JP2897775B2 - Sterilization method of experimental animal feed by high energy electron beam irradiation - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to a method for sterilizing feed for clean laboratory animals by irradiation with high energy electron beams.

[Background Art] Clean animals (including sterile animals) such as mice and rats intended for use in animal experiments are sterilized not only in the breeding environment but also in the feed for breeding (experimental animal feed). Alternatively, sterilization has been conventionally performed due to environmental restrictions of breeding in a clean state. The methods used to sterilize feeds include high-pressure steam sterilization, radiation sterilization using gamma rays from cobalt-60 (hereinafter referred to as gamma-ray sterilization), and gas sterilization using ethylene oxide gas. Although it is a sterilization method, the gas sterilization method is practically hardly used at present because of its high carcinogenicity due to residual gas.Sterilization treatment by high-pressure steam sterilization or gamma-ray sterilization is performed. Have been done.

In the case of the high-pressure steam sterilization method, in the sterilization step, first, a laboratory animal feed contained in a container such as a bag or a can is placed in an autoclave. Next, after the inside of the autoclave is evacuated, steam is supplied and the temperature is maintained at 121 ° C. for 20 minutes. Thereafter, exhaust steam and vacuum evacuation are performed, and finally, hot air supply and drying are performed to complete the sterilization process.

On the other hand, in gamma ray sterilization, sterilization is performed according to the following procedure. The experimental animal feed previously packed in a plastic bag or can is placed in a cardboard box of a fixed size, and the cardboard box is further placed in a carton case. Next, the case is placed on a conveyor and moved to a cobalt-60 irradiation room, and a predetermined amount of gamma rays are irradiated in the irradiation room to sterilize the case. Gamma ray sterilization conditions for a cobalt-60 irradiation facility currently operating for sterilization on a commercial scale are as follows: The dose required for sterilization is 10 KGy to 50 KGy, and irradiation is performed at room temperature and in air at room temperature. The irradiation time (sterilization time) required for one carton case in a normal commercial scale facility (500,000 Ci) is about 2.0 hours for 10KGy irradiation and about 10 hours for 50KGy irradiation.

The outline of both the high-pressure steam sterilization method and the gamma-ray sterilization method, which are the current sterilization methods, has been described above. Each method has advantages and disadvantages.

The high-pressure steam sterilization method is advantageous in that sterilization costs can be reduced from the economic point of view, and that the equipment is simple and easy to maintain and manage. It is disadvantageous in that it becomes harder or more brittle), the vitamin components in the feed deteriorate, and the preference for sterile feed decreases. Further, after sterilization, moisture due to steam remains in the feed, which shortens the storage period of the feed. On the other hand, in the gamma ray sterilization method, contrary to the former sterilization method, there is almost no change in physical properties of the feed after sterilization, the deterioration of vitamin components is small, and sterilization does not affect the storage period of the feed, etc. However, on the other hand, there are disadvantages such as high sterilization cost and difficulty in maintaining and managing a gamma irradiation facility.

As described above, gamma ray sterilization is superior to high-pressure steam sterilization in terms of changes in the physical properties of feed, deterioration of vitamin components, and palatability due to sterilization, but this is quantitatively determined by experiments. Is shown in According to the experimental report by Kuwahara et al. (The 31st Annual Meeting of the Society of Experimental Animal Science, 1984), in physical property changes, the hardness of feed changed by up to several tens of percent before and after autoclaving, whereas before and after irradiation in gamma sterilization. It is clear that there is almost no change in hardness. Also, regarding vitamin degradation, especially vitamin
Degradation of B ₁ is while hardly with gamma sterilization, significantly deteriorates in the high-pressure steam sterilization, it has been found that more than half as compared with the untreated deteriorates. In addition, it has been found that the palatability is 2-3 times higher for gamma-ray sterilized breeding than for steam-sterilized feed. However, gamma ray sterilization, which is an excellent sterilization method, is a drawback of this sterilization method as described above in terms of sterilization processing capacity, maintenance and management of gamma ray irradiation equipment, sterilization cost, etc., and these are inferior to high-pressure steam sterilization method. It is.

As shown the current irradiation conditions in gamma sterilization,
Since irradiation of one carton case requires 2.0 hours or more, the gamma-ray sterilization method significantly reduces the productivity of the sterilization process, and the sterilization cost is necessarily high. Further, since the half-life of cobalt-60 is about 5.3 years, the activity of cobalt-60 decreases with time, that is, the amount of gamma irradiation per unit time decreases, and the irradiation time (sterilization time) increases. As the time increases, the sterilization capacity is increasingly reduced, further increasing the cost of sterilization. For example, in the case of a 500,000 Ci commercial irradiation facility, the irradiation time required for 10 kg for 50KGy irradiation at the time of completion of the facility will be doubled to 20 hours after about 5 years, and the productivity of sterilization will elapse over time. Together with it. However, in commercial irradiation facilities, in order to keep the irradiation time (sterilization processing time) as constant as possible, cobalt-60 is normally replenished regularly. At present, such replenishment of such a radiation source is also a problem. Since the gamma-ray source Cobalt-60 is not produced in Japan, all irradiation facilities in Japan rely on imports from the Canadian Atomic Energy Authority (AECL), resulting in supply instability and safety during transportation. In addition to securing a large amount of radiation, the cost of the radiation source itself is high, and the maintenance cost of the irradiation facility is high, and the sterilization processing capacity is low. .

Despite being an excellent sterilization method, gamma ray sterilization is not so widespread due to the problems described so far, and currently only accounts for 5 to 6% of the total sterilized feed. As a proposal, a sterilization method by electron beam irradiation is shown,
No specific irradiation conditions are presented, and no special features are disclosed, so that the irradiation conditions, productivity, etc. are considered to be comparable to those of gamma-ray sterilization, and its practical use is not yet known. .

[Problems to be Solved by the Invention] At present, with the development of superior new drugs or the establishment of advanced technologies in the field of biotechnology constantly progressing, the requirements for the quality of experimental animals will become increasingly severe in the future. That is easy to guess. To meet such demands, breeding in a clean environment, including aseptic conditions that are more controlled than ever, is necessary.
The challenge is to establish a new sterilization method that does not change the physical properties due to sterilization and does not deteriorate the components, and that can fully meet this requirement, and that is excellent in productivity and easy to maintain. is there.

[Means for Solving the Problems] As a novel sterilization method having the advantages of gamma ray sterilization from cobalt-60 and the advantages of high-pressure steam sterilization, the present inventors have used electron beams. We have found electron beam sterilization. Especially in the present invention, in order to obtain productivity as in the high-pressure steam sterilization method, the electron beam exceeds 5 MeV and exceeds 10 MeV.
It is characterized by utilizing high-energy electron beams up to V for sterilization. Conventionally, low-energy electron beams of less than 1 MeV or medium-energy electron beams of up to 5 MeV have been used to improve plastics and rubber products, or to sterilize medical devices. The present invention provides unprecedented knowledge and techniques regarding sterilization of experimental animal feed, because there was no target use case and the effect of high-energy electron beams on vitamins, etc. was not clear. Things.

An electron beam is one of radiations similar to gamma rays in terms of chemical and physical effects in terms of radiation effects on substances, but gamma rays are radiations that differ greatly in terms of penetrative power to an irradiated object. The difference is that radioisotopes are the source of gamma rays, whereas electron beams are radiation that is generated electrically by an electron accelerator.
Therefore, it can be said that the electron beam irradiation facility does not require replenishment of the radiation source as in the gamma ray irradiation facility, and operates the accelerator only at the time of irradiation, thereby facilitating the maintenance and management of the facility.
Furthermore, while gamma rays from cobalt-60 are radiation with a constant energy of 1.25 MeV, electron beams can freely control the magnitude of energy and current by the performance of the electron accelerator, which means that productivity can be controlled. It becomes possible to set irradiation conditions according to an object to be irradiated.
In addition, the use efficiency of gamma rays is low because the generated gamma rays irradiate the entire space, whereas the use efficiency of electron beams is high because electrons are emitted only to the target object. Productivity grows. The present inventors have focused on such characteristics of the electron beam and have found a method of sterilizing experimental animal feed using the electron beam.

The electron beam sterilization method by the present inventors is a method in which an object to be irradiated is placed on a conveyor, carried into an electron beam irradiation chamber, and sterilized by electron beam irradiation, similar to the gamma ray sterilization method. Since the average specific gravity of the experimental animal feed is in the range of 0.5 to 0.7, in consideration of economy, the sterilization method assumes that the irradiation electron energy is in the range of 5 MeV to 10 MeV, preferably 8 MeV to 10 MeV. There is a feature.

Here, the maximum value of the energy is set to 10 MeV due to the restriction on activation by an electron beam, and the maximum value is set to 10 MeV due to the danger that activation may occur if the electron beam is irradiated with more energy. The magnitude of the energy of the irradiation electrons is proportional to the transmission distance in the object for the object having the same specific gravity. FIG. 1 shows the relationship between the energy of the electron beam and the transmission distance of the electron beam in the irradiated sample having an average specific gravity of 1.0. The transmission distance of an electron beam into a sample having a specific gravity d = the transmission distance (cm) of an electron beam in an irradiated sample having an average specific gravity of 1.0 ÷ the specific gravity d. For electrons with 5 MeV energy, the transmission distance is about 2.2 to 3.1 cm (average specific gravity 0.7 to
For irradiated electrons having an energy of 10 MeV, it is about 4.6 to 6.5 cm (corresponding to an average specific gravity of 0.7 to 0.5). Therefore, in the electron beam sterilization according to the present invention, assuming that the front and back surfaces of a cardboard box packed with laboratory animal samples (average specific gravity: 0.5) are sequentially irradiated, the thickness that can be sterilized is larger than 8 cm from FIG. It becomes 16cm.

Next, regarding the current value of the irradiation electrons, the inventors have at most
It is limited to 10 mA, preferably 1 mA to 5 mA. The higher the current value of the irradiation electrons, the higher the irradiation dose per unit time and the higher the productivity.However, for an electron beam having an energy exceeding 5 MeV, which is a feature of the present invention, a current value larger than 10 mA is used. Giving electrons is more difficult than the acceleration principle of an electron accelerator that produces electrons, and it takes 1 mA to produce a stable electron beam in the above energy range.
A current value of 5 mA or more and 5 mA or less is preferable. About 10 minutes or less is sufficient for the irradiation time of the electron beam as described above.

Assuming that a commercial electron accelerator capable of irradiating 10 MeV irradiation electrons with a current value of 1 mA is assumed from the range of irradiation electron energy and current value that is preferable in the present invention, specific gravity is 0.
For irradiated object 5, the processing amount per hour becomes 0.97 m ³ in 4.85M ³ or 50KGy irradiation at 10KGy irradiation. Therefore, the amount of processing is found to be 500,000 Ci cobalt-ray source sterilization 0.2 m ^3, with about 25 times extremely productive when compared to 0.04 m ³ at 50KGy at 10KGy gamma sterilization. Taking into account the fact that the cost of an electron beam sterilization facility is less than that of a gamma ray sterilization facility of equivalent capacity, the cost of the electron beam sterilization according to the present invention will be significantly lower than that of the gamma sterilization.

The embodiments of the present invention will be described in the following order. In the embodiments, all the electron beams extracted from the linear accelerator are used.
There are currently two types of electron accelerators, one of which is an electrostatic (DC) accelerator and the other is a linear (AC) type. The former is difficult in principle to produce high-energy electron beams.
Accelerators that accelerate electrons up to eV energy have been manufactured, but accelerators that accelerate electrons with energies exceeding that have not been realized. On the other hand, in the case of a linear accelerator, as described above, although the magnitude of the current value is limited by the acceleration principle, the electron energy can be accelerated infinitely in principle, and to the desired energy level It can accelerate electrons. Therefore, the present inventors use a linear accelerator in the embodiment because an electron beam having an energy exceeding 5 MeV is required. Actually 10MeV, 50μ
Irradiation is performed using the electron beam of A, and after irradiation, the sterilization effect, vitamin deterioration, and change in physical properties are evaluated. The evaluation results show that the sterilization method according to the present invention is satisfactory in all aspects of sterilization effect, vitamin deterioration, and changes in physical properties, as specifically shown in Examples, and these points are equivalent to the gamma-ray sterilization method. It has been confirmed that the sterilization method has advantages. Efficiency of productivity is improved and maintenance is easy by the electron beam which is 8 times the energy of the gamma ray and the irradiation dose rate is 100 to 1000 times, and is easy to maintain. The fact that the same sterilization effect as that of gamma rays was obtained is a novel finding found by the present invention. The present invention is based on this new finding.

Example 1 Six polyethylene bags (38 × 27.5 cm) each having a thickness of 90 μm were prepared, and a solid feed (wheat, bran, corn, soybean meal, white fish meal, beer) of 14 mm × average length 18 mm for mice and rats was prepared for each. After adding yeast, alfalfa meal, mineral mixture, and vitamin mixture), seal the mouth of the bag with a policyr and arrange the feed in the bag so that the feed is lined up flatly. (30 × 35 × 10 cm), piled up in six layers, covered with a cardboard cover, and subjected to electron beam sterilization.

FIG. 2 shows an indicator with indicator bacteria and a radiochromic film for dose measurement wrapped with aluminum foil on the surface of each stage bag to evaluate the sterilization state and irradiation dose after sterilization. So attached.

The electron accelerator used was a linear accelerator, the irradiation electron energy was set to 10 MeV, and the current value was set to 50 μA. The conveyor speed was set to 10 cm / min since one side of the cardboard box was irradiated with 30 KGy from above, the cardboard box was placed on the conveyor, and electron beam irradiation was performed. Irradiation is about 4
Minutes, and after that, sterilization, evaluation of dose distribution, examination of deterioration of vitamin components, and the effects of irradiation of general components in feed were also evaluated.

Table 1 shows the evaluation results of the average absorbed dose at each measurement point and the survival rate of the indicator bacteria. As can be seen from Table 1, the absorbed dose is 20 KGy or more up to the fourth row from the top, whereas 5
It turned out that it was less than 10KGy at the first stage. In addition, it was found that the survival of the indicator bacterium was not observed up to the fourth stage from the top in correspondence with this result. Therefore, in the case of single-sided irradiation,
It was confirmed that the thickness of the cardboard box was sterilizable up to about 6 cm. Next, Table 2 shows the results of the sterility test of the feed itself. As a result of conducting the test with 20 pellets, it was found that most of the pellets were positive in the fifth and sixth stages. Therefore, since this result corresponds to the evaluation result obtained in the evaluation of the indicator bacterium, it was clarified that the sterilization effect on the feed could be confirmed only by the evaluation result of the indicator bacterium. Table 3 shows the evaluation of the deterioration of the vitamin components in the feed. The degradation of vitamins up to the fourth stage, where the absorbed dose is 20 KGy or more, was small in each vitamin component, as in the case of gamma-ray sterilization, and was found to be a sterilization method superior to high-pressure steam sterilization. In addition, as shown in Table 4, the general components in the feed were hardly affected by electron beam irradiation as shown in Table 4, and it was found that electron beam sterilization was a desirable method.

Example 2 A feed was packed in a cardboard box in the same manner as in Example 1, and both sides of the cardboard box were irradiated at an irradiation dose of 30 KGy. The same electron beam accelerator as in Example 1 was used, and the setting conditions for energy and current were the same as in Example 1.
In addition, after irradiating one side of the cardboard box on both sides,
Once the beam was stopped and inverted, the other side was irradiated. Tables 5 and 6 show the absorbed dose, the survival rate of the indicator bacteria, and the evaluation results of the sterility test. As can be seen from Table 5,
The two-sided irradiation showed that the survival of the indicator bacteria became zero at all measurement points. Therefore, it was confirmed that there was no problem in terms of transparency when the thickness was about 10 cm in the case of double-sided irradiation with a 10 MeV electron beam.

Example 3 A feed was packed in a cardboard box in the same manner as in Example 1, and one side of the cardboard box was irradiated with an irradiation dose of 20 KGy. The same electron beam accelerator as in Example 1 was used, and the same energy and current values were set. However, irradiation was performed at a conveyor speed of 15 cm / min. Table 7 shows the evaluation results of the absorbed dose and the survival rate of the indicator bacteria. Similar to the results obtained in Examples 1 and 2, the sterilization effect was sufficiently confirmed even at an irradiation dose of 20 KGy. However, because of single-sided irradiation, the residual ratio of indicator bacteria is zero as in Example 1 at a depth of 6 cm from the surface with respect to the depth. Table 8 shows the deterioration evaluation of the vitamin components. Also a matter of course, the deterioration of the vitamin B ₁ in the irradiation of 20KGy it was found that almost does not occur.

Example 4 After filling the entire box in a bare state without feeding the feed into a cardboard box of the same size as that of Example 1, a 1-cm-wide rectangular cardboard with 9 pieces of radiochromic film attached thereto was used as shown in FIG. 20KGy under the same conditions as in Example 3
Was irradiated on one side, and the deep dose distribution was measured. FIG. 4 shows the measurement results. According to this, since the absorbed dose is 10 KGy or more up to a depth of 8 cm, it can be understood that the absorbed dose is 20 KGy in the entire region up to a thickness of 16 cm assuming double-sided irradiation. With this absorbed dose of 20 KGy, it has been found that there is no problem with the sterilizing effect as shown in Example 3. Therefore, in consideration of the fact that the feed is actually packed loose as in this example, in the case of the experimental animal feed, 16 cm is irradiated by the electron beam irradiation with the energy of 10 MeV.
It was confirmed that sterilization was possible up to the thickness.

[Brief description of the drawings]

FIG. 1 shows the relationship between the energy of an electron beam, the electron beam in an irradiated object having an average specific gravity of 1.0, and the transmission distance. Fig. 2 shows a 6-tiered feed wrapped with aluminum foil (a piece of paper and radiochromic film with indicator bacteria attached) at positions A, B and C on the surface of each feed for electron beam sterilization. This shows the state in which it is performed. Fig. 3 shows the deep dose distribution obtained by inserting a box of 1 cm wide rectangular paper with nine pieces of radiochromic film into a cardboard box, after packing the box in a bulk state without putting the feed in a bag. This shows the state for measuring. FIG. 4 shows the measurement results of the deep dose distribution of FIG.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Masakazu Furuta, Inventor 5-5--24-206, Toyosaki, Kita-ku, Osaka-shi, Osaka (72) Inventor Kazushige Otohata 3-34-5, Chuo, Musashimurayama-shi, Tokyo (72) Inventor: Tomio Suwa, 471-4, Kawagata-shi, Saitama Prefecture (56) Reference: JP-A-63-189152 (JP, A) JP-A-63-65865 (JP, A) JP-A 60-24847 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) A61L 2/08-2/14 A23K 1/00-1/24 A23L 3/00 A23L 3/26-3/30

Claims (2)

(57) [Claims] 1. The energy of irradiated electrons is 5 to 10 Me per cubic centimeter of feed for clean laboratory animals.
V, a sterilization method that improves productivity by irradiating a high-energy electron beam with a current value of 0.03 to 10 mA for a short time. 2. Irradiation electron energy: 8 to 10 MeV, current value: 1
The sterilization method according to claim 1, wherein the high-energy electron beam of 5 mA is irradiated for a short time within 10 minutes.