JPH0118743B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for effectively sterilizing packaging materials made of thermoplastic resin films with radiation while suppressing the odor generated from the packaging materials to an almost non-problematic level. Methods for sterilizing packaging materials include chemical sterilization methods using hydrogen peroxide, ethylene oxide gas, and the like, as well as physical sterilization methods that involve irradiation with ionizing radiation such as ultraviolet rays, gamma rays, and electron beams. Although these physical sterilization methods do not have the problem of chemical sterilization methods in which the sterilizer remains and impairs safety, they are not without problems. For example, ultraviolet sterilization has the disadvantage that it has weak sterilizing power and can only sterilize the surface of packaging materials. On the other hand, sterilization using ionizing radiation (hereinafter simply referred to as radiation) such as gamma rays and electron beams has the problem of requiring relatively large irradiation equipment, but it has a large sterilizing power and is suitable for food packaging materials. It is also expected to be used as a sterilization method. However, this radiation sterilization method has a problem in that the packaging material itself deteriorates due to the use of high-energy radiation. Particularly when considering the use of thermoplastic resin films as food packaging materials, odor generation becomes more of a problem than a decrease in strength. This odor is thought to be caused by crosslinking and decomposition of the thermoplastic resin due to radiation irradiation, and in particular by low boiling point substances produced by the decomposition. This tendency is particularly strong when polyolefin resins are used. In order to prevent the generation of decomposition odor when polyolefin resin packaging materials are irradiated,
Methods have been devised to provide irradiation in the absence of oxygen by sealing the packaging material with an oxygen scavenger. However, even with this method, when a composite material with a resin having oxygen barrier properties is subjected to radiation sterilization in a rolled state, oxygen cannot be sufficiently removed and the generation of decomposition odor cannot be prevented. In view of the above-mentioned circumstances, the present invention provides a method for radiation sterilization of packaging materials made of thermoplastic resin films without generating almost any decomposition odor by controlling the irradiation dose depending on the material. The purpose is to According to research conducted by the present inventors, it has been found that the decomposition odor caused by radiation sterilization has an important relationship with the amount of low molecular weight oxides produced during radiation irradiation, particularly carboxylic acids with a boiling point of 200°C or less. In other words, when a packaging material made of a thermoplastic resin film is irradiated with ionizing radiation, the amount of low molecular weight carboxylic acid generated is 0.0025% of the weight of the thermoplastic resin film.
If the amount is below, the decomposition odor will hardly be a problem, but if it exceeds this, the decomposition odor will be felt. Therefore, depending on the type of thermoplastic resin film constituting the packaging material, the relationship between the irradiation dose and the amount of low molecular weight carboxylic acid generated should be understood in advance, and the amount of low molecular weight carboxylic acid generated should be determined in advance. By irradiating radiation while suppressing the irradiation dose to a range below the limit amount, radiation sterilization can be performed without substantially generating decomposition odor. The radiation sterilization method of the present invention is based on such knowledge, and more specifically, when a packaging material made of a thermoplastic resin film is sterilized by irradiating it with ionizing radiation, the boiling point generated from the film is It is characterized in that ionizing radiation is irradiated at a dose such that the total weight of carboxylic acids at 200° C. or less is 0.0025% or less of the weight of the thermoplastic resin film. The above condition of 0.0025% or less of the amount of carboxylic acid needs to be satisfied for each individual film when a plurality of thermoplastic resin films are used to constitute one packaging material (see Example 4 below). The present invention will be explained in more detail below. The thermoplastic resin constituting the packaging material film used in the present invention is not particularly limited.
For example, polyolefin resin, polyamide resin, polyester resin, etc. can be used.
Among these, polyamide resins, polyester resins, and the like are preferable thermoplastic resins from the viewpoint of less generation of carboxylic acid due to radiation irradiation and less decomposition odor. However, according to the present invention, the most attractive thermoplastic resins for use include polyolefins, typically polyolefins, copolymers of olefins, and other copolymerizable monomers containing olefins as a main component. It is a polyolefin resin selected from copolymers with and mixtures with other thermoplastic resins containing these as main components. This is because these olefin resins have excellent properties such as heat sealability, thin film formability, thin film strength, and transparency required for packaging materials. This is because it is often used as a laminated film with other thermoplastic resins such as. Although these polyolefin resins have desirable properties as packaging materials as described above, they tend to generate a decomposition odor when irradiated with radiation. Therefore, the present invention has great significance in controlling the irradiation dose and suppressing the generation of decomposition odor. Specific examples of polyolefin resins include high-pressure polyethylene, medium-low pressure polyethylene, polyethylene ionomer, polypropylene, ethylene-butene-1 copolymer, and ethylene-vinyl acetate copolymer. For these thermoplastic resin films, the amount of carboxylic acids with a molecular weight of 200 or less generated by radiation irradiation can be determined by any method that can capture and quantify the low molecular weight carboxylic acids generated. As shown in Examples and Comparative Examples, a combination of capture by deep cooling with liquid nitrogen and quantification by gas chromatography is a suitable example. According to the present invention, the method of irradiating the packaging material made of the above-mentioned thermoplastic resin film with radiation is basically not particularly limited. The packaging material can be irradiated in one piece or in a stack, filled or unfilled. Furthermore, the packaging material during irradiation can be placed in an inert atmosphere, but it is usually preferable to keep it in an air atmosphere and control the amount of carboxylic acid generated by adjusting the irradiation dose because it is convenient. As the radiation to be irradiated, high-energy rays generally known as ionizing radiation are used, and specifically gamma rays and electron beams are preferably used, but X-rays and the like may also be used. As for the irradiation method, in addition to point beam irradiation, linear or curtain-like irradiation methods are also used. According to the present invention, the radiation dose is controlled in accordance with the amount of low molecular weight carboxylic acid generated from the thermoplastic resin film. Therefore, if the amount of primary bacteria adhering to the thermoplastic resin film is excessive, the sterilization effect may not be achieved to the desired extent by radiation irradiation. In such cases, the preparation and storage conditions of the packaging material consisting of thermoplastic resin film and the contents should be controlled;
Alternatively, preliminary sterilization may be performed using a relatively weak sterilization method such as heat sterilization or UV sterilization to suppress the initial amount of bacteria below a certain level, and finally radiation sterilization may be performed. Hereinafter, the present invention will be explained in more detail with reference to Examples and Comparative Examples. Example 1 20g of high-pressure polyethylene (referred to as high-pressure polyethylene A) film (thickness: 40μ) with a volume of 3
Place it in a glass container and seal it with air at normal pressure.
A dose of 2.0 Mrad of gamma rays was irradiated to kill ¹⁰⁶ Bacillus subtilis bacteria. Due to this γ-ray irradiation, the glass sealed container was filled with odor substances generated from the high-pressure polyethylene. Next, in order to capture the entire amount of odorous substances produced, this glass sealed container was immersed in a water bath at about 100°C to volatilize the entire amount of odorous substances from the high-pressure polyethylene film. Nitrogen gas was introduced at a rate of 30 c.c./min over a period of 48 hours, and the exhaust gas was introduced into a U-tube deep cooled with liquid nitrogen. As a result, odorants were captured in liquid or solid state. The captured odorous substances were then introduced into a gas chromatography mass spectrometer (Hitachi GC-MS apparatus M-80) equipped with a Ucon oil column, and the odorous substances were identified. As a result, these odorous substances contain a large number of saturated and unsaturated hydrocarbons, with particularly characteristic and strong odor substances such as acetic acid, propionic acid, butyric acid, and valeric acid with boiling points below 200°C. Carboxylic acid was found. On the other hand, by similar gas chromatography operation,
The amount of carboxylic acid with a boiling point of 200°C or less was determined. Furthermore, repeat the above operation and increase the γ-ray irradiation dose to 0.5, 1.0,
1.5, 2.0, and 2.5 Mrad, and the series of operations was repeated for different types of high-pressure polyethylene B, ethylene-vinyl acetate copolymer C, polyamide, and polyester. The results are summarized in Table 1 below. On the other hand, a sensory test was conducted using a similar thermoplastic resin film. That is, 20g of the same type of thermoplastic resin film as in the above test was placed in a wide-mouth bottle (capacity:
200 ml) with air to deliver a predetermined dose of γ.
After irradiation, the package was opened and its odor was determined.
The results are also shown in Table 1 below.

[Table] × is an example of a product judged to have a strong odor 2 80μ film of low density polyethylene (Mirason 16P, manufactured by Mitsui Polychemical Co., Ltd.) and ethylene-vinyl acetate copolymer (Mirason ACE-30N, manufactured by Mitsui Polychemical Co., Ltd.) ) 80μ film is stacked on top of each other and sealed on all sides to make a bag called a back-in-box (size: 470m/m x 470m/m, capacity: 10
) was created. Separately, add 1.0 Mrad γ to the above low density polyethylene.
When irradiated with radiation, the total weight of carboxylic acids with a boiling point below 200°C was 0.0015% of the film weight. On the other hand, the total amount of carboxylic acid generated in the ethylene-vinyl acetate copolymer at 1.0 Mrad is 0.0016%. Therefore, the bag obtained above was irradiated with gamma rays at 1.0 Mrad,
Immediately use a dedicated aseptic filling machine (DN-AB filling machine,
Filled with 10 sterilized milk (manufactured by Dai Nippon Printing)
When stored at room temperature for 10 days and then subjected to a sensory test, no abnormal odor was detected. On the other hand, each bag contained 10 to 40 bacteria.
This bacterium can be sufficiently sterilized by 1.0 Mrad of γ-ray irradiation. Example 3 A bag (generally called a back-in-box inner bag) made by overlapping 80 μm low-density polyethylene (Mirason 16P manufactured by Mitsui Polychemicals) and 80 μm low-density polyethylene (LK30 manufactured by Mitsubishi Yuka) films and sealing them on all sides. The size is 470m/m
x470m/m capacity10). Separately, the above-mentioned low-density polyethylene was irradiated at 1.5 Mrad to determine the total amount of carboxylic acids with a boiling point of 200° C. or lower, and it was found to be 0.0019% for the former and 0.0023% for the latter. Therefore, this bag was irradiated with gamma rays at 1.5 Mrad,
When the container was filled in the same manner as in Example 2 and a sensory test was performed after storage, no abnormal odor was detected. Example 4 (Comparative Example) A similar back-in-box inner bag was made using the same resin film as in Example 3. Separately, 2.0 Mrad for the low density polyethylene used.
When the total amount of carboxylic acids with a boiling point of 200°C or less was measured after irradiation, values of 0.0025% and 0.003% were obtained. This bag was irradiated with γ-rays at 2.0 Mrad and filled in the same manner as in Example 1, and when a sensory test was conducted after storage, unlike in Example 3, an off-flavor was clearly felt from the milk inside.

Claims (1)

[Claims] 1. When sterilizing a packaging material made of a thermoplastic resin film by irradiating it with ionizing radiation,
The total weight of carboxylic acids with a boiling point of 200°C or less generated from the film is greater than the weight of the thermoplastic resin film.
A radiation sterilization method characterized by irradiating ionizing radiation at a dose of 0.0025% or less. 2. The method of item 1 above, wherein the thermoplastic resin film is a polyolefin resin film. 3. The method of item 1 or 2 above, wherein the packaging material is pre-sterilized by a separate method and then irradiated with ionizing radiation.