Method for controlling a cooking process in a cooking appliance and cooking appliance

Abstract

Method for controlling a cooking process in a cooking appliance (10) wherein a heat flux per surface area of ​​a product to be cooked is determined, this heat flux per surface area is integrated over the cooking time and the cooking process is terminated when this heat flux integral reaches a predetermined value.

Description

[0001] The invention relates to a method for controlling a cooking process in a cooking appliance and to a cooking appliance.

[0002] In many cooking processes, the cooking time (or cooking temperature) must be adjusted to the load in the appliance. Prior art already considers using weight sensors, optical image recognition, etc., to detect the load in the cooking chamber and to change various parameters depending on the load. This is intended to ensure that, for each selected cooking process, the product being cooked has the same consistency at the end of the process, regardless of the load. An example of such a method can be found in EP 2 098 788 A2.

[0003] It is also known that analyzing the temperature of the cooking chamber atmosphere after the oven door has been closed can provide information about the load. For example, the temperature of the cooking chamber atmosphere drops much more sharply after the chamber is fully loaded with a frozen product than it does when only a few products have been added. Furthermore, with a small new load, the cooking chamber atmosphere can be heated back up to the target temperature much more quickly than with a maximum load. Depending on the recorded temperature profile, the cooking time or temperature can then be adjusted accordingly, although the necessary changes must be determined beforehand through testing. The adjustment of the cooking parameters depends on both the product and the process.Furthermore, the results depend on the specific appliance type. Extensive test series would therefore be necessary to determine the appropriate adjustments to process parameters for all combinations of products, cooking processes, and different appliance types. Additionally, reliable load detection based on temperature profiles is only possible if load changes are always performed in the same way. If preheating varies, the door is opened for only a very short or a particularly long time for loading, or the door is opened during cooking to change the load, the load status can no longer be reliably detected, and the cooking process will not always produce perfect results.

[0004] EP 0 735 449 B1 discloses a method for operating an oven that solves the problem of varying load conditions by performing a calibration process for each new load of the oven at maximum capacity. During this process, the temperature profile is recorded – that is, the temperature profile the heating element can maintain even at maximum load. In operation, this exact temperature profile is then replicated for each new load. This ensures that the target temperature is maintained throughout the entire baking process. However, this method also means that baking time is "wasted" when the oven is lightly loaded, as the heating element must operate below its maximum capacity in such cases.Furthermore, this method is only suitable for a specific product and does not provide any options for variation that would allow for consideration of other cooking process parameters (such as the core temperature of the product being cooked).

[0005] From DE 42 31 365 C2 a cooking method is known in which a temperature integral is determined to control the cooking process.

[0006] From EP 2 031 306 A1 a cooking method is known in which the core temperature is integrated.

[0007] From DE 198 39 008 C2 a cooking process is known in which an integral value is calculated on the basis of the temperature, which is intended to be a characteristic value for the heat energy.

[0008] The object of the invention is to create a cooking method and a cooking device with which different cooking processes can be adapted to different products in a simple and reliable manner without load detection, so that a predetermined state of the product is achieved as reliably as possible at the end of the cooking process.

[0009] To solve this problem, the invention provides a method for controlling a cooking process in a cooking appliance, in which a heat flow per surface area of ​​the product to be cooked is determined, this heat flow per surface area is integrated over the cooking time, and the cooking process is terminated when this heat flow integral reaches a predetermined value. The invention also provides a cooking appliance with a cooking chamber, a heating device, and a control unit, wherein the control unit includes an integrator that can integrate a heat flow per surface area of ​​the product to be cooked over time, and an evaluation circuit that can control the cooking process depending on the integrated values ​​of the heat flow per surface area.

[0010] The invention is based on the fundamental idea of ​​determining the heat flow per unit surface area of ​​the food being cooked, also known as the specific heat input into the product being cooked, as the key parameter for controlling the cooking process. This implements the understanding that ultimately, the specific heat input is the decisive parameter with which all deviations of the actual cooking process from the previously defined theoretical cooking process are virtually automatically detected. For example, if the door of the cooking appliance is left open for an excessively long time during loading, causing the temperature of the cooking chamber atmosphere to drop, this leads to a reduction in the specific heat input into the product being cooked. The same applies to a significant drop in temperature in the cooking chamber after it has been loaded to its maximum capacity with, for example, frozen products: the specific heat input into the product is reduced.Condensation processes on the product being cooked, as well as varying flow velocities in the cooking chamber atmosphere, also influence the specific heat input to the product. Instead of recording or predicting the various individual parameters and then estimating their effect on the cooking process, as is done in the prior art, the invention focuses solely on the specific heat input, which is integrated for each individual cooking process. As soon as the value of the heat flow integral obtained in this way reaches a value specifically predetermined for the respective product, it is assumed that sufficient energy has been transferred from the cooking chamber atmosphere to the product, and the cooking process is considered complete.

[0011] The specific heat input or heat flux per unit surface area of ​​the food being cooked can be determined from the product of an assumed heat transfer coefficient α for the current cooking process and a driving temperature difference. The assumed heat transfer coefficient α takes into account both the consistency of the product being cooked and the influence of the cooking chamber atmosphere, i.e., the airflow velocity and humidity.

[0012] The driving temperature difference is preferably the difference between a cooking medium temperature T M and a temperature T O It uses a parameter that represents the surface temperature of the product being cooked. In this way, one of the key parameters for heat flow per unit surface area into the product being cooked is taken into account.

[0013] The cooking medium temperature T can be used as the temperature. MThe temperature used is measured downstream of the heating device, viewed in the direction of flow of the cooking medium. This allows the use of a temperature sensor typically located at the outlet of the heating device.

[0014] To obtain more precise values ​​for the cooking medium temperature, the cooking medium temperature T can be used. M The temperature used is that measured downstream of the heating element, viewed in the direction of flow of the cooking medium, minus a loss value. This approach takes into account that the temperature of the cooking medium in the cooking chamber is lower than the temperature it has when leaving the heating element.

[0015] Half of the cooking chamber cooling ΔT can be considered a loss value. GR The following values ​​are applied: ΔT GR = P HZ / (V*ρ(T)*c p ), where P HZthe heating power of the cooking appliance, V the volume flow rate through the heating device, ρ(T) the density of the cooking medium in the cooking chamber and c p The isobaric specific heat capacity of the cooking medium. It has been found that with this cooking chamber cooling ΔT GR The heat losses that occur in practice can be depicted quite precisely.

[0016] Surface temperature T OThe boiling point, dew point, an average between the boiling point and dew point, or an average surface temperature between a starting temperature and the boiling point can be assumed. Firstly, the respective values ​​can be appropriately set depending on the product being cooked. Secondly, it has been found that the exact value of the surface temperature is usually not critical. An error in the assumed surface temperature is, to a first approximation, independent of the load. Thus, while it does affect the value for the specific energy input, it does not, to a first approximation, lead to a relative change in the cooking process parameters. This is because the target value of the heat flow integral to be achieved for the specific product is determined beforehand, taking into account the (potentially erroneous) surface temperature T. Ois determined. This may result in a difference between the actual heat flow and the calculated heat flow. However, this error is carried over identically to all calculations and therefore has no (or at least no noticeable) effect on monitoring an actual cooking process relative to the theoretical cooking process.

[0017] To improve process accuracy, it may be possible to adjust the set surface temperature T. O The cooking time varies. This allows for particularly precise measurement of the heat flow integral. The surface temperature T O This can be determined and recorded experimentally beforehand.

[0018] According to one embodiment of the method, the heat transfer coefficient is assumed to be constant. It has been found that even with this simplification, quite good results can still be achieved.

[0019] Alternatively, the heat transfer coefficient can be varied depending on the fan speed. This better reflects the change in heat transfer resulting from a change in the flow velocity of the cooking medium atmosphere in the calculation.

[0020] In principle, it is possible to theoretically determine the value of the heat flow integral at which the cooking process ends. However, it is preferable to determine this value experimentally beforehand. This allows for a particularly precise consideration of the practical differences between various types of cooking appliances and the actual conditions during the cooking process.

[0021] According to one embodiment of the invention, the value of the heat flow integral at which the cooking process is terminated is determined depending on the desired degree of browning of the product being cooked. This allows the user to define different degrees of browning for the product being cooked, which can then be achieved very precisely using the method according to the invention.

[0022] According to one embodiment, the heat flow integral is reduced by a predetermined value as soon as a predetermined cooking time is exceeded. This prevents the product being cooked from drying out due to an excessively long cooking time.

[0023] According to one embodiment of the method according to the invention, a target profile for the heat flow integral is specified, and during the cooking process, a deviation between the current target value of the heat flow integral and the current actual value is determined. Depending on this deviation, the heat flow per unit surface area of ​​the food being cooked is then corrected. In this embodiment of the invention, the cooking process is not merely extended if the energy input into the product being cooked is slower than planned, but rather an attempt is made to increase the energy input during the cooking process itself. This can be achieved by increasing the cooking medium temperature and / or by increasing the fan speed. Conversely, the energy input can be reduced during the cooking process if it is determined at a certain point that more energy has already been introduced into the product being cooked than intended.

[0024] According to a preferred embodiment, a core temperature integral is taken into account when correcting the heat flow per surface area of ​​the food being cooked. This integral, based on the driving temperature difference, particularly at the beginning of the cooking process, allows for a prediction of the core temperature of the product being cooked. This embodiment makes it possible not only to optimize the cooking process with regard to the browning of the product, but also to control the cooking process so that a desired core temperature is reached in the product at the anticipated end of the cooking process.

[0025] Preferably, a target core temperature profile is specified, and deviations between this target profile and the actual core temperature are monitored during the cooking process. This allows the cooking process to be adjusted based on any deviations between the actual and target core temperature profiles. For example, if the core temperature rises faster than expected, the cooking medium temperature can be increased to achieve the desired degree of browning more quickly than originally planned (and thereby limit the further rise in core temperature, which depends primarily on the cooking time and only minimally on the cooking medium temperature). Conversely, the cooking medium temperature can be decreased if it is detected that the core temperature is rising more slowly than expected.This allows more time for heat transfer into the interior of the product being cooked before the surface reaches the desired degree of browning.

[0026] Particularly precise monitoring of the core temperature is possible when a core temperature probe is used to determine the actual core temperature. This actual core temperature is then compared to a predetermined target core temperature, and the heat flow per unit surface area of ​​the food is adjusted accordingly. The values ​​obtained in this way are much more precise than those obtained through calculation.

[0027] According to one embodiment of the invention, the moisture content of the cooking medium is taken into account when determining the heat flow integral. For example, in a cooking process aimed at browning the product, reducing the moisture content of the cooking medium can lower the product's surface temperature. This leads to increased energy absorption, thus potentially accelerating browning. Similarly, in cooking processes primarily aimed at achieving a specific core temperature, the cooking process can be influenced as desired by altering the moisture content of the cooking medium.

[0028] The invention is described below with reference to various embodiments, which are illustrated in the accompanying drawings. These show: - Fig. 1 schematically a cooking appliance according to the invention; - Fig. 2. the course of the heat flow integral during different cooking processes with different loads; and - Fig. 3 schematically shows the course of the heat flow integral and the core temperature during different cooking processes with different loads.

[0029] In Fig. Figure 1 schematically shows a cooking appliance 10 intended for professional use in large-scale catering establishments, restaurants, canteens, etc. It contains a cooking chamber 12, accessible from the outside by opening a door 14. The cooking chamber accessories 16, such as baking trays, grill plates, baking tins, or racks on which the products to be cooked are placed, can be arranged within the cooking chamber (schematically indicated here).

[0030] To create a desired cooking chamber atmosphere, a heating device 18 and a fan wheel 20 are provided, which heat and circulate the atmosphere present in the cooking chamber 12 (also referred to as the cooking medium). A steam module can also be integrated into the heating device 18 to bring the humidity of the cooking medium to a predetermined value.

[0031] Other components such as a ventilation system for the cooking chamber 12 to the outside atmosphere, a quenching box, etc. are not shown here for the sake of clarity.

[0032] The cooking appliance 10 also includes a control unit 22, which receives signals from, among other things, a temperature sensor 24, located directly downstream of the heating element 18, and a humidity sensor 26, located inside the cooking chamber 12. The control unit 22 controls, among other things, the heating element 18 and a drive motor 28 for the fan 20. Furthermore, a user interface 30 is provided, which includes an input window 32 and an output window 34. The input window allows the user to preselect a specific cooking process, such as the product to be cooked and the desired browning level, while the output window can display, for example, the remaining cooking time of the current process or indicate which of the various shelf levels in the cooking chamber contains the products whose cooking process has just been completed.The input window and the output window can also be combined into a multifunctional unit. Additionally, the control unit 30 can be designed to emit acoustic signals, for example, a confirmation tone as input feedback or a signal tone when a cooking process is complete.

[0033] The control unit 22 includes, among other things, an integrator 36, with which the specific heat input into a product to be cooked in the cooking chamber 12 can be integrated over the cooking time, as well as an evaluation circuit 38, which can control various parameters of the cooking process depending on the integrated values ​​supplied by the integrator 36.

[0034] The integrator 36 integrates the specific heat input into the product being cooked over the cooking time during a cooking process. "Specific heat input" is the amount of energy absorbed per unit area of ​​the surface of the product being cooked per unit time. The integrator takes into account a heat transfer coefficient α, which is stored for various predefined cooking processes (i.e., for each product and its different cooking states). The assumed heat transfer coefficient α is further modified depending on other parameters, in particular the rotational speed of the fan 20. Regarding the dependence of the heat transfer coefficient α on the rotational speed of the fan 20, it can be assumed that the air velocity is proportional to the fan speed. Based on this, the heat transfer coefficient to be applied can be estimated using approximation formulas.

[0035] Furthermore, the integrator takes into account a driving temperature difference, which can generally be assumed to be the difference between a temperature T M of the cooking medium and a temperature T O on the surface of the product being cooked. The temperature of the cooking medium can be determined relatively reliably. As a first approximation, the value detected by the temperature sensor 24 can be used. More precise values ​​are obtained if the cooling of the cooking medium in the cooking chamber 12 is also taken into account, which can be determined based on the power that the heating device 18 must provide to keep the temperature in the cooking chamber constant. It is particularly preferred if the temperature of the cooking atmosphere is taken as the average between the temperature "in front" of the cooking chamber and "behind" the cooking chamber, so that an average value for the cooking medium temperature is obtained.

[0036] The surface temperature T OThe temperature of the product being cooked can theoretically be directly detected by a suitable sensor, such as an infrared sensor, and provided to the integrator. However, it has been found that simplifying assumptions can be made without significantly affecting the outcome of the cooking process. For example, it has proven sufficient to assume a constant surface temperature between the starting temperature and the boiling point. With more complex control methods, the surface temperature profile of the product being cooked can be experimentally recorded beforehand and stored as a curve. The integrator then uses the previously measured surface temperature value for each point in the cooking process.

[0037] Additionally, the signal from humidity sensor 26 can be taken into account, since the humidity of the cooking chamber atmosphere affects the surface temperature T. O of the product to be cooked.

[0038] In the evaluation circuit 38, a value for the integrated specific heat input over the cooking time (hereinafter referred to as the "heat flow integral") is stored for each cooking process that the cooking appliance 10 can perform. This value is set to the end of the respective cooking process. This value can be determined experimentally for each product to be cooked with different loads in the cooking chamber 12, for the different properties of the finished product (e.g., surface browning or core temperature), and for the different appliance types. In practice, it should be sufficient to carry out these experiments only for specific loads and products and then determine the value of the heat flow integral for the cooking processes that were not experimentally tested by interpolation or extrapolation.

[0039] In Fig. Figure 2 shows the different values ​​of the heat flow integral for an exemplary cooking process that focuses primarily on the desired degree of browning (here, baking bread rolls). These values ​​were experimentally determined beforehand for different degrees of browning. A value of 35 was chosen for a "light" browning, a value of 70 for a "medium" browning, and a value of 100 for a "dark" browning. This diagram also shows the heat flow integral as a function of cooking time for different batch sizes. Line d1 represents the heat flow integral in a cooking process for one tray of bread rolls intended to achieve a "dark" browning. Line d3 represents the heat flow integral when using three trays, and line d6 represents the heat flow integral when using six trays.The same applies to the lines assigned to the cooking process for the browning level "medium" and for the browning level "light".

[0040] The cooking process is complete when the heat flow integral reaches the predetermined threshold. For example, when loading six trays of bread rolls to achieve a "light" browning level, the cooking process is complete after approximately 1,000 seconds. Cooking processes with three trays or just one tray end somewhat sooner.

[0041] It is also evident that the cooking time depends hardly on the desired degree of browning, but noticeably on the number of trays used. For example, the cooking process for a "dark" browning finishes at approximately the same time when six trays are used, as does the cooking process for a "medium" browning finish, also using six trays. Put simply, this is because, regardless of the desired degree of browning, roughly the same amount of energy must be supplied to achieve the same fully baked state inside the roll. The different degrees of browning are achieved through different temperatures during the cooking process; for a "dark" browning, a higher cooking medium temperature is used (at least for a certain portion of the cooking process).However, this higher temperature does not affect the time at which the inside of the bread rolls is fully baked, as heat conduction within the bread roll is limited by the boiling point of the liquid contained in the dough.

[0042] Fig. 3 refers to a cooking process that uses a desired core temperature of the product as a guide parameter, as is often the case when cooking meat. The values ​​are shown in Fig. 3. The course of the heat flow integral over time and the course of the core temperature over time. Curve KT1 denotes the course of the core temperature when loaded with one sheet, while KT3 denotes the course of the core temperature when loaded with three sheets and KT6 when loaded with six sheets. The curves for the values ​​of the heat flow integral are accordingly labeled WI1, WI3, and WI6.

[0043] Comparing the loading conditions with only one tray to those with three trays, it becomes apparent that the core temperature rises somewhat more slowly with three trays than with one. This is due to the increased heat dissipation in the cooking chamber and the resulting cooling of the cooking chamber atmosphere. Because of the lower temperature of the cooking chamber atmosphere, the heat flow integral also rises more slowly with three trays than with one. Consequently, the value of 80°C used here for the end of the cooking process is reached later with three trays than with one. Nevertheless, the cooking process is still managed to achieve the same core temperature of 90°C.

[0044] Under certain load conditions in the cooking chamber, the food being cooked may absorb more heat than the heating element can provide. This results in a longer cooking time because the cooking chamber atmosphere (at least for part of the cooking process) cannot reach the desired temperature. In these cases, conflicting objectives arise, which can vary depending on the product and require different compromises. For example, the problem might be that a desired browning is desired without the core temperature exceeding a certain value. Roast beef is an example of this; here, priority would be given to the core temperature, while compromises would have to be made regarding the desired browning.Another problem can be that it takes so long to reach the desired core temperature that there is a risk of the product drying out. If the product is not critical with regard to the core temperature, the cooking process is stopped prematurely to prevent drying out.

[0045] An example of how such cases can be taken into account is shown in Fig. 3. It can be seen that the value for the heat flow integral, which represents the end of the cooking process, is defined as a function of the cooking time. Once the cooking process reaches a duration of 600 seconds, the integral value decreases. With a cooking time of 700 seconds, the value has dropped from the initial value of 80 to 50. This ensures that, for cooking processes lasting longer than a predetermined value, a compromise is maintained between the achieved core temperature and, in this case, the browning of the product. In the illustrated example of a load of six trays, it can be seen that the cooking process ends as soon as the heat flow integral reaches a value of 60.This means that the cooking process is ended at a point when the product being cooked has not yet received as much energy as originally intended, but still in time before the core temperature has reached excessively high values ​​and the product being cooked has dried out.

[0046] The in Fig. The three plotted core temperature values ​​can either be directly measured by a core temperature probe or are based on assumptions that estimate the core temperature profile depending on the cooking time. This allows for the compensation of temperature differences, particularly those caused by the load, during the initial phase of a cooking process. The maximum value for the surface temperature T O The temperature is just below the boiling point.

[0047] The particular advantage of the method according to the invention is that it is simpler to implement than load detection. Furthermore, the method according to the invention results in fewer trials during process development, since the appropriate extension of the cooking time is determined automatically and does not need to be determined individually through trials with different loads. In addition, differences in equipment are compensated for. Another advantage is that different initial conditions, such as incorrectly adjusted gas burners or malfunctioning heating elements, are also taken into account when extending the cooking time. Changes in fan speed or varying reversal frequencies are also considered. Finally, it is advantageous that the browning of the product being cooked and the core temperature can be coordinated.

Claims

[1] Method for controlling a cooking process in a cooking appliance (10) wherein a heat flux per surface area of ​​a product to be cooked is determined, this heat flux per surface area is integrated over the cooking time and the cooking process is terminated when this heat flux integral reaches a predetermined value. [2] Method according to claim 1, characterized by , that the heat flux per cooking surface is determined from the product of an assumed heat transfer coefficient α for the current cooking process and a driving temperature difference. [3] Method according to claim 2, characterized by , that the driving temperature difference is the difference between a cooking medium temperature T M and a temperature T O is which represents the surface temperature of the product being cooked. [4] Method according to claim 3, characterized by , that the cooking medium temperature T MThe temperature used is measured behind a heating device, viewed in the direction of flow of the cooking medium. [5] Method according to claim 3, characterized by , that the cooking medium temperature T M The temperature used is measured behind a heating device, viewed in the direction of flow of the cooking medium, minus a loss value. [6] Method according to claim 5, characterized by , that the loss value is half of a cooking chamber cooling ΔT GR is set, which is calculated as follows: ΔTGR=PHZ / (V*ρ(T)*cp). where P HZ the heating power of the cooking appliance is, V is the volume flow through the heating device, ρ(T) is the density of the cooking medium in the cooking chamber, and c p is the isobaric specific heat capacity of the cooking medium. [7] Method according to any one of claims 3 to 6, characterized by , that the surface temperature T OThe boiling point, the dew point temperature, an average between the boiling point and the dew point temperature, or an average surface temperature between a starting temperature and the boiling point is assumed. [8] Method according to claim 7, characterized by that the assumed surface temperature T O The cooking time varies. [9] Method according to any one of claims 3 to 6, characterized by that the surface temperature T O is determined experimentally beforehand. [10] Method according to any one of claims 2 to 9, characterized by , that the heat transfer coefficient is assumed to be constant. [11] Method according to any one of claims 2 to 9, characterized by , that the heat transfer coefficient varies depending on the speed of a fan. [12] Method according to any one of the preceding claims, characterized by, that the value of the heat flow integral at which the cooking process ends was determined experimentally beforehand. [13] Method according to claim 12, characterized by , that the value of the heat flow integral at which the cooking process is terminated is determined depending on the desired degree of browning of the product to be cooked. [14] Method according to claim 13, characterized by , that the value of the heat flow integral is reduced by a predetermined value as soon as a predetermined cooking time is exceeded. [15] Method according to one of claims 13 and 14, characterized by , that a target curve of the heat flow integral is specified and during the cooking process a deviation between the current target value of the heat flow integral and the current actual value is determined and the heat flow per surface area of ​​the food being cooked is corrected depending on the deviation. [16] Method according to claim 15, characterized by, that when correcting the heat flow per cooking surface, a core temperature integral is taken into account, which, based on the driving temperature difference, especially at the beginning of the cooking process, enables a prediction of the core temperature of the product to be cooked. [17] Method according to claim 12, characterized by , that the value of the heat flow integral at which the cooking process is terminated is determined depending on a specified target core temperature of the product to be cooked. [18] Method according to claim 16, characterized by , that a target core temperature profile is specified and that during the cooking process a deviation between the target core temperature profile and the actual core temperature profile is monitored. [19] Method according to claim 18, characterized by, that a core temperature probe is used to determine the actual core temperature, that the actual core temperature is compared with a predetermined target core temperature, and that the heat flow per surface area of ​​the food is corrected depending on the deviation. [20] Method according to any one of claims 15 to 19, characterized by , that the cooking medium temperature is changed to correct the heat flow per surface area of ​​the food being cooked. [21] Method according to any one of claims 15 to 20, characterized by , that the fan speed is changed to correct the heat flow per surface area of ​​the food being cooked. [22] Method according to any one of the preceding claims, characterized by , that the moisture content of the cooking medium is taken into account when determining the heat flow integral. [23] Cooking appliance (10) designed to carry out the method according to one of the preceding claims, comprising a cooking chamber (12), a heating device (18) and a control unit (22), wherein the control unit (22) includes an integrator (36) that can integrate a heat flux per surface area into a product to be cooked over time, and an evaluation circuit (38) that can control the cooking process depending on integrated values ​​of the heat flux per surface area.

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