JPS6339225B2 - - Google Patents

Description

[Detailed description of the invention]

[Detailed Description of the Invention] The present invention relates to a method for sterilizing a sealed package, and more particularly to an improvement in a method for sterilizing a sealed package using microwaves. In recent years, attempts have been made to apply microwave heating to heat sterilization of packaged foods. Microwaves have the characteristic of passing through packaging materials with low dielectric loss, such as plastics, with little heating. However, the heating mechanism of microwaves is that when the dipole inside the food is placed in an alternating current electric field, the time of the electric field increases. Rotational vibrations of dipoles occur in response to changes in energy, and heat is generated by the friction of molecules, so foods with large dielectric loss can be efficiently heated from the inside. Due to this feature,
Microwaves are widely used in the food industry, and are put to practical use in drying foods, thawing frozen foods, and sterilizing them. For example, heat sterilization
Application examples include sterilization of ham sausages, prevention of mold growth in pastries, and sterilization of squid delicacies to prevent mold. However, in these known methods, the microorganisms to be sterilized have extremely low heat resistance and can be completely killed in about 10 to 20 minutes at 80 to 90 degrees Celsius.
There was no need to specifically measure the temperature of the product because a sterilization effect could be obtained if the temperature of the product was raised to a temperature close to that level. However, some microorganisms that cause spoilage of packaged foods have a high heat resistance that cannot be killed by heating below 100℃, and like canned and bottled retort foods, they must be treated at high temperatures of 100℃ or higher. The same applies to microwave heat sterilization, which makes it impossible to produce packaged foods that can be stored for a long time. For this reason, it is legally required for packaged foods that are heat sterilized at temperatures above 100°C to record the exact temperature during the heat treatment. Nevertheless, none of the known methods and devices for microwave sterilization discloses an effective temperature measurement method. Temperature measurement methods currently used when heat sterilizing packaged foods and food products using microwaves include temperature-sensitive labels or thermopaints that change color at a certain temperature, discoloration of glycine-glucose mixed solutions, and methods of measuring protein coagulation. Freezing point, etc. are used, but these methods only determine the maximum temperature reached and the total heat history during the process; unlike traditional temperature measurement methods that use thermocouples and thermistors, these methods do not measure the heating time. It has been impossible to continuously measure the heating temperature over time, and it has been impossible to control the degree of sterilization in terms of the temperature-time relationship. For this reason,
Sterilization of packaged foods by microwave heating at temperatures of 100°C or higher has not been put to practical use because it has been impossible to reliably control whether the packaged foods have been sterilized. Therefore, it is an object of the present invention to provide a method for sterilizing foods filled and sealed in containers by microwave heating while accurately measuring the heating temperature over time. Another object of the present invention is to accurately measure the heating history of foods even under the influence of microwaves during microwave heat sterilization, and to produce heat sterilized packaged foods of good quality that can withstand long-term storage. We are here to provide you with a method. According to the present invention, microwaves are irradiated to a sealed package formed by filling an article to be sterilized in a container that is transparent to microwaves, and the temperature of the sealed package under microwave irradiation is changed over time. The progress is measured using a phosphor temperature sensor based on the temperature dependence of the intensity in the fluorescence wavelength region of the phosphor, and the temperature measured by the phosphor sensor is 100°C or higher, and the formula F=∫ ^t _p log ^-1 (T-121.1/Z) dt ...(1) In the formula, log ^-1 means anti-log, T represents the temperature (℃) measured by the phosphor sensor, and t represents the micro It represents the time (minutes) after the start of microwave irradiation, and Z is the temperature dependence value of microorganisms, the so-called Z value (℃). A method for sterilizing a sealed package is provided, the method comprising continuous irradiation. First, the meaning of the above formula (1) will be explained. Log ^-1 is commonly used in the field of heat sterilization and is usually referred to as anti-log. What this means is that the thermal lethality time curve (usually thermal
The following equation can be obtained from the Death time curve (TDT curve). log t=121.1−T/Z...(2) In this equation (2), t is the lethal time (minutes) at the heating temperature T (℃), and Z is the TDT curve [vertical axis: lethal time (minutes, Logarithm), horizontal axis: heating temperature (°C)] indicates the temperature (°C) required to reduce the lethal time to 1/10, and is an indicator of the strength of the heat resistance of the bacteria. Equation (2) is usually expressed and used as the following equation. t=log ^-1 121.1-T/Z...(3) The reciprocal of t, 1/t, is the lethality rate at that heating time. Letting this be L, the following equation is obtained. L=1/t=1/log ^-1 121.1-T/Z =log ^-1 T-121.1/Z...(4) In the total sterilization process (from 0 to t), the integral value (F) of total sterilization is F =∫ ^t _p L dt =∫ ^t _p log ^-1 T-121.1/Zdt. The Z value of bacteria that spoil food varies slightly, but most are close to 10 (℃), so there is no particular problem in representing it with this value. Furthermore, since the Z value of Clostridium botulinum that requires the most attention is 10 (°C), it can be seen that this value may be used as a representative value. In the present invention, the meaning of F being 3.2 or more is that in heat sterilization packaging, it is necessary to kill at least Clostridium botulinum spores as bacteria to be sterilized, and the heat resistance of this bacterium is exactly F = 3.12. ,
In the present invention, it is set as 3.2 based on this. In the present invention, the significance of measuring the temperature T with a fluorescent sensor instead of a conventional thermocouple or thermistor is as follows. One of the important features of the present invention is that the time course of the temperature of a sealed package under microwave irradiation is measured using a phosphor temperature sensor, which measures the light intensity in the fluorescence wavelength region of the phosphor. The purpose is to measure based on temperature dependence. First, the usual method of measuring the temperature of a sealed package under heat sterilization over time is as follows:
This method uses a thermoelectric material or thermistor as a temperature sensor, but when heat sterilizing with microwaves, there is radio wave leakage from the temperature sensor, and the electric field is concentrated at the tip of the temperature sensor, making it difficult to accurately determine the temperature of the product. Measurement becomes difficult. In contrast, according to the invention, when a phosphor temperature sensor is used to measure the temperature of a sealed package under microwave irradiation,
Since the electric field does not concentrate on the sensor and the temperature can be extracted in the form of a fluorescent signal, the problem of radio wave leakage is effectively resolved, and the temperature of the sealed package can be measured accurately and over time. It becomes like this. The fluorescent temperature sensor used in the present invention and the principle of temperature measurement using it are disclosed in US Patent No.
4075493 and 4215275, this is used to measure the temperature of a sealed package under microwave sterilization conditions, and the time course of the measured temperature is measured by microwave irradiation. This was completely unknown prior to the present invention. In the present invention, any container can be used to fill the article to be sterilized as long as it is transparent to microwaves and can be sealed. Such a container may be a flexible container such as a bag, or a rigid container such as a cup or a wide mouth bottle. These containers are made of materials with low dielectric loss, such as thermoplastic or thermosetting plastics, such as olefin resins such as polyethylene and polypropylene; polyester resins such as polyethylene terephthalate; and nylon 6.
It is preferable to use a container made of polyamide resin such as polyamide resin; polycarbonate; polyvinyl chloride, vinylidene chloride resin, etc. Other containers made of glass can also be used. Items to be sterilized to be filled into containers include, but are not limited to, liquid or pasty foods and beverages, such as coffee, black tea; straight fruits such as lemon juice, orange juice, plum juice, budoji youth, and strawberry juice;・Fruit juice drinks including processed fruit juice drinks such as juice or nectar; Vegetable juice drinks including tomato juice and various vegetable juices; lactic acid bacteria drinks; cooked curry, cooked hash, stews such as borscht and beef stew, and gravy such as meat sauce Types: Sweet and sour pork, sukiyaki, Happo-na, Chinese-style boiled vegetables, boiled asparagus, beans, cream-stewed tuna, etc., boiled vegetables, fish, and meat; Consomme soup, potage soup, miso soup, pork soup, kenchin soup, etc. Soup; rice foods such as rice, red rice, fried rice, gomoku rice, pilaf, and kayu; noodles such as spaghetti, soba, udon, Chinese soba, and macaroni; compound seasoning for fried rice soup or Chinese soba soup; red bean Favorite foods such as zenzai, shiruko, sweet bean, mitsumame, pudding, jelly, and water yokan; Processed seafood and farmed products such as meatballs, hamburgers, corned beef, ham, sausages, grilled fish, Kunsei, bacon, and kamaboko; Mandarin oranges, peaches, Fruit products such as pineapple, cherries, and olives; seasonings such as mustard oil, sauce, vinegar, mirin, dressing, mayonnaise, ketchup, cooking oil, miso, lard, and ketchup; tofu, diyam,
Examples include luxury goods such as butter and margarine. The contents can be filled into the container either cold or hot, and the container can be sealed by a method known per se, such as heat sealing, seaming, and sealing. Incidentally, when sealing the container, oxygen in the head space of the container can be removed by steam injection, nitrogen substitution, or the like. In the present invention, the phosphor temperature sensor uses a layer of phosphor whose light intensity in the fluorophore wavelength range changes depending on the temperature of the phosphor. Suitable phosphors are rare earth oxysulfides, the compounds of which may be doped with rare earth elements. These phosphors have the following formula M ₂ O ₂ S・nX, where M is a rare earth element such as lanthanum, gadolinium, or yttrium, X represents a doping component made of the rare earth element, and n is 0.01 to It is a number that is 10 atomic %, and is expressed as . In order to extract the temperature as a fluorescence signal from the above-mentioned fluorescent sensor, an ultraviolet light source and a fluorescence detection mechanism are provided, and the fluorescent sensor is connected to the ultraviolet light source and the fluorescence detection mechanism by optical fibers, respectively. Thus, ultraviolet light from the light source is applied to the phosphor layer through the optical fiber to excite the phosphor, and the fluorescence with an intensity characteristic of the temperature of the phosphor layer is transmitted through the optical fiber to a fluorescence detection mechanism. . The signal from this fluorescence detection mechanism is
In a signal processing mechanism, based on the temperature dependence of the light intensity, it is converted into a temperature indicating signal and displayed as temperature. In order to measure the temperature of the package to be heat sterilized, the fluorescent sensor is fixed to the package. In FIG. 1 showing this fixing method, a hole is made in the wall of a rigid sampling container 1, a packing ground 2 made of polytetrafluoroethylene is fitted into the hole, and a probe made of optical fiber is inserted through the packing ground 2 made of polytetrafluoroethylene. 3 and held so that the fluorescent temperature sensor 4 is located at the center of the package. In addition, in the case of a flexible container such as a pouch, as shown in FIG. , the probe holder 5 ensures that the sensor 4 is still located at the center of the package. In the present invention, unlike normal heat sterilization, the food inside the container is uniformly heated throughout, so it is not necessarily necessary to position the temperature sensor at the center of the package. It should be understood that the temperature of the product can be measured fairly accurately, for example, even if the fluorescent temperature sensor is located close to the container wall, or even if the temperature sensor is in contact with the container wall. be. In the present invention, heat sterilization using microwaves can be performed by any method. The microwaves have frequencies of 915 and 2450 MHz, but electromagnetic waves with other frequencies can of course also be used.
Microwave irradiation is easily performed by introducing microwaves into the sterilization chamber from a microwave generator such as a magnetron through a waveguide, and at this time, the microwaves are uniformly irradiated into the sterilization chamber. For this purpose, a fan can be provided at the waveguide outlet or even at a suitable location in the sterilization chamber. The output of microwaves varies significantly depending on the amount of heat required for sterilization, but in general, the output is set so that sterilization of one package by microwave irradiation is completed in 30 seconds to 30 minutes. It is desirable to specify. Of course, the microwave irradiation may be performed continuously or intermittently to keep the product temperature within a certain range. Microwave irradiation can be carried out continuously or batchwise. In the case of a continuous type, the package may be placed on a conveyor belt or other transport mechanism and sent continuously or intermittently into a sterilization chamber under microwave irradiation; in the case of a batch type, a predetermined amount is placed in the sterilization chamber. It is best to pack it in a package and irradiate it with microwaves. The atmosphere in the sterilization chamber may be at normal pressure or may be under air pressure. For example, when the packaging container is a pressure-resistant container or when the sterilization temperature is relatively low, the sterilization chamber atmosphere may be at normal pressure. Of course, if the sterilizing atmosphere is pressurized so as to cancel out all or part of the vapor pressure inside the package, damage to the packaging container or the sealed portion during sterilization can be effectively prevented. Generally, it is desirable to pressurize the sterilization chamber to about 1.2 to 3.0 kg/cm ² gauge. When performing continuous microwave sterilization by pressurizing the sterilization chamber,
It is preferable to provide seals at the entrance and exit of the sterilization chamber. In the present invention, it is desirable that the sealed package introduced into the microwave sterilization chamber be preheated in advance in order to shorten the heating time to the sterilization temperature. Generally, it is convenient for this purpose to maintain the temperature of the sealed package at 60 to 90°C. For this purpose, hot filling of the contents or preheating of the package with hot air or hot steam is used. According to the present invention, by including the above-mentioned sealed package for sterilization temperature sampling in the group of sealed packages to be microwave sterilized, it is possible to accurately measure the time course of product temperature under microwave irradiation conditions. can be measured. That is, according to this method, only a fluorescent temperature sensor and an optical fiber probe, which are not affected by microwaves, are located in the sterilization chamber, and these temperature measuring instruments are not exposed to hot water during sterilization. The temperature of the product under microwave sterilization can be accurately measured as the intensity of fluorescent light without being affected by hot steam, pressure, etc. In the present invention, the temperature measured by the fluorescent sensor is 100°C or higher and the formula F=∫ ^t _p log ^-1 (T-121.1/Z) dt where T is the temperature measured by the fluorescent sensor The temperature (°C) is expressed, t is the microwave irradiation time, and Z is the temperature dependence value of microorganisms. Continue microwave irradiation for a sufficient time so that the integral value (F) expressed by is 3.2 or more. This is also extremely important. As already mentioned above, it is known that the effectiveness of killing microorganisms in packaged foods depends on the combination of temperature and time. In the present invention, the temperature of the packaged food under microwave sterilization is measured moment by moment, and this temperature is 100°C or more, preferably 105 to 130°C,
In addition, by calculating the correlation between temperature and time as the integral value (F) of the above formula, and continuing microwave irradiation until this integral value (F) exceeds a certain value, it can be stored for a long time. Moreover, it becomes possible to produce heat-sterilized packaged foods of good quality. Calculation of this integral value (F) can be automatically performed by inputting the signal of the time after the start of microwave irradiation and the temperature detection signal at this time into a computer and performing the calculation. In the above formula, the temperature-dependent value of the microorganism is the so-called Z value, which can vary within the range from 7 in the case of Bacillus coagulans to 14 in the case of Bacillus subtilis. In the present invention, it is desirable that the sealed package after the microwave irradiation is forcibly cooled in order to prevent quality deterioration due to so-called overcooking of the contents. This cooling can be performed using, for example, a cooling medium such as cold air or cold water, and can be performed under pressure as in the case of sterilization in order to prevent damage to the packaging container due to internal pressure. Generally, it is preferable to cool the product to a temperature of 50°C or less within a period of 4 to 10 minutes. It is immediately clear from the following experimental examples that according to the present invention, the temperature of a sealed package under microwave irradiation can be accurately measured and a sterilized package with long shelf life can be obtained. Let's become. That is, a transparent plastic bag-like container contains glycine and glucose each at a concentration of 1/15 molar.
A package was prepared by filling with 7.7% flour solution and sealing. In addition, some of these sealed packages include:
A commercially available thermolabel for temperature indication was attached. Furthermore,
When preparing the above-mentioned sealed package, a phosphor temperature sensor and a chromel-alumel (CA) thermocouple were attached in the positional relationship shown in FIG. 2 to prepare a sampling package. The four types of sealed packages thus prepared were irradiated with 2450 MHz microwaves under air pressure, and after 4 minutes had elapsed, they were cooled under air pressure and the product temperature was measured. The results are shown in Table 1.

【table】

[Table] The method using the present invention and the C-A thermocouple can quantitatively show the temperature-time relationship compared to the other two methods, but the method using the C-A thermocouple has the same content. This shows that the temperature rises faster than that measured using the method of the present invention, even though the temperature was measured using the method of the present invention. Therefore, we determined the conditions under which the integral value (F) was 3.2 from the heating and cooling curves obtained from these two methods, and when we processed 50 bags under these conditions and conducted a storage test, we found that C
-Many changes were observed when temperature was measured using A thermocouple. In order to understand the reason for this, we examined the temperature curve using a C-A thermocouple and found that, as shown in Figure 3, when using a thermocouple, electricity to the magnetron was stopped and microwave oscillation was stopped. It was found that the thermoelectromotive force suddenly dropped instantly. This falling phenomenon occurs because the electric field concentrates at the tip of the thermocouple, causing the tip to become extremely heated.The thermoelectromotive force after the falling phenomenon is considered to be the correct temperature; It was discovered that the temperature of the product could not be measured. On the other hand, in the present invention, it has become clear that the temperature rise and fall are stable regardless of whether or not the magnetron is energized, and can be measured extremely accurately. Similarly, the results of product temperature measurements obtained using other measurement methods are shown, but the measurement method using thermolabels and glycine-glucose solutions does not cause discoloration or browning unless the temperature reaches a certain level, and when cooled, It can be seen that while it is not clear what state the temperature-time relationship is in, it is clear that the method of the present invention can accurately grasp each point. Table 2 shows the results of the sterilization state and finished quality when the product temperature at the center of the package was measured using the method of the present invention, and when microwave treatment was performed with the same degree of sterilization using other methods. As a result, it can be seen that the packaged food whose temperature was measured by the method of the present invention has been sterilized well and is of good quality because it has not undergone excessive thermal history. However, the quality of packaged foods deteriorates due to deterioration and excessive heating, and the method of the present invention is extremely effective in sterilizing packaged foods at temperatures of 100°C or higher using microwave heating as the main heat source. It shows.

[Table] (f) Number of lost bags
(g) Example of number of test bags 1 Minced meat 200g, onion 50g, raw breadcrumbs 20g
Mix well and make a 14mm thick raw hamburger steak.
Fry in a frying pan until both sides are golden brown.
Heat-resistant spores of Clostridium and Sporogenus NCA-pA-3679 are added to this hamburger.
It was inoculated into the center of the hamburger using a syringe at a concentration of 10 ³ /g. This inoculated hamburger steak was filled into a transparent retort pouch (manufactured by Toyo Seikan Co., Ltd., polyester x polypropylene, 130 x 170 mm), and the batch air pressurized microwave sterilizer (manufactured by Toyo Seikan Co., Ltd., model
H40-C-40M/W) Output 800W, frequency 2450M
Hz, steam-hot water combination type), air pressurization 1.5
Microwave heating was carried out under Kg/cm ² pressure. For temperature measurements, a 1000A thermometer manufactured by Luxtron in the United States was used. The sensor part was loaded into the pouch according to the method shown in Figure 2. Microwave irradiation for 5 minutes, then put into the refrigerant for 4 minutes.
When immersed for a minute, a good temperature-time curve was obtained with a maximum temperature of 125°C. When the integral value (F) was calculated based on this curve, F=3.2 was obtained.
Twenty bags of spore-inoculated samples were kept in a constant temperature room at 50°C for two months, but none of them deteriorated.

[Table] An example of calculating the F value in this example is shown in Table 3 below. Example 2 Bacillus subtilis spores (Fo = ¹⁰³ /g) were added to a potato salad prepared by mixing 20% potato dice, 50% potato mash, 10% onion, 12% carrot, and 8% corn. 3.0) was added. 100g of this spore-inoculated salad was placed in a transparent molded container (manufactured by Toyo Seikan Co., Ltd., trade name: Lamicon Cup, polypropylene x EVAL x polypropylene).
The container was filled and a lid of the same configuration was heat-sealed and sealed. For temperature measurement, we used a 1000A thermometer made by Luxtron in the United States.
I used a mold. The sensor part was fixed to the container using Teflon gland packing. Example 1
Using the same air-pressurized microwave heating device as used in , heating was performed under 2.5 Kg/cm ² pressure for 5.5 minutes, followed by cooling with cooling water for 5 minutes. The temperature measurement yielded an accurate mountain-shaped temperature-time curve with the peak reached at 126°C. When the integral value F was calculated from this temperature-time curve, it was found to be F=3.1. Seventeen bags of microwave-heated samples were kept in a constant temperature room at 37°C for two months, but none of them deteriorated. Example 3 170 g of powdered soup (corn soup made by Ajinomoto) dissolved in a predetermined amount of water was inoculated with Clostridium and Sporogenus spores at a concentration of 10 ³ /g (Fo = 2.8). A paper cup (manufactured by Tokan Kogyo Co., Ltd., trade name: PC Cup) laminated with polyethylene was filled, and a lid of the same composition was heat-sealed. The temperature measurement method was carried out in accordance with Example 2. Using the air pressurized microwave heating device described above, the product was irradiated with microwaves for 4 minutes and then cooled with cold air at -5°C for 10 minutes.The measured product temperature was extremely good, and this condition was F = 2.9. . Heat sterilized packaged food
When 25 pieces were stored at 37°C for 1.5 months, none of them deteriorated.

[Brief explanation of drawings]

Figure 1 is a sectional view showing how the temperature sensor is attached to a rigid container, Figure 2 is a sectional view showing how the temperature sensor is attached to a flexible container, and Figure 3 is a diagram showing the temperature detected using a thermocouple and microwaves. FIG. 3 is a diagram showing the relationship with irradiation time. 1, 1a is a container, 2 is a packing ground, 3 is a probe, 4 is a fluorescent temperature sensor, 5
indicate probe holders, respectively.

Claims (1)

[Claims] 1. Irradiating microwaves to a sealed package formed by filling an article to be sterilized in a container that is transparent to microwaves, and determining the temperature of the sealed package under microwave irradiation for a period of time. The process is measured using a phosphor temperature sensor based on the temperature dependence of the intensity in the fluorescence wavelength region of the phosphor, and the temperature measured by the phosphor sensor is 100°C or higher, and the formula F= ∫ ^t _p log ^-1 (T-121.1/Z) dt ...(1) In the formula, log ^-1 means antilog, T represents the temperature (℃) measured by the phosphor sensor, and t It represents the time (minutes) after the start of microwave irradiation, and Z is the temperature dependence value of microorganisms, the so-called Z value (℃). A method for sterilizing a sealed package, characterized by continuing irradiation with. 2 Connect the phosphor temperature sensor attached to the sealed body to the ultraviolet light source and the fluorescence detection mechanism with probes made of optical fibers, irradiate the phosphor with ultraviolet light through the probe, and measure the intensity of the generated fluorescence through the probe. 2. The method according to claim 1, wherein the temperature is measured by detecting the temperature using a fluorescence detection mechanism.