JP6764167B2 - How to inactivate norovirus in oysters - Google Patents

Description

The present invention relates to a method for inactivating norovirus contained in oyster shell meat.

Oysters are often eaten raw, preferring their unique flavor and texture, not only in Japan but also overseas. In recent years, many raw oysters (raw oyster shellfish) have been offered at restaurants such as oyster bars. In addition, you can easily buy fresh raw oysters by mail order on the Internet, and you can easily enjoy raw oysters at home. However, oyster shells can contain human norovirus, and some consumers are worried about food poisoning from eating raw oysters.

Among the noroviruses classified into the genus Norovirus of the Caliciviridae family, those that infect humans are human noroviruses. The infection route of human norovirus is when shellfish such as raw oysters contaminated with human norovirus are ingested without sufficient heating, or when a person infected with human norovirus cooks without washing their hands sufficiently. It is known that the food is contaminated and the food is ingested. Infection with human norovirus causes symptoms such as vomiting, diarrhea, and abdominal pain. Hereinafter, human norovirus is simply referred to as norovirus.

Norovirus is known to be sensitive to heat. The treatment of applying heat to oysters to raise the temperature (heat treatment) is considered to be an effective means for deactivating the activity of norovirus (hereinafter referred to as inactivation). According to the website of the Ministry of Health, Labor and Welfare (Non-Patent Document 1), in the case of foods such as bivalves (for example, oysters) that may be contaminated with norovirus, it is desirable to heat the center at 85 ° C to 90 ° C for 90 seconds or longer. There is a description. However, in reality, as described above, oysters are preferably eaten in a large amount in the raw state, and a method of inactivating norovirus by a method other than heating is strongly desired.

It is known that norovirus can be inactivated by performing high-pressure treatment with ultra-high pressure as a method other than heating. For example, Non-Patent Document 2 describes that norovirus could be inactivated as a result of high-pressure treatment of oyster shellfish inoculated with norovirus at 300 MPa to 600 MPa. It has also been shown that the higher the pressure of the high pressure treatment, the greater the amount of norovirus that could be inactivated. However, in this report, the oysters subject to high-pressure treatment are not in a frozen state (for example, in a state of minus several tens of degrees), but in a raw state, that is, in a non-frozen state.

Ministry of Health, Labor and Welfare "Q & A about Norovirus", September 1, 2016 search, Internet, http://www.mhlw.go.jp/stf/seisakunitsuite/bunya/kenkou_iryou/shokuhin/syokuchu/kanren/yobou/040204-1. html Mu Ye, et al., Inactivation of Human Norovirus in Contaminated Oysters and Clams by High Hydrostatic Pressure, Applied and Environmental Microbial, p.2248-2253, April 2014.

Currently, there is a possibility that attractive raw oysters are not sufficiently eaten due to concerns about food poisoning caused by norovirus. If the norovirus in oyster shells can be significantly inactivated in some way and the concern about food poisoning of norovirus can be reduced or eliminated, more raw oysters will have the same appearance, flavor and texture as raw oysters. It is expected that the preserved oysters will be accepted by more consumers. In other words, if we can provide oysters that are safe and have the same appearance, flavor, and texture as raw oysters, it is expected that oyster consumption will further increase. As a result, further development of the oyster industry and further development of producers, fabricators, etc. can be expected.

As described above, high-pressure treatment of oysters is a method that can be expected to inactivate norovirus in oyster shells without heating (Non-Patent Document 2).

However, if high-pressure treatment is performed at a higher pressure that can completely inactivate norovirus, the oyster shell will turn white (damage the appearance), and the flavor and texture will deteriorate, resulting in a commercial value as an oyster. Will disappear. On the other hand, if the pressure is lowered, the norovirus cannot be sufficiently inactivated. Therefore, it is not possible to sufficiently inactivate norovirus in the shellfish while maintaining the appearance, flavor, and texture of the shellfish even if the oysters are simply subjected to high-pressure treatment.

As a method for inactivating oyster norovirus, a method that can be easily, in large quantities, and at low cost can be realized at an actual oyster processing plant or the like, instead of an experimental level method. By realizing this, it will be possible to provide more consumers with safe and secure oysters that maintain shellfish, flavor, and texture that are close to the raw state.

An object of the present invention is to significantly inactivate norovirus in oyster shell meat. Alternatively, it is an object of the present invention to significantly inactivate norovirus in oyster meat while maintaining the appearance, texture and flavor of oyster shell meat. Alternatively, it is an object of the present invention to provide a method that can be easily carried out in a large amount at an oyster processing plant or the like at a low cost.

The method of the present invention comprises a step of putting a group of oysters in a frozen state into a high-pressure processing apparatus while maintaining the frozen state, and a high-pressure treatment of the group of oysters in the frozen state in the high-pressure processing apparatus. It is characterized by including a step of inactivating norovirus in the shell meat of each oyster while thawing each oyster, and a step of taking out the oyster group after the high pressure treatment from the high pressure treatment apparatus.

According to the tests and studies of the present inventors, as will be described in detail later, the pressure of oysters in a frozen state (for example, oysters at minus several tens of degrees) is higher than that in the case of high-pressure treatment of oysters in a raw state. It has been found that treatment can enhance the inactivation of norovirus. In particular, it has been found that if frozen oysters are subject to high-pressure treatment, norovirus can be inactivated at lower pressures. In other words, the combination of freezing and high-pressure treatment has a unique technical significance. For example, the appearance, texture, and flavor of oyster shellfish are maintained, and the norovirus in the shellfish is inactivated. It becomes possible to make it. In that case, it is desirable to select an appropriate pressure to be applied in the high-pressure treatment according to the season, situation, needs, and the like. The time of the high-pressure treatment may be determined in consideration of other purposes of the high-pressure treatment, at least as long as the substantial inactivation of norovirus can be achieved.

Against the above background, in the above method, the frozen oysters are placed in the high-pressure processing apparatus, and the frozen oysters are subjected to the high-pressure processing in the high-pressure processing apparatus. As a result, each oyster is thawed, and at the same time, the norovirus is substantially inactivated substantially. Depending on the pressure conditions and the pressure holding time, it is possible to maintain the appearance, texture, and flavor of the oyster shellfish. This is a big advantage. Moreover, since a large high-pressure processing device is used, a large amount of oysters can be processed at once, and efficient and stable processing can be expected. The effects of high-pressure treatment are expected to include inactivation of norovirus, defrosting function, exfoliation of oyster shells and shellfish (also called dehulling), and killing of pathogens.

The oysters can be added to the high pressure processing equipment manually or mechanically. The frozen oysters may be shelled oysters or oysters as stripped meat (shellfish-only oysters), as will be described later. In the former case, high-pressure treatment can simultaneously achieve peeling of the oyster shell and shellfish. Desirably, a large number of oysters are placed in a predetermined case (container) together with seawater or fresh water before being charged into the high pressure treatment apparatus. The high-pressure processing device is preferably a device capable of increasing or decreasing the pressure in the pressure chamber containing the oyster group.

All or part of the plurality of steps constituting the above method may be automated. That is, all or part of those steps may be performed manually.

In the above method, it is also preferable that the pressure at the time of the high pressure treatment is 250 MPa or less. Under such pressure conditions, it is usually possible to better maintain the appearance, texture and flavor of the oyster shellfish. The lower limit of the pressure range is defined as the pressure as long as the inactivation of norovirus can be achieved, and examples thereof include 120 MPa.

In the above method, it is also preferable that the time for the high-pressure treatment is several minutes or several tens of minutes or less. As long as the inactivation of norovirus can be achieved, if the high-pressure treatment time is shortened, the damage to the oyster shell meat can be reduced and the production efficiency can be increased.

Desirably, in the step of putting the oysters in the high-pressure treatment device, the frozen oyster group is put in a case, and after seawater or fresh water is injected into the case, the case is put in the high-pressure treatment device. The case is a container, which is also called a bucket, tray, etc.

In the above method, when seawater is injected into the case, the pressure during the high-pressure treatment is increased or the time required for the high-pressure treatment is increased as compared with the case where fresh water is injected into the case. You can make it longer. Further, in the above method, when seawater is injected into the case, the higher the salt concentration of the seawater, the higher the pressure at the time of the high pressure treatment or the longer the time at the time of the high pressure treatment. , May be.

In the above method, each oyster in the frozen oyster group is, for example, an oyster in which a shell and shell meat are connected by an adductor muscle, and in that case, the shell and shell meat of each oyster are subjected to the high-pressure treatment. Is peeled off. Alternatively, each oyster in the frozen oyster group is an oyster composed of a shell and shell meat from which the shell and shell meat have been peeled off. Alternatively, each oyster in the frozen oyster group is an oyster consisting only of shellfish whose shell and shellfish have been peeled off.

In the above method, each oyster in the frozen oyster group is peeled by high-pressure treatment of each oyster before freezing with the high-pressure treatment device, and the peeled oysters are frozen. Is also suitable. According to this configuration, the high-pressure treatment apparatus used for peeling is reused in the above-mentioned high-pressure treatment.

The method of the present invention includes a step of freezing the landed oyster group, a step of storing the frozen oyster group in a storage, and a step of taking out the frozen oyster group from the storage and freezing the frozen oyster group. Each oyster is thawed by a step of putting the oyster group in the state into a high-pressure processing device while maintaining the frozen state and by high-pressure processing the oyster group in the frozen state in the high-pressure processing device. It is characterized by including a step of inactivating the norovirus in the shellfish of each oyster and a step of taking out the oyster group after the high pressure treatment from the high pressure treatment apparatus and shipping them.

It can be said that the high-pressure processing device itself, which operates under predetermined conditions for realizing the above applications, is also a feature of the present application.

Further, as a modification or application example, application of the above method to fish and shellfish other than oysters can be considered.

Since the present invention is based on a verification test using human norovirus directly without using an alternative virus such as feline calicivirus, which has been widely used for the inactivation test of human norovirus, the credibility of the present invention. It can be said that the sex is even higher.

According to the present invention, the high-pressure treatment of frozen oysters can promote the inactivation of norovirus in the oyster shell meat. Alternatively, norovirus can be significantly inactivated even when high pressure treatment is performed at a low pressure that can maintain the appearance, texture, and flavor of oyster shellfish. Alternatively, since it is a simple method of high-pressure processing oysters in a frozen state, it can be easily carried out in a large amount at a processing plant or the like at a low cost. Alternatively, a new oyster product called thawed raw oyster can be realized.

It is a block diagram which shows an example of the apparatus in the processing plant in embodiment of this invention, and the flow of processing of oysters. It is a flowchart which shows an example of the flow of processing of an oyster in embodiment of this invention. It is a side view of the high pressure processing apparatus which shows the outline of the structure of the high pressure processing apparatus in embodiment of this invention. It is a figure which shows the result of having examined the amount of norovirus contained in the shell meat of each oyster after high pressure processing of raw oysters from Momonoura, Ishinomaki-shi and frozen oysters under different pressure conditions. It is a figure which shows the result of having examined the amount of norovirus contained in each oyster after high pressure processing of raw oysters from Hiroshima and frozen oysters under different pressure conditions. It is a flowchart which shows the flow of processing of an oyster in another embodiment of this invention.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a block diagram showing an example of an apparatus in the processing plant 10 in the present embodiment and a flow of processing oysters. As shown in FIG. 1, in the processing plant 10, a freezer 12 for freezing oysters, a storage 14 for storing frozen oysters, and high-pressure processing of frozen oysters. There is a high-pressure processing device 16 and the like. In the present embodiment, the frozen oysters are subjected to high-pressure treatment to thaw the oysters and inactivate the norovirus in the oyster shell meat.

The thick black arrows in FIG. 1 indicate the flow of oyster processing, and (a1) and (a2) added to the thick line indicate the processing when oysters arrive, and (b1) to (b4) indicate. Indicates that the oysters are processed at the time of shipment. Further, FIG. 2 is a flowchart showing a flow of processing oysters. Hereinafter, the processing of oysters will be specifically described with reference to FIGS. 1 and 2.

The landed oysters are transported to the processing plant 10 by truck or the like. The oyster group means a plurality of oysters, and the number of oysters indicated by the oyster group may be the same or different in each step of the process described below. As shown in FIG. 1, the oyster group carried to the processing plant 10 is placed in the refrigerator 12 at the processing plant 10 and frozen (S100 in FIG. 2). The freezer 12 is, for example, a freezer capable of rapidly freezing oysters. The oysters are placed in the refrigerator 12 and frozen at, for example, −25 ° C. to −35 ° C.

Then, the oyster group frozen in the refrigerator 12 is taken out from the refrigerator 12 and transported to the storage 14. Then, the frozen oyster group is stored in the storage 14 (S102 in FIG. 2). This is the process to be performed when the oysters are received. In the storage 14, temperature control is performed so that the frozen state of the oyster group can be maintained until the oysters are shipped.

The number of oysters stored in the storage 14 can be assumed to be large, for example, hundreds of thousands or more, or smaller, medium or small. Further, the period of storage in the storage 14 can be assumed to be, for example, several months to one year, but it varies depending on the situation.

At the time of shipment of oysters, the oyster group is taken out from the storage 14 and put into the high-pressure processing device 16 (S104 in FIG. 2). When the oyster group is put into the high-pressure processing device 16, for example, several tens of frozen oysters are put into the case 18, and the case 18 is put into the high-pressure processing device 16. In FIG. 1, three cases 18b, 18c, and 18d are depicted in the high-pressure processing apparatus 16. The high-pressure processing apparatus 16 of the present embodiment can simultaneously process the three cases 18b, 18c, and 18d at high pressure, and can simultaneously process a large amount of oysters at high pressure. The case 18 may also be called a bucket, a tray, or the like.

In the high-pressure treatment apparatus 16, the frozen oyster group in the case 18 is subjected to high-pressure treatment by applying water pressure (S106 in FIG. 2). As a result, the norovirus in the oyster shell meat is inactivated while the frozen oysters are thawed. In particular, in the present embodiment, the frozen oysters are subjected to high-pressure treatment, which greatly promotes the inactivation of norovirus. The effect of inactivating norovirus will be described in detail later. The configuration of the high-pressure processing apparatus 16 and the details of the high-pressure processing will also be described later.

In the present embodiment, the high-pressure treatment of the high-pressure treatment apparatus 16 causes thawing of oysters, inactivation of norovirus in the oyster shell meat, and cutting of the adductor muscle connecting the oyster shell and the shell meat. That is, the oyster shell and the shellfish are peeled off (unshelled) at the same time (S106 in FIG. 2).

When the high-pressure processing in the high-pressure processing device 16 is completed, the case 18 is taken out from the high-pressure processing device 16, that is, the oyster group is taken out from the high-pressure processing device 16 (S108 in FIG. 2). Each oyster in this group of oysters is a thawed, raw oyster, in which norovirus is significantly inactivated. Further, as will be described later, the oyster is able to maintain the appearance, texture and flavor of the oyster shell meat by performing the high pressure treatment at a lower pressure.

Then, the oysters are taken out from the case 18, and the oysters with shells (however, the shells and the body are peeled off) are carefully packed in a foamed styrol or the like and shipped to the market.

Next, the configuration of the high-pressure processing apparatus 16 will be described. As the high-pressure processing apparatus 16, for example, a horizontal ultra-high-pressure processing apparatus “FOOD FRESHER” manufactured by Kobe Steel can be used. Hereinafter, the high-pressure processing apparatus manufactured by Kobe Steel will be described as the high-pressure processing apparatus 16 of the present embodiment.

FIG. 3 is a side view of the high-pressure processing device 16 showing an outline of the configuration of the high-pressure processing device 16 in the present embodiment. FIG. 3 is partially transparent so that the configuration can be seen. As shown in FIG. 3, the high-pressure processing apparatus 16 includes a press frame 24, a pressure chamber 26 (also referred to as a pressure vessel) housed in the press frame 24, a high-pressure pump 28, and a water tank 30 containing fresh water. It is provided with an operation panel 32 for giving an operation instruction and a pressure setting of the high pressure processing apparatus 16.

The water tank 30 is connected to the high pressure pump 28, and the high pressure pump 28 is connected to the pressure chamber 26. The high-pressure treatment device 16 is capable of increasing the water pressure in the pressure chamber 26 by supplying fresh water from the water tank 30 and pressure from the high-pressure pump 28. The size of the pressure chamber 26 is, for example, 100 liters.

When the operation of the high pressure processing device 16 is started via the operation panel 32, the water pressure in the pressure chamber 26 rises to the pressure set by the operation panel 32. Then, a high-pressure treatment is applied to the frozen oyster group in the case 18 placed in the pressure chamber 26. The time for performing the high-pressure processing is set on the operation panel 32, and when the time elapses, the pressure is automatically reduced and the operation of the high-pressure processing device 16 is stopped.

As shown in FIG. 3, a transport table 38a is provided on the inlet side (right side of FIG. 3) of the press frame 24, a case 18a is placed on the transport table 38a, and the frozen oyster 40 is placed in the case 18a. Is put in. As described above, about several tens of frozen oysters 40 can be put in one case 18a, but the number of oysters 40 can be more than several tens depending on the size of the oysters 40 and the like. It may become. When the frozen oysters 40 are put into the case 18a, a process of fastening the shells of the oysters 40 with a binding band (cable tie treatment) may be performed. This is a treatment for preventing the shellfish from slipping out of the shell because the shell of the oyster 40 and the shellfish are peeled off when the high-pressure treatment is performed. When the oysters 40 are stuck to each other due to freezing, they may be put in the case 18a in the state of being stuck to each other.

After the frozen oyster group is placed in the case 18a, seawater or fresh water is injected into the case 18a. Then, the case 18a advances on the transport table 38a and is charged into the pressure chamber 26 of the press frame 24.

The high-pressure processing apparatus 16 in the present embodiment has a configuration in which three cases 18 can be accommodated in the pressure chamber 26 and they can be simultaneously subjected to high-pressure processing. Therefore, a large amount of oysters can be processed at high pressure at the same time. FIG. 3 shows a state in which a group of oysters in a frozen state is put in and cases 18b, 18c, and 18d in which seawater or fresh water is injected are put into the pressure chamber 26. When the high pressure treatment is started, the inlet lid 34 and the outlet lid 36 of the pressure chamber 26 are closed, the inside of the pressure chamber 26 is in a high pressure state, and the oyster group in the case 18 is subjected to the high pressure treatment. Then, when the high pressure treatment is completed, the inlet lid 34 and the outlet lid 36 of the pressure chamber 26 are opened, and then the case 18 to be subjected to the high pressure treatment is put into the pressure chamber 26, so that the treated case 18 is pressed into the press frame. It is pushed out to the exit side (left side in FIG. 3) of 24. At this time, the processed case 18 is extruded onto the transport table 38b (in FIG. 3, the case 18e is extruded onto the transport table 38b). Then, each processed oyster is taken out from the case 18 on the transport table 38b. Each of these oysters is a oyster that has been thawed, the norovirus has been significantly inactivated, and the shell and shellfish have been exfoliated.

Next, the pressure at the time of high-pressure processing of oysters will be described.

The present inventors performed high-pressure treatment on raw oysters at different pressures, and investigated the state of oyster shellfish and the state of norovirus inactivation in the shellfish. As a result, it was found that, for example, when high pressure treatment is performed at a pressure of 300 MPa or more, whitening may occur in the shellfish. It was also found that it is highly possible that norovirus cannot be sufficiently inactivated even if the raw oysters are subjected to high-pressure treatment of, for example, less than 300 MPa.

On the other hand, when the present inventors performed high-pressure treatment on frozen oysters, they noticed that a large amount of norovirus was inactivated even at a low pressure. Although details will be described later, norovirus in the shellfish was sufficiently inactivated even when the frozen oysters were subjected to high-pressure treatment at 250 MPa or less, for example. Further, since the high pressure treatment was performed at a low pressure of 250 MPa or less, the appearance, texture and flavor of the shellfish after the high pressure treatment were also good. The high-pressure treatment pressure required to sufficiently inactivate norovirus depends on conditions such as the time of high-pressure treatment, the type and size of oysters, the temperature of oysters, and the amount of norovirus contained in shellfish. It is considered to be. Therefore, it is assumed that the norovirus can be sufficiently inactivated even at a pressure of about 120 MPa.

Against the background as described above, in the present embodiment, the frozen oyster group is subjected to high pressure treatment at a pressure of 120 MPa to 250 MPa. However, the magnitude of the pressure may be other than that as long as the norovirus can be sufficiently inactivated while maintaining the appearance, texture and flavor of the oyster shellfish. It is desirable to select an appropriate pressure to be applied in the high-pressure treatment according to the conditions such as the time for high-pressure treatment, the type and size of oysters, the temperature of oysters, and the amount of norovirus contained in shellfish.

Next, the time required for high-pressure processing of oysters will be described.

The time for high pressure treatment of oysters is another purpose of high pressure treatment as long as the norovirus in the oyster meat can be sufficiently inactivated while maintaining the appearance, texture and flavor of the oyster meat. The decision will be made in consideration of the thawing of oysters. Alternatively, it is determined according to conditions such as the pressure applied by high-pressure treatment, the type and size of oysters, the temperature of oysters, and the amount of norovirus contained in shellfish. According to the tests of the present inventors, as will be described later, the frozen oysters are subjected to high-pressure treatment at 250 MPa for 5 minutes, for example, so that the appearance, texture and flavor of the oyster shells are maintained. , It has been found that norovirus can be sufficiently inactivated. Therefore, in the present embodiment, the time for high-pressure processing is set to, for example, 5 minutes. However, as described above, the time is appropriately adjusted according to other purposes of the high pressure treatment and the above conditions.

It should be noted that the medium injected into the case 18 before the case 18 is charged into the high-pressure processing apparatus 16 is seawater or fresh water, and the pressure for the high-pressure treatment and the time for the high-pressure treatment as follows. May be changed. When seawater is injected into the case 18, the pressure during the high-pressure treatment is increased or the time during the high-pressure treatment is lengthened as compared with the case where fresh water is injected into the case 18. Alternatively, the pressure during the high pressure treatment is increased and the time during the high pressure treatment is lengthened. This is because it is considered that norovirus in oyster shellfish is difficult to be inactivated when the salt content in the shellfish is high, so when seawater is injected into Case 18 (the salt content in the shellfish becomes high, Or, if it is maintained), the pressure during high-pressure treatment will be higher, the time will be longer, and the norovirus will be sufficiently more than when fresh water is injected (when the salt content in the shellfish will be lower). It inactivates. Further, for the same reason, when seawater is injected into the case 18, the pressure and time of the high-pressure treatment may be changed as follows according to the salt concentration of the injected seawater. The higher the salinity of the injected seawater, the higher the pressure during the high-pressure treatment or the longer the time during the high-pressure treatment. Alternatively, the pressure during the high pressure treatment is increased and the time during the high pressure treatment is lengthened.

Next, the effect of inactivating norovirus in oyster shell meat by high-pressure treatment of oysters with the high-pressure treatment device 16 will be described.

Figure 4 shows the results of high-pressure treatment of oysters landed in Momonoura, Ishinomaki City, Miyagi Prefecture in January 2016 under different pressure conditions, and then examining the amount of norovirus contained in the shellfish of each oyster. It is a graph which shows.

The graph of FIG. 4 is obtained as follows. First, at the same time, a plurality of oysters landed at the same place were frozen in a freezer 12 to be in a frozen state. Among them, some oysters were slowly thawed to a raw state, and other oysters were kept frozen. Then, each raw oyster was subjected to high pressure treatment at a different pressure for 5 minutes, and similarly, each frozen oyster was subjected to high pressure treatment at a different pressure for 5 minutes. Then, 1 gram of the midgut gland of the shellfish of each oyster was taken out, and the amount of norovirus contained therein was examined by a real-time PCR method. Then, a graph was created by associating the value of the pressure applied to the oyster (pressure value) with the amount of norovirus. If the midgut gland of one oyster shellfish did not reach 1 gram, the midgut gland of the oyster shellfish subjected to high pressure treatment under the same conditions was added to make 1 gram.

In FIG. 4, "raw" means raw oysters (slowly thawed frozen oysters), and "frozen" means frozen oysters. Further, "no high pressure treatment" means that the high pressure treatment was not performed. In other words, "without high-pressure treatment" is a study of the amount of norovirus in oyster shell meat without high-pressure treatment of oysters. The amount of norovirus on the vertical axis of the graph of FIG. 4 is the copy number per gram of the midgut gland and is represented by a logarithmic value.

As shown on the left side of FIG. 4, the amount of norovirus contained in the midgut gland of shellfish decreases as the pressure of high-pressure treatment of raw oysters (frozen oysters slowly thawed) increases. However, even if high pressure treatment of 250 MPa or more is performed, the effect of sufficient inactivation of norovirus cannot be confirmed. On the other hand, as shown on the right side of FIG. 4, norovirus was substantially not detected in the frozen oysters by performing the high pressure treatment of 180 MPa or more. That is, the norovirus can be sufficiently inactivated by performing the high-pressure treatment of 180 MPa or more.

Since the amount of norovirus contained in the midgut gland depends on the individual oyster, it is assumed that norovirus will be detected even when frozen oysters are treated under high pressure at 180 MPa, but this was taken into consideration. Even so, it can be seen that there is a clear difference in the amount of norovirus that can be inactivated between raw oysters and frozen oysters. That is, it can be seen that the high-pressure treatment of frozen oysters promotes the inactivation of norovirus in the oyster shell meat. By using frozen oysters, norovirus in shellfish can be sufficiently inactivated even when the pressure of high-pressure treatment is low. Further, in FIG. 4, the norovirus can be sufficiently inactivated by the high-pressure treatment for 5 minutes.

FIG. 5 is a table showing the results of high-pressure treatment of 180 MPa and 250 MPa on oysters landed in Hiroshima City, Hiroshima Prefecture, and then examining the amount of norovirus contained in the shell meat of each oyster.

The table of FIG. 5 was obtained as follows. First, oysters were randomly selected from several types of oyster groups that were stored frozen and contained a large amount of norovirus. Among them, some oysters were slowly thawed to a raw state, and other oysters were kept frozen. Then, each raw oyster was subjected to high pressure treatment at a different pressure for 5 minutes, and similarly, each frozen oyster was subjected to high pressure treatment at a different pressure for 5 minutes. Then, 1 gram of the midgut gland of the shellfish of each oyster was taken out, and the amount of norovirus contained therein was examined by a real-time PCR method. Here, the norovirus was divided into a GI gene group and a GII gene group, and the amount of each norovirus was examined. Then, a table was created by associating the pressure value (pressure value) applied to the oysters with the amount of norovirus in the GI gene group, and associating the pressure value applied to the oysters with the amount of norovirus in the GII gene group. For the midgut gland of one oyster shellfish that did not reach 1 gram, the midgut gland of the oyster shellfish that had been subjected to high pressure treatment under the same conditions was added to make 1 gram.

In FIG. 5, the amount of norovirus in the GI gene group is shown on the upper side, and the amount of norovirus in the GII gene group is shown on the lower side. "Raw" means oysters that have been slowly thawed from frozen oysters, and "frozen" means oysters that have been frozen. In addition, the amount of norovirus in each of the first, second, and third times indicates the amount of norovirus detected in each of the three times of high-pressure treatment using different oysters under the same conditions. The amount of norovirus in the table of FIG. 5 is the copy number per gram of the midgut gland and is represented by a real value.

As shown in FIG. 5, in both the GI gene group and the GII gene group, the case where the frozen oysters were subjected to the high-pressure treatment was higher than the case where the frozen oysters were subjected to the high-pressure treatment. You can see that the amount of norovirus is low. In raw oysters, there are many noroviruses of the GI gene group and noroviruses of the GII gene group that cannot be inactivated even when subjected to high-pressure treatment at 250 MPa. On the other hand, the frozen oysters can sufficiently inactivate the norovirus of the GI gene group and the norovirus of the GII gene group even by high-pressure treatment of 250 MPa or less. It is known that oysters from Hiroshima generally contain more norovirus in shellfish than oysters from Momoura, but even such oysters from Hiroshima are in a frozen state. By applying high-pressure treatment to oysters, norovirus can be significantly inactivated. Further, in FIG. 5, the norovirus can be significantly inactivated by the high-pressure treatment for 5 minutes.

As described above, according to the present embodiment, the high-pressure treatment of frozen oysters can promote the inactivation of norovirus in shellfish. The synergistic effect of freezing and high-pressure treatment can promote the inactivation of norovirus in shellfish. Alternatively, the norovirus can be sufficiently inactivated even when the high pressure treatment is performed at a low pressure that can maintain the appearance, texture, and flavor of the oyster shellfish. Alternatively, since it is a simple method of high-pressure processing oysters in a frozen state, it can be easily carried out in a large amount at a processing plant or the like at a low cost. It can be carried out using existing equipment in a processing plant or the like.

This makes it possible to provide consumers with attractive raw oysters that have sufficiently reduced or eliminated the risk of food poisoning of norovirus while maintaining the appearance, texture, and flavor of oyster shellfish. Become. It is predicted that more thawed raw oysters will be accepted by more consumers, and as a result, further development of the oyster industry and further development of producers, processors, etc. can be expected.

In the embodiment described above, the oysters frozen by the refrigerator 12 are temporarily stored in the storage 14. However, the oysters frozen by the refrigerator 12 may be directly transported to the high-pressure processing device 16 and put into the high-pressure processing device 16 without being stored in the storage 14.

In the embodiment described above, the frozen oyster in which the shell and the shellfish are bonded by the adductor muscle is placed in the high-pressure processing device 16, but the shell and the shellfish have been peeled off (the adductor muscle has been cut). Frozen oysters composed of shellfish and shellfish, or frozen oysters composed only of shellfish whose shells and shellfish have been peeled off may be placed in the high-pressure processing apparatus 16. In this case, the shell and the shell meat are peeled off (cutting the scallop) in advance, either manually or by using the high-pressure processing device 16.

Here, a case where the oyster shell and the shellfish are peeled off in advance by using the high-pressure processing apparatus 16 will be described in detail. FIG. 6 shows the flow of processing of the embodiment in which the oyster shell and the shellfish are peeled off in advance by using the high-pressure processing apparatus 16.

As shown in FIG. 6, first, the oysters landed in S200 are carried to the processing plant 10.
High pressure processing is performed by the high pressure processing device 16. By this high-pressure treatment, the oyster shell and the shell meat are peeled off (the scallop is cut). Then, in S202, the peeled oyster group is frozen in the refrigerator 12. The oysters placed in the refrigerator 12 include oysters in which shells and shellfish are combined and oysters containing only shellfish. Then, after freezing in the freezer 12, the frozen oysters are transported to the storage 14. Then, the oysters are stored in the freezing state in the storage 14 until shipping (S204). This is the processing at the time of arrival.

Then, at the time of shipment of the oysters, the oysters are taken out from the storage 14 and put into the high-pressure processing device 16 (S206). The oyster here is a frozen oyster composed of shells and shellfish, or a frozen oyster composed of only shellfish.

Then, the oysters are subjected to high-pressure treatment with the high-pressure treatment device 16 to thaw the oysters and inactivate the norovirus in the shell meat of the oysters (S208). The high-pressure processing device 16 is the same as the high-pressure processing device 16 used for the process of peeling the oyster shell and the shellfish in S200. Then, after the high-pressure treatment, the oysters are taken out from the high-pressure treatment device 16 (S210), and each oyster (oyster with a shell or oyster with only meat (shellfish)) is carefully boxed in a foamed styrol or the like and put on the market. Will be shipped.

As described above, the same high-pressure processing device 16 is used to perform the oyster peeling process at the time of arrival, the oyster thawing process at the time of shipment, and the norovirus inactivation process. The rate can be increased. In addition, the scale of equipment required for a series of processes can be reduced.

10 processing plant, 12 refrigerator, 14 storage, 16 high pressure processing equipment, 18, 18a, 18b, 18c, 18d, 18e case, 24 press frame, 26 pressure chamber, 28 high pressure pump, 30 water tank, 32 operation panel, 34 inlet lid, 36 outlet lid, 38a, 38b carrier, 40 oysters.

Claims (9)

The process of putting a group of oysters in a frozen state into a high-pressure processing device while maintaining the frozen state,
A step of inactivating norovirus in the shellfish of each oyster while thawing each oyster by high-pressure processing the frozen oyster group in the high-pressure treatment apparatus.
A step of taking out the oyster group after the high-pressure treatment from the high-pressure treatment device, and
Only including,
The pressure during the high pressure treatment is 120 MPa or more and 250 MPa or less.
A method characterized by that. In the method according to claim 1 ,
In the step of putting the oysters in the high-pressure treatment device, the frozen oyster group is put in a case, and after seawater or fresh water is injected into the case, the case is put in the high-pressure treatment device.
A method characterized by that. In the method according to claim 2 ,
When seawater is injected into the case, the pressure during the high-pressure treatment is increased or the time during the high-pressure treatment is lengthened as compared with the case where fresh water is injected into the case.
A method characterized by that. In the method according to claim 2 or 3 ,
When seawater is injected into the case, the higher the salt concentration of the seawater, the higher the pressure during the high-pressure treatment or the longer the time during the high-pressure treatment.
A method characterized by that. In the method according to any one of claims 1 to 4 ,
Each oyster in the frozen oyster group is an oyster in which the shell and the shell meat are connected by an adductor muscle.
By the high pressure treatment, the shell of each oyster and the shellfish are peeled off.
A method characterized by that. In the method according to any one of claims 1 to 4 ,
Each oyster in the frozen oyster group is an oyster composed of a shell and shell meat from which the shell and shell meat have been peeled off.
A method characterized by that. In the method according to any one of claims 1 to 4 ,
Each oyster in the frozen oyster group is an oyster consisting only of shellfish whose shell and shellfish have been peeled off.
A method characterized by that. In the method according to claim 6 or 7 .
Each oyster in the frozen oyster group is peeled by high-pressure treatment of each oyster before freezing with the high-pressure treatment device, and each peeled oyster is frozen.
A method characterized by that. The process of freezing the landed oysters and
The process of storing frozen oysters in a storage and
A step of taking out the frozen oyster group from the storage and putting the frozen oyster group into a high-pressure processing apparatus while maintaining the frozen state.
A step of thawing each oyster and inactivating norovirus in the shellfish of each oyster by high-pressure processing the frozen oyster group in the high-pressure processing apparatus.
The process of taking out the oyster group after the high-pressure treatment from the high-pressure treatment device and shipping them, and
Only including,
The pressure during the high pressure treatment is 120 MPa or more and 250 MPa or less.
A method characterized by that.