GB2578437A

GB2578437A - A can and treatment method of the contents using electric fields

Info

Publication number: GB2578437A
Application number: GB1817432.6A
Authority: GB
Inventors: Jones Edward; Stuart Ellis Bell Robert; John Maloney Norman
Original assignee: C Tech Innovation Ltd
Current assignee: C Tech Innovation Ltd
Priority date: 2018-10-26
Filing date: 2018-10-26
Publication date: 2020-05-13
Also published as: GB201817432D0

Abstract

A can 1 for foodstuffs is detailed which comprises a metal side wall 2 with a high dielectric strength isolating surface or layer 8,4, and metal end(s). A central part of an end(s) has an electrically conductive surface or layer and a peripheral part of an end(s) has an electrically isolating high dielectric strength surface or layer, and a conformable sealing layer. The sealing layer may itself be made from a high dielectric strength electrically isolating material, the end(s) and the side wall and the sealing layer 9 co-formed in a double seam seal (figure 1). An electric field can be applied to result in joule pasteurizing or sterilizing of the contents of the can. The can may be subjected to a voltage of up to 5000V at one end to produce the heating effect. A combination of voltages can be applied to cause joule heating and electroporation.

Description

A can and treatment method of the contents using electric fields Technical Field [0001] This invention relates to a metal can and a method of treating and heating food-stuffs in a metal can by applying electric fields. Background Art [0002] Canned food is in widespread use because it has a long shelf life at ambient temperature. The cans are sealed, and the contents heat treated in the pack after sealing. Typical thermal treatments seek to achieve FO (equivalent exposure time at 121°C) of 6 minutes or longer. This ensures that there are low bacterial loads resulting in the long shelf life. Metal cans are also very effective as an oxygen barrier.

[0003] There are various types of cans in use with different geometries and sealing arrangements. One type of can in common use is the round cylindrical three-part can. This design is used for a great variety of foodstuffs including vegetables, soups, meat and dairy products.

[0004] Three-piece cans are constructed from flat sheets of metal, usually steel. The circular ends are stamped out and the body are formed by rolling a flat sheet into a cylinder and welding the seal. Coatings may be applied to surfaces of the three parts before the parts of the can are joined together. Standard coatings may be polymeric or metallic. The purpose of the coatings is to reduce the rate of corrosion on the internal and external surfaces of the can and to create a barrier coating to prevent tainting of the contents by the can material and to preserve the cosmetic clean and untarnished appearance to the inside of the can.

[0005] Typically, the end of the can is joined to the can side using a double seal to create an open vessel. The double seam seal is a method whereby the two edges are joined together by first rolling and then folding the seam resulting in a multi-layered joint. A polymer sealant may be locally applied to one of the surfaces before the join is made to prevent the formation of any air-gaps or leak-paths in the joint. This open part-finished can is then filled with the foodstuff. The end piece is then placed on the can and the can sealed using the same double-sealing method.

[0006] Two-piece cans where the body and one end are a single part and the filling-closing end is a separate part are also in very common use. Often the preformed closed end part is formed by a deep drawing and wall ironing (D&I) process. Two-piece cans for food and beverage preservations also use a double seam seal at the filling-closing end.

[0007] Two-piece and three-piece cans can be rectangular oval or squared in shape (plan view).

[0008] The sealed can is then heat treated to sterilise or stabilise the foodstuff. This is commonly done by means of a retort containing hot water or other liquid or for higher temperatures by means of a pressurised system using steam or other heating means. Typically, this may involve heating a can until the contents reach 121°C with a cycle time of between 10 minutes and 60 minutes.

[0009] Retorts may be static or rotating or employ movement and shaking of cans and containers to agitate the contents to speed up the heat transfer within. Retorts may operate as batch processes or continuous processes. Retorts which rotate, and which shake the containers achieve faster cycle times compared to retorts where the product is static however they are more effective with low viscosity contents which are readily mixed and for contents which are not damaged or degraded by the shaking. Shaker type retorts at oscillation rates (e.g. 60 to 100 double strokes per minute) subjects the contents to 1200 to 2000 vigorous shakes for a sterilisation process heating to say 121°C and taking ten minutes. This is damaging to the contents and impairs product quality particulate identity and product rheological properties. Static retorts have cycle times of up to 60 minutes which is damaging due to over processing of much of the contents. Retorts may be continuous through flow processes which have means of introducing and removing cans from a pressurised environment by means of pressure-retaining inlets and outlets.

[0010] The canning processes which are in common use have cycle times that are determined by the physical constraints of heat transfer from the heating medium through the can walls and into the body of the foodstuff. This heat-transfer is by conduction and this places a lower limit on the processing time. Approaches to reduce this time such as shaking are described but they are only partially successful. It would be advantageous to reduce the processing time required to better to retain the food properties such as flavour and nutritional content and colour and texture.

[0011] Joule heating is known as a means of heating foodstuffs by passing an electrical current directly through the foodstuff. The heating is a consequence of the internal electrical resistance of the foodstuff. Joule heating is known for heating foodstuffs in industrial processing. It has been considered also for heating foodstuffs within packaging, but no method has existed to successfully heat contents by this means within an all metal can.

[0012] The commonly used grades of can metal have various coatings some of which have limited electrically insulating properties, but they are not sufficiently mechanically robust assuredly to maintain electrical insulation after the seam has been formed.

[0013] US 2013675 A (AMERICAN CAN CO) 10/09/1935 discloses a means of adapting a metal can for the purposes of electrically heating the contents by means of inserting a separate lining with electrically insulating properties inside of the can body. This insulation lining is extended into the two end seams so that one end is electrically insulated from the can body. Whilst this does achieve the objective of electrically isolating the contents of the can from the walls of the can it is not a practical or suitable means for use in high volume canning because the physical integrity of insulating layer that is required to withstand the seal rolling and folding process means that the insulating lining layer is of an unsuitable specification for the walls of the can. (Low voltages are employed which will not heat or preserve the full range of foodstuffs commonly found in cans and which will not have any significant electroporation effect) [0014] US 3863048 A (MORTON C BUCKLEY) 28/01/1975 describes an electric resistance heater cooker for a food package. In this case the purpose of the device is to cook a portion of packaged food placed into the device and the device does not heat up an enclosed or canned portion of food.

[0015] US 3877360 A (ELECTRO FOOD) 15/04/1975 describes a container such as a tin can with an arrangement of electrodes that allows the contents to be heated by means of joule heating. In this case the can or container includes a channel which can be opened to the outside and an electrode inserted without contacting the foodstuff. The channel can then be withdrawn if required and then an electric current passed through the foodstuff. The second electrode is formed by the outer metal wall of the container or by a sleeve member inserted into a peripheral chamber in the container.

[0016] US 3886290 A (NAT ELECTRO COOK CORP) 27/05/1975 describes an arrangement for the ohmic heating of foodstuffs that are combinations of materials with high and low conductivities, such as Frankfurter or hot-dog sausages and bread buns.

[0017] US 4100302 A (LECTROFOOD CORP) 11/07/1978 US patent 4,100,302 describes an apparatus for the electrical heating of foodstuffs such as ground meat patties, sausage, pizza topping, kielbasa, blintzes, egg rolls, cold cuts, cold cuts-cheese combination, and cold cut-chopped liver combination.

[0018] US 2013/062332 A (THE OHIO STATE UNIVERSITY) 14/03/2013 describes a rectangular prism-shaped food packet formed of a multilayer laminate material that includes a metallic foil layer sandwiched between interior and exterior polymer layers and an arrangement to allow electrical connections to electrodes within the package.

[0019] US 2013/186879A A (WOLFGANG TILZ ET AL) 25/07/2013 US patent 2013/0186879 disclose various means of electrically heating multiple cans, including conveyor systems and systems for simultaneously treating multiple cans, the objective being to increase the throughput of such a system.

[0020] DE ALWIS, AAP, et al. The Use of Direct Resistance Heatiing in the Food Industry. Journal of the Food Industry. 1990, vol.11, no.1, p.3-27. review the industrial use of direct resistance heating for food processing including its application to in-can sterilisation. The review notes that none of the methods described have reached commercial application and further notes that none of them anticipated or addressed the issue of the electrolysis of the metallic electrodes. It is necessary to resolve this issue to pass the current through the food.

[0021] It can be seen from the above-mentioned examples that joule heating has been considered for use with packaged food, but none addresses the mechanical and electrical requirements to achieve the simultaneous hermetic sealing and electrical breakdown strength through a double seamed can seal and thereafter the required but different electrical breakdown strength required with voltage gradient along the internal surface of the can.

Summary of invention

[0022] According to the present invention a can comprises a metal side wall with a high dielectric strength isolating surface or layer, and metal end(s), a central part of an end(s) which has an electrically conductive surface or layer, a peripheral part of an end(s) which has an electrically isolating high dielectric strength surface or layer, and a conformable sealing layer which may itself be made from a high dielectric strength electrically isolating material, the end(s) and the side wall and the sealing layer co-formed in a double seam seal.

[0023] In this specification and appended claims "high dielectric material" is taken to mean a material whose dielectric strength is adequately to insulate the can ends from to isolate the can ends from the side wall in a double seam seal. Materials with strengths of lkV/mm to 2MV/mm provide a range of materials suitable for use in different situations.

[0024] In one embodiment at least one can lid and can base have an additional electrically conducting layer their interior walls [0025] In another aspect of the invention a method of joule heating the contents of a can, the can comprising a metal side wall, an electrically conducting can lid and an electrically conducting can base each having an interior wall and an exterior wall, at least one of the electrically conducting lid and base being sealed to the side wall with a conformable electrically isolating layer formed between at the least one electrically conducting can lid and can base and the side wall, includes the step of connecting the lid and base of the can to an electrical supply applying an electric field to the food contained in the can.

[0026] The treatment may be solely thermal using joule heating applying low electric fields continuously or the treatment may be nonthermal using electroporation applying high electric fields for very short durations or it may have the characteristics of both applying moderate electric fields or it may involve a combination of these treatments together or sequentially.

[0027] Other features of the invention are set out in the accompanying description and claims.

[0028] This invention enables joule heating to be realised with a metal can constructed substantially from conventional materials commonly used to construct can bodies. This overcomes the heat transfer limitations associated with conventional in-can pasteurisation and sterilisation methods and significantly reduces the cycle time with the effect of improving the quality of the product. As a result, the heating time for the sealed can be reduced very significantly.

[0029] Surprisingly we have found that co-forming the double seamed seal with the additional layer of insulating material in the region of the seam and combining material and surface properties formed thereby created a seam between an end and the wall of a can with the mechanical robustness required of the isolation in the seam as well as the required hermetic sealing. This solves the problem of achieving the electrical insulation required without having to introduce a non-standard thick lining material into can whether as a separate piece or as a thick coating on the can material [0030] The invention allows a joule heating sterilisation cycle heating to 132°C in 30 seconds with hold of just 30 seconds, providing sterilisation performance equivalent to a static retort cycle time of up to 60 minutes to reach the required temperature throughout the contents and then to hold the contents at that temperature -i.e. 60 times faster. Shaking retorts have a cycle time of about 10 minutes, so this invention allows a sterilization cycle for food in a can 10 times faster. The joule heating processing method used in the invention, furthermore, may realise a continuous operation sterilisation process for contents in cans. It is not subject to interruptions for loading, unloading, longer heat up times and cooling times prevalent with batch retort processes.

[0031] Metal cans will distort outwards "peaking" under excessive positive differential pressure (internal pressure -external pressure). Standard cans resist this peaking up to differential pressures of 2.3 bar. The saturation temperature of steam corresponding to 2.3 bar is 137 °C. It is feasible particularly with vacuum filling to sterilise cans unsupported up to temperatures approaching this. However, in a further embodiment of the invention externa support is provided for the cans while being processed if required or if cycles demand such.

[0032] As the electric field strength applied to the contents increases effects alter from predominately joule heating with a very mild electroporation effect to increasing electroporation effects at higher electric fields. Such so called "non-thermal" electroporation is also effective for microbial inactivation. These nonthermal methods generally employ moderate and high electric fields applied for very short periods of time. It is known that these effects will combine in a synergistic way. Electroporation alone will not kill spores but high temperature will. Electroporation at higher temperatures however becomes much more effective and requires lower electric fields compared to that needed when it is conducted at lower temperatures. This invention provides a method to utilise joule heating or electroporation of any combination of these effects within a metal can.

[0033] "Electroporation", in the context of this invention, means the application of a high strength electric field to the contents of a can, according to the invention, for short period sufficient to disrupt the cell structure of microorganisms that may be present to deactivate them either alone or in combination with a high temperature.

[0034] The electrical and mechanical requirements for the can seam seal required are particularly onerous for the high voltages required to apply high electric fields and surface treatments and materials are combined to achieve the required electrical breakdown strength.

[0035] The invention provides for the possibility of performing the treatment process as part of the canning process on one machine or for the heat treatment process to be conducted separately.

Brief description of drawing

[0036] The invention will be more fully described with reference to the accompanying drawing which shows a section through part of the side and part of the lid or the base of a can.

Description of embodiments

Electrical considerations [0037] The invention requires that the side wall of a can be electrically isolated from at least one of the base or lid ends. Direct electrical mechanical contact with an end is a simple method to apply voltage to an end and thereby apply an electric field to the contents which delivers energy to the contents when an electric current flow through the resistive contents. Can bodies may be tin plated steel uncoated or steel with a variety of internal coatings. Some standard internal coatings are polymer and these can possess some small resistance to electrical breakdown at 4V to 6V as is sometimes used to test the integrity of the coating between two localised test points. The coatings are not capable of and are not designed for electrical breakdown resistance at elevated voltages. In conventional cans, there is no electrical isolation across the body flanges through the double seam and onto the external wall.

[0038] Figure 1 illustrates part of a can according to the invention and shows a cross section through the seal formed between the side-wall of a can and the lid according to the present invention. A can 1 has a side wall 2 of a can, typically a cylindrical shape and formed of metal. Typically, metal cans are tin plated steel, tin-free steel or aluminium. The can side wall 2 has an exterior coating 3 applied to the metal of the side wall, usually typically a lacquer of some sort, and intended to reduce corrosion. The material of the side wall 2 and coatings 2 are conventional and commonly used in cans. Electrical isolation to the inside wall of 2 is achieved at 47 This can be a conventional layer, or a treatment or an electrically isolating method as described below. The can 1 has a metal end piece 5. For a three-piece can there would be two end pieces, for a two piece can one end piece. For a cylindrical can the end piece(s) is/are normally a circular disc(s), for a can which is another shape, say square or rectangular in cross section, the end piece(s) 5 will be appropriately shaped. The exterior wall of the end piece 5 has a coating 6, normally the same as coating 3. The interior wall of the end piece optionally has a conductive layer 7 to what would be the flat central part 11 of the end piece 5. An electrically isolating layer 8, covers the area between the electrically conducting layer 7 and the rim 10 of the end piece.

[0039] The end of the side wall 2 is are formed into an external flange 12 which bent downwards to form a hook 13 [0040] The end piece 5 is shaped to extend from the flat central portion 11 to extend over the top of hook 13, downwards and then upwards to form a second hook 14 which engages in hook 13.

[0041] A seal material 9 is coated on the inside of hook 13 and 14 and over the electrically isolating layer 8. The electrically isolating layers 4 and 8 extend around the ends of hooks 13 and 14 to ensure complete isolation of the end piece 5 from the side walls 2. The can 1 with the overlapping hooks 13 and 14 which are compressed together against the seal material 9 to form the double seam seal 15.

[0042] Any isolation material for the isolation 8 and 4 will be stretched as the double seam seal is being formed. Therefore, the selected isolation material needs to have good dielectric strength and good ductility properties such that the required electrical isolation is not compromised by the seaming process.

[0043] The thickness of the isolating layer 7 is determined by the dielectric strength properties of the selected material or materials and the voltage that will be applied across the material. The greater the dielectric strength the thinner the isolation layer can be. Likewise, the greater the voltage the thicker the isolation layer must be. The electrical isolation must be guaranteed following the stretching and deformation of the mating parts during the seaming process; if the required electrical isolation is not present after seaming then electrical breakdown and arcing may occur at the seam.

[0044] As an alternative to separate sealant and isolation layers some isolation materials can perform both roles.

[0045] Many polymers and coatings and surface treatments can be used to impart electrical isolation. Many polymers have good dielectric strength i.e. a high resistance to electrical breakdown. This electrical breakdown strength is commonly expressed in Vim or kV/mm. Some typical material breakdown strengths are shown in Table 1.

Table 1

Material kV/mm Material kV/mm Polypropylene 50 Fused silica 500 PTFE 60 A1203 400 Polyimide 200 Diamond 1000 [0046] For a typical 400m1'300 x 401' size can 73mm diameter by 103mm long Table 2 shows some estimated voltages, breakdown strength vs thickness for different contents bulk initial electrical conductivity and for different heating times for 120 °C temperature rise.

Table 2

Conductivity Initial S/m 4°C Heating time secs Voltage rms kV/mm kV/mm kV/mm (thickness 0.02mm) (thickness 0.1mm) (thickness 0.2mm) 0.01 120 120 1506 106 21 11 0.01 120 30 3012 213 43 21 1 120 120 151 11 2 1 1 120 30 301 261 4 2 3 120 120 87 6 1 1 3 120 30 174 12 2 1 [0047] It is expected from calculated powers and for expected contents conductivities and fill arrangement that voltages up to 3 kV could be utilised for purely joule heating of very low conductivity contents. For most foods the applied voltage will be less than 1 kV for purely joule heating. Higher voltages can be applied with great advantage for very short durations in a non-joule heating manner to make use of electroporation effects which occurs when high electric fields are applied to the contents.

Mechanical consideration for the seal [0048] Further considerations to take in account for the selection of an isolation material and seal material is the maximum thickness that can be incorporated into the double seam. The isolating layer that can be applied is limited by the small dimensions and space and tight tolerances required and afforded between the end and the body part through the double seam. Space and voids may be in the order of 0.2mm. If the isolating layer is too thick the seam will be compromised due to the inability to form the desired proven double seam configuration tightly enough with the required overlap. The seam is likely to be weak and prone to unfold. Too thin a layer will not achieve the hermetic sealing and the sealing and filling of voids which are both required through the geometry of the double seam. Too thin a layer of insulation will not possess the required breakdown strength for the treatment of the contents [0049] The selected isolation layer may be of any form such as a film or layer or an applied coating or a surface treatment or combinations of these. The electrical isolation properties of the layers 4 are preferably variably along the voltage gradient through the contents, most robust at the double seam 15 and where the voltage is high, reducing to no breakdown strength at a ground potential end save that typical for the standard barrier coating integrity test (resistance to 4V to 6V applied). The electrical properties can come from a pre-existing layer or treatment on one or both components parts of the double seam seal or via an application of additional material to impart the required electrical breakdown strength along the can.

Hermetic sealing consideration [0050] The selection of the electrical isolating and the other materials or treatments for the seal must be such so that the seal materials conform to fill the gaps and voids in the double seam seal to create a hermetic and pressure resisting seal.

[0051] A can end piece (s) forms an electrical conduction surface or electrode by which an electric field is applied to the contents of the container causing an electric current to flow in the contents from an energy source external to the container.

[0052] The end electrode to contents contact surface layer 7 should comprise a material which is compatible with the contents and which does not impair the contents or render them unsuitable, unusable and in the case of foods unsafe or inedible. Electrolysis reactions and dissolution of electrode materials must be controlled to acceptable levels. Control to acceptable limits is achieved by a) using proprietary materials or coatings which have acceptable levels of electrolysis or dissolution or which prevent these effects altogether and b) by using voltage waveforms and frequency modifications which prevent electrolysis and dissolution or c) by using a combination of these methods. Materials which are acceptable with respect to dissolution include carbon and carbon forms as or within the conductive surface. Alternative materials which prevent electrolysis and dissolution are dimensionally stable anode materials an example of which is Titanium with mixed noble metal oxides coating. Another example is a boron doped diamond coating. Voltage waveforms which prevent electrolysis are bipolar waveforms and, in particular, where the frequency of the waveforms is higher eg > 1kHz and better > 10 kHz. In general electrolysis and dissolution can be prevented by using such elevated frequencies but for low frequencies dissolution must be limited to materials acceptable for dissolution or alternatively must result in an acceptable level of release into or effect on the contents when it does occur or alternatively for low frequencies materials which prevent electrolysis of dissolution must be used.

[0053] A further option for an electrically conductive part or layer acting as an electrode is that conductive part or surface of the can end may comprise a polymer with conductive properties e.g. a polymer incorporating a conductive filler such as carbon nanotubes of a conductive polymer such as polypyrolle containing a conductive filler such as carbon or carbon nanotubes.

[0054] Materials which are not restricted, or which are generally considered as safe for ingestion may form part of the electric conductive properties. These include Tin, Magnesium, Titanium, Carbon, forms of carbon, Silicon.

[0055] Conforming seal materials 9 may include, elastomers, polymers, caulks, low shore strength formulations, RTV compounds, multiple component curing compounds, epoxies, potting compounds and the like [0056] Composite preformed material combinations may be used.

[0057] Composite applied materials and coating and treatments to raw materials may be employed.

[0058] Other material combinations to achieve the sealing performance bringing together the characteristic to conform and survive the seam sealing process.

[0059] A variation for power transfer to the container may include generation of a voltage at the ends non-contacting electromagnetic means.

[0060] The heating power delivered to the contents may be in the range 100W to 10 kW, more typically 2 kW to 8 kW.

[0061] The voltage applied to the can for joule heating may be in the range 50 V to 5000 V, more typically up to 1000 V [0062] The heating, microbial inactivation, preservation action, texture or rheology effects on the contents can be augmented or intensified by application of a defined waveform shape or on-off timing of the applied voltage or by the combination at any time with the application of moderate (100 V/cm to 500V/cm) electric fields or high (500 V/cm to 50000 V/cm) electric fields. Preferably an additional higher electric fields or faster rise time voltages would be applied when the contents are already at high temperatures (> 50 C), since micro-organisms are already highly stressed at elevated temperatures meaning they are more easily killed or inactivated by an additional stress. Additional short duration of applied high fields while the contents are hot means lower than normal electric fields for this treatment need be applied due to the cumulative and synergistic stress applied to the microorganism. A lower thermal input plus a lower intensity nonthermal effect that that which would be required from either alone can be effective for the required preservation. In this way a more optimal energy preservation process can be realised.

Example

[0063] 400 ml tin cans were formed, containing 350g of a foodstuff. The bottom end piece is attached and sealed. No isolation is provided in this. The can is filled and then the top end as described in figure 1 is attached and sealed to the can side walls by forming a double seam seal.

[0064] The can achieves the isolating layer using a combination of two materials applied to those parts of the can side walls can top end piece prior to contact and seal formation. A layer of MomentiveTM RTV118 silicone rubber adhesive is first applied. A layer of polyimide TESA 51408 Tape is added to the silicone layer. The very high mechanical strength of the tape cushioned and co-formed through the seal with the softer sealing material ensures the flange edges contain the required isolation thickness and integrity. The combination of both materials meets the required mechanical and electrical properties.

[0065] The thickness of the internal coating of the cans may vary between applications and can manufacturers. A typical internal polymer layer is insufficient for electrical isolation and breakdown and corrosion is observed when heating voltages are applied.

[0066] The can end pieces were formed from standard canning material. The inside faces of the end pieces ends were formed of Klinger Graphite SLS. This material is pure graphite reinforced with a stainless-steel foil. This layer is in direct electrical contact with the metal of the can end which itself is in electrical contact with the external power circuit by means of a temporary contact point applied to the outer surface of the can end.

[0067] Once filled and sealed at both ends the can was placed in a horizontal orientation and electrical connections made to both ends. The horizontal orientation ensures that the foodstuff is in contact with both ends of the can. An alternating current of 8A at 130 V was then passed between the two ends of the can, the temperature of the contents of the can reaching 90°C. The time taken to reach this temperature was 2 minutes.

[0068] The invention provides a high dielectric strength electrical isolation layer of small thickness and high strength is combined and used along with a conformable sealing material both of which are then co-formed within the dimensions of a canning double seam seal such that the electrical isolation through the seal extends all the way through the seal from the outside to the inside and mates with an electrically isolating material on the body walls inside the can.

[0069] A conductive surface which is resistant to electrolysis and dissolution and is food compatible is incorporated into an electrically isolated seamed end piece as the means to pass electric current and apply an electric field to the contents for of microbial and enzymatic inactivation and pasteurisation and sterilisation and preservation or texture modification by thermal and/or nonthermal means.

[0070] The electrical isolation may derive from an applied material which may be a polymer application film or coating. The electrical isolation may come in whole or part through an applied electrically non-conducting material. The electrical isolation properties may also come in whole or in part via a surface treatment or chemical treatment method of the parts of the can. This could be for example anodising for Aluminium or passivating or plating or other material deposition or transformation method to impart electrical isolation.

[0071] The sealing and electrical isolation may be applied to one end piece or both end pieces of a three part can or to the single end piece of a two-part can or at one end or both ends of a three-piece can.

[0072] The electrical isolation layer through the double seam seal shall have an electrical breakdown strength in the range 1 kV/mm to 2000 kV/mm (or higher) which resists electrical breakdown at the voltages required to treat the contents in the desired time.

[0073] The electrical isolation material should have the strength to survive the double seam formation and compression operations.

[0074] The can side walls can also be electrically isolating capable or resisting breakdown of continuously applied voltages for joule heating ranging from 5000 Vat the high voltage end to OV at the low voltage end. As such the breakdown strength of the electrical isolation material or layer may vary along the can wall.

[0075] The electrical isolation achieved between the ends of the can for a sealed empty container should be such so that when filled only the contents heat when a voltage (typically in the range 50 V to 5000 V) is applied to the ends.

[0076] The electrical isolation and sealing to the ends and of the sides may comprise a single component or material or multiple components or materials or treatments, both on the internal and the external surfaces of the ends and the body as required.

[0077] The electrical isolation provided for between the can ends and the body includes the interface between the double seam seal end hook of the side wall is sometimes measured as the countersink depth.

[0078] The electrical isolation material or treatment method for the seal may be on either the end pieces on the side wall or both. A combination of material or treatments on both may be used to achieve the required electrical isolation.

[0079] The sealing material for the seal may be on either the end of the can or on the body although a combination of material on both may be used to achieve the required electrical isolation.

[0080] The treatment and materials and method for achieving the electrical isolation may be applied locally to specific areas of the can ends and the side walls or they may extend to greater areas or the whole of the surface of the can wall.

[0081] The treatment and materials and method for achieving the sealing may be applied locally to specific areas of the can ends and the side walls or they may extend to greater areas particularly of the side walls.

[0082] The treatment and material giving rise to the electrical isolation may be applied to the bulk sheet raw materials for the container at the time of manufacture of the material or at the time of formation of the individual container parts.

[0083] The treatment and material giving rise to the sealing may be applied to the bulk sheet raw materials for the container at the time of manufacture of the material or at the time of formation of the individual container parts.

[0084] The voltage coupling and electric field application to the end pieces can also be achieved by physical contacting means or by non-contacting electromagnetic means.

[0085] The active electrical conduction area at the end can comprise all or part of the contact area between the end and the internal product.

[0086] The shape of the end pieces can be tuned so that the conduction path through the can contents is varied such that the heating of the contents can be influenced, for example, so that the central volume heats preferentially to the peripheral volume.

[0087] Electrical isolation is created between the opposite can ends where this electrical isolation breakdown value is tailored to the voltage requirements pertaining along the resistive heating path taking into account the voltage drop present. This may be by means of reduced isolating layer thickness or reduces breakdown strength properties of the layer.

[0088] Electrical isolation between the end and the body of the can for the purposes of the invention where one end of the can may be at an elevated voltage requiring isolation from the body with the other end at low/ground potential requiring no isolation.

[0089] There may be high breakdown strength electrical isolation at one end, the high voltage end of the can but little or no electrical isolation at the other end, the low-voltage/ground potential end, and the required high breakdown strength electrical isolation being also present between the side walls and the end piece at the high voltage end.

[0090] The breakdown strength at the high voltage end may be high enough so as to allow Moderate Electric Field and High Electric Field waveforms to be applied for short durations and potentially just for one or two pulses.

[0091] The heat treatment of contents of a can may be reapplied or topped up during a rework cycle.

[0092] The electrode and breakdown strength capability may also be utilised to allow standard thermal processing inactivation to be augmented by application of higher electric field waveforms with increasing nonthermal inactivation effects to the contents separately or at the same time as heating waveforms.

[0093] Applied waveforms may take any waveform shape and adopt any frequency and any on-off characteristic and any electric field.

[0094] The in-can heating and heat transfer may be accelerated using movement, vibration, and/or ultrasound (airborne or contacting) or combinations of these.

[0095] The surface properties of the conducting surface of the can ends in combination with the applied electric field waveform prevent electrolysis and dissolution of this conducting surface and of the underlying material.

[0096] The side wall and ends can have a surface layer which itself is conformable and forms a hermetic seal which fill the gaps and voids in the double seam seal when they are co-formed.

[0097] The applied voltage or a voltage generated at an end shall have any waveform and be applied at any frequency and any on-off characteristic for the purpose of causing joule heating and or electroporation of the contents. Examples of waveforms which are not ecxclusive of other combinations include, sine wave, square wave, sawtooth, waveform with very fast rise time such as 10 ns, monopolar and bipolar waveforms, waveforms with a DC bias.

[0098] Electroporation of the contents f the can may be achieved by applying a very high voltage up to 400000V for a very short time of the order of microseconds to an end having a very high electrical breakdown strength which might be up to of 2000 kV/mm for such a process. The high voltage generates electrical stress and trans-membrane electric field at cell membranes of any microorganisms in the can causing electroporation.

[0099] An alternating electric field can be applied to the contents via the end pieces of the can.

[00100] The treatment of the can contents repeated or topped up during a rework cycle.

[00101] A can with an overall electrical isolation to the ends and of the sides due to a high electrical breakdown strength on one surface or multiple surfaces. [00102] The hermetic sealing to the ends of a can with the overall seal due to one component or multiple components within the double seam.

[00103] The electrical isolation material or treatment method for the electrical isolation the may be a material or treatment applied to either the end of the can or on the wall including a combination of materials or treatments on either or both.

[00104] The sealing material for the seal may be a material on either the end of the can or on the can wall including a combination of material on both.

[00105] The electrical isolation may be applied locally to specific areas of the ends and the body or they may extend to greater areas or all of the surfaces of the can ends and body.

[00106] The sealing material may be applied locally to specific areas of the ends and the body or may extend to greater areas or all of the surfaces of the can ends and body.

[00107] The electrical isolation may be applied to the bulk sheet raw materials for the can at the time of manufacture of the bulk material or at the time of formation of the individual can parts.

[00108] The sealing material may be applied to the bulk sheet raw materials for the can at the time of manufacture of the material or at the time of formation of the individual can parts.

[00109] The voltage coupling and electric field application to the ends of the container can be achieved by physical contacting means or by non-contacting electromagnetic means.

[00110] The active electrical conduction area at the end can comprise all or part of the contact area between the end and the internal product.

[00111] The conductivity of conduction path from the end may be modified such that the heating of the contents can be influenced for example by the shape of the end so that the central volume heats preferentially to the peripheral volume.

[00112] Electrical isolation between the end and the body of the can for the purposes of the invention where one end of the can may be at an elevated voltage requiring isolation from the body with the other end at low/ground potential requiring no isolation.