Disinfection method for plastic devices

Abstract

Cleaning and disinfection of multi-use plastic materials used in the food industry can be accomplished by using microwave radiation in combination with an electrolyte solution, thereby achieving at least 99.99% killing of microorganisms present on and inside the plastic device. Cleaning and disinfection of plastic trays used in security check screening can also be accomplished using microwave radiation in combination with an aqueous solution, thereby achieving at least 99.9% killing of pathogenic microorganisms present on the plastic tray.

Claims

1. A method for disinfecting a multiuse plastic device used in food industry, in which harmful vegetative microorganisms may grow, comprising: contacting and/or washing the plastic device with an electrolyte solution comprising one or more of (a) dissolved salt and/or (b) an alkali or an acid, such that the plastic device is wetted, wherein the multiuse plastic device is selected from double walled load carriers, containers with an inner volume of 8 to 2,000 liters, and conveyor belts and wherein the multiuse plastic device includes damaged parts, internal surfaces, and hollow spaces that are insufficiently sterilized by the electrolyte solution and contain entrapped water; and transferring the thus wetted plastic device into a microwave oven treatment chamber, in which the plastic device and the entrapped water are subjected to microwave radiation treatment in a sufficient time to eliminate essentially all vegetative microorganisms present on and in the plastic device; wherein the microwave radiation treatment time is less than about 7 minutes; and wherein the microwave radiation is formed by 6-36 magnetrons, each having an effect of 0.5-5 kW.

2. The method according to claim 1, wherein the electrolyte solution further comprises (c) a bubble forming agent.

3. The method according to claim 1, wherein the microwave treatment is from about 30 seconds to about 5 minutes.

4. The method according to claim 1, wherein the electrolyte solution is formed by at least one dissolved salt selected from the group consisting of NaCl, KCl, Na.sub.2SO.sub.4, MgCl.sub.2, Na.sub.2HPO.sub.4, NaH.sub.2PO.sub.4, K.sub.2HPO.sub.4, KH.sub.2PO.sub.4, seawater, and a mixture thereof.

5. The method according to claim 4, wherein the amount of dissolved salt(s) is from about 5 g/L to about 50 g/L.

6. The method according to claim 1, wherein the alkali is NaOH and/or KOH.

7. The method according to claim 1, wherein the acid is HCl or H.sub.3PO.sub.4.

8. The method according to claim 1, wherein the electrolyte solution contacts the plastic device by spraying, brushing, pressure washer or immersing.

9. The method according to claim 1, wherein the microwave radiation treatment is performed in a microwave oven which has a uniform electromagnetic radiation throughout the treatment chamber.

10. The method according to claim 1, wherein the multiuse plastic device comprises load carriers, containers, pallets, boxes, trays, shovels, or conveyor belts.

11. The method according to claim 1, wherein the multiuse plastic devices have damage(s) in the surface or internal parts, in which there is an increased risk of growth of microorganisms.

12. The method according to claim 1, wherein at least 99.99% of vegetative microorganisms initially present on and/or in the plastic device are killed.

13. The method according to claim 1, further comprising expanding the electrolyte solution, heated by the microwave radiation treatment, to colder areas of the plastic device with a foaming agent.

14. A method for disinfecting a conveyor belt used in food industry, in which harmful vegetative microorganisms may grow, comprising: demounting of the conveyor belt, washing and/or contacting the conveyor belt parts with an electrolyte solution comprising one or more of (a) dissolved salt and/or (b) an alkali or an acid, and placing the thus wetted conveyor belt parts in a plastic container and transferring the container, containing the conveyor belt parts, into a microwave oven, in which the conveyor belt parts are subjected to microwave radiation treatment in a sufficient time to eliminate essentially all vegetative microorganisms present on the conveyor belt parts.

Description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

(1) The method according to the first aspect of the invention is of significance to food industry and particularly to the seafood industry that transport fresh seafood and meat product in large load carriers. The load carriers are made of plastic, due to rough handling are prone to damage, and consequently risk of growth of pathogen bacteria in the damaged areas. The method of the invention is also significant to the food industry where the surfaces of conveyor belts come directly into contact with food product. Such surfaces of conveyor belts are commonly made of plastic material, and hence subjected to the risk of growth of pathogen bacteria in damaged areas, and/or in areas of hinged couplings/links of modular transport belts.

(2) The present method of disinfecting multiuse plastic devices used in food industry essentially includes the steps of contacting the plastic devices with a solution comprising least an electrolyte (ions) and transferring the thus wetted plastic device into a microwave generator in which the plastic device is subjected to microwave radiation treatment for a sufficient time to eliminate essentially all vegetative microorganisms present on and inside the plastic device. The present invention utilizes the fact that moist heat kills microorganisms more effectively than dry heat. In addition, microwaves especially heat water and humid environment, therefore the energy supplied is converted to heat in locations where most of the microorganisms are present. As microwaves penetrate through plastic material, without significantly heating the plastic material, water or liquid containing areas and cavities inside the plastic device will also be rapidly heated, so that any potential pathogenic bacteria are killed. The ions in the electrolyte solution increases the heating rate significantly, therefore even small droplets will be heated fast enough to kill the bacteria before they evaporate. In addition, the number of bacteria being killed increases due to increasing osmolarity of the electrolyte solution as the water is evaporating. The electrolyte solution may also comprise a foaming agent which aids the disinfection process by bubble formation during the microwave treatment, thus moving the heated electrolyte solution to colder areas, and thereby increasing the areas being treated, including any hollow space between two walls in damaged areas if present.

(3) It should be understood that by disinfecting in this context, it is meant a method that removes essentially all nucleating vegetative cells initially present on and in the plastic device. By essentially all, it is meant by at least 99.99% of nucleating vegetative cells initially present on and in the plastic device.

(4) The term vegetative cells as employed herein denote any bacterium or unicellular alga that is actively growing or forming spores. Including but not limiting to Bacillus cereus (gram-positive bacteria), Clostridium perfringens (gram-positive bacteria), C. botulinum (gram-positive bacteria), Listeria monocytogenes (facultative anaerobic bacteria), Escherichia coli (gram-negative bacteria), Salmonella spp. (gram-negative bacteria), and Vibrio parahaemolyticus (gram-negative bacteria).

(5) In the present context the term multiuse plastic device should be understood to denote any multiuse plastic equipment used in the food industry, which are subjected to strict hygiene requirements. The term multiuse plastic device should be understood to denote any multiuse plastic equipment used in the food industry, which are normally used several times, and whose surfaces repeatedly come into contact with new food products. Such plastic devices include large load carriers, containers, pallets, boxes, trays, shovels, conveyor belts and other plastic equipment used in the food industry for handling, transporting and temporary storing of food products, semi-finished or finished food products. Some of the plastic devices are only used internally within the production facility, while other, especially plastic load carriers, containers and pallets, may be used for transport between a receiving facility and a food processing facility, and possibly also the end market. Load carriers or containers may be designed to fit on standard European or International size, e.g. standard European sizes 1200800 mm, 800600 mm pallets as well as standard 12001000 mm pallets. Other common sizes within manufacturing and shipping are 600400 mm and 400300 mm, which fit and can easily be stacked on a pallet measuring 1200800 mm Inner volumes of load carriers or containers varies from typically 8-10 L to 2000 L. In the present context, the term conveyor belt or transport belt should be understood to denote conventional conveyor belts of different lengths, widths and designs used in the food processing industry, including hinged modular belts. Relevant in this context are conveyor belts of plastic materials. The multiuse plastic devices for the food industry are typically made of polyethylene of different density, generally known in the art. There are several densities of polyethylene, e.g. high density (HDPE), medium density (MDPE), low density (LDPE) and very low density (VLDPE). The polyethylene products are conventionally produced by rotational moulding techniques, injection molding or extruding. The multiuse plastic devices may also comprise other materials such as polyurethane. Thus the present invention is especially suitable for plastic devices made of polyethylene and/or polyurethane, although not limited to said plastic materials.

(6) In the present context large should be understood as denoting load carriers and plastic devices including the above dimensions. Large should also be understood to include load carriers and containers with inner volumes from about 8 L to about 2000 L. Plastic conveyor belt parts, that comes into contact with food products and therefore frequently need to be sanitized and disinfected may have various dimensions and forms, however due to the large number of parts such conveyor belt parts are considered to be large items in the present context. It should be noted that the load carriers and other plastic devices used in the food industry, which can be treated according to the present invention application, may have other dimensions than specified above.

(7) The present invention will be explained by disinfection of load carriers of the type large plastic containers of 500-1000 liters, where the plastic containers have double walls. Such containers are extensively used in the seafood industry as well as other food product industries (e.g. meat, chicken). In order for the plastic containers to have the sufficient strength when stacked above each other when they are filled with fish or other food products, the containers are normally produced with double walls where the distance between the inner and outer wall is typically 2-3 cm. The space between the walls is filled with air or an isolating material such as polyurethane, which also provides extra stability and strength. The containers are made of polyethylene which is acid, alkali and corrosion resistant. Said materials (polyethylene and polyurethane) are heated by microwave radiation, but much slower than water and water containing accumulations, such as residues of fish, food, etc. Microwave treatment of such large items might in a worst case scenario cause parts of the load carrier overheat and melt or ignite. This puts a limit to the time and intensity of the microwave treatment, while at the same time the aim of a destroying/killing rate of vegetative microorganisms in and on the load carrier of least 99.99% should be obtained.

(8) Before the disinfecting treatment of the load carrier, the container should be washed in order to remove stains, dried residues, etc. If the container has larger damages, such as punctures, any contained dirty water should be emptied from hollow spaces before the cleaning. The cleaning may be performed according to normal procedures using wash water, scrubbing and rinsing. The washing may be performed by manual methods using brushes and/or pressure washer or by automatic methods. In an automatic method, e.g. an industrial washing machine, washing may be performed by spraying washing solution by means of nozzles, followed by rinsing with water. The washing may be performed by using conventional cleaning agents approved for use in the food industry, in amounts (concentration of cleaning agent in wash water) recommended by the producer of the cleaning agents. Advantageously the washing is performed by using a washing solution containing electrolytes (ions of salts) and/or alkali (e.g. caustic soda or caustic potash) and/or acid (e.g. hydrochloric acid or phosphorus acid). The washing solution may also comprise a foaming agent (e.g. soap approved for use in food industry), especially if the plastic device are dirty and have solidified layers of food remnants. By washing with the said washing solution comprising electrolytes and optionally a bubble forming agent, a wet film is left on the plastic device, and in damaged areas (holes, scratches, etc.). This wet film contains remains of the electrolyte solution in an amount optimal for the microwave treatment, and the wet plastic devices may thus be directly transferred to the microwave oven for microwave treatment.

(9) The electrolyte solution (wash water) employed herein is an aqueous solution comprising a dissolved salt and/or an alkali or an acid. Examples of suitable salt are NaCl, KCl, Na.sub.2SO.sub.4, MgCl.sub.2, Na.sub.2HPO.sub.4, NaH.sub.2PO.sub.4, K.sub.2HPO.sub.4, KH.sub.2PO.sub.4 or seawater, or a mixture thereof. Said salts are food compatible and any salt left behind on the plastic device after evaporation may even prevent new bacterial growth. Suitable alkalis are NaOH and KOH which are also food compatible. The concentration of dissolved salt in water, forming the electrolyte solution, may be from about 0.5% to about 5.0% (5 g/L to 50 g/L). Tests showed that effective killing of microorganisms, and increased heating rate, was achieved with a salinity of about 1%. The salinity of seawater is about 3.5%. Thus, suitable salinity in the electrolyte solution may be from about 1% to 3.5% (10 g/L to 35 g/L.

(10) Alkali (bases) may kill microorganisms in itself, however if alkali is present in the electrolyte solution, it should be in a very dilute form. During the microwave treatment, as water evaporates, the concentration of the alkali increases, thereby the killing effect of microorganisms increases. Concentration of alkali in the electrolyte solution may be from about 0.1% to about 2% (in water). The inventors found that effective killing of microorganisms and a fast heating rate during microwave treatment was achieved with concentration of alkali between about 0.4% and about 1.5% in the electrolyte solution. It is however desired to reduce the usage of chemicals, therefore the concentration of alkali should advantageously be in the lower area, e.g. about 0.2% to about 0.5%.

(11) Instead of an alkali, an acid may be used. Suitable acids are e.g. hydrochloric acid (HCl) and phosphorus acid (H.sub.3PO.sub.4), which are food compatible. Correspondingly as the alkali, acid may kill microorganisms in itself. If acid is present in the electrolyte solution, it should be in a very dilute form, thus during the microwave treatment as the water evaporates the concentration increases which increases the microorganism killing effect. Concentration of acid in the water, forming the electrolyte solution, should be from about 0.1% to about 2%, e.g. from about 0.4% to about 1.5%. Due to the desire of reduced usage of chemicals, the concentration of acid should be about 0.2% to about 0.5%. Also as the water evaporates the concentration of the alkali (e.g. caustic soda) or acid increases to a level that leads to killing of bacteria, which may be important in very small droplets.

(12) The electrolyte solution may also comprise foaming agents, i.e. bubble forming substance. Such foaming agent may be any soap approved for use in the food industry. The amount of said foaming agent can be according to recommended amounts indicated by the soap producer. Advantageously the amount of foaming agent is in the lower recommended range, as layers of soap may remain on the plastic surface after disinfection. Such remaining soap products should be rinsed off before reusing a plastic device for food product. Presence of a bubble forming agent in the electrolyte solution is on the other side positive during the disinfecting treatment in the microwave oven, since bubbles are produced during the microwave treatment as the water starts boiling. The bubbles expand the volume of remaining wash solution preset especially in damaged places, such as hollow areas and in scratches, so that heat is moved over larger areas.

(13) The electrolyte solution suitable for washing and microwave treatment is a diluted solution, and does not contain any strong sanitizing and/or disinfecting chemicals that require additional precautions regarding handling, storage of use beyond normal protective equipment for industrial washing procedures, such as gloves and eye protection. The present electrolyte solution does not contain harmful substances, such as oxidizing chemicals or disinfectants, which may put the workers and/or the environment at risk.

(14) After washing and rinsing the damaged plastic devise, e.g. a load carrier, including drainage of any entrapped liquid in hollow spaces, the load carrier is treated by microwave radiation. As indicated above, the load carrier should be wet after the washing, containing a film of the electrolyte solution, especially at damaged areas in the surface and/or any hollow parts, before the treatment in the microwave oven. The wet load carrier is thus subjected to microwave radiation for a sufficient amount of time to disinfect all parts of the surface and internal parts of the load carrier. Microwaves kill microorganisms by heating in the same way as conventional heat treatment. The rate of killing is a function of time and temperature. The difference is that by using microwave radiation, the heating occurs much faster and primarily in the water phase, both on the outside and the inside of the device, compared to conventional heating means. Total energy consumption is therefore significantly lower than for conventional heating. There is, however, a risk that when using microwaves for heating, large temperature gradients over relatively short distances may be created, and microorganisms can survive in cold and dry spots in the load carriers. Consequently, quick heating rate and uniform distribution of microwaves for sufficient heating is important to achieve satisfactory results. This implies the recommended method to disinfect wet load carriers right after washing and by using a microwave system that provides a homogenous microwave field.

(15) The microwave-heated systems for performing the present disinfecting method should generate a homogeneous microwave field. By a homogenous microwave field, it is meant that the microwave oven should distribute the microwaves as uniform as possible such that cold spots are as few or as unlikely as possible. It is difficult to calculate the exact radiation distribution inside the oven as the objects to be treated and the amount of water and biological material varies. To estimate the radiation distribution in a microwave oven statistical methods are therefore used. A feasible probability distribution is the Chi-quadrat distribution, which expresses the sum of energy from n independent sources, where the parameter n is referred to as the number of degrees of freedom. A microwave oven may generate different standing waves of cold and warm zones inside the oven, these are called modes. Each mode supplies electromagnetic fields inside the microwave chamber in different positions. Different modes can be present at the same time or they may be shifted consecutively e.g. by using movable reflector or stirrer. Large industrial microwave ovens usually have a greater number of modes which increases the number of degrees of freedom, and gives more uniform microwave distribution in the oven. The number of degrees of freedom n can be estimated to:

(16) $n=2\frac{{E\left(P\right)}^2}{\mathrm{VAR}\left(P\right)}$

(17) Where P is the energy delivered to several places in the oven (e.g. measured by temperature sensors), and the expectation is E(P) and the variance is VAR(P). In an oven with 64 degrees of freedom it is 99.99% probable that the radiation intensity on a random position inside the oven is at least 47.5% of the average radiation intensity. It should however be understood that objects inside the microwave oven, including load carriers of plastic, will in varying degree affect the oven by absorbing energy and develop heat. Therefore the inventors performed tests by treating commercial load carriers (plastic trays, plastic pallets and plastic containers). It was found that microwave ovens with a hexagonal treatment chamber and a plurality of magnetrons were suitable for performing the microwave treatment according to the present invention, thereby achieving a killing rate of microorganisms of at least 99.99%. Microwave are a form of electromagnetic radiation with wavelengths ranging from about one meter to one millimeter, with frequencies between 300 MHz (1 m) and 300 GHz (1 mm). The microwave radiation used in microwave ovens usually have frequency of between 915 MHz to about 25 GHz. A standard microwave frequency suitable for the present microwave system is e.g. 2.45 GHz.

(18) For logistical reasons, and to be able to treat large plastic items, the microwave oven for carrying out the present disinfection method should be a large industrial microwave oven, having a treatment chamber that can accommodate large load carriers/containers with dimensions as specified above. Such microwave ovens may be equipped with several magnetrons, which ensure a homogenous distribution of electromagnetic radiation in the treatment chamber. Examples of such ovens are microwave systems with 6-36 magnetrons, e.g. at least 8-16 magnetrons, each having e.g. an effect of 0.5-5 kW, e.g. 1-3 kW, the chamber may e.g. have a volume of 3-8 m.sup.3. The treatment chamber may be designed as a rectangular box, hexagonal box, or any other suitable shape which allows a uniform distribution of electromagnetic radiation, while having capacity of accommodating a desired amount of plastic devices to be treated. It should be noted that microwave ovens suitable for carrying out the present method may be different from the said examples, and the skilled person should readily be able to choose suitable microwave systems adapted for the plastic objects that is to be treated. It should also be noted that in order to ensure sufficient distribution of microwaves in the treatment chamber, thereby achieving uniform heating of objects having different shapes and volumes, temperature sensors should be used to monitor the treatment of different objects, thereby finding suitable radiation intensity and treatment time for different objects. Tests performed by the inventors showed that microwave treatment time of less than 5-7 minutes was sufficient. Depending on the object to be disinfected, the treatment time may be as low as a few seconds, e.g. about 20 seconds, or about 30 seconds. Treatment of large plastic devices will generally be from about 1 minute to about 5 minutes. As stated above, treatment time and intensity (total effect from magnetrons) should be identified for different objects, to ensure sufficient heating time of all parts of the object, and to avoid overheating of any parts as such overheating may destroy the plastic devices. It is possible to identify suitable treatment time and microwave intensity, and possibly position in the treatment chamber, for different standard items, thereby by using recorded treatment requirements for the different items it is possible to treat the devices in a reliable and efficient way.

(19) In a method for disinfecting a plastic conveyor band, the dismantled plastic parts should be washed according to conventional practice, and as described above using wash water that contains at least one electrolyte. The said plastic parts of the conveyor band may be washed by any suitable methods, either manually or automatic. The conveyor band may be washed by e.g. water jetting or flushing, while running, before being taken apart for microwave treatment. The plastic conveyor belt parts, wetted by the electrolyte solution, as described above, is thereafter transferred into the microwave oven, which may be of the same type as explained above, and subjected to microwave radiation in a sufficient time to achieve the desired rate of killing of the vegetative cells. Treatment time of less than 5 minutes will in most cases be sufficient to kill at least 99.99% of all microorganisms present on the plastic parts. It should be noted that in order to ensure suitable treatment time and intensity, temperatures sensors should be used to monitor the progress of the treatment thus ensuring that desired temperatures are achieved in a homogenous way, and without overheating any parts.

(20) To carry out the disinfecting treatment of the plastic conveyor belt parts in a particularly efficient way, a plurality of wetted (by the electrolyte solution) conveyor belt parts may be placed in a load carrier/plastic container suitable for accommodating said parts, and that can withstand microwave radiation, the container may be of the type and of the material as described herein. This method for disinfecting the parts of the conveyor band results in a significantly amount of saved time compared to the traditional methods for sanitizing such conveyor bands, while at the same time achieving a very high degree of disinfection, meeting the goal of killing at least 99.99% of the microorganisms (vegetative cells). The present method efficiently disinfects all parts of a conveyor belt, including damaged parts, internal surfaces and hollow spaces, which might be difficult to sanitize using conventional washing and disinfection methods.

(21) In an automated process to disinfect multiuse plastic devices, the plastic devises can be washed in a washing machine, where the wash solution is the electrolyte solution as described herein. After washing and rinsing the plastic object, still wetted by the electrolyte solution, especially in damaged areas, are directly transferred, e.g. by a transport band or rolling conveyor, into a microwave oven for treatment. The microwave oven may be equipped with two doors, thus after microwave treatment, the plastic device can be further moved by the transport band or rolling conveyor to a temporary storage, e.g. for repairing any damages on the plastic device.

(22) In order to make the present invention more readily understood and show disinfection treatment effect of the present method, reference is made to the following examples, which are intended to be illustrative only and not intended to be limiting the scope.

(23) The method according to the second aspect of the invention, to disinfect plastic trays used for screening at security checkpoints, is a simple, effective and reliable method of disinfecting such plastic trays. The method has a killing rate of at least 99.9%, such as 99.99%, and may even completely eliminate pathogenic microorganisms, including viruses, present on the plastic tray before the treatment. The present method is realized without usage of strong disinfectant and strong chemicals, and represents an environmental friendly method of disinfecting the plastic trays.

(24) It should be understood that by disinfecting in this context, it is meant a method that kills/eliminates essentially all vegetative microorganisms, including viruses, initially present on the plastic tray. By essentially all, it is meant by at least 99.9, or more advantageously at least 99.99% of vegetative microorganisms and viruses initially present on the plastic tray.

(25) The term pathogenic microorganism as employed herein should be understood to include microorganisms that can produce disease. Pathogenic microorganism includes pathogenic bacteria, viral pathogens, pathogenic fungi, prionic pathogen, parasites and algal pathogen. Microorganism should be understood to also include viruses in the present context. Pathogenic bacteria in the present context refers principally to pathogenic vegetative bacteria.

(26) In the present context the term plastic tray should be understood to denote any plastic tray, bin, box or container used for holding personal belonging for screening at security checkpoints. Such plastic trays are also commonly denoted security trays. Plastic security trays come in different sizes, some examples of commercial security trays have sizes 530360110 mm; 550390140 mm; and 635525110 mm. The plastic trays can typically be made of polyethylene materials, polypropylene material, or other suitable plastic material for such purposes, generally known in the art.

(27) The present method of disinfecting plastic security trays used for screening at security checkpoints essentially includes the steps of contacting the plastic tray with an aqueous solution, and transferring the thus wetted plastic tray into a microwave generator in which the plastic device is subjected to microwave radiation treatment for a sufficient time to eliminate essentially all pathogenic microorganisms present on the plastic tray. The aqueous solution for wetting the plastic tray may be water, such as tap water, or the aqueous solution may comprise added electrolytes (ions). The present invention utilizes the fact that moist heat kills microorganisms more effectively than dry heat. As microwaves penetrate through plastic material without significantly heating the plastic material, water or liquid containing areas, also in damaged areas inside the plastic tray, will be rapidly heated, thus any microorganisms are killed. Ions (electrolytes) in the aqueous solution increases the heating rate during microwave treatment significantly, therefore even small droplets will be heated fast enough to kill the microorganisms before the droplets evaporate. In addition, the number of microorganisms being killed increases due to increasing osmolarity of the electrolyte aqueous solution as the water is evaporating. The aqueous solution may also comprise a foaming agent which aids the disinfection process by bubble formation during the microwave treatment, thus expanding the heated solution to colder areas, and thereby efficiently and quickly increasing the areas being treated.

(28) Before the microwave treatment of the plastic tray, the tray is contacted with an aqueous solution, which may comprise (a) dissolved salt(s) and/or (b) diluted alkali or diluted acid, and/or (c) a foaming agent. If the plastic tray is soiled it should be washed, e.g. by using the said aqueous solution. The washing may be performed by manual methods using e.g. brushes, rinsing and/or pressure washer or by automatic methods, e.g. an industrial washing machine. Examples of suitable salts dissolved in the aqueous solution are NaCl, KCl, Na.sub.2SO.sub.4, MgCl.sub.2, Na.sub.2HPO.sub.4, NaH.sub.2PO.sub.4, K.sub.2HPO.sub.4, KH.sub.2PO.sub.4, or a mixture thereof. Said salts are non-toxic and any salt left behind on the plastic tray after evaporation will not be harmful. The concentration of dissolved salt in water, forming an aqueous electrolyte solution, may be from about 0.5% to about 5.0% (5 g/L to 50 g/L), e.g. from about 1% to 3.5% (10 g/L to 35 g/L.

(29) Alkali (bases) may kill microorganisms in itself when the concentration is relatively high, however if alkali is present in the aqueous solution according to the present method, it should be in a very dilute concentration. Suitable alkalis are NaOH, KOH, or mixtures thereof. During the microwave treatment, as water evaporates, the concentration of the alkali increases, thereby the killing effect of microorganisms increases. Concentration of alkali in the aqueous solution may be from about 0.1% to about 2% by weight. Effective killing of microorganisms and a fast heating rate during microwave treatment may be achieved with concentration of alkali between about 0.4% and about 1.5% by weight in the aqueous solution. It is however desired to reduce the usage of chemicals; therefore the concentration of alkali, if present, should advantageously be in the lower area, e.g. about 0.2% to about 0.5% by weight.

(30) Instead of an alkali, an acid may be used. Suitable acids are e.g. hydrochloric acid (HCl) and phosphorus acid (H.sub.3PO.sub.4). Correspondingly as the alkali, acid may kill microorganisms in itself at relatively high concentration. If acid is present in the aqueous solution, it should be in a very dilute concentration. During the microwave treatment as the water evaporates the acid concentration increases which increases the microorganism killing effect. Concentration of acid in the aqueous solution, forming an electrolyte solution, should be from about 0.1% to about 2% by weight, e.g. from about 0.4% to about 1.5% by weight. Due to the desire of reduced usage of chemicals, the concentration of acid, if present, should be about 0.2% to about 0.5% by weight. As the water evaporates the concentration of the acid or alkali increases to a level that leads to killing of microorganisms, which may be important in very small droplets.

(31) The aqueous solution may also comprise at least one foaming agent, i.e. bubble/froth forming substance. The foaming agent may be a soap. The amount of said foaming agent can be according to recommended amounts indicated by the soap producer. Advantageously the amount of foaming agent is in the lower recommended range, to reduce the amount of any remaining foaming agent on the plastic tray surface after disinfection treatment. Presence of a froth forming agent in the aqueous solution may have positive effect during the disinfecting treatment in the microwave oven, since bubbles are produced during the microwave treatment as the water starts boiling. The froth expands the volume of aqueous solution present on the plastic surface, so that heat is moved over larger areas. The aqueous solution may be water, water comprising at least one foaming agent, water comprising at least a foaming agent together with electrolyte(s) from (a) dissolved salt(s) as specified above, and/or (b) diluted alkali or diluted acid, as specified above, or water comprising (a) dissolved salt(s) as specified above, and/or (b) diluted alkali or diluted acid, as specified above.

(32) Hence, the aqueous solution suitable for washing/wetting the plastic tray and microwave treatment does not contain any strong sanitizing and/or disinfecting chemicals that require additional precautions regarding handling, storage of use. The present aqueous solution does not contain harmful substances, such as oxidizing chemicals or disinfectants, which may put the workers and/or the environment at risk.

(33) After washing and rinsing the plastic security tray, the plastic tray is treated by microwave radiation. As indicated above, the plastic tray should still be wet after the washing/wetting, thus having a film of the aqueous solution on the surface, before the treatment in the microwave chamber. The wet plastic security tray is subjected to microwave radiation for a sufficient amount of time to disinfect all parts of the surface. Heating by microwaves radiation kill microorganisms by heating in the same way as conventional heat treatment. The rate of killing is a function of time and temperature. The difference is that by using microwave radiation, the heating occurs much faster compared to conventional heating means, and primarily in the water phase, both on the outer surface and the inside of the plastic tray, in case of any damaged areas. Total energy consumption is therefore significantly lower than for conventional heating. There is, however, a risk that when using microwaves for heating, temperature gradients over relatively short distances may be created, and microorganisms can survive in cold and dry spots on the plastic tray. Consequently, quick heating rate and uniform distribution of microwaves for sufficient heating is important to achieve satisfactory results. This implies the recommended method to disinfect wet plastic trays immediately after washing/wetting and by using a microwave system that provides a homogenous microwave field in the treatment chamber.

(34) The microwave-heated system for performing the present disinfecting method should thus generate a homogeneous microwave field. By a homogenous microwave field, it is meant that the microwave oven should distribute the microwaves as uniform as possible such that cold spots are as few or as unlikely as possible. It may be challenging to calculate the exact radiation distribution inside a microwave oven as the objects to be treated and the amount of water and biological material varies. To estimate the radiation distribution in a microwave oven, statistical methods are therefore used. A feasible probability distribution is the Chi-quadrat distribution, which expresses the sum of energy from n independent sources, where the parameter n is referred to as the number of degrees of freedom. A microwave oven may generate different standing waves of cold and warm zones inside the oven, these are called modes. Each mode supplies electromagnetic fields inside the microwave chamber in different positions. Different modes can be present at the same time or they may be shifted consecutively e.g. by using movable reflector or stirrer. Large industrial microwave ovens usually have a greater number of modes which increases the number of degrees of freedom and gives more uniform microwave distribution in the oven. The number of degrees of freedom n can be estimated to:

(35) $n=2\frac{{E\left(P\right)}^2}{\mathrm{VAR}\left(P\right)}$

(36) Where P is the energy delivered to several places in the oven (e.g. measured by temperature sensors), and the expectation is E(P) and the variance is VAR(P). In an oven with 64 degrees of freedom it is 99.99% probable that the radiation intensity on a random position inside the oven is at least 47.5% of the average radiation intensity. It should however be understood that objects inside the microwave oven will in varying degree affect the oven by absorbing energy and develop heat. Microwave ovens with a hexagonal treatment chamber and/or a plurality of magnetrons are especially suitable for performing the microwave treatment according to the present invention, because such system give a very uniform distribution of radiation intensity. It should be noted that the microwave chamber may have other shape than hexagonal, such as a rectangular shape, pentagonal or square shape. Microwave are a form of electromagnetic radiation with wavelengths ranging from about one meter to one millimeter, with frequencies between 300 MHz (1 m) and 300 GHz (1 mm). The microwave radiation used in microwave ovens usually have frequency of between 915 MHz to about 25 GHz. A standard microwave frequency suitable for the present microwave system is e.g. 2.45 GHz.

(37) For logistical reasons, and to be able to treat relatively large plastic security trays, possibly more than one, or a plurality, simultaneously, the microwave oven for carrying out the present disinfection method can be a large industrial microwave oven, having a treatment chamber that can accommodate large plastic security trays, or a plurality of trays, with dimensions as specified above. Such microwave ovens may be equipped with several magnetrons, which ensure a homogenous distribution of electromagnetic radiation in the treatment chamber. Examples of such ovens are microwave systems with 6-36 magnetrons, e.g. 6-24, or 6-12 magnetrons, each having e.g. an effect of 0.5-5 kW, e.g. 1-3 kW. The treatment chamber may have a volume of about 0.15-3 m.sup.3. The treatment chamber may be designed as a rectangular box, hexagonal box, square box, or any other suitable shape which allows a uniform distribution of electromagnetic radiation, while having capacity of accommodating a desired number of plastic trays to be treated. If several plastic trays are treated simultaneously, such as 10-30 trays, these should conveniently be placed e.g. on a rack system. It should be noted that microwave ovens suitable for carrying out the present method may vary in dimensions and effects, and the skilled person should readily be able to choose suitable microwave systems adapted for the size and amounts of plastic security trays that is to be treated. It should also be noted that in order to ensure sufficient distribution of microwaves in the treatment chamber, thereby achieving uniform heating of plastic trays having different sizes and volumes, temperature sensors may be used to monitor the treatment of different sizes of security trays, thereby finding suitable radiation intensity and treatment time.

(38) Tests performed by the inventors showed that microwave treatment time for a plastic security tray can be less than 30 seconds, e.g. less than 25 seconds. Depending on the size of the plastic tray to be disinfected, the treatment time may be about 7-20 seconds, e.g. 10-20 seconds. Treatment of more than one plastic tray simultaneously will generally involve longer treatment time compared to treatment of only one plastic tray, since the total amount of water that needs to be heated is greater when a larger number of trays are treated at the same time. The treatment time of several trays (e.g. 20-30 trays) at the same time may therefore be about 1-3 minutes. However, the treatment time for each plastic security trays will be small, probably less than 10 seconds for each plastic tray when several are treated at the same time. As stated above, treatment time and intensity (total effect from magnetrons) can be identified for different sizes and number of security trays, to ensure sufficient heating time of all parts of the tray, and to avoid overheating of any parts as such overheating may destroy the plastic trays. It is possible to identify suitable treatment time and microwave intensity, and possibly position in the treatment chamber, for different standard tray sizes, thereby by using recorded treatment requirements for the different security trays it is possible to treat the trays in a reliable and efficient way.

(39) In an automated or semi-automated process to disinfect plastic security trays, the plastic trays can be transported, e.g. on a conveyor line to a first zone, where the plastic security tray is wetted with the aqueous solution, e.g. in a washing machine, by rinsing, by spraying or dipping. After wetting the plastic tray, and while still wet, the plastic trays are directly transferred, e.g. by a conveyor line, into a microwave chamber for treatment with microwave radiation. The microwave chamber may be equipped with gates (e.g. sliding or hinged doors) at one or both ends, thus after microwave treatment, the disinfected plastic device can be further moved by the conveyor line to a collection point of the security trays, or directly returned to the security checkpoint for use. As described above, several plastic trays can be treated in the microwave chamber at the same time. In a practical method this may involve a first zone where the trays are wetted by the aqueous solution, the wetted trays may be placed on a rack and transferred into the microwave chamber for treatment. Another automated or semi-automated process may involve use of machine of robots handling and transferring the plastic trays between zones.

(40) The following is a basis for estimating the killing rate of microorganisms (vegetative bacteria): if it is assumed that 99.5% of the water on a plastic security tray is present as volumes of 0.5 ml or more, and that in these volumes 99.999% of all microorganisms are killed during the microwave treatment while the remaining water volume is distributed in the form of small drops (0.05-0.5 ml) with 98% killing of the bacteria, the proportion of bacteria that survive the treatment can be calculated as:
100%.Math.(N.sub.0.Math.0.995.Math.0.00001+N.sub.0.Math.0.005.Math.0.02)/N.sub.0=0.00099%+0.01%=0.01099%0.01%

(41) N.sub.0=number of living cell before treatment. This corresponds to a killing of 99.99% as the target of the present method.

(42) The present method for disinfecting plastic security trays by the usage of an aqueous solution in combination with microwave radiation is quick and reliable, and ensures that plastic security trays, that are handled by a large number of people, may not become a source of spreading diseases. The method according to the present invention also considerably reduces, and may even eliminate, the usage of strong sanitizing and/or disinfecting agents and chemicals, which is an important advantage since it is an increasing problem that microorganisms develop resistance to disinfectants and antiseptic agents. Since the problem of resistance bacteria is expected to increase in the future it is extremely important to develop disinfection methods that reduces and even avoids the usage of disinfecting chemicals. Attention is also directed to avoid spreading of resistant bacteria and microorganisms, which is especially a risk at security checkpoints at airport where travelers may bring pathogenic microorganisms to several locations. The method for disinfecting by the usage of microwave radiation, according to the present invention, involves a very short treatment time in a microwave oven, thus overheating and hot spots in the plastic trays are avoided. Short treatment time is also important for logistic reasons.

(43) The present method of disinfecting plastic security trays may be used for disinfecting plastic trays which are used in screening at any type of security checkpoints, e.g. at airports, cruise ship boarding, train stations, and buildings (such as offices, museums, tourist attractions, courthouse, schools and governmental buildings, etc.) with security checkpoints.

EXAMPLES

(44) Determination of Disinfection Effect on Load Carriers

(45) Indicator bacterial cells (Enterococcus faecium NCIMB 2699) were grown in 50 ml GBB medium in culture flasks. The cells were harvested, centrifuged (3220g, 15 min., 20 C.) and washed once with KPS-buffer, before resuspension in 1/10 of original culture volume in pure water or KPS-buffer and transferred to sterile plastic reagent tubes with screw cap.

(46) Growth Medium and Dilution Buffer:

(47) GBB-medium (per liter): Glucose H.sub.2O, 1.1 g; K.sub.2HPO.sub.4, 3.68 g; KH.sub.2PO.sub.4, 1.32 g; Lab Lemco powder, 1.0 g; Peptone, 5.0 g; yeast extract, 2.0 g; NaCl, 5.0 g; RO-water, 1000 ml, pH=7.2.

(48) KPS-buffer (per liter): K.sub.2HPO.sub.4, 0.87 g; KH.sub.2PO.sub.4, 0.68 NaCl, 9.0 g; RO-water, 1000 ml, pH=7.2.

(49) The germ number in the samples was determined as most probably number (MPN) in a 96 well plate-based method. The sample was diluted in a tenfold dilution series in KPS-buffer, and from each relevant dilution 50.1 ml sample was transferred to 5 wells containing 0.1 ml double concentration GBB-medium (all components added in double concentration) on the 96 well plate. The well plate was packed in plastic and incubated overnight at 37 C. Wells with visible growth (turbidity and cell sediment) were registered as positive and most probable germ number was determined from tables arranged by Robert Blodgett (s.a) and available at US Food and Drug Adm. (BAM Appendix 2: Most Probable Number from Serial Dilutions, October 2010, url: www.fda.gov/Food/FoodScienceResearch/LaboratoryMethods/ucm109656.htm).

(50) The tests were carried out on load carriers with approximate external dimensions (hbl) 100 cm80 cm80 cm, total inner volume about 330 L. Holes were drilled to different depths at several places in walls and bottom of load carriers, and vials with bacterial suspension (volumes from 15-400 l) were placed in the drilled holes. The microwave system used in the experiment was a microwave oven with a hexagonal chamber and equipped with 36 magnetrons, thus ensuring a very homogeneous dispersion of microwaves inside the oven. The microwave oven was equipped with infrared camera to monitor the heat generation during the treatment of the load carriers. The oven was also equipped with fiber optic sensors for logging of temperatures in different places in and on the load carrier and in bacterial suspensions during the treatment. In this way, the heating of various fluids in the microwave as a function of location and different liquid volumes was tested. During treatment, the temperature in the bacterial suspensions and load carriers was measured and the degree of killing of the indicator bacteria was determined as a reduction in germ numbers, table I and table II. The bacterial suspensions in table I were suspended in pure water while the bacterial suspensions in table II were suspended in KPS-buffer. In addition, a detergent ADDI SU 930 was added to some vials in table I and table II. ADDI SU 930 (producer Lilleborg AS) is a strong alkaline detergent for foam and high pressure cleaning in the food industry comprising 10-30% NaOH, 1-5% ethanol, 1-5% alkyl-glucoside, 1-5% di-methyl-amine-oxide.

(51) TABLE-US-00001 TABLE I Test results where the bacteria were suspended in pure water. Samples where the goal of 99.99% killing was achieved is highlighted. Susp. Germ number Approximate 95% Sample volume after treatment confidence interval Survival no. (l) (MPN/ml) (MPN/ml) (%) Other observations B 1 400 5.1 .Math. 10.sup.7 (1.7-12.0) .Math. 10.sup.7 0.6 B 2 400 2.8 .Math. 10.sup.6 (1.0-6.9) .Math. 10.sup.7 0.4 B 3 400 9.9 .Math. 10.sup.7 (3.0-30.0) .Math. 10.sup.7 1.2 B 4 135 5.9 .Math. 10.sup.8 (1.6-16.3) .Math. 10.sup.8 7.3 B 5 135 .sup.2.6 .Math. 10.sup.10 .sup.(0.7-8.1) .Math. 10.sup.10 324 B 6 135 3.6 .Math. 10.sup.8 (1.1-11.1) .Math. 10.sup.8 4.5 B 7 45 7.3 .Math. 10.sup.9 (2.2-22.2) .Math. 10.sup.9 92 B 8 45 7.3 .Math. 10.sup.9 (2.2-22.2) .Math. 10.sup.9 92 Ampoule almost dry after treatment B 9 45 3.8 .Math. 10.sup.8 (1.3-8.9) .Math. 10.sup.8 4.7 Ampoule almost dry after treatment B 10 15 .sup.1.1 .Math. 10.sup.10 .sup.(0.3-3.1) .Math. 10.sup.10 133 Ampoule almost dry after treatment B 11 15 .sup.1.1 .Math. 10.sup.10 .sup.(0.3-3.1) .Math. 10.sup.10 133 Ampoule almost dry after treatment B 12 15 3.6 .Math. 10.sup.9 (1.0-11.3) .Math. 10.sup.9 45 Ampoule dry after treatment BV 13.sup.A 45 <3 .Math. 10.sup.1 <0.00001 Ampoule dry after treatment BV 14.sup.A 45 2.2 .Math. 10.sup.4 (0.7-4.4) .Math. 10.sup.4 0.006 B 15.sup.B 45 4.9 .Math. 10.sup.9 (1.6-9.8) .Math. 10.sup.9 61 Ampoule almost dry after treatment .sup.AAn additional 45 l wash solution (1.5% ADDI SU 930) was added to the ampoule. .sup.BA long ampoule (4.5 ml total volume) to ensure that the sample was deep into the thickest part of the load carrier.

(52) TABLE-US-00002 TABLE II Test results where the bacteria were suspended in KPS-buffer. Samples where the goal of 99.99% killing was achieved is highlighted. Susp. Germ number Approximate 95% Sample volume after treatment confidence interval Survival no. (l) (MPN/ml) (MPN/ml) (%) Other observations B 1 400 2.0 .Math. 10.sup.4 (0.5-5.5) .Math. 10.sup.4 0.0002 Ampoule dry after treatment B 2 400 8.5 .Math. 10.sup.3 (3.5-25.0) .Math. 10.sup.3 0.0001 Ampoule dry after treatment B 3 400 2.8 .Math. 10.sup.3 (0.9-6.5) .Math. 10.sup.3 0.00003 Ampoule dry after treatment B 4 135 <2.2 .Math. 10.sup.3 <0.00003 Ampoule dry after treatment B 5 135 9.6 .Math. 10.sup.3 (3-26) .Math. 10.sup.3 0.0001 Ampoule dry after treatment B 6 135 2.4 .Math. 10.sup.4 (0.7-7.4) .Math. 10.sup.4 0.0003 Ampoule dry after treatment B 7 45 2.9 .Math. 10.sup.7 (0.8-8.9) .Math. 10.sup.7 0.4 Ampoule almost dry after treatment B 8 45 5.1 .Math. 10.sup.7 (1.5-16) .Math. 10.sup.7 0.6 Ampoule almost dry after treatment B 9 45 1.1 .Math. 10.sup.8 (0.3-3.3) .Math. 10.sup.8 1.4 Ampoule almost dry after treatment B 10 15 1.5 .Math. 10.sup.8 (0.5-47) .Math. 10.sup.8 1.9 Ampoule dry after treatment B 11 15 2.2 .Math. 10.sup.8 (0.7-67) .Math. 10.sup.8 2.8 Ampoule dry after treatment B 12 15 7.3 .Math. 10.sup.8 (2.3-16.7) .Math. 10.sup.8 9.2 Ampoule dry after treatment BV 13.sup.A 45 5.1 .Math. 10.sup.3 (1.5-15.6) .Math. 10.sup.3 0.00006 Ampoule dry after treatment BV 14.sup.A 45 5.1 .Math. 10.sup.3 (1.5-15.6) .Math. 10.sup.3 0.00006 Ampoule dry after treatment B 15.sup.B 45 1.8 .Math. 10.sup.8 (0.5-4.9) .Math. 10.sup.8 2.2 Ampoule almost dry after treatment .sup.AAn additional 45 l wash solution (1.5% ADDI SU 930) was added to the ampoule. .sup.BA long ampoule (4.5 ml total volume) to ensure that the sample was deep into the thickest part of the load carrier.

(53) The results show that small volumes and cells suspended in pure water increased bacterial survival rate (see Table I and Table II). However, the presence of the alkaline detergent ADDI SU 930 resulted in a greatly increased bacterial killing. An interesting and highly unexpected observation was that the presence of ions from dissolved salt in the water (KPS-buffer) promotes killing of bacteria, see Sample No. B1-B6 in Table II.

(54) Experiments in laboratory scale and industrial plant showed that the killing of the indicator organism E. faecium NCIMB 2699 was dependent on the volume of fluid the bacteria were suspended in and the ionic content in the fluid. At very small volumes (<100 l) the killing was less effective, and especially if the bacteria were suspended in clean water. In practice, however, it is expected that most of the water in and on a load carrier after washing and cleaning will be present in larger water collections in the damaged areas, rather than small water droplets (a drop of water is typically about 50 microliter). It is also reasonable to expect that the number of bacteria per water collection will be somewhat proportional to the volume of liquid. Thus, the overall killing rate for a large load carrier will be strongly affected by the distribution between small and large water volumes.

(55) The following is a basis for estimating the killing rate: if it is assumed that 99.5% of the water in and on a load carrier is present as volumes of 0.5 ml or more, and that in these volumes 99.999% of all vegetative bacteria are killed during the microwave treatment (see results in Table II) while the remaining water volume is distributed in the form of small drops (0.05-0.5 ml) with 98% killing of the bacteria, the proportion of bacteria that survive the treatment can be calculated as:
100%.Math.(N.sub.0.Math.0.995.Math.0.00001+N.sub.0.Math.0.005.Math.0.02)/N.sub.0=0.00099%+0.01%=0.01099%0.01%

(56) N.sub.0=number of living cell before treatment. This corresponds to a killing of 99.99% as the target of the present method.

(57) By the method described herein, it is thus provided a method that can be used for disinfecting multiuse plastic devices, including large multiuse plastic devices, which are prone to damages on the surface or internal parts, wherein potentially photogenic bacteria may grow. The present method may be used for treating said multiuse plastic devices before repairing any damaged parts. The present method may also be part of a hygiene routine in a food processing facility, hence ensuring a clean and sanitized production line for the food processing, thereby minimizing the risk of infecting and spreading harmful bacteria to food product.

(58) Having described preferred embodiments of the invention it will be apparent to those skilled in the art that other embodiments incorporating the concepts may be used. These and other examples of the invention illustrated above are intended by way of example only and the actual scope of the invention is to be determined from the following claims.