TECHNICAL BACKGROUND AND PRIOR ART The quality of food products and other perishables are highly dependent on storage con- ditions such as the temperature and the storage time from production or packing until it finally reaches the end consumer. The deterioration processes are faster when the tem- perature is raising due to increasing biochemical reaction rates, and therefore the quality of perishable goods declines more rapidly at high temperatures than at low temperatures.

Examples of perishable goods which need to be stored under conditions such that a par- ticular temperature limit is not exceeded or at least not exceeded for longer than a prede- termined period of time, include fresh food products, chilled food products and food prod- ucts that have been pre-cooked or processed by freezing, irradiation, partial cooking, freeze drying or steaming. If such products are not stored under appropriate temperature conditions then there is a danger of propagation of microorganisms which are injurious to human health or of spoilage organisms. Further examples of products which may need to be stored under appropriate temperature conditions are certain pharmaceuticals which would otherwise deteriorate.

Currently only date marking is applied for the insurance of storage quality. By date mark- ing onty, no information is given to the consumer or others about the storage conditions to which the product has been exposed, hence the purchasers of susceptible products are not able to determine whether the product has been stored under appropriate temperature conditions during the time of storage. Relying on date marking as a sole quality criterion

presupposes that the perishable product has been stored under appropriate conditions throughout the entire storage period. The end consumer will e. g. not be able to determine if a frozen product has been thawn during transportation and subsequently frozen before being put on sale. To be on the safe side, producers of perishable goods often use date marking with a wide safety margin, hence products which are actually still suitable for consumption or use are often discarded.

Therefore, there is a continuing interest in the monitoring of the time and temperature to which storage sensitive products have been exposed in e. g. food distribution chains from factory to consumer.

By supplying a perishable product with a time-temperature indicator which follows the in- dividual product from packing to sale, the producer, the grosser, the retailer and the con- sumer will have a better product control than they have currently. By the use of a time- temperature indicator, the true shelf life of the products can be monitored, which means that discarding can be retarded until the applied time-temperature indicator has detected that storage conditions have not been appropriate and/or that the recommended storage time has been exceeded.

Time-temperature indicators may be classified as either partial history or full history indi- cators depending on their response mechanism. Partial history indicators will not respond unless a threshold temperature has been exceeded, while full history indicators respond independently of a temperature threshold and provides a cumulative response to the time and storage conditions to which the time-temperature indicator (and hence the product) has been exposed.

Thus, EP 505 449 B1 discloses an example of a partial history time-temperature indicator comprising a fusible material such as polycaprolactone triol, polyethylene glycol C1 4 alkyl ether and polyvinyl alcool, which flows when a given threshold temperature is exceeded and re-solidifies when exposed to temperatures below the same temperature. The fusible material flows in a substrate and an indicator system produces a physically detectable change in the substrate when the fusible material flows therein.

Partial history time-temperature indicators such as the above described, do not provide a direct measure of time-temperature history. This is most important, since the degradation

of perishables depend on the time exposure to particular temperatures. For example, food exposed for a period of time to one temperature may degrade to the same extent if ex- posed to a shorter period of time at a higher temperature. Hence there are several ad- vantages in using full history time-temperature indicators.

However, there are a number of general requirements that a full history time-temperature indicator should fulfil. These include that the indicator gives a continuous and cumulative response to time and temperature, and that the response to time and temperature is gen- erated gradually and is a function of both time and temperature. The response to time and temperature should be irreversible to prevent the time-temperature indicator from being reset. Preferably it should also be capable of indicating the time-temperature history within a wide temperature range.

The indicator should furthermore in conjunction with a perishable product, show the real condition of this product, and e. g. reflect the storage conditions to which the product has been exposed and be able to show if a frozen product has been defrosted for a period of time. It should also be conveniently activated so that pre-usage storage of the indicator is not a problem, and the response to time and temperature should be given in a visually and easily interpretable manner. Finally, and importantly, it should be non-toxic and not pose any threat to human health.

According to present invention there is now provided a full history time-temperature indi- cator which fulfil all of the above mentioned requirements. The response given by the time-temperature indicator according to the invention is easily read by the human eye, and in conjunction with a product it gives a measure of the storage conditions to which the product has been exposed by giving a cumulative response to time-temperature expo- sure.

SUMMARY OF THE INVENTION Accordingly, the invention relates in a first aspect to a full-history time-temperature indi- cator system which is capable of exhibiting a time-temperature dependent and visually detectable chemical reaction. The system comprises at least one immobilised and at least one mobile reactant, wherein the reactants are contained in separate adjacent compart-

ments, and the reactants initially are separated by a sealing to prevent contact between the reactants. The rate of mobility of the mobile reactant is time-temperature dependent and the system is activatable by removing the sealing between the compartments whereby the mobile reactant in a time-temperature dependent manner is brought into contact with the immobilised reactant, resulting in a visually detectable reaction signal that indicates the time-temperature history.

In a further aspect there is also provided a method for monitoring the time-temperature history of a product, comprising defining a typical time-temperature history for said prod- uct which results in a desired or an undesired change of the product.

The method comprises the construction of a system as defined above which provides a visually detectable reaction signal indicating that the desired or the undesired change of the product is imminent, associating the product with the constructed system, activating the system and following the visually detectable reaction signal during storage of the product.

In a still further aspect, the invention pertains to a method for producing a full-history time- temperature indicator system. The method comprises the selection of at least two reac- tants which, when brought into contact, provides a visually detectable chemical reaction, immobilising one of the reactants in a medium, which permits the other of the reactants to become mobile, enclosing the mobile reactant and the medium containing the immobilised reactant in adjacent compartments separated by a breakable sealing.

DETAILED DISCLOSURE OF THE INVENTION Thus, in one aspect the present system contemplates a full-history time-temperature indi- cator system which is capable of exhibiting a time-temperature dependent and visually detectable chemical reaction.

The system comprises, as it is mentioned above, at least one immobilised and at least one mobile reactant, wherein the reactants are contained in separate adjacent compart- ments. The reactants are initially separated by a sealing to prevent contact between the reactants. The rate of mobility of the mobile reactant is time-temperature dependent and the system can be activated by removing or breaking the sealing between the compart-

ments whereby the mobile reactant in a time-temperature dependent manner is brought into contact with the immobilised reactant, resulting in a visually detectable reaction signal that indicates the time-temperature history.

In one useful embodiment of the system according to the invention, the reactants are metal compounds from the transition metal group (IB-VIIIB), metal compounds from the groups IIIA, IVA and VA in the periodic table, such as ions from the group consisting of Fe, Mn and Al or metals from the rare-earth elements (Lanthanoids).

The reactants may also be a metal reacting or metal binding compound.

The chemical reaction between the reactants can result in a redox process and/or the formation of e. g. a complex, a chelate and a compound with low solubility (precipitate).

In a presently preferred embodiment of the invention as described herein, the mobile re- actant is Fe (CN) 64-, however also the use of Fe as mobile reactant is also contemplated.

The ion Fe (CN) 64~ can e. g. be derived from K4Fe (CN) 6 and the ion Fe can e. g. be achieved from FeCI3.

When using the presently preferred mobile chemical reactant, Fe (CN) 6, in the system, a strongly blue coloured compound is formed when Fe3+ is the immobilised reactant. The blue coloured compound has low solubility and is formed by the reaction: 3Fe (CN) 64- + 4Fe3+- Fe4 [Fe (CN) 6] 3 Also a strongly blue coloured complex can be formed by the reactants by the reaction: Fe (CN) 6 + Fe + K" KFe [Fe (CN) 6] Both the above reactions are substantially irreversible, and none of the mentioned reac- tants or the formed products are poisonous to human health and hence do not represent a hazard to consumers.

The material for immobilising the reactant can e. g. be a gel based on a hydrocolloid, such as an alginate, a carrageenan, an agar, a pectin, a starch, a gum, a cellulose, a protein or

other organic polymers having reacting functional groups. The concentration of the hydro- colloid in the gel is advantageously in the range of 0.1 to 20% such as in the range of 0.1 to 2% by weight. The gel can further comprise an alcohol such as glycerol. Prior to activa- tion of the system the immobilised reactant is typically present at a concentration which is in the range of 0.01 to 1 M, such as about 0,1 M. The immobilisation of the reactant is typi- cally performed by slowly adding a 0.1 M FeCts solution to a mixture comprising alginate, glycerol and water, while stirring vigorously. However, it is also within the scope of the in- vention to use a particulate polymeric matter such as a resin as material for immobilisa- tion, e. g. a cation or anion exchange resin.

Prior to activation of the time-temperature indicator system, the mobile reactant is present in the system at a concentration which is in the range of 0.01 to 1 M, such as about 0,1 M, in a mixture comprising alginate, glycerol and water.

The time-temperature indicator system according to the invention can advantageously be a system wherein the reactants are contained in a cylinder element consisting of two compartments separated by a sealing. Such a cylinder element can be made of different materials such as glass and polymeric materials. Such a polymeric material can e. g. be polyethylene. The sealing between the two compartments may be provided by bending the cylinder element to occlude the transition between the compartments or it may be pro- vided by a barrier such as thin polymer film. The barrier may also be provided by means of a material such as e. g. a wax, which is solid within a certain temperature range, but flows when a given threshold temperature is exceeded.

The system is typically activated by breaking or removing the sealing between the two compartments containing the reactants. The breaking can e. g. be performed by means of exposing the sealing to mechanical stress, irradiation or heat.

However, it is also contemplated that the reactants can be contained in a disc shaped car- rier element comprising a central portion containing one reactant and a peripheral portion containing the other reactant. The disc shaped carrier element can be made of e. g. a polymer wherein a part of the carrier element is a gel wherein one reactant is immobilised.

Additionally, also a system wherein the reactants are contained in a rectangular strip shaped element is within the scope of the invention. Such a strip shaped element is con- veniently made of a cellulosic or a polymeric material.

The system according to the invention is capable of indicating the time-temperature his- tory within a temperature range having a difference between the upper and lower limit which is at the most 50°C preferably in the temperature range of-20°C to 30°C such as in the range of-20°C to 4°C. However the system according to the invention is also capable of indicating the time-temperature history in the temperature range of 30°C to 90°C.

In a further aspect of the invention, there is provided a method for monitoring the time- temperature history of a product, comprising defining a typical time-temperature history for the product which results in a desired or an undesired change of the product, constructing a system according to the invention which, when subjected to such a typical time- temperature history, provides a visually detectable reaction signal which indicates that the desired or the undesired change of the product is imminent, associating the product with the thus constructed system, activating the system and following the visually detectable reaction signal during storage of the product.

Such products include e. g. food products, chemical products, pharmaceutical products, cosmetics or biological materials. Typically such food products are products which are fresh, frozen, preserved or dehydrated, and typical biological materials are products like e. g. diagnostic reagents, plants, seeds and semen.

It is also within the scope of the invention that the above system can be associated with a container for the products described above. Typical containers are e. g. cans, trays, bags and jars. The association of the system to such containers can be provided by means of an adhesive layer on the system by which the system will be substantially irremovable when associated with the container. The association of the system to the container can be constructed in such a way that if the system is attempted to be removed from the con- tainer by which it is associated, it will break or be destroyed. By this it can prevented that the system is abused.

In yet a further aspect of the invention, there is provided a method for producing a system according to the invention, comprising selecting at least two reactants which, when brought into contact, provides a visually detectable chemical reaction, immobilising one of the reactants in a medium which permits the other of the reactants to become mobile, en- closing the mobile reactant and the medium containing the immobilised reactant in adja- cent compartments separated by a breakable sealing.

There is also provided a container associated with a system according to the invention, wherein the system is associated with an inner or outer surface of the container or wherein the system is integrated in the container material. The system can e. g. be asso- ciated with the inner surface of a polymer film used for the wrapping of fresh meat, or the system can e. g. be integrated in a container for food such as a tray for meat packaging or a milk container.

The invention will now be described by way of illustration in the following non-limiting ex- amples.

EXAMPLE 1 Preparation of the mobile and the immobilised reactant The mobile reactant is a 0.1 M solution of K4Fe (CN) 6 in a mixture of 0.64 % (w/w) alginate, 47.29 % (w/w) glycerol, 52.06 % (w/w) water.

The immobilised reactant is prepared by slowly adding a 0.1 M FeCts solution in a mixture of 47% (w/w) glycerol and 53% (w/w) water to a mixture of 0.64% (w/w) alginate, 47.29% glycerol and 52.06% (w/w) water. The amount of 0.1 M FeC ! s sotution is 17% (w/w) of final weight. The mixing is done at room temperature by stirring vigorously. By this method a viscous liquid is achieved wherein small gel lumps containing immobilised Fe are equally dispersed.

EXAMPLE 2 Time-temperature indicator prepared as an ampoule For preparing a time-temperature indicator as an ampoule, two transparent cylinders made of polystyrene, each of them closed in one of the ends were applied. One of the cylinders (No. 1) was able to fit into the other cylinder (No. 2) by having an outer diameter which was equal to the inner diameter of cylinder No. 2.

The mobile reactant was prepared as described in Example 1 and contained K4Fe (CN) 6 at a concentration of 0.1 M. The mobile reactant was transferred to cylinder No. 1.

The immobilised reactant containing Fe ions immobilised in the alginate gel was pre- pared as described in Example 1 and subsequently transferred to cylinder No. 2.

The two cylinders were assembled by gently pressing them together, carefully avoiding formation of air bubbles in the cylinders, and by this the chemical reaction was started.

The time-temperature indicator was incubated and regularly inspected for coloration of the gel. The mobile reactant, the Fe (CN) 64--ions, diffused from the liquid into the gel where it reacted with the Fe3+-ions immobilised therein which was visualised by the gel becoming strongly blue coloured. The extent of the coloration was an indication of the exposure to both time and temperature, corresponding to a 1. order reaction.

EXAMPLE 3 Colour development as function of time at 4°C in a strip shaped time-temperature indica- tor A time-temperature indicator was prepared as a strip wherein the reactants were prepared as described in the above example 1.

The colour development in the part of the strip-shaped time-temperature indicator con- taining the immobilised Fe was monitored. The colour development was measured in percentages as the length of the strongly blue coloured zone compared to the total length of the part of the time-temperature indicator containing the immobilised reactant. The time-temperature indicator was kept at a constant temperature of 4°C for 76 days.

The results of the above experiment is presented in the below Table 3.1.

TABLE 3.1 Days Colour development (%) 0 0 1950 38 75 57 88 76 93

It is apparent from the above results that the time-temperature indicator responds well to the exposure of time at a constant temperature.

EXAMPLE 4 Colour development as function of time at 4°C and-20°C Two identical time-temperature indicators were prepared as described in the above ex- ample 3, and the colour development was measured at 4°C and-20°C, respectively.

The results of this experiment is presented in Table 4.1.

TABLE 4.1 Days Colour development Colour development +4°C (%)-20°C (%) 0 0 0 10.9 50 8 21.8 74 15 32.7 86 22 43.6 92 28 As can be seen from the above results, the colour development is not only time depend- ent but also temperature dependent. After 43.6 days 92% of the strip held at 4°C was blue coloured, whereas only 28% of the strip held at-20°C was blue coloured.

There was an inverse correlation between the diffusion rate of the mobile reactant and the ambient temperature. The lower the temperature, the slower the mobile reactant was moving and hence the slower the time-temperature indicator was blue coloured. This en- hances the reproducibility and hence the predictability of the response.