JP7769376B2 - Method and device for estimating the heated state of food - Google Patents

Description

The present invention relates to a method and device for estimating the heating state of food.

Foods served in sealed containers, such as foods in retort pouches or low-temperature cooked foods, are heated for processing and sterilization purposes after being sealed during production. There is a need for a method to evaluate changes in the processing state of food due to this heating while it is still in a sealed state.

For example, Non-Patent Document 1 discloses that there is a high correlation between the impedance value of a meat sample during retort sterilization and the energy required to break the meat sample (i.e., the hardness of the meat sample). Patent Document 1 also proposes a method for estimating the processing state of food using a CPE index calculated from the impedance spectrum.

Japanese Patent Application Laid-Open No. 2017-169813

Takumi Okamoto, Kiyoshi Kawai, Yoshio Hagura, "Study on unopened and non-destructive measurement of the hardness of foods enclosed in retort pouches," Abstracts of the 16th Annual Meeting of the Japan Society for Food Engineering (2015), Japan Society for Food Engineering, July 27, 2015, p. 45

It is known that the electrical properties of materials depend on the measurement temperature, and the measured impedance value is also affected by this measurement temperature dependency. The methods disclosed in Patent Document 1 and Non-Patent Document 1 do not take into account the influence of this measurement temperature dependency, and therefore there is room for improvement.

One aspect of the present invention aims to realize a method for estimating the heating state of food using impedance that is less affected by measurement temperature dependence.

In order to solve the above-mentioned problems, one embodiment of the present invention provides a method for estimating the heated state of food, which includes a first measurement step of measuring a target impedance, which is the impedance at a predetermined measurement temperature, of a target food sealed in a non-metallic container; a second measurement step of measuring a reference impedance, which is the impedance at the measurement temperature, of a reference food, which is the same type of food as the target food but is sealed in the non-metallic container and has different heating conditions than the target food; and an estimation step of estimating the heated state of the target food from the difference between the target impedance and the reference impedance.

In one aspect of the method for estimating the heated state of food according to the present invention, the target food may be the reference food that has been heated.

In one aspect of the method for estimating the heated state of food according to the present invention, the heating conditions for the reference food may be such that the reference food is heated to a temperature higher than the heating conditions for the target food, so that the reference food is in a state of more advanced thermal denaturation than the target food.

In one aspect of the present invention, a method for estimating the heated state of food may include measuring the target impedance at each of the multiple measurement temperatures during heating of the target food in the first measurement step, measuring the reference impedance for each of the multiple measurement temperatures in the second measurement step, and obtaining the difference value for each measurement temperature in the estimation step, and estimating the heated state of the target food from the multiple difference values.

In one aspect of the method for estimating the heated state of food according to the present invention, the target food may be a food containing at least one type selected from the group consisting of meat, potatoes, and grains.

In order to solve the above-mentioned problems, one embodiment of the present invention provides a food heating state estimation device that includes electrodes and a control device. The control device includes a first measurement unit that acquires a measurement value of target impedance, which is the impedance at a predetermined measurement temperature of a target food sealed in a non-metallic container, measured by the electrodes; a second measurement unit that acquires a measurement value of reference impedance, which is the impedance at the measurement temperature of a reference food that is the same type of food as the target food and sealed in a non-metallic container but has different heating conditions from the target food, measured by the electrodes; and an estimation unit that estimates the heating state of the target food from the difference between the target impedance and the reference impedance.

One aspect of the present invention makes it possible to realize a method for estimating the heating state of food using impedance, which is less susceptible to measurement temperature dependence.

FIG. 1 is a block diagram schematically illustrating a configuration of an estimation device according to an embodiment. FIG. 1 is a flow diagram illustrating an example of an estimation method according to an embodiment. FIG. 10 is a flow diagram illustrating another example of an estimation method according to an embodiment. In the estimation method according to one embodiment, the left diagram shows the relationship between the temperature change and the impedance when the sample is heated, and the right diagram shows the difference in the impedance of the sample before and after heating. For the sample used in the estimation method according to one example, the left graph shows the DSC measurement results, and the right graph shows the normalized enthalpy. For the sample used in the estimation method according to one embodiment, the left diagram shows the load-displacement curve, and the right diagram shows the elastic strain energy. FIG. 10 is a diagram showing the relationship between the difference in impedance before and after heating and the elastic strain energy for a sample used in an estimation method according to an embodiment. FIG. 10 is a graph showing the change in impedance over time during the process of heating raw pork to 75° C. and cooling it twice in succession. This figure shows the change in resistance over time during the heating and cooling process of potato starch, which was repeated twice in succession at 75°C, 60°C, or 50°C. FIG. 1 shows the change in capacitance over time for four types of starch, each heated to 75° C. and cooled twice in succession.

One embodiment of the present invention will be described below with reference to the drawings. It should be noted that the present invention is not limited to the above-described embodiments and examples, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments or examples are also included within the technical scope of the present invention.

1. Overview of the present invention
A method for estimating the heated state of food according to one embodiment of the present invention (hereinafter referred to as "this estimation method") is a method for estimating the heated state of food using measured impedance values.

It is known that impedance measurements are affected by measurement temperature dependence. For example, when comparing impedance measured at two different temperatures, it is preferable to take into account the effect of measurement temperature dependence on each measurement, but it is difficult to accurately grasp the effect of measurement temperature dependence. Therefore, because conventional methods do not take into account the measurement temperature dependence of impedance, there is room for improvement in the accuracy of the results of estimating the heated state of food.

In this estimation method, the impedance of the target food, whose heated state is to be estimated, and the impedance of the reference food used for comparison are each measured at the same measurement temperature. When measurements are taken at the same measurement temperature, the influence of measurement temperature dependency on each impedance measurement value is thought to be nearly identical. Therefore, by obtaining the difference between these two impedances, an index that shows purely the difference in heated state between the target food and the reference food can be obtained, without including the influence of measurement temperature dependency of impedance.

This estimation method can effectively reduce food waste, thereby contributing to the achievement of Sustainable Development Goals (SDGs), such as Goal 2 "Zero Hunger" and Goal 12 "Responsible Consumption and Production." For example, in the manufacturing process of sealed foods such as low-temperature heated foods or retort pouch foods, inspection samples are opened, inspected, and then discarded. With this estimation method, accurate inspection can be performed by estimating the heating state without opening the sample, so no samples are wasted. Furthermore, in the development process of such foods, there is no need to open many samples during heating in order to find the heating conditions that result in the optimal heating state.

Below, we will provide specific examples of an estimation device that can implement this estimation method and the estimation method itself.

[2. Estimation device]
As shown in Figure 1, an apparatus 1 for estimating the heated state of food according to one embodiment of the present invention includes at least an electrode 11 for measuring the impedance of food and a control device 31. Note that "estimating the heated state of food" refers to estimating the degree to which food has been thermally denatured, but the heated state of food also includes a state in which the food has not been thermally denatured.

(2-1. Configuration of the Estimation Device)
At least two electrodes 11 are provided so as to sandwich food 12, the object of impedance measurement. Electrode 11 may be, for example, a flat electrode or a flat, mesh-like electrode. If electrode 11 is flat, the area that can sandwich food 12 is increased, allowing the impedance of the entire food 12 to be measured. Furthermore, by forming electrode 11 into a mesh-like shape, flat electrode 11 can be configured in a shape that does not interfere with heating of food 12.

The food 12 is food sealed in a non-metallic container. In this specification, the term "food 12" is intended to collectively refer to the target food 12a, whose heating state is to be estimated in this estimation method, and the reference food 12b, which is used to obtain the impedance difference value.

The food 12 is not particularly limited as long as its impedance changes due to thermal denaturation, and is preferably a food containing protein or starch, for example. Protein changes its impedance when it aggregates due to thermal denaturation caused by heating. Starch changes its impedance when it gelatinizes due to thermal denaturation caused by heating.

Such food products 12 may be, for example, foods containing at least one selected from the group consisting of meat, potatoes, and grains. Examples of meat foods include livestock meat, such as beef, pork, and poultry. Examples of potato and grain foods include potatoes, sweet potatoes, wheat, and corn, as well as foods containing starch obtained from these.

The non-metallic container used to seal the food 12 may be any container that does not interfere with the measurement of the impedance of the food 12 by the estimation device 1. Examples of non-metallic containers include heat-resistant containers made of synthetic resin, such as low-temperature cooking packs or retort pouches, or glass containers. Non-metallic containers may also be containers that contain metal in part, such as a glass container body with a metal lid. When using such a container that contains metal, the estimation device 1 can measure the impedance of the food 12 by placing the food 12 between the electrodes 11 so that the metal part is not positioned between the electrodes 11.

The electrode 11 and food 12 are placed in an incubator 13. The incubator 13 is a component that adjusts the temperature of the food 12. The incubator 13 may be any device that can adjust the temperature of the food 12 measured during impedance measurement, but it is preferable that it is also capable of cooking the food 12. If the incubator 13 is capable of cooking the food 12, the heating state of the food 12 can be confirmed by impedance measurement using the estimation device 1 while cooking the food 12.

The incubator 13 may be a device that heats the air inside, or a device that heats the liquid inside, like a thermostatic bath. From the perspective of accurately adjusting the impedance measurement temperature, the incubator 13 is preferably a thermostatic circulating bath. Furthermore, when cooking the food 12 using the incubator 13, the incubator 13 may be a low-temperature cooker or a retort sterilizer, etc. Furthermore, the incubator 13 may autonomously adjust the temperature, such as heating, or may be configured to be able to adjust the temperature using a computer 30 having a control device 31. The computer 30 will be described later.

The electrodes 11 are connected to an LCR meter 20. The LCR meter 20 is a device that measures the impedance of the food 12. The LCR meter 20 measures the impedance of the food 12 by applying an AC signal of a predetermined frequency between the two electrodes 11 that sandwich the food 12 and acquiring a response signal.

The impedance measurement value measured by the LCR meter 20 may be expressed as the magnitude of the impedance |Z| (unit: Ω), or may be expressed by various indices included in the impedance. Examples of such indices include, but are not limited to, resistance R (unit: Ω), reactance X (unit: Ω), and capacitance C (unit: F). The estimation device 1 can estimate the heating state of the target food 12a using any of these indices as the impedance measurement value.

The estimation device 1 includes a computer 30 having a control device 31. The computer 30 measures the impedance of the food 12 via the LCR meter 20 and performs processing using the obtained impedance measurement value. Note that the computer 30 may also have the functions of the LCR meter 20. The computer 30 may be a general-purpose computer such as a PC (Personal Computer) or server, or may be a dedicated computer having dedicated logic circuits, etc., used to execute the estimation method.

The computer 30 includes a control device 31 and a storage device 35. The storage device 35 is a component that stores various data acquired or generated by the computer 30. Examples of the storage device 35 include a hard disk drive (HDD), a solid state drive (SSD), or a read-only memory (ROM).

(2-2. Control Device)
The control device 31 is a component that comprehensively controls each part of the computer 30. The control device 31 includes a first measurement unit 32, a second measurement unit 33, and an estimation unit .

The first measuring unit 32 acquires a measurement value of the target impedance measured by the electrodes 11 via the LCR meter 20. The "target impedance" is the impedance of the target food 12a sealed in a non-metallic container at a predetermined measurement temperature. The "predetermined measurement temperature" is not particularly limited, but is preferably a temperature within the temperature range of the cooking temperature to be performed on the target food 12a.

The second measurement unit 33 acquires the measurement value of the reference impedance measured by the electrode 11 via the LCR meter 20. The "reference impedance" is the impedance of the reference food 12b sealed in a non-metallic container at the same measurement temperature as the measurement temperature of the target impedance.

Reference food 12b is the same type of food as target food 12a. Furthermore, reference food 12b is a food whose heating conditions, i.e., the conditions under which it was heated up until the time of impedance measurement, differ from those of target food 12a. "Different heating conditions" means that target food 12a and reference food 12b differ in at least one of the following conditions: heating temperature, rate of temperature rise during heating, and holding time at each temperature during heating. For reference food 12b, it is sufficient that the heating conditions are known, and it is preferable that the degree of thermal denaturation (heated state) is known.

When the target food 12a and the reference food 12b are "foods of the same type," this means, for example, that the target food 12a and the reference food 12b may be the same food (same individual), or they may be foods made from the same ingredients but not the same individual. Here, "the same food" refers to a case where the target food 12a and the reference food 12b are the same but differ only in the heating conditions. Specifically, for example, the state of the target food 12a before heating may be defined as the reference food 12b; in other words, the target food 12a may be the heated reference food 12b.

An example of a case where the target food 12a and the reference food 12b are made from the same ingredients is when the types of ingredients and processing conditions other than the heating conditions of the target food 12a and the reference food 12b are the same.

The estimation unit 34 obtains the difference between the target impedance and the reference impedance, and estimates the heating state of the target food 12a from this difference. The difference obtained here is an index that indicates the difference in heating state between the target food 12a and the reference food 12b in a form that is not affected by the measurement temperature dependency of the impedance measurement value. Specific processing examples by the estimation unit 34 are described below along with each step of this estimation method.

[3. Estimation method]
(3-1. Obtaining food impedance before and after heating)
An example of this estimation method will be described below with reference to Fig. 2. Fig. 2 shows an example in which the state of food 12 before heating is regarded as reference food 12b, and the state after heating is regarded as target food 12a, and the target impedance and reference impedance are respectively acquired. Here, an example is described in which this estimation method is implemented using estimation device 1, but the use of estimation device 1 is not essential for implementing this estimation method.

First, the user of the estimation device 1 places food 12 between the electrodes 11 (S1). At this time, the food 12 is unheated and in a raw state before thermal denaturation. Next, the second measurement unit 33 measures the impedance of the food 12 before heating (reference food 12b) using the electrodes 11 and acquires the measured value as the reference impedance (S2, second measurement step). After acquiring the reference impedance, the user of the estimation device 1 begins heating the food 12 using the incubator 13 (S3). Note that heating of the food 12 may begin before acquiring the reference impedance. In this case, the second measurement unit 33 may use the impedance of the food 12 acquired first as the reference impedance.

After heating the food 12, the heating by the incubator 13 is stopped, and the heated food 12 (target food 12a) is cooled (S4). At this time, when the temperature of the food 12 reaches the same temperature as the measured temperature of the reference impedance in S2, the first measurement unit 32 measures the impedance of the heated food 12 (target food 12a) using the electrodes 11 and obtains the measured value as the target impedance (S5, first measurement step). In this way, in this estimation method, the second measurement step may be performed before the first measurement step.

Furthermore, the term "same temperature" here does not require that the temperatures be exactly the same; it is sufficient that the temperatures are close enough that the effect of the impedance's temperature dependence on measurement can be practically ignored. In other words, the measurement temperature of the target impedance and the measurement temperature of the reference impedance are "same" if, for example, the difference between these measurement temperatures is within 2°C, preferably within 1°C, and more preferably within 0.5°C.

Next, the estimation unit 34 obtains the difference between the reference impedance and the target impedance and estimates the heating state of the food 12 after heating (S6, estimation step). At this time, the obtained difference value serves as an indicator of the progress of thermal denaturation of the food 12 due to heating performed after the reference impedance was obtained. Because the reference impedance and the target impedance are measured at the same temperature, these difference values are not affected by the measurement temperature dependency of the impedance and accurately indicate the heating state of the food 12 after heating.

The estimation unit 34 may estimate the heating state of the food 12 after heating, for example, such that the larger the obtained difference value, the stronger the thermal denaturation state. Furthermore, the currently obtained difference value may be compared with a difference value stored in the storage device 35 that was previously obtained using this estimation method for a food with a known degree of thermal denaturation, to estimate the heating state of the food 12 after heating.

In this example, the target food 12a and the reference food 12b are the same food. Therefore, there is no need to prepare the reference food 12b in advance and obtain its impedance. By measuring the impedance while heating the food 12, an accurate indicator of the heating state of the food 12 after heating can be obtained.

(3-2. Obtain reference impedance in advance)
Another example of this estimation method will be described below with reference to Fig. 3. Fig. 3 shows an example in which a reference impedance of a reference food 12b that has been thermally denatured by heating is acquired in advance, and then a target impedance of a target food 12a that is to be separately cooked by heating is acquired.

First, the reference impedance is obtained through steps S11 to S13. The user of the estimation device 1 places the reference food 12b between the electrodes 11 (S11) and heats the reference food 12b using the incubator 13 (S12). At this time, it is preferable that the heating conditions for the reference food 12b are such that the reference food 12b is in a state of more advanced thermal denaturation than the target food 12a after the planned heating.

For example, the heating conditions for the reference food 12b are preferably conditions in which the food is heated to a higher temperature than the heating conditions for the intended target food 12a. Alternatively, the heating conditions for the reference food 12b may be conditions in which the food is heated to the same temperature as or higher than the heating conditions for the target food 12a, but for a longer time than the heating conditions for the target food 12a.

Next, the second measurement unit 33 measures the impedance of the reference food 12b using the electrodes 11 at multiple measurement temperatures while the reference food 12b is cooling after heating, and obtains the measurement values for each measurement temperature as the reference impedance (S13, second measurement step). At this time, the second measurement unit 33 may, for example, obtain the reference impedance every time the temperature changes by a predetermined value (e.g., 1°C), or may obtain the reference impedance every predetermined time (e.g., 30 seconds) regardless of the temperature change.

Next, the user of the estimation device 1 places the target food 12a between the electrodes 11, replacing the reference food 12b (S14), and heats the target food 12a using the incubator 13 (S15). The first measurement unit 32 measures the impedance of the target food 12a using the electrodes 11 at least once while the target food 12a is being heated, and obtains the measured impedance as the target impedance (S16, first measurement step).

The estimation unit 34 determines whether there is a reference impedance measured at the same temperature as the measurement temperature of the target impedance obtained in S16 (S17). Note that the first measurement unit 32 may measure the impedance of the target food 12a only if there is a reference impedance measured at the same measurement temperature as the temperature at that time. In this case, the estimation unit 34 may skip the processing of S17.

If it is determined that a reference impedance measured at the same temperature as the measurement temperature of the target impedance exists (Yes in S17), the estimation unit 34 acquires the difference value between the target impedance and the reference impedance at the same measurement temperature. The estimation unit 34 then estimates the heating state of the target food 12a at that measurement temperature from the acquired difference value (S18, estimation step). Note that if it is determined that a reference impedance measured at the same temperature as the measurement temperature of the target impedance does not exist (No in S17), the estimation unit 34 may skip the processing of S18.

The control device 31 then determines whether heating of the target food 12a has finished (S19). If the target food 12a is to be further heated (No in S19), the control device 31 may return to S15 and obtain the target impedance measured at a measurement temperature different from the previous measurement. In this case, the first measurement unit 32 measures the target impedance at multiple measurement temperatures while heating the target food 12a. If it determines that heating of the target food 12a has finished (Yes in S19), the control device 31 terminates the processing of this estimation method.

In this example, the estimation unit 34 acquires the difference value between the target impedance and the reference impedance for each of the multiple measurement temperatures. The estimation unit 34 may individually evaluate the acquired difference values to estimate the heating state of the target food 12a at each measurement temperature.

The estimation unit 34 may also estimate the heated state of the target food 12a from multiple difference values. That is, the estimation unit 34 may estimate the heated state of the target food 12a based on changes in multiple difference values. In this example, as heating progresses, it is thought that the difference value becomes smaller as the degree of thermal denaturation of the target food 12a approaches the degree of thermal denaturation of the reference food 12b. Therefore, the estimation unit 34 may, for example, estimate the heated state of the target food 12a if it is further heated from that point on by evaluating changes in multiple difference values up to that point.

As in this example, this estimation method may use a reference food 12b that has been preheated and thermally denatured, and measure the reference impedance at multiple measurement temperatures while the reference food 12b is cooling or reheating after heating. Because the reference food 12b is thermally denatured, the changes in the reference impedance obtained at each measurement temperature primarily reflect changes due to the influence of measurement temperature dependency, rather than changes due to thermal denaturation. Therefore, by obtaining the difference between this reference impedance and the target impedance, changes in the heating state of the target food 12a due to heating can be evaluated while effectively eliminating the influence of measurement temperature dependency.

The reference impedance obtained from the reference food 12b in steps S11 to S13 may be used repeatedly as long as the target food 12a is the same type of food. In other words, for example, if reference impedance data is already stored in the storage device 35, the user of the estimation device 1 may skip steps S11 to S13 and execute the estimation method from S14.

In addition, the processes of S11 to S13 may be performed after the processes of S14 to S16. In other words, the reference impedance may be obtained by the second measuring unit 33 after the target impedance is obtained by the first measuring unit 32.

[4. Software implementation example]
The functions of the estimation device 1 (hereinafter referred to as the "device") can be realized by a program that causes a computer to function as the device, and a program that causes a computer to function as each control block of the device (particularly each part included in the control device 31).

In this case, the device includes a computer having at least one control device (e.g., a processor) and at least one storage device (e.g., a memory) as hardware for executing the program. The functions described in each of the above embodiments are realized by executing the program using this control device and storage device.

The program may be stored non-transitory on one or more computer-readable storage media. These storage media may or may not be included in the device. In the latter case, the program may be supplied to the device via any wired or wireless transmission medium.

Furthermore, some or all of the functions of each of the control blocks can be realized by logic circuits. For example, integrated circuits incorporating logic circuits that function as each of the control blocks are also included in the scope of the present invention. In addition, the functions of each of the control blocks can also be realized by, for example, a quantum computer.

Furthermore, each process described in each of the above embodiments may be executed by AI (Artificial Intelligence). In this case, the AI may run on the control device, or on another device (such as an edge computer or cloud server).

[Experimental Example 1. Evaluation of the heated state of pork]
The results of the estimation of the heating state when this estimation method was used with pork as the target food were compared with the results of evaluation using the conventional method.

(1-1. Materials and methods of this estimation method)
Commercially available, uncooked pork (domestic, tenderloin) was used as food product 12. The tendons and fat were trimmed from the pork, leaving only the lean meat. The pork was then sliced into approximately 5 mm pieces with the cut surface perpendicular to the muscle fibers to prepare samples. Approximately 50 g of the sample was spread evenly in a retort pouch (NCF2-6, three-sided seal bag, nylon 15 μm/polypropylene 60 μm, 120 mm x 200 mm, Cow-Pack Co., Ltd.) to an area of approximately 100 mm x 100 mm. The retort pouch was then cut open to form a 120 mm x 150 mm bag. A thermocouple for temperature measurement was attached to the center of the exterior of the retort pouch. In this experiment, the temperature on the side of the sample and the temperature at the center were considered to be the same.

The estimation device used was the estimation device 1 shown in Figure 1. Specifically, the two electrodes 11 were parallel plate electrodes (150 mm x 120 mm x 2 mm, stainless steel φ6 mm punched plate, 40.2% aperture ratio). The incubator 13 was a stainless steel pot connected to an electric heater and filled with silicone oil (KF-96-50CS, Shin-Etsu Chemical Co., Ltd.). The LCR meter 20 was an LCR meter (3532-80, Hioki E.E. Corporation). Ground wires were connected to the stainless steel pot, electric heater, and LCR meter to eliminate external noise.

The retort pouch containing the sample was sandwiched between parallel plate electrodes, and the magnitude of the sample's impedance |Z| (unit: Ω) was measured. In this Experimental Example 1, unless otherwise specified, "impedance" refers to the magnitude of impedance |Z|. Impedance measurements were taken over time at 30-second intervals. The measurement frequency was 24,370 Hz. The sample was heated using an electric heater until it reached the target temperature (65°C, 75°C, 85°C, 95°C), after which the temperature was maintained for 6 hours, after which heating using the electric heater was stopped and the temperature was allowed to decrease. This measurement was conducted in a laboratory at approximately 25°C. Three measurements were taken for each temperature condition.

(1-2. Materials and methods of conventional methods)
Two conventional methods were used: (a) a method for evaluating the enthalpy change of a sample using a differential scanning calorimeter (DSC), and (b) a method for evaluating the hardness of a sample using elastic strain energy as an index. Both of these conventional methods require direct measurement of the food product, and are not methods that can evaluate the condition of the food product while it is sealed in a container.

(a) The enthalpy change of the sample was evaluated as follows. Approximately 20 mg of the sample under each condition used in the impedance measurements described above was removed from the retort pouch, sealed in an aluminum pressure pan, and set in a DSC (DSC-60Plus, Shimadzu Corporation). Unheated raw pork samples were also prepared. Distilled water and indium were used for temperature calibration of the DSC, and indium was used for calorific calibration. α-alumina was used as the reference material. The DSC measurement temperature range was 20 to 100°C, and the heating rate was set to 5°C/min. To investigate the effect of the sample's thermal history on the DSC heating curve, the sample was heated to a certain temperature (first scan), cooled to 5°C, and then heated to the same temperature again (second scan). Three measurements were performed for each condition, and the results were analyzed using the software provided with the DSC (LabSolutions TA, Shimadzu Corporation).

(b) The hardness of the samples was evaluated as follows. The samples under each condition used in the impedance measurements described above were removed from the retort pouches, and the hardness of the samples was measured using a plunger penetration test. A small materials testing machine (EZ-SX, Shimadzu Corporation) was used for the plunger penetration test. A cylindrical plunger with a diameter of 5 mm was attached to the load cell (pressure-sensing part) of the materials testing machine, and the plunger was inserted into the sliced surface of the sample (in the direction of the muscle fibers) at 10 mm/min. Ten measurements were performed on each sample. The area up to the yield point was calculated as the elastic strain energy from the load-displacement curve obtained from the measurements, and the hardness of the samples was evaluated. The results were analyzed using the software provided with the materials testing machine (TRAPEZIUM X, Shimadzu Corporation).

(1-3. Results of this estimation method)
The results of this estimation method are shown in Figure 4. The diagram indicated by reference numeral 401 in Figure 4 shows the measured impedance values from the start of heating of the sample (40°C) until the target temperatures (65°C, 75°C, 85°C, 95°C) were reached and then the temperature was lowered to 40°C. The diagram indicated by reference numeral 402 in Figure 4 shows the difference value Δ|Z| (unit: Ω) between the impedance measured at the start of heating of the sample as the reference impedance and the impedance measured at the point when the temperature reached 40°C after heating and lowering as the target impedance.

As shown in the diagram indicated by the reference numeral 401, the impedance of the sample increased as it was heated. Generally, the impedance of meat tends to decrease due to deterioration or destruction of the cells that make it up, but the measurement results in this study showed the opposite trend. When the sample was heated, the impedance tended to increase, presumably due mainly to the aggregation of proteins caused by thermal denaturation of the sample, rather than due to cell deterioration.

Furthermore, as shown in the diagram indicated by the reference numeral 402, the difference value Δ|Z| increased as the maximum heating temperature increased. This suggests that the difference value indicates the heating state of the sample, as a proportional relationship is observed between the degree of heating and the magnitude of the difference value Δ|Z|. Furthermore, the measurement temperature dependence of the impedance is eliminated by obtaining the difference value Δ|Z| of the impedance obtained at the same measurement temperature. Therefore, the obtained difference value Δ|Z| is considered to be an accurate indicator of the degree of thermal denaturation of the sample.

(1-4. Results of conventional method)
Figure 5 (a) shows the evaluation results of the enthalpy change of the samples. The diagram indicated by reference numeral 501 in Figure 5 shows the DSC measurement results of each sample, and the diagram indicated by reference numeral 502 shows the evaluation results of the normalized enthalpy. The normalized enthalpy is the value obtained by calculating the enthalpy of the endothermic reaction at 40 to 100°C for each sample and dividing it by the enthalpy of the unheated sample (Raw).

As shown in the diagram indicated by reference numeral 501, an endothermic reaction peak indicating thermal denaturation of the protein was observed in the unheated sample at approximately 50°C. Furthermore, samples heated to each target temperature did not exhibit an endothermic reaction peak at temperatures below the target temperature. Furthermore, as shown in the diagram indicated by reference numeral 502, the more strongly heated the sample, the smaller the normalized enthalpy value. As such, normalized enthalpy correlates with the degree of thermal denaturation of the sample, and is therefore an indicator of the heating state of the sample.

Figure 6 also shows the results of the hardness evaluation of the (b) samples. Reference numeral 601 in Figure 6 indicates the load-displacement curve for each sample. In the diagram indicated by reference numeral 601 in Figure 6, the cross indicates the breaking point, which is the peak top, the solid arrow indicates the yield point, and the hollow arrow indicates a point that is not a clear yield point but shows slight yield. Furthermore, the diagram indicated by reference numeral 602 in Figure 6 shows the elastic strain energy.

As shown in the diagram indicated by the reference numeral 601, for samples heated to approximately 65°C, the breaking energy (the area up to the breaking point on the load-displacement curve) and elastic strain energy are nearly the same. On the other hand, for each sample heated to 75°C or higher, the proportion of elastic strain energy in the breaking energy is small. The smaller this proportion, the less elastic the properties.

Furthermore, as shown in the diagram indicated by the reference numeral 602, when elastic strain energy was compared as an indicator of the physical properties of the samples, the lower the heating temperature of the sample, the greater the elastic strain energy. In this way, elastic strain energy shows a negative correlation with the degree of thermal denaturation of elastic fibers, actin, etc. in the sample due to heating, and serves as an indicator of the heating state of the sample.

(1-5. Correlation between the results of this estimation method and the results of the conventional method)
We investigated the correlation between the impedance difference value Δ|Z| obtained by this estimation method and the evaluation index results obtained by the conventional method. Figure 7 shows the comparison results between the difference value Δ|Z| of each sample obtained by this estimation method (see the diagram indicated by reference numeral 402 in Figure 4) and the elastic strain energy obtained using the same sample (see the diagram indicated by reference numeral 602 in Figure 6).

As shown in Figure 7, the higher the heat treatment temperature of the sample, the larger the difference value Δ|Z| and the smaller the elastic strain energy tended to be. In other words, it became clear that there is a negative correlation between the difference value Δ|Z| and the elastic strain energy for the heating state of the sample.

Furthermore, when comparing the graph indicated by reference numeral 402 in Figure 4 with the graph indicated by reference numeral 502 in Figure 5, a negative correlation was also observed between the difference value Δ|Z| and the normalized enthalpy for the heating state of the sample.

As described above, the difference value Δ|Z| obtained by this estimation method has been shown to be an index for evaluating the heating state of food, similar to the elastic strain energy and normalized enthalpy that have traditionally been used to evaluate the processing state of food. This estimation method is extremely useful in that it does not require opening the non-metallic container in which the food is sealed, and can non-destructively evaluate the heating state of food with the same accuracy as conventional methods.

Experimental Example 2. Impedance measurements over time when pork is heated twice.
In the above-mentioned Experimental Example 1, a reference impedance was obtained from an unheated sample to evaluate the heating state of the sample after heating. In this Experimental Example 2, similar to Experimental Example 1, pork is used as a sample, and the impedance of the sample after thermal denaturation is examined.

(2-1. Materials and Methods)
The sample and impedance measurement method were the same as in Experimental Example 1. In Experimental Example 2, the sample was heated and cooled twice. Specifically, the sample was heated to approximately 75°C, the heating was stopped, and the temperature was lowered to approximately 30°C (first heating and cooling). The same sample was then heated again to approximately 75°C, the heating was stopped, and the temperature was lowered to approximately 37°C (second heating and cooling). The magnitude of impedance |Z| was obtained over time at 30-second intervals from the start of the first heating to the end of the second cooling. Note that in Experimental Example 2, as in Experimental Example 1, "impedance" refers to the magnitude of impedance |Z| unless otherwise specified.

(2-2. Results)
As shown in Figure 8, the impedance of the sample was approximately 9100 Ω at the start of the first heating, gradually decreased to around 65°C, and then rapidly increased to approximately 10500 Ω. From there, a gradual increase to approximately 11000 Ω was observed during the temperature decrease after the first heating was stopped. When the second heating was subsequently performed, the impedance of the sample gradually decreased with heating to 10500 Ω, the same as when the first heating was stopped, and then a gradual increase again to approximately 11000 Ω was observed during the second temperature decrease after the heating was stopped.

In these results, the rapid increase in impedance during the first heating is thought to be due to thermal denaturation of the sample. On the other hand, after the first heating was stopped, it is thought that there were almost no changes in the heating state of the sample due to heating or cooling, at least in the temperature range below the maximum temperature of approximately 75°C. This suggests that the slight changes in impedance observed during the first cooling, the second heating, and the cooling represent changes due solely to the measurement temperature dependence of impedance. This result indicates that the sample after the first heating was stopped is useful as a reference food for correcting the measurement temperature dependence of impedance.

For example, a sample taken after the first heating cycle has stopped can be used as a reference food, and reference impedances can be obtained at multiple measurement temperatures during cooling or reheating. Difference values can then be obtained from the measured impedance values of the target food using the multiple reference impedances obtained. The multiple difference values obtained in this way are considered useful as an accurate indicator of changes in the heating state of the target food, with the influence of the measurement temperature dependency of the impedance removed.

Experimental Example 3. Impedance measurements over time when various starches are heated twice.
Starch was used as a sample to study the change in impedance during two heating and cooling treatments similar to those in Experimental Example 2.

(3-1. Materials and Methods)
The impedance measurement method was the same as in Experimental Example 1. The difference between the estimation device 1 used in Experimental Example 3 and Experimental Example 1 was that two electrode plates (150 mm x 120 mm x 6 mm) with a heat sink structure were used as the electrodes 11, and silicone oil was circulated inside the electrode plates. The function of the incubator 13 was realized by heating the silicone oil flowing inside the electrode plates using a constant temperature circulation layer (ECOLINE RE104, Lauda). The two electrode plates were placed inside a polystyrene foam shielding box (310 mm x 380 mm x 142 mm) lined with aluminum foil. The aluminum foil of the shielding box was connected to the ground terminal of the LCR meter to block external noise.

Four types of starch were used as samples: potato starch, corn starch, wheat starch, and sweet potato starch. For each starch, a 30% by mass starch-xanthan gum suspension was prepared using a 0.15% by mass xanthan gum aqueous solution to prevent precipitation, and approximately 15 g of this suspension was filled into a retort pouch (15 cm x 12 cm) to prepare the sample.

Impedance measurements were taken at 80 logarithmically spaced points between 1000 Hz and 1 MHz, and the change in resistance Rp or capacitance Cp at a frequency of 269,400 Hz was observed as an example. Resistance Rp and capacitance Cp are each a type of physical quantity included in impedance. Impedance and sample temperature were repeatedly measured every 3.5 minutes.

During the impedance measurement, the sample was heated and cooled twice. Specifically, for the potato starch sample, the sample was heated to approximately 80°C, approximately 65°C, or approximately 50°C, then the heating was stopped and the temperature was allowed to cool to approximately 30°C (first heating and cooling). The same sample was then heated again to the same temperature, the heating was stopped, and the temperature was allowed to cool to approximately 30°C (second heating and cooling). The resistance Rp was measured over time from the start of the first heating and cooling to the end of the second heating and cooling.

Furthermore, for each of the four types of starch samples, the sample was heated to approximately 80°C, the heating was stopped, and the temperature was allowed to cool to approximately 30°C (first heating and cooling). Then, each sample was heated again to approximately 80°C, the heating was stopped, and the temperature was allowed to cool to approximately 30°C (second heating and cooling). The capacitance Cp was measured over time from the start of the first heating and cooling to the end of the second heating and cooling.

(3-2. Results)
9 shows the results of measuring the resistance Rp of potato starch sample after heating and cooling twice, where the graph 901 shows the results for heating to about 80°C, the graph 902 shows the results for heating to about 65°C, and the graph 903 shows the results for heating to about 50°C.

As shown in Figure 9, just as in Experimental Example 2, when pork was used as the sample, when potato starch was used as the sample, the resistance Rp changed significantly during the first heating due to changes in the heating state of the sample. This is also evident from the fact that the higher the maximum temperature reached during heating, the greater the change in resistance Rp. The gradual change in resistance Rp after the first heating was stopped is thought to be due solely to the effect of dependence on the measurement temperature.

As in the case of pork, this demonstrates that it is possible to estimate the heating state of potato starch using the impedance difference value, using the sample after the first heating cycle has stopped as the reference food. Furthermore, it was shown that the measured impedance value used to estimate the heating state is not limited to the magnitude of impedance |Z|, and that the resistance Rp included in the impedance can also be used.

Figure 10 shows the results of measuring the capacitance Cp for four types of starch samples, which were heated and cooled twice. The graph indicated by reference numeral 1001 in Figure 10 shows the results for corn starch, the graph indicated by reference numeral 1002 shows the results for wheat starch, the graph indicated by reference numeral 1003 shows the results for sweet potato starch, and the graph indicated by reference numeral 1004 shows the results for potato starch.

As shown in Figure 10, even for starches other than potato starch, the capacitance Cp changes significantly during the first heating due to changes in the heating state of the sample, and after the first heating is stopped, the capacitance Cp changes only due to the influence of the measurement temperature dependency. Therefore, as with potato starch, it was shown that for various starches other than potato starch, it is possible to estimate the heating state using the impedance difference value by using the sample after the first heating is stopped as a reference food. Furthermore, it was shown that there is no problem if the measured impedance value used to estimate the heating state is the capacitance Cp included in the impedance.

REFERENCE SIGNS LIST 1 Estimation device 11 Electrode 12 Food 13 Incubator 20 LCR meter 30 Computer 31 Control device 32 First measurement unit 33 Second measurement unit 34 Estimation unit 35 Storage device

Claims (6)

a first measurement step of measuring a target impedance, which is the impedance at a predetermined measurement temperature, of a target food sealed in a non-metallic container;
a second measurement step of measuring a reference impedance, which is the impedance at the measurement temperature, of a reference food that is the same type of food as the target food and is sealed in the non-metallic container but has heating conditions different from those of the target food;
and estimating the heated state of the target food from a difference value between the target impedance and the reference impedance. The estimation method described in claim 1, wherein the target food is a heated version of the reference food. The estimation method according to claim 1 or 2, wherein the heating conditions for the reference food are such that the reference food is heated to a temperature higher than the heating conditions for the target food, such that thermal denaturation is more advanced in the reference food than in the target food. In the first measurement step, the target impedance is measured at each of the plurality of measurement temperatures during heating of the target food;
In the second measurement step, the reference impedance is measured for each of the plurality of measurement temperatures;
The estimation method according to claim 1 , wherein the estimation step acquires the difference value for each of the measured temperatures, and estimates the heated state of the target food from the plurality of difference values. The estimation method described in any one of claims 1 to 4, wherein the target food is a food containing at least one type selected from the group consisting of meat, potatoes, and grains. A food heating state estimation device comprising an electrode and a control device,
The control device
a first measurement unit that acquires a measurement value of a target impedance, which is the impedance of a target food sealed in a non-metallic container at a predetermined measurement temperature, measured by the electrodes;
a second measurement unit that acquires a measurement value of a reference impedance, which is the impedance at the measurement temperature of a reference food that is the same type of food as the target food and sealed in the non-metallic container but has heating conditions different from those of the target food, measured by the electrodes;
an estimation unit that estimates the heating state of the target food from a difference value between the target impedance and the reference impedance.