METHOD AND INDICATOR FOR INDICATING THAWING

Abstract: A method of indicating whether a food article has undergone a freeze-thaw sequence by applying to the food article an indicator including a first material (3) enclosed within microcapsule walls (4), and a second material (5) normally separated from the first material by the microcapsule walls. The two materials include substances which produce a color change when brought into contact with each other, but which are normally separated from each other by the microcapsule walls. The first material further includes a liquid, preferably water, which when frozen expands sufficiently to rupture the microcapsule walls (Fig. 26), such that when the first material is thawed, it comes into contact with the second material producing a color change indicating that the food article has undergone a freeze-thaw sequence.

Description: METHOD AND INDICATOR FOR INDICATTNG THAWING FIELD OF THE INVENTION The present invention relates to a method for indicating whether an article, particularly a perishable food article, has undergone a freeze-thaw sequence, and also to indicators for use in such method. BACKGROUND OF THE INVENTION Various kinds of food substances are kept in a frozen condition in order to inhibit spoilage by bacterial or other microbial growth. Certain frozen foods, particularly sea and meat products, are very susceptible to spoilage when the temperature increases above the freezing point even for short periods. Thus, a temporary power failure in a freezer may cause thawing, and thereby spoilage of the food article, which may not be noticed when the article is refrozen by the resumption of the power. This may occur in the consumer's freezer, but may also occur anywhere along the distribution channel until the food article reaches the consumer. SUMMARY OF THE INVENTION An object of the present invention is to provide a simple, low cost, and effective method for indicating whether an article has undergone a freeze-thaw sequence so as to alert a consumer of the possible spoilage of the food article. Another object of the invention is to provide an indicator for use in such method. According to one aspect of the present invention, there is provided a method of indicating whether an article has undergone a freeze-thaw sequence, comprising: applying to the food article an indicator including: a first material enclosed within microcapsule walls, and a second material normally separated from the first material by the microcapsule walls; the first and second materials including substances which produce a color change when brought into contact with each other, but which are normally separated from each other by the microcapsule walls; the first material further including a liquid which, when frozen, expands sufficiently to rupture the microcapsule walls such that when the first material has been frozen and is subsequently thawed, it passes through the ruptured microcapsule walls and comes into contact with the second material to produce a color change indicating that the food article has undergone a freeze-thaw sequence. According to further features in the preferred embodiments of the invention described below, the liquid which expands when frozen is water. Also, the indicator includes a rigid housing enclosing the first and second materials and preventing manual rupture of the microcapsule walls. According to still further features in the described preferred embodiments, the color change substance in the second material is an acid-sensitive leucodye, and the first material provides a weak acid environment Embodiments of the invention are described below wherein the leucodye (color former) is dissolved in an organic solvent which is water immiscible, the weak acid environment is a water solution of citric acid, and the capusule walls are polyurea According to another aspect of the present invention, there is provided a freeze-thaw indicator for application to articles, particularly food articles to provide an indication in accordance with the above method of whether the article has undergone a freeze-thaw sequence As will be described more particularly below, the foregoing method and indicator provide a simple, low-cost and effective means to indicate whether an article, particularly a perishable food article, has undergone a freeze-thaw sequence, and thereby to alert the consumer that the article may have become spoiled Further features and advantages of the invention will be apparent from the description below BRIEF DESCRIPTION OF THE DRAWINGS The invention is herein described, by way of example only, with reference to a number of examples set forth below and also with reference to the accompanying drawings, wherein: Fig. 1 schematically illustrates one form of freeze-thaw indicator constructed in accordance with the present invention; Fig. 2a illustrates one of the microcapsules in the indicator of Fig. 1 in the normal condition of the capsules; Fig. 2b illustrates the microcapsule of Fig. 2a, but in its ruptured condition resulting from the expansion of the water within the encapsulated material caused by freezing; Figs. 3a, 3b and 3c illustrate three phases in the operation of the indicator of Fig. 1 ; Fig. 4 schematically illustrates the indicator of Fig. 1 incorporated in a housing applied to a perishable food article to prominently display the word "SPOILED" when the indicator has undergone a freeze-thaw sequence; Fig. 4a is an enlarged sectional view along IVa. Va of Fig. 4; and Fig. 4b illustrates a modification in the construction of Fig. 4a. DETAILED DESCRIPTION OF THE PRESENT INVENTION With reference first to Fig 1 , there is illustrated a freeze-thaw sequence indicator, generally designated 2, including a plurality of microcapsules of a first material 3 enclosed within microcapsule walls 4 dispersed within a second material 5, and all enclosed within a transparent rigid housing 6 The first material 3 and the second material 5 both include substances which produce a color change when brought into contact with each other, but which substances are normally separated from each other by the walls 4 of the microcapsules in which the first material is encapsulated The first material 3 includes a liquid which, when frozen, expands sufficient to rupture the microcapsule walls 4, such that when the first material 3 is thawed, it comes into contact with the second material 5 producing a color change indicating that the indicator has undergone a freeze-thaw sequence Housing 6 enclosing the two materials 3 and 5 is rigid to prevent manual rupture of the capusle walls 4, and is transparent to permit viewing the color change when it occurs The foregoing is schematically illustrated in Figs 3a-3c Thus, Fig 3a illustrates the normal condition of the indicator, wherein microcapsules of the first material 3 are dispersed in the second material 5 but are separated therefrom by the microcapsule walls 4 enclosing the first material 3, Fig 3b illustrates the condition wherein freezing of the indicator, particularly the first material 3 within the microcapsule walls 4, causes the first material to expand and therefore to rupture the microcapsule walls, whereas Figs 3c illustrates the condition wherein subsequent thawing of the indicatorn causes the first material 3 to come into direct contact with the second material 5 and thereby to produce the color change indicating that the indicator, and thereby the article to which the indicator has been attached, has undergone a freeze-thaw sequence This indicator remains even if the indicator, and the food articles including the indicator, are subsequently re-frozen Fig 4 illustrates the freeze-thaw sequence indicator 2 applied to a packaged food article, generally designated 10, such as a fish or meat product normally sold in a frozen condition and to be retained in a frozen condition until ready for consumption or cooking In the case illustrated in Fig 4, the housing 11 for the indicator is shaped to spell the word "SPOILED", so that when the color-change is produced by the freeze-thaw sequence, the food article will be irreversibly marked "SPOILED", and thereby alert the consumer Preferably, the two color- change materials 3, 5, are normally colorless or transparent so that the word "SPOILED" is not prominently noticeable until a color change has been produced by the food article undergoing the freeze-thaw sequence As one example, the rigid housing 11 containing the two materials 3, 5, may be made of two transparent plastic sheets 11a, 11b, sealed along their peripheries, as shown in Fig 4a Fig 4b illustrates another example wherein the transparent rigid housing, therein designated 11', containing the two color-change materials 3, 5, is in the form of a transparent glass or plastic tube which is sealed at its opposite ends Preferably, the color former substance of the second material 5 is a leucodye, and the substance of the first (encapsulated) material 3 provides a weak acid environment and includes water which expands when frozen to rupture the microcapsule walls. Leucodyes is the name given to colorless chemical substances that produce a color change when reacting with a specific chemical agent. For example, certain substances containing e.g., lactones, will color when reacting with weak acids. Such leucodyes are well known from their use in carbonless paper, in color copiers based on the Cycolor process developed by Mead Corporation of Dayton, Ohio, and in radiation-sensitive indicators such as described in US Patent 5,206, 118, incorporated herein by reference. In the Cycolor process the leucodyes are dissolved in in dispersed acrylic monomers that are then encapsulated with a wall made of another polymer and dispersed in a phenolic resin containing zinc salicylate which forms an acidic environment. Once the microcapsules are crushed by mechanical force, their colourless dye content spills out and interacts with the weak acidic environment to form a colored dye. In the carbonless paper technique, the leucodye in an oily phase is encapsulated, while the continuous phase in which the microcapsules are dispersed is the weak-acid watery phase. In the present invention the two phases are reversed; that is, the encapsulated material is the water plus weak acid, and the continuous water immiscible phase in which the capsules are dispersed contains the leucodye. In addition, both color former materials used for producing the color change are selected to be edible, or at least non-toxic if ingested in the quantity included in the indicator so as to be useable with food products. Thus, a preferable weak acid environment for the encapsulated material is a water solution of citric acid or acetic acid Further, in carbonless paper, the two color former substances are brought into contact by the application of external force to rupture the microcapsule walls In the present invention, the microcapsule walls are ruptured by the freezing of the core liquid, preferably water, which is expanded when frozen Microencapsulation is a well established technology used in a variety of applications In general, creation of microcapsules is based on the polarity between the two immiscible phases, oil and water, which in turn enables the dispersion of one phase in the form of tiny droplets in the other phase A preferred example of a process that can achieve the desired microcapsular structure is called "Interfacial Polymerization", which can be used to encapsulate aqueous solutions dispersed in oil or polymers The resultant microcapsule walls are of polyurea, and its structural features include the ability to create a non-permeable continuous wall that can be very thin Such a process of producing water containing microcapsules enclosed by polyurea walls is described in detail by Yamane, Ohshima and Kondo in the Journal of Microencapsulation Volume 9, pp 279-286, 1992, incorporated herein by reference When the watery content of these microcapsules is frozen, the inside volume is increased due to the state change of the water from liquid into ice The increase of volume of the microcapsule content causes the microcapsules walls to rupture If a weak aqueous acid solution, or a dispersion of acidic salt in water, is the content of such microcapsules, this content will not start to flow into the surrounding phase while in the freezing state, but once the temperature increases above the melting point, the solution or dispersion from the inside of the ruptured microcapsules flows into the surrounding continuous phase containing the leucodye to produce the color change, which is irreversible The continuous phase can be, for example, a solution of leucodyes in organic solvents preferably an aromatic oil or ester containing benzene rings For example, there could be used one of the organic solvents described in the above cited US Patent 5,206,118 which is edible and which is used in food products In one preferred embodiment, a color former such as ones made and sold commercially by Hilton-Davis of Ohio, USA or Mitsubishi of Japan, is dissolved in an organic solvent, such as an ester which constitute the water immiscible phase The aqueous phase is composed of granules of citric acid dissolved in water in a concentration of up to 30% The aqueous phase is then added to an oily medium, and by controlled fast stirring, is dispersed into tiny droplets These watery droplets are then encapsulated by a reaction between, for example, amines and isocyanate, the first present in the water phase and the other present in an oily phase These two substances will interact on the surface of the droplets to form a thin polyurea shell, which will constitute the microcapsular wall normally preventing contact between the weak acid inside the microcapsules and the leucodye in the continuous oil phase A preferred leucodye is one made by Hilton-Davis Corporation of Dayton, Ohio and sold under the trademark "Copikem" This material is available in the form of white powder and is soluble in various organic solvents. A 0.3% solution of "Copikem" (Reg.TM) in an ester is preferred for the continuous-phase material 5; whereas a solution of citric acid (0.5% in alcohol, or 33% in water) is preferred for the encapsulated material 3. EXAMPLES Water-loaded polyurea microcapsules were prepared by making use of the interfacial polymerization reaction between tetraethylenepentamine (TEP) in an aqueous phase and toluylenediisocyanate (TDI) in an organic solvent as described in the above-cited 1992 publication by Yamane, Ohshima and Kondo, except that (a) the oil phase included a leucodye, namely that supplied under the trademark "Copikem" (Reg.TM) by Hilton-Davis Corporation of Dayton, Ohio, dissolved in an aromatic solvent or ester, and (b) the aqueous phase including a weak acid, namemly, a 30% water solution of citric acid. The two phases were thoroughly mixed to produce the water-loaded polyurea microcapsules, with the leucodye within the continuous oil phase, and the weak acid within the dispersed water phase. Upon freezing, the microcapsule walls were ruptured, such that upon thawing, the water phase came into contact with the leucodye oil phase, to produce the color change indicating the freeze-thaw sequence. Instead of citric acid, there may be also used acetic acid, or other weak acids. Another way to achieve the structure described above and to permit the use of ethanol, an edible material, instead of the water immiscible organic solvent, follows more closely the method of Yamane, Ohshima and Kondo. The polyurea encapsulated microcapsules are prepared with cyclohexane in the water immiscible phase, and the diluted citric or acetic solution in the aqueous phase. The chemical substances which form the microcapsule walls after dispersion are tetraethylenepentamine included in the aqueous phase, and toluyediisocyanate included in the cyclohexane of the water immiscible phase. Following the formation of the microcapsules they are separated from the liquid by centrifugation, and placed in 95% ethanol in which 0.3% of the leucodye (Copikem, Reg TM) was previously dissolved. In this way, not only the water immiscible phase is edible, but also the process allows placing microcapsules of predetermined size in the indicator Yamane et al. have shown that the temperature of freezing decreases with decrease of the microcapsular size. Centrifugation allows cropping of batches of microcapsules with each batch containing microcapsules of the same size, but different batches containing different sizes Those familiar with the art will appreciate that using this method permits creating a variety of indicators with a range of freezing temperatures. It will be appreciated that the foregoing represent merely preferred examples of the invention, that many other variations and modifications may be made

Claims: CLAIMS A method of indicating whether an article has undergone a freeze-thaw sequence, comprising, applying to the article an indicator including: a first material enclosed within microcapsule walls, and a second material normally separated from said first material by said microcapsule walls, said first and second materials including substances which produce a color change when brought into contact with each other, but which are normally separated from each other by said microcapsule walls; said first material further including a liquid which, when frozen, expands sufficiently to rupture said microcapsule walls such that when said first material has been frozen and is subsequently thawed, it passes through said ruptured microcapsule walls and comes into contact with said second material to produce a color change indicating that the food article has undergone a freeze-thaw sequence. The method according to Claim 1 , wherein said liquid which expands when frozen is water The method according to either of Claims 1 or 2, wherein said indicator includes a rigid housing enclosing said first and second materials and preventing manual rupture of said microcapsule walls. The method according to Claim 3, wherein said rigid housing spells out the word "SPOILED", which word becomes prominently displayed by said color change when the food article has undergone a freeze-thaw sequence The method according to any one of Claims 1 -4, wherein said color change substance in said second material is an acid-sensitive leucodye, and said first material provides a weak acid environment The method according to Claim 5 wherein said weak acid environment is a water solution of citric acid The method according to any one of Claims 1 -9 wherein said microcapsule walls are of polyurea A freeze-thaw indicator for application to an article to provide an indication of whether the article has undergone a freeze-thaw sequence, comprising a first material enclosed within microcapsule walls, and a second material normally separated from said first material by said microcapsule walls, said first and second materials including substances which produce a color change when brought into contact with each other, but which are normally separated from each other by said microcapsule walls, said first material further including a liquid which, when frozen, expands sufficiently to rupture said microcapsule walls such that when said first material has been frozen and is subsequently thawed, it passes through said ruptured microcapsule walls and comes into contact with said second material to produce a color change indicating that the food article has undergone a freeze-thaw sequence The indicator according to Claim 8, wherein said liquid which expands when frozen is water The indicator according to either of Claims 8 or 9, wherein said indicator includes a rigid housing enclosing said first and second materials and preventing manual rupture of said microcapsule walls The indicator according to Claim 10, wherein said housing is of rigid plastic sheet material sealed around its periphery The indicator according to Claim 11 , wherein said housing is a glass tube sealed at both ends The indicator according to any one of Claims 8-12, wherein said rigid housing spells out the word "SPOILED", which word becomes prominently displayed by said color change when the food article has undergone a freeze- thaw sequence The indicator according to any one of Claims 8-13, wherein said color change substance in said second material is an acid-sensitive leucodye, and said first material provides a weak acid environment The indicator according to Claim 14, wherein said weak acid environment is citric acid The indicator according to any one of Claims 8-15, wherein said microcapsule walls are of polyurea 17. The indicator according to Claim 16, wherein said microcapsule walls are the reaction product of an amine included in said first material, and an isocyanate included in said second material which reacts with said amine when the two materials are mixed to produce said polyurea microcapsule walls enclosing said first material and normally separating it from said second material. 18. The method of indicating whether a food article has undergone a freeze-thaw sequence substantially as described with reference to any of the disclosed examples. 19. An indicator for indicating whether a food article has undergone a freeze-thaw sequence substantially as described with reference to any of the disclosed examples.

Inventors: CARMON AMIRAM, CARMON, AMIRAM
Applicants: CARMON AMIRAM, CARMON, AMIRAM
Application year: 1997
Main classification: G01K11
IPC codes: F25D29, G01K11