WO2009000034A1

WO2009000034A1 - Doneness indicator

Info

Publication number: WO2009000034A1
Application number: PCT/AU2008/000924
Authority: WO
Inventors: Robert John Steele
Original assignee: Commonwealth Scientific And Industrial Research Organisation
Priority date: 2007-06-25
Filing date: 2008-06-25
Publication date: 2008-12-31

Abstract

The present invention provides a device for indicating exposure to a pressure, said device comprising a layer of force-sensitive material which provides a signal when exposed to pressure, wherein said force-sensitive material is sealed between first and second polymeric layers. A package for a foodstuff or other material prone to microbial spoilage which comprises said device and methods of using said device for measuring a maximum level of pressure within a chamber are also provided.

Description

DONENESS INDICATOR

FIELD OF THE INVENTION

The present invention relates to high pressure processing (HPP) of foodstuffs and other materials. In particular, the invention relates to devices and methods for indicating whether HPP of, for example, foodstuffs such as fruit juices or sliced smallgoods, has been performed or sufficiently completed (i.e. such that the food has been processed).

BACKGROUND OF THE INVENTION

It has been known for almost a hundred years that processing certain foodstuffs by exposure to pressures greater than 100 MPa can prevent spoilage through inactivation of certain microorganisms (including spoilage-causing and pathogenic bacteria), while maintaining the foods in a "near-fresh" condition (see Hite (1899) and Bridgman (1911)). However, technological constraints prevented the widespread application of such processing in the food industry until the late 1980s. Since then, commercial scale equipment has become available which is capable of routinely treating foods at pressures in the order of 600-700 MPa, and consequently, HPP now provides a range of different foodstuffs to the retail market, many of which retain much of their natural freshness or "unprocessed character". In particular, HPP-treated foods which are now commercially available include fruit juices, sliced smallgoods, fruit jams, fruit jellies, rice, salsa, guacamole and oysters.

However, while there is increasing acceptance of HPP treatment of foodstuffs, the usual batch type nature of such treatments does raise the possibility of inadvertent mixing up of processed batches of foodstuffs with unprocessed batches of foodstuffs. Such errors, obviously, have serious consequences in terms of food poisoning risks and liability and significant financial losses through product recalls and erosion of product goodwill or reputation. It is therefore likely that further take-up of HPP by the food industry will be subject to regulatory requirements for plans and measures to prevent, or at least minimise, the risk that foodstuffs intended for HPP treatment have been sufficiently treated prior to distribution from the processing plant. Devices and methods for indicating whether HPP of foodstuffs and other materials subject to microbial spoilage has been performed or sufficiently completed (i.e. such that the food has been processed) would therefore be of considerable assistance in minimising the risks associated with HPP which are outlined above.

So-called "doneness indicators", which measure the completeness or "doneness" of thermal processing of foodstuffs have been used for many years in the food industry (particularly, in the food canning industry), and modern versions of these indicators, utilising irreversible thermochromic inks, provide a quick and convenient visual assessment of the level of doneness. Translation of such technology to HPP however (i.e. where the device is required to indicate whether HPP of, for example, foodstuffs has been performed or sufficiently completed) has not, to the knowledge of the applicant, been previously achieved in a form that would be viable and useful to the food industry. This is, in part at least, due to the harsh conditions that such a device is required to endure within a high pressure chamber.

Minerich and Labuza (2003) recognised the need for doneness indicators for HPP and, in attempting to find a solution to the problem mentioned above, they investigated the use of properties of some metals such as copper, which has a face-centred cubic crystal lattice structure and a consequent plastic attribute such that it will increase in density when subjected to high pressure, to provide a pressure indicator. In particular, these authors produced a compacted copper tablet under isostatic or uniaxial compression, such that the density of the tablet increased in a non-linear fashion when subjected to high pressure, and tested this tablet as an indicator of pressure by measuring the density of the tablet before and after HPP. However, without a visual assessment of exposure to pressure (i.e. in a manner akin to that of modern doneness indicators for thermal processing), to allow for a quick and routine check that HPP has been performed or sufficiently completed, use of the copper tablet indicator has been limited.

In Minerich and Labuza (2003), it is claimed that a microencapsulated ink product sold under the trade name PressureX^® Tactile Pressure Indicating Film (Sensor Products Inc, NJ, USA) is capable of recording pressure within a high pressure chamber (eg a high hydrostatic pressure (HHP) chamber) of up to 125 MPa. However, in order to achieve inactivation of a sufficient range of microorganisms responsible for microbial spoilage of foodstuffs and other materials, HPP requires that a minimum pressure of 300 MPa be achieved in the pressure chamber (i.e. a pressure of at least 300 MPa is required to inactivate all of spoilage-causing and pathogenic bacteria, yeasts and moulds) and, accordingly, the use of PressureX Tactile Pressure Indicating Film in the manner described by Minerich and Labuza (2003) is unsuitable for commercial application. In any case, it is the view of the present applicant that the use described by these authors would be prone to errors and loss of functionality since the ink and developer within the film are water sensitive and would be likely to leach out into the aqueous medium contained within the HHP chamber.

Contrary to the teaching of Minerich and Labuza (2003), the present applicant has developed a means of using a force sensitive material, such as a microencapsulated ink product, to measure pressure in a HPP chamber in excess of 125 MPa.

SUMMARY OF THE INVENTION

In a first aspect, the present invention provides a device for indicating exposure to a pressure, said device comprising a layer of a force-sensitive material which provides a signal when exposed to pressure, wherein said force-sensitive material is sealed between first and second polymeric layers.

The selection of polymeric material(s) for the first and second polymeric layers, together with selection of the thickness of said layers, can be used to usefully modulate or tune the device of the invention. For instance, selection of a "hard" polymeric material(s), such as polyethylene terephthalate, for the first and second polymeric layers, will result in a device which will register changes in pressure (i.e. produce signal) quite quickly relative to a device of equivalent construction using a "soft" (i.e. more highly extensible) polymeric material(s), such as polyethylene, for the first and second polymeric layers.

In a second aspect, the present invention provides a package for a foodstuff or other material prone to microbial spoilage, said package comprising a container including a wall with at least one translucent or transparent section and a device according to the first aspect, wherein said device is affixed to an inner surface of the wall such that the device is visible through said at least one translucent or transparent section.

In a third aspect, the present invention provides a method of measuring a maximum level of pressure within a chamber, the method comprising the steps of:

(i) placing a device according to the first aspect within said chamber,

(ii) generating pressure within said chamber,

(iii) reading the signal provided by the force-sensitive material, and

(iv) determining from said signal reading the maximum level of pressure generated in said chamber.

In a fourth aspect, there is provided a method of preparing a device according to the first aspect, wherein said method comprises the steps of:

(i) providing the force sensitive material;

(ii) providing the first and the second polymeric layers;

(iii) sealing the force-sensitive material between the first and second polymeric layers.

BRIEF DESCRIPTION OF THE FIGURES

Figure 1 shows a schematic representation of a doneness indicator in accordance with an embodiment of the present invention.

Figure 2 shows a doneness indicator prepared in accordance with one embodiment of the present invention. The indicator has been exposed to a pressure of 600 MPa in an isobaric high pressure chamber.

Figure 3 shows a doneness indicator prepared in accordance with another embodiment of the present invention. The left-hand image shows the untreated indicator. The right hand side shows the indicator after being exposed to a pressure of 200 MPa. Figure 4 shows a plot of the effect of pressure against the colour developed in a PressureX^® Tactile Pressure Indicating Film.

Figure 5 provides the hardnesses of some polymers and materials that could potentially be used to attenuate the pressure transmission in devices of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The applicant has found that a device comprising a force-sensitive material such as a microencapsulated ink product may be constructed in such a way as to provide a useful indication of pressure exposure. Such a device has application as a "doneness indicator" for HPP treatments; that is, as an indicator as to whether such HPP treatments have been performed or sufficiently completed (i.e. such that treated foodstuffs or other materials prone to microbial spoilage such as pharmaceutical or veterinary preparations, have been subjected to a pressure sufficient to achieve inactivation of a sufficient range of microorganisms responsible for microbial spoilage of foodstuffs and other materials).

In a first aspect, the present invention provides a device for indicating exposure to a pressure, said device comprising a layer of force-sensitive material which provides a signal when exposed to pressure, wherein said force-sensitive material is sealed between first and second polymeric layers.

The selection of polymeric material (s) for the first and second polymeric layers, together with selection of the thickness of said layers, can be used to usefully modulate or tune the device of the invention. For instance, selection of a "hard" polymeric material(s), such as polyethylene terephthalate, for the first and second polymeric layers, will result in a device which will register changes in pressure (i.e. produce signal) quite quickly relative to a device of equivalent construction using a "soft" (i.e. more highly extensible) polymeric material(s), such as polyethylene, for the first and second polymeric layers. That is, the "soft" polymeric materials would be used for the measurement of very high pressures whereas the harder polymeric materials would be used for the measurement of lower pressures. The structure of the device of the invention can be such that when pressure such as isostatic pressure is applied, compression of the structure can result in the application of vectorial force onto the force-sensitive material thereby resulting in the production of a signal, preferably a visual signal (eg colour development).

The force-sensitive material is preferably selected from microencapsulated ink products such as products having microencapsulated ink contained within a matrix and associated support film (see, for example, "Microencapsulation", Curt Thies, in Kirk- Othmer Encyclopedia of Chemical Technology, John Wiley & Sons, Inc (1995)) such as is found in "carbonless paper" and the abovementioned indicator film sold under the trade name PressureX^® Tactile Pressure Indicating Film. When pressure is applied to a device comprising this kind of force-sensitive material, the force generated by compression of the device results in the production of a colour signal (eg red colour) by causing rupture of the microcapsules encapsulating the ink substance and subsequent reaction with a developer compound (typically an acid) found within the support film. The colour produced preferably changes in intensity in a manner proportional to the pressure applied to the device.

Other examples of force-sensitive materials suitable for use in the device of the present invention include materials comprising a laminated sandwich of a film incorporating a leuco-dye substance adjacent to a developer film such that as the pressure of the chamber is increased the dye diffuses into the developer layer and is converted to a coloured form.

In a further embodiment, the pressure sensitive material can be a capacitor formed from a laminate comprising two layers of conducting material which are separated by a nonconducting layer. The signal in that case is the change in capacitance generated between the two conductive layers as the two conductive layers are forced closer together by the applied pressure. Preferably, the two conducting layers are aluminium. Preferably, the non-conducting layer is a polymer. Accordingly, in a preferred form the capacitor is an aluminium foil laminate. In another embodiment, the force-sensitive material may be a piezoelectric material which generates an electrical signal when subjected to a force.

The first and second polymeric layers, which are preferably film layers having a thickness typically in the range of 10 to 100 μm, may consist of the same or different polymeric materials. The use of different polymeric materials for each of the first and second polymeric layers allows for further tuning or modulation of the device according to the present invention. Accordingly, in a preferred embodiment each of the first and second polymeric layers consists of different polymeric materials.

Suitable polymeric materials include epoxy phenolic resins, polyethylene, polyethylene terephthalate, polycarbonate, polypropylene, polystyrene, polyvinyl chloride, polyvinyl alcohol, nylon, polyurethane, ethylene vinyl alcohol, cellophane, ethyl cellulose, or other self supporting film.

Preferably, the first and second polymeric layers are impermeable to moisture, in particular the aqueous medium commonly contained within a HPP chamber.

Further, it is preferable that the first and second polymeric layers are, at least, translucent so that a visual signal may be readily observed or otherwise detected. More preferably, the first and second polymeric layers are transparent.

Together with the force-sensitive material, the device of the present invention preferably comprises a layer of a suitable support material (e.g. polyethylene terephthalate) sealed between the first and second polymeric layers. Alternatively, the device of the present invention may be itself affixed to a layer of a support material.

The support material also serves as a means to usefully modulate or tune the device of the present invention. Again, selection of a "hard" polymeric material(s), such as polyethylene terephthalate, for the support material, will result in a device which will register changes in pressure (i.e. produce signal) quite quickly relative to a device of equivalent construction using a "soft" (i.e. more highly extensible) polymeric material(s), such as polyethylene, for the support material. Preferably, the device of the invention is capable of indicating exposure to pressures of at least 125 MPa, more preferably at least 300 MPa, and most preferably at least 700 MPa.

In an embodiment of the present invention, wherein PressureX^® Tactile Pressure Indicating Film is used as the force-sensitive material, the device is preferably adapted so that exposure of the force-sensitive material to moisture is prevented (i.e. the device provides a moisture impermeable barrier around the force-sensitive material). Further, the device of such an embodiment is, preferably, restricted to use at temperatures of < 4O⁰C.

In another embodiment, the device may be incorporated into a package by including the device within a label or barcode. In the latter case, the device may function so as to allow the barcode to be read by a barcode reader only after the device has been exposed to the required HPP pressure.

The structure of a particularly preferred embodiment of the device of the invention is shown in Figure 1. The depicted device comprises a layer of a force-sensitive indicator material on a layer of a support material sealed between a layer of a first polymer and a layer of a second polymer. The depiction in Figure 1 of plugs at each end of the device is intended to indicate a water-tight seal. In use, the depicted device is incorporated into a package for a foodstuff or other material prone to microbial spoilage, and the package then placed in a HPP chamber for HPP treatment. Examples of how the device may be incorporated into the package include affixing the device to an outer or inner surface of a wall of a package (eg a container for a foodstuff) adjacent to a transparent section of the wall such that the device (and, more particularly, any signal generated by the device) is visible. Following completion of the HPP treatment, the device incorporated into the package may be visually inspected to confirm that a signal has been produced that is indicative of a pressure (eg 125 to 700 MPa) having been generated in the HPP chamber.

The intensity of the signal produced by a device according to the present invention upon exposure to pressure is, preferably, proportional to the maximum pressure applied to the force-sensitive material. Accordingly, the present invention allows for the production of a device which is capable of measuring very high pressures by providing a structure which attenuates the pressure or force applied to the force-sensitive material. Such a structure may be provided by a device which further comprises one or more additional polymeric layers sealed within the first and second polymeric layers and which are layered over the pressure sensitive device. Such one or more additional polymeric layers may attenuate the force applied to the force-sensitive material and thereby enable higher pressures to be measured than would otherwise be achieved. Such one or more additional polymeric layers may the same as, or different to, the polymeric material(s) used in the first and second polymeric layers. In addition, each of said one or more additional polymeric layers may be made of the same or different polymeric materials as the other additional polymeric layers. Suitable polymeric materials include epoxy phenolic resins, polyethylene (eg. HDPE and LDPE), polyethylene terephthalate (ie PET), polycarbonate, polypropylene (eg BOPP), polystyrene, polyvinyl chloride, polyvinyl alcohol, nylon, polyurethane, ethylene vinyl alcohol (ie EVA), cellophane, ethyl cellulose, or other self supporting film.

For instance, in one embodiment, the one or more additional layers may be a third and a fourth layer. The third later may be polyethylene terephthalate. The fourth layer may be selected from the group including HDPE (high density polyethylene), EVA (ethyl vinyl alcohol), BOPP (biaxially oriented polypropylene), polystyrene, and polyethylene terephthalate (PET).

In a second aspect, the present invention provides a package for a foodstuff or other material prone to microbial spoilage, said package comprising a container including a wall with at least one transparent section and a device according to the first aspect, wherein said device is affixed to an inner surface of the wall such that the device is visible through said at least one transparent section.

In a third aspect, the present invention provides a method of measuring a maximum level of pressure within a chamber, the method comprising the steps of;

(i) placing a device according to the first aspect within said chamber, (ii) generating pressure within said chamber,

(iii) reading the signal provided by the force-sensitive material, and

The method is preferably capable of measuring a maximum level of pressure generated in the chamber of at least 125 MPa, more preferably at least 300 MPa, and most preferably at least 700 MPa.

(i) providing the force sensitive material;

(ii) providing the first and the second polymeric layers;

The sealing of the force-sensitive material between the first and second polymeric layers can be achieved by any suitable means including lamination of the first and second polymeric layers so as to sandwich the force-sensitive material between them. Other suitable sealing means include simple heat sealing of the films. Preferably, the force-sensitive-material is sealed by lamination of the first and second polymeric layers.

The first and the second polymeric layers may be separate layers or can be comprised in a single article such as a laminating pouch, for example, a polystyrene 1 lOμ laminating pouch (ibico^® desktop finishing systems).

Table 1 and Figure 5 set out some typical hardnesses of polymeric materials that are used in the Examples or which could be included the polymeric layers of the present invention. As discussed above, the use of "softer" polymeric materials allows for higher pressures to be measured and the use of "harder" polymeric materials allows for lower pressures to be measured.

Table 1 Properties of polymers used in the Examples

In order that the nature of the present invention may be more clearly understood, preferred forms thereof will now be described with reference to the following non- limiting examples.

EXAMPLES

Example 1

A doneness indicator according to the present invention was prepared by laminating a section of PressureX^® Tactile Pressure Indicating Film between two layers of polyvinyl chloride (PVC) film.

The indicator was placed in an AVURE 2 litre isobaric high pressure chamber in water and the pressure within the chamber raised to 600 MPa and held for 300 seconds. The colour of the indicator changed from white to a deep red colour and is shown in Figure 2.

Example 2

A doneness indicator according to the present invention was prepared by laying a section of carbonless carbon paper on to a sheet of cast polyethylene terephthalate and sealing the structure within two polyethylene containing layers of a Cryovac™ packaging material (consisting of LLDPE₅ EVA and PVdC). Carbonless copy paper, non-carbon copy paper, is used to make a simultaneous copy of an original, handwritten document. It consists of a sheet(s) of paper that may be coated on the bottom and/or the top with micro-encapsulated dye or ink and/or a reactive clay. At least one sheet is coated with micro-encapsulated dye. The same sheet or another sheet contains a clay that reacts with the dye to form a permanent mark. The back of the middle sheet is also coated with the dye. The lowermost sheet is coated on the top surface with the clay with no coating applied to the back side. When sufficient force is applied to the sheets, the micro-capsules break and spill their dye.

The indicator was placed in an AVURE 2 litre isobaric high pressure chamber in water and the pressure within the chamber raised to 200 MPa and held for 30 seconds. The colour of the indicator changed from white to a dull grey colour and is shown in Figure

3.

Example 3

A doneness indicator according to the present invention was prepared by laying a section of PressureX^® Tactile Pressure Indicating Film on to a sheet of cast polyethylene terephthalate and affixing a sheet of 23 μm HDPE film over the top of the pressure indicating film and sealing the structure within two polyethylene layers as detailed in example 2.

The indicator was placed in an AVURE 2 litre isobaric high pressure chamber in water and the pressure within the chamber raised to 200 MPa and held for 30 seconds. The colour of the indicator changed from white to a pink colour with the following colour intensity L=74.55, a=32.48, b=15.22. The colour coordinates for untreated indicator film was L=93.85, a=-0.83, b=15.4.

Example 4

A doneness indicator according to the present invention was prepared by laying a section of PressureX^® Tactile Pressure Indicating Film on to a sheet of cast polyethylene terephthalate and affixing a sheet of 40μm EVA film over the top of the pressure indicating film and sealing the structure within two polyethylene layers as detailed in example 2.

The indicator was placed in an AVURE 2 litre isobaric high pressure chamber in water and the pressure within the chamber raised to 200 MPa and held for 30 seconds. The colour of the indicator changed from white to a pink colour with the following colour intensity L=81.78, a=19.10, b=11.04.

Example 5

A doneness indicator according to the present invention was prepared by laying a section of PressureX^® Tactile Pressure Indicating Film on to a sheet of cast polyethylene terephthalate and affixing a sheet of 25 μm BOPP film over the top of the pressure indicating film and sealing the structure within two polyethylene layers as detailed in example 2.

The indicator was placed in an AVURE 2 litre isobaric high pressure chamber in water and the pressure within the chamber raised to 200 MPa and held for 30 seconds. The colour of the indicator changed from white to a pink colour with the following colour intensity L=75.69, a=31.56, b=15.24.

Example 6

A doneness indicator according to the present invention was prepared by laying a section of PressureX^® Tactile Pressure Indicating Film on to a sheet of cast polyethylene terephthalate and affixing a sheet of 12μm cast PET film over the top of the pressure indicating film and sealing the structure within two polyethylene layers as detailed in example 2.

The indicator was placed in an AVURE 2 litre isobaric high pressure chamber in water and the pressure within the chamber raised to 200 MPa and held for 30 seconds. The colour of the indicator changed from white to a pink colour with the following colour intensity L=71.09, a=38.58, b=17.56. Example 7

A doneness indicator according to the present invention was prepared by laying a section of PressureX^® Tactile Pressure Indicating Film on to a sheet of cast polyethylene terephthalate and affixing a sheet of polystyrene film over the top of the pressure indicating film and sealing the structure within two polyethylene layers as detailed in example 2.

The indicator was placed in an AVURE 2 litre isobaric high pressure chamber in water and the pressure within the chamber raised to 200 MPa and held for 30 seconds. The colour of the indicator changed from white to a pink colour with the following colour intensity L=70.14, a=46.07, b=15.54.

Example 8

A doneness indicator according to the present invention was prepared by laying a section of PressureX^® Tactile Pressure Indicating Film on to a sheet of cast polyethylene terephthalate and affixing a sheet of PVdC coated cellophane film over the top of the pressure indicating film and sealing the structure within two polyethylene layers as detailed in example 2.

The indicator was placed in an AVURE 2 litre isobaric high pressure chamber in water and the pressure within the chamber raised to 200 MPa and held for 30 seconds. The colour of the indicator changed from white to a pink colour with the following colour intensity L=76.39, a=39.40, b=12.32.

Example 9

A doneness indicator according to the present invention was prepared by laying a section of PressureX Tactile Pressure Indicating Film on to a sheet of cast polyethylene terephthalate and affixing a sheet of 40μm EVA film over the top of the pressure indicating film and sealing the structure within two polyethylene layers as detailed in example 2. The indicator was placed in an AVURE 2 litre isobaric high pressure chamber in water and the pressure within the chamber raised to 200 MPa and held for 30 seconds. The colour of the indicator changed from white to a pink colour with the following colour intensity L=81.78, a=19.10, b=11.04.

Example 10

A doneness indicator according to the present invention was prepared by laying a section of PressureX^® Tactile Pressure Indicating Film on to a sheet of cast polyethylene terephthalate and no film over the top of the pressure indicating film and sealing the structure within two polyethylene layers as detailed in example 2.

The indicator was placed in an AVURE 2 litre isobaric high pressure chamber in water and the pressure within the chamber raised to 200 MPa and held for 30 seconds. The colour of the indicator changed from white to a pink colour with the following colour intensity L=83.94, a=21.26, b=35.43.

Example 11

A doneness indicator according to the present invention was prepared by laying a section of PressureX^® Tactile Pressure Indicating Film on to a sheet of cast polyethylene terephthalate and affixing a sheet of adhesive film (sectioned from the adhesive part of a Post-It^® note) over the top of the pressure indicating film and sealing the structure within two polyethylene layers as detailed in example 2.

The indicator was placed in an AVURE 2 litre isobaric high pressure chamber in water and the pressure within the chamber raised to 200 MPa and held for 30 seconds. The colour of the indicator changed from white to a pink colour with the following colour intensity L=92.49, a=2.74, b=17.88. It was noted that the adhesive layer severely attenuated the colour development in the indicating film. This serendipitously demonstrated the effect of the polymer properties such as hardness, glass transition temperature and density had on the extent of colour development or pressure transduction of the encapsulated ink or related structure. Some properties of the polymers used are tabulated from published data in Table 1. The relationship of hardness is shown in Figure 5 which was considered an important factor in the colour development.

Example 12

A doneness indicator according to the present invention was prepared by laying a section of PressureX^® Tactile Pressure Indicating Film between two sheets of cast polyethylene terephthalate 15μm and polystyrene HOμ laminating pouch (ibico^® desktop finishing systems). The indicator was sealed into the pouch with a standard office laminator.

The indicator was placed in an AVURE 2 litre isobaric high pressure chamber in water and the pressure within the chamber raised to 200 MPa and held for 30 seconds. The colour of the indicator changed from white to a pink colour in a similar fashion to previous examples outlined above.

Example 13

A doneness indicator according to the present invention can be used to calculate the pressure to which it has been exposed from the colour reading obtained.

In this experiment, doneness indicators were prepared by sealing a piece of a PressureX^® Tactile Pressure Indicating Film between two layers of ethylene vinyl alcohol (EVA) film. The doneness indicators were then sealed in a commercially available laminated polypropylene based retort pouch before subjecting them to different pressures within an AVURE 2 litre isobaric high pressure chamber. The colour signal corresponding to each particular pressure was measured using a Hunter Lab colour sphere (Hunter Associates Laboratory, Inc., 9529 Lee Highway, Fairax, Va. 22030) (Anon., 1979).

The graph of Figure 4 plots the effect of pressure along the abscissa against the colour developed in the Pressurex^® Tactile Pressure Indicating Film. The extent of colour change can be calculated as the square root of the sum of the differences between the L, a & b readings squared using the Hunter lab colour sphere. Y= sqrt( (L₁-L₂)² +(ai-a₂)² + (b_rb₂)²)

wherein:

Y is the extent of colour change in the Pressurex^® Tactile Pressure Indicating Film'

L is the lightness parameter;

a is the red/green colour balance parameter; and

b is the blue/yellow colour balance parameter.

Throughout this specification the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

All publications mentioned in this specification are herein incorporated by reference. Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed in Australia or elsewhere before the priority date of each claim of this application.

It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive. REFERENCES

Anon. Technical Bulletin PBI 18 1979. A recommended practice for measuring the color of transparent plastic bottles. The Society of Plastic Industries (1979).

Bridgman, PW (1911). Water, in the liquid and five solid forms, under pressure. Proceedings of the American Academy of Arts and Sciences (pp. 441-558).

Boston, MA: American Academy of Arts and Sciences.

Hite, BH (1899). The effect of pressure in the preservation of milk (ppl5). Morgantown: West Virginia Agricultural Exp Station Bulletin.

Minerich and Labuza (2003). Development of a pressure indicator for high hydrostatic pressure processing of foods. Innovative Food Science and Emerging

Technologies 4 (2003) 235-243.

Claims

1. A device for indicating exposure to a pressure, said device comprising a layer of force-sensitive material which provides a signal when exposed to the pressure, wherein said force-sensitive material is sealed between first and second polymeric layers.

2. A device according to claim 1 wherein the signal generated is a change in colour of the force-sensitive material.

3. A device according to claim 2 wherein the force-sensitive material is selected from microencapsulated ink products.

4. A device according to claim 2 wherein the force sensitive material comprises a laminated sandwich of a film incorporating a leuco-dye substance adjacent to a developer film such that when the device is exposed to the pressure the dye diffuses into the developer layer and is converted to a coloured form.

5. A device according to claim 1 wherein the force-sensitive material is a capacitor.

6. A device according to claim 1 wherein the force-sensitive material is a piezoelectric material which generates an electrical signal when subjected to the pressure.

7. A device according to any one of claims 1 to 6 wherein the first and second polymeric layers each independently comprise one or more polymeric materials selected from the group consisting of an epoxy phenolic resin, polyethylene, polyethylene terephthalate, polycarbonate, polypropylene, polystyrene, polyvinyl chloride, polyvinyl alcohol, nylon, polyurethane, ethylene vinyl alcohol, cellophane, ethyl cellulose, or other self supporting film.

8. A device according to any one of claims 1 to 7 wherein each of the first and second polymeric layers consists of different polymeric materials.

9. A device according to any one of claims 1 to 8 wherein the force-sensitive material is sealed between the first and second polymeric layers by lamination of the first and second polymeric layers so as to sandwich the force-sensitive material between them.

10. A device according to any one of claims 1 to 9 wherein the intensity of the signal produced by the device upon exposure to the pressure is proportional to the maximum pressure applied.

11. A package for a foodstuff or other material prone to microbial spoilage, said package comprising a container including a wall with at least one transparent section and a device according to any one of claims 1 to 10, wherein said device is affixed to an inner surface of the wall such that the device is visible through said at least one transparent section.

12. A method of measuring a maximum level of pressure within a chamber, the method comprising the steps of;

(i) placing a device according to any one of claims 1 to 10 within said chamber,

(ii) generating pressure within said chamber,

(iii) reading the signal provided by the force-sensitive material, and

13. A method of preparing a device according to any one of claims 1 to 9, wherein said method comprises the steps of:

(i) providing the force sensitive material;

(ii) providing the first and the second polymeric layers;