or the plate (in any section) so this effects the heat transfer rate from a hot area of the plate to a cooler area (since heat transfer rate is proportional to cross sectional area, temperature gradient and thermal conductivity of the material). Even with induction heating there tend to be areas of greater heating, so theoretically for even heating one wan ts a thicker plate to get an even surface temperature.

The low heat capacity of the plates 18, 23, with the dimensions described above, may enable them to be heated from 20C to 200C in a time of 60s to 90s, which is typically quicker than apparatus with thicker heating plates, whilst maintaining uniform and well controlled heating.

One or both of the heating plates 18, 23 may have a shaped recess in its contact surface, for example a pattern or logo cut into its contact surface, for example by machining it to a depth of 1 - 1.5mm. This tends to reduce heating of the food product in the area of the recess which can leave a corresponding area of reduced browning on the cooked product.

The power circuit board 26 in combination with the induction coils 17, 22 may produce an oscillating magnetic field which acts upon the ferrous heating plates 18, 23 causing them to heat up. This method of heating is fast and energy efficient. The speed and efficiency of heating allows the temperature of the heating plates to be accurately controlled.

The moveable heating plate assembly may be supported by two pivot brackets 12 which in turn may be positioned to move outside the width of the body 1 and may be fixed to opposite ends of a pivot axle 10 which may pass through the body 1 and may be supported on rotary bearings 9 secured to the sides of the body 1. The pivot axle 10 where it passes within the body 1 , may have an actuator lever 8 rigidly attached to it. Rotary springs 1 1 may be mounted on the pivot axle giving rise to a rotary torque acting to close the heating plates 18 , 23. Each spring 1 1 may have one end 1 1A connected to the actuator lever 8 and the other end 1 1B connected to a fixed point on the body 1. Therefore, as in the arrangement shown in Figure 1 , the actuator lever 8 may be fixed relative to the movable plate assembly. In this case movement of the actuator lever 8 between its open position at one extreme of travel and its closed position at another extreme of travel results in corresponding moveme nt of the movable heating plate 18. However in a modification to this arrangement, the actuator lever 8 may be connected to the pivot brackets 12 and hence to the movable plate assembly via the spring 1 1 , so that the lever 8 can rotate relative to the movable plate assembly, and can be moved from its open position to its closing position in which it will act on the spring 1 1 , so that the spring 1 1 will apply a closing force to the movable plate assembly, as will be described in more detail below with reference to Figure 3. For example the second end 1 1B of the spring 1 1 may be connected to the pivot bracket 12 instead of to the body 1. Movement of the movable plate assembly relative to the lever 8 may be limited, for example by two stops 8 A, 8B .

An adjustable mechanical device 7 such as a set screw may be fitted which may contact a point, for example on an inclined surface 34, on an extension 12A of the pivot bracket 12, to limit the movement of the pivot bracket 12and the travel of the moveable ferrous heating plate 18. This controls the maximum compression of the food product, whether the movable plate assembly is rotatably connected to the lever 8 directly or via a spring 1 1 . A limit switch 6 may be provided to detect when the plates are fully closed. For example the switch 6 may be arranged to detect when the pivot bracket 12 or its extension 12A reaches a closing position at one extreme of its travel. That closing position may be set by the adjustable device 7.

A mechanical positioner 14A may be provided, for example at the bottom of the two heating plates 18, 23 and between them, so that it can support a food product located between them. The positioner 14A may be a metal flap of stainless steel or other food safe material which can be made to rotate , for example by an electrically powered actuator 13, between a supporting position as shown in Figure 1 in which it will support the food product and a release position in which it allows the food product to slide downwards out from between the heating plates 18, 23.

At the rear of the body 1 a back plate 4 may be located. This may act as a mounting for an electrically driven actuator 5 which may be connected to the actuator lever 8 and controlled by the control PCB 3. The actuator 5 may therefore be arranged to rotate the actuator lever 8 thereby to open and close the grill. However where the actuator lever 8 is connected to the pivot bracket 12 via the spring 1 1 as described above, it will be appreciated that movement of the position of the movable heati ng plate 18 will be determined by the position of the actuator lever 8, the force of the spring 1 1 , and the resistance to movement of the product between the plates 18, 23. .

At the top of the unit the control PCB (printed circuit board) assembly 3 may be fitted with a cooling fan 2. The control PCB (printed circuit board) assembly 3 may contain a microprocessor embedded with a suitable program for controlling the operating sequence and the ferrous plate temperatures.

In operation the food product may be lowered between the heating plates 18 , 23 and is held in position by the mechanical positioner 14A. The movable plate assembly may be closed, for example by the actuator 5. Once full closure is detected by the limit switch 6 the grilling cycle may be started. At the end of the grilling cycle the heating plates 18, 23 may be opened, for example by operation of the actuator 5 initiated by the control PCB assembly 3. At the same time the control PCB assembly 3 may operate the rotary actuator 13 moving the positioner 14 downwards allowing the food product to fall by gravity out from the base of the machine.

Referring to Figure 2, which shows a further embodiment of the invention in which most of the components are the same as those in the first embodiment and are indicated by the same reference numerals , the mechanical positioner 14 may be a metal strip of stainless steel or other food safe material which extends along the length of the grill, for example between the lower edges of the two heating plates 18, 23 and may be held in place by four support arms or strips 27. Two of the support strips 27 may be arranged at each end of the positioner 14, each with its lower end connected to the positioner 14 and its upper end connected, for example to a respective one of the brackets 19, 24, at the upper edge of the heating plates 18, 23. This allows the mechanical positioner 14 to rise and fall as the heating plates 18 , 23 open and close.

In operation, the food product may be placed onto the positioner between the heating plates 18, 23 and is held in position by the mechanical positioner 14 while the grill is open. Then as the grill is closed, the positioner 14 may move downwards so that the product is fully inserted between the heating plates 18, 23 when the grill is closed. At the end of the grilling cycle the heating plates 18 , 23 may be opened, for by operation of the actuator 5 initiated by the control PCB assembly 3. The mechanical positioner 14 may rise so lifting the food product to allow removal from the machine.

As described above with reference to Figure 1 , in a modification of the grill of Figure 2, the actuator lever 8 may be connected to the pivot brackets 12 and hence to the movable plate assembly via the spring 1 1 , so that the lever 8 can rotate re lative to the movable plate assembly, and can be moved to apply a closing force to the movable plate assembly via the spring 1 1.

Referring to Figure 3 the apparatus may comprise an adaptation of the traditional contact grill concept. In this case it may be designed for manual operation, but can be adapted for semi or fully automatic operation .

The apparatus may therefore comprise an upper plate assembly 138 which may have a metal plate 140 which is heated, for example by a resistive electric heating element 139. The upper plate assembly may be held by outer support arms or brackets 141 which may be connected to the main body 132 so that they can be freely rotated about an axis 143 to open and close the grill. The upper plate assembly 138 may have a handle 137 fitted, for example to its front edge, for manual opening. The weight of the upper plate assembly 138 may be counterbalanced by a spring or counterweight so it does not exert a force on the food product.

The lower plate 135 may be mounted on the main body 132 and may also heated by a resistive electric element 134. The temperature of the plates may be detected by a temperature measuring sensor 136 which may for example be clamped to one or more of the plates. The temperature sensor 136 may be connected to the control board 133 which may for example be mounted within the main body 132 of the grill at the front. The control board 133 may be arranged to monitor the temperature of the plates and control the supply of power to the heating element 134 so as to maintain the plate temperature at a target value or within a target range during the grilling cycle. A manual lever 142 may be rotatably mounted on the main body 132, for example on a common axis of rotation with the support arms 141 of the upper plate assembly. The manual lever 142 may be connected by a rotary or other type of spring 144 to the support arms 141 of the upper plate assembly so that the spring 144 can be pre-loaded by rotating the manual lever 142 to apply a rotational load or biasing force to the upper plate assembly. The manual lever 142 may be movable between an opening position as shown in Figure 3 in which, when the spring 144 is relaxed, the upper plate assembly 138 is in a raised or open position, and a closing position in which the spring 144 is biasing the upper plate assembly downwards. The manual lever 142 may be held in position in the opening position or the closing position by a manual catch. The spring 144 may be adjustable connected to either the support arm 141 or the lever 142 so that the closing force provided by the spring is adjustable. For example the manual lever 142 may have two or more holes 142A formed in it so that the spring can be connected to the manual lever 142 in different positions, thereby selecting the preload that will be applied to the food product for a given plate position when the manual lever 142 is in its closing position.

A lever arm 145 may be rigidly connected to the outer brackets 141 and an adjustable compression limiting device, for example in the form of a stop 130, may be fitted which limits the movement of the lever arm 145, anticlockwise as seen in Figure 3 , and therefore limits the travel of the moveable heating plate 140 towards the stationary heating plate 135. This controls the minimum spacing between the heating plates 135, 140 and therefore the maximum compression of the food product. A limit switch 131 or other sensor may also be provided so as to detect when the lever arm 145 reaches the stop 130, i.e. so as to sense full closure of the grill. A further limit switch or other sensor may be also provided so as to sense when the manual lever 142 reaches its closing position. This further sensor may be connected to the control PCB 133 so as to provide a starting signal to start heating of the food product. In operation the food product may be inserted between the plates 135, 140 and the plates closed by moving the manual load lever 142 to its closing position. This applies a closing load to the upper plate assembly 138 so that the food product is compressed. If the food product is sufficiently soft, for example if it is fully thawed, the food product may be compressed so that the lever arm 145 meets the compression limiting device 130 and actuates the limit switch 131. A signal may then be passed from the limit switch 131 to the control board 133 signalling full closure of the grill. The control board 133 may be arranged to respond full closure of the grill by starting a final grilling cycle in which the temperature of the plates 135, 140 is controlled so as to cook the product.

However, if the food product is insufficiently soft, for example if it is still frozen, the closure of the grill by downward movement of the upper plate assembly may be prevented by the food product, and the spring 144 may be held in torsion applying a closing force the upper plate assembly while the lever arm 145 is held away from the stop 130 and the limit switch 131. Therefore the control PCB 133 may receive signals from the sensors indicating that the manual lever 142 is in the closing position but the grill is not fully closed. It may then be arranged to operate in a defrosting cycle, and to remain in the defrosting cycle until the limit switch 131 senses full closure of the grill, at which point it is arranged to start the grilling cycle.

When the grilling cycle is completed the load lever 142 may be released and the upper plate assembly may be lifted using the handle 137. The food product may be removed manually with tongs.

The heating arrangement of the embodiment of Figure 3 may be modified so as to correspond to that of Figures 1 and 2. Similarly the heating arrangement of the embodiment of Figures 1 and 2 may be modified so as to correspond to that of Figure 3.

It will be appreciated that the temperature of the plates can be controlled in various ways in the defrosting cycle and in the grilling/cooking cycle. For example, in the defrosting cycle the temperature of the plates may be maintained below 100°C so as to avoid the boiling off of water from the food product. The temperature may be kept constant for example at 90°C during the defrosting cycle, or it may be ramped up from a starting temperature to a higher final temperature over the course of the defrosting cycle. While the time of the defrosting cycle may be fixed, in at least some cases it is preferable for the duration of the defrosting cycle to be determined by the position of the heating plates, i.e. the degree of closure of the grill, and optionally also by the sensed temperature of the plates. For example the defrosting cycle may be finished and the cooking cycle started only when both full closure of the grill is sensed, and a target plate temperature is reached, or simply on full closure of the grill as described above. In a still further modification the position of the movable plate is monitored as it closes, using a position sensor in the form of a linear or rotary transducer, and the temperature of the plates is varied, for example increased, in a continuous or stepped manner as the grill closes and the spacing between the heating plates decreases towards the fully closed position.

During the main cooking cycle the temperature may be simply kept at a higher temperature of for example 150 °C or 200°C for a fixed period, or the duration of the cooking cycle may be varied depending on the sensed temperature of the plates during the cooking cycle. In either case the timings and temperatures may be determined by selecting one of a number of programs , via the user input, before the grill is closed, for example specific programs being provided for the type of bread, the type of filling, the number of products, the degree of freezing of the product, or any combination of those. Whether the program is selected by an operator or simply programmed into the grill, the plate temperature, and/or the duration of the heating, and/or the heating power supplied to the plates, may be controlled in a variety of ways to ensure consistent cooking or defrosting of the product, to compensate, for example, for whether or not the grill is already hot when the cooking or defrosting cycle is started, or the temperature of the product when it is placed in the grill.

For example, an ideal cooking cycle for a food product might require the temperature to increase during a warm up phase from room temperature to 200°C over 30s and then cooking at a constant temperature of 200°C for two minutes. The amount of heating in the ideal cycle may for example be defined as the temperature in degrees Centigrade multiplied by the number of seconds of cooking at that temperature. For example the heating provided as the temperature rises at a constant rate from 25°C to 200°C over 30s can be defined as (25+(200-25)/2)x30 = 3375°Cs. However, depending on the heating power of the heater, and the number and temperature of products in the grill, the heater might not be able to provide enough heat to follow the ideal cycle. The controller may therefore be arranged to monitor the temperature of the plates as they rise to 200 °C to determine how many such heating units are provided during the warm-up phase of the cycle, and then adjust the length of the constant temperature part of the cycle so that the total number of heating units over the whole cycle is the same as for the ideal cycle.

It will be appreciated that these units of heating are generally defined as units of temperature multiplied by units of time, and the units of temperature (or indeed the units of time) may be defined in various ways. While °C may be used, which therefore uses 0°C as the baseline, the scale may be shifted so as to use the number of °C above a different threshold temperature, such as 50°C or 100°C. The threshold temperature may be chosen for a particular product as the temperature at which cooking, rather than just warming, starts to occur. In this case the amount of time taken to reach the threshold temperature would not affect the duration of the cooking cycle which would only be defined for temperatures above the threshold temperature, and might typically include a warm up phase and a constant temperature phase, or a number of stepped temperature changes with warm up phases between them. For example the cycle might be arranged to warm up from 50 °C to 150 °C over 30s, then cook at a cooking temperature of 150 °C for 1 minute, then warm up to 200 °C over 15s, and then cook at a browning temperature of 200 °C for 15s. The duration of each warm up phase might be monitored and the lengths of the fixed temperature phases adjusted accordingly so as to achieve the ideal total amount of heating. Referring to Figure 4, the heating process outlined above may be achieved using an algorithm, which may be programmed into the controller, for example the PCB 3. At a first step 400, the heating of the induction coil 22 is switched on , triggered for example by the manual lever 142 of the embodiment of Figure 3 being moved to its closing position, or by actuation of the actuator 5 in the embodiments of Figure 1 or Figure 2. The plates may be cold or may have been pre-heated at this point. At step 402 the temperature T of the plates is checked by checking the signal from the temperature sensor 21. If it is below the threshold temperature T _th at which the food product in the apparatus will start to cook, no immediate action is taken and the algorithm loops back, rechecking the temperature until it reaches T _th . At that point, the algorithm progresses to step 404 at which a variable H indicative of the total heating of the product is increased by the product of the measured temperature T and a time interval Δΐ. At the start of the cooking process H is set to zero, and in general terms it will be increased for each cycle of the algorithm after the threshold temperature is reached. It will be appreciate that the time interval Δΐ is arbitrary and may or may not correspond to the actual time for each cycle of the algorithm.

At the next step 406 the value of H is compared to a value H _M AX which defines a total amount of heating or cooking of the product. If H does not yet equal H _max , which it clearly will not initially, then the algorithm proceeds to step 408 where the plate temperature T is compared with a maximum plate temperature T _max . If the maximum plate temperature is exceeded then at step 410 the induction coil 22 is switched off and the algorithm then returns to step 402. If the maximum plate temperature is not exceeded then, at step 412 the plate temperature T is compared to a minimum plate temperature T _min , which is just below T _min to provide hysteresis in the control of plate temperature T. If the plate temperature T is below T _min then the induction coil 22 is switched on again at step 414 and the algorithm then returns to step 402.

If at step 412 the plate temperature T is above T _min then the algorithm then returns to step 402. If at any point, at step 406, the heating variable H reaches the value H _max then at step 406 the induction coil 22 is switched off and the cooking process ends.

Referring to Figure 5 it will be appreciated that when the algorithm is started, the plate temperature T will keep increasing until at some time t j the threshold temperature T _th is reached. At that point, cooking will be deemed to have started and the measure if cooking H will be incremented on each further cycle of the algorithm. The plate temperature T will continue to increase until, at a time t ₂ it reaches the maximum temperature T _max . After that the temperature will be maintained within the normal cooking temperature range between T _max and T _min until cooking is completed at a further time t ₃ .

Still referring to Figure 5, in a modification to the control process described above, the algorithm may further define a browning temperature T _br which needs to be maintained for the product to be browned. This may be higher or lower than the normal maximum cooking temperature T _max . The algorithm may define a fixed time that the plate temperature T needs to be maintained at the browning temperature. For example, the algorithm may be arranged to heat the product to the browning temperature for a fixed browning period t _br after the main cooking process is complete, for example after H has reached H _max . The cooking cycle may therefore only end at a further time t ₄ after the browning temperature has been maintained for the required browning period. Alternatively the browning temperature may be equal to the maximum normal cooking temperature, and the algorithm may assume that sufficient browning has taken place unless H _max is reached after less than a predetermined period at the maximum normal cooking temperature (or within the normal cooking temperature range).

The various parameters of the cooking process, such as the normal cooking temperature T _max , the total heating H _max or the cooking time, and the browning temperature and browning time t _br may be variable, and may be set by a user, for example using the user input 36. For example the user input may allow a user to input a cooking temperature T _max and a cooking time, and the controller may calculate from the input values the total heating H _max and T _min . In this case, while the input value of T _max may be used by the algorithm, for example as shown in Figure 4, the input cooking time will not exactly set the cooking time, but will instead be used to calculate the total required heating H _max which will then be used by the algorithm to determine the cooking time depending on the measured plate temperature T. A value of the threshold temperature T _th may also be input by the user or may be fixed or determined by the controller. The values of the input parameters may be input in various ways. For example they may be displayed on the user input and increased or decreased by the user pressing 'up' or 'down' arrow indicators, or may be input be rotatable knobs.

If the grill is used without a defrost cycle, and is just used for cooking, then the temperature of the plates at the start of the cooking cycle might vary significantly depending on whether or not it has been used before. The duration of the warm up phase may therefore be modified, for example using a similar type of heating calculation, to ensure that the correct total heating is provided over the whole cooking cycle.

The apparatus may further include a safety feature which is arranged to detect the presence of an unsuitable item placed or trapped between the plates which might damage the apparatus or cause a fire hazard. This may make use of the plate position sensor 131 in the embodiment of Figure 3, or a similar sensor in the embodiments of Figures 1 and 2. For example, as described above, the controller 3 may be arranged to distinguish between frozen and unfrozen products by measuring the time between the start of the heating of the product and the full closure of the plates. The controller may use this time period also to detect the presence of an unsuitable object between the plates. For example, a maximum time limit of 3 or 5 seconds for this time period may be used to indicate that the product is frozen . If a frozen product is detected, the cooking cycle may be chosen so as to maintain a thawing temperature (which could be the threshold temperature T _th ) for a first period, or for a first total amount of heating H as described above, and then to move on to the cooking temperature as shown in Figure 5. If the plates are still not closed after a higher maximum time limit, for example of two to three minutes , that can be used to indicate that the product is not a frozen food product but an unsuitable item, or that the apparatus is malfunctioning in some way. The controller may therefore be arranged, if the longer time limit is reached without the plates fully closing, to output an alarm signal and/or to turn off the heating elements. This might be appropriate, for example, in situations where the apparatus is available for use by members of the public, such as in hotel rooms.