Temperature indicator for deep-frozen products

An apparatus for checking deep-frozen food, which irreversibly indicates if an additional storage temperature has been exceeded, is described. A flat plastic bag having transparent walls, into which eutectic mixture having a liquid/solid phase transition in the range from 0.degree. to -50.degree. C. has been introduced, together with a magnetic dispersion consisting of finely divided magnetic pigments, has a heat-conducting connection to the material to be frozen. A magnetic stripe which contains a magnetically recorded pattern and which remains connected to the bag until the indicating apparatus together with the material to be frozen has been cooled below the limiting temperature is stuck to the bag. The magnetic stripe is then removed. If the permissible limiting temperature is exceeded, the optically or magnetically readable structure produced by the magnetic stripe in the indicator bag is eliminated, permitting easy checking of the deep-frozen food. The present invention permits many potential variations by combination of a plurality of applied magnetic stripes.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an apparatus for irreversibly indicating 
whether the maximum storage temperature of products stored in the frozen 
or deep-frozen state has been temporarily exceeded, consisting of a 
container which has a heat-conducting connection to the stored product and 
contains an inorganic eutectic mixture or an organic compound which has a 
melting point in the range from about 0.degree. C. to -50.degree. C., the 
container containing clearly indicating parts. 
In the storage of foods in the frozen state, a storage temperature of at 
least -16.degree. C. must be maintained if a relatively long shelf life is 
to be ensured. If, on the other hand, a temperature of -10.degree. to 
-15.degree. C. is exceeded, the shelf life of many foods is no longer 
ensured; the frozen material is considered to have thawed although it 
still appears frozen on the outside. 
Usually, freezers are equipped with built-in thermometers which indicate 
the temperature in a certain part of the freezing space, or a conventional 
thermometer is placed on or next to the frozen material and is removed 
from the freezer for readings. In these cases, there is no guarantee that 
the temperature of the frozen material will be measured exactly. Moreover, 
air convection in the freezing space may be hindered by stacking and 
packing of the frozen material, so that the minimum temperatures required 
for ensuring the shelf life of the foods may be exceeded in certain parts 
of the freezing space, even when the equipment functions satisfactorily. 
The user of the frozen material cannot detect the fact that the maximum 
storage temperature has been exceeded, so that he cannot counteract 
spoilage of the food by removing it in good time and using it immediately. 
The danger that deep-frozen food will be thawed without this being noticed 
is particularly great, for example, when a relatively large amount of 
material to be chilled is introduced in a warm state into the freezer. 
It is therefore desirable to have a temperature indicator which shows, for 
every food pack or for a group of packs, whether they have been 
sufficiently chilled during their entire storage time. 
2. Description of the Related Art 
Temperature-indicating apparatuses of the generic type stated at the outset 
are disclosed, for example, in DE 25 47 638 and 32 43 031. In the 
first-mentioned Laid-Open Application, the indicator element is a contact 
layer of absorptive material, over which a layer of a dye soluble in a 
liberated liquid is arranged, over which layer in turn an indicating layer 
of absorptive material is present. When the frozen material partially 
thaws, cellular liquid is released at its surface and is absorbed by the 
contact layer. The liquid thus absorbed dissolves part of the colour layer 
and the coloured liquid is then absorbed by the indicator layer whose 
discoloration provides evidence of reduction in quality due to 
interruption of the cooling chain. 
The last-mentioned application describes a container having transparent 
walls, for example a flat bag, which contains a eutectic mixture, those 
walls of the container which face the frozen material bearing a 
recognizable inscription on their inner surface. When the temperature 
falls below the critical temperature, the inscription is invisible; it 
become visible only when the critical temperature is exceeded. This 
publication also states that an absorptive material can be placed between 
the frozen material and the container described, which absorptive material 
absorbs the liquid taken up on exceeding the critical temperature and in 
this way detects when the permissible temperature has been exceeded. 
The two abovementioned temperature indicators have a complicated design; 
moreover, it is not ensured that the frozen product does not come into 
contact with the indicator medium and thus become damaged. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an indicator apparatus 
of the generic type stated at the outset, which does not have the 
above-mentioned disadvantages and irreversibly indicates if the 
permissible temperature is exceeded, the indication being presented 
optically in a simple manner or being detected by another means. 
We have found that this object is achieved, according to the invention, by 
an apparatus for irreversibly indicating whether the permissible storage 
temperature of products stored in the frozen state has been temporarily 
exceeded, consisting of a container which has a heat-conducting connection 
to the stored product and contains, in its interior, a medium having a 
melting point in the range from about 0.degree. to -50.degree. C. and 
clearly indicating parts, wherein finely divided magnetically soft or 
magnetically hard pigments have been introduced into the medium and the 
container is exposed, during the liquid/solid or solid/liquid phase 
transition, to an external magnetic field which leaves behind optically or 
magnetically readable information in the medium. Further novel features 
are evident from the subclaims, the description and the drawing.

The indicator apparatus (2) which will be described in more detail is 
inserted into the covering (3), for example a shrink-film of the frozen 
material, at any point of the surface (4) of the frozen material (1). The 
temperature indicator (2) is preferably a flat bag (5) which consists of a 
flexible transparent plastic and contains, as a medium, an inorganic 
eutectic mixture or an organic compound, which have a melting point in the 
range from 0.degree. to -50.degree. C., preferably from -10.degree. to 
-25.degree. C. 
DETAILED DESCRIPTION OF THE INVENTION 
The essential feature of the invention is (see FIG. 2) that finely divided 
magnetically hard or magnetically soft pigments (6) are dispersed in the 
stated medium. Under the action of an external magnetic field, the 
magnetic pigments in the dispersion can be moved or collected at certain 
locations or concentrated. The external magnetic field used may be, for 
example, a magnetic stripe (7) or permanent magnet (not pictured) which 
contains, magnetically recorded, a bar code pattern (8) or alphanumeric 
characters (not pictured) or a graphics pattern, for example a company 
logo (not pictured), and which is in close contact with the magnetic 
dispersion in the bag, for example is stuck to the outside of the latter. 
The magnetic field of the magnetic stripe or permanent magnet causes the 
magnetic pigments to arrange themselves in conformity with the pattern of 
the magnetic stripe or permanent magnet. 
If the carrier liquid of the magnetic dispersion and any additional 
auxiliary components, for example dispersants and stabilizers or the 
additives stated further below, are now chosen so that a liquid/solid 
phase transition occurs in the temperature range described, or at least 
the dispersion sets, the ordering of magnetic pigments is retained even 
after removal of the magnetic recording field, ie. of the magnetic stripe 
described above, and can, for example, be easily recognized visually with 
the naked eye as information or a defined pattern. 
It is therefore merely necessary to maintain the external magnetic field 
during cooling of the material below the critical temperature and of the 
temperature indicator described and then to remove said magnetic field (8) 
after the temperature has fallen below the critical temperature. The 
ordered state of the solidified magnetic dispersion is maintained as long 
as the temperature does not exceed a certain value or does not exceed a 
certain value-time product. 
However, if this is the case, for example on failure of the freezer, the 
viscosity of the magnetic dispersion is again reduced and the pattern 
vanishes owing to thermal excitation of the pigments. This provides the 
desired visual indication in the event that the critical temperature is 
exceeded. 
However, it is also possible to scan the solidified magnetic state with a 
detector which responds to magnetic fields, for example magnetically 
recorded bar code information with a magnetically responding bar code 
reader. In this case, the bag containing the magnetic dispersion need not 
have transparent walls (5). 
The magnetic temperature-sensitive dispersion described above can be 
produced simply and economically. Suitable dispersion liquids are 
substances which change their liquid/solid state of aggregation in the 
temperature range from 0.degree. to -50.degree. C. or at least exhibit a 
pronounced change in their viscosity. For example, eutectic mixtures, 
disclosed in the abovementioned DE 32 43 031, are suitable for this 
purpose, such as ammonium chloride, calcium nitrate, ammonium nitrate, 
sodium nitrate, potassium chloride or mixtures thereof together with 
water. Organic compounds, such as diethyl succinate, octyl caprylate, 
heptyl caprylate, hexyl laurate and others, are also suitable. 
Finely divided magnetic pigments (6) obtained according to the prior art by 
precipitation or by a ceramic method are dispersed in this liquid. For 
example, .gamma.-Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, CrO.sub.2, metal 
powders or alloys are suitable for this purpose. The magnetic behavior of 
the pigments described above (magnetically soft or magnetically hard) can 
be adjusted as desired by means of their structure and their composition. 
They should be chosen so that a sufficiently high magnetic force can be 
exerted on them for migration in the dispersion liquid, that their mutual 
attraction in the deep-frozen state is not too high so that separation 
during thawing as a result of thermal excitation is ensured, and that they 
are capable of producing good optical contrast for visual detection. 
To achieve the abovementioned conditions, the magnetic pigments can be 
incorporated into low molecular weight or high molecular weight 
dispersants. Such dispersants are disclosed in, for example, DE 39 08 425. 
Furthermore, the magnetic pigments may be dispersed in polymeric binders; 
examples of these are likewise disclosed in the abovementioned DE 39 08 
425. 
To keep the distance between the two films of the bag which holds the 
dispersion composition small and constant, a correspondingly large 
colorless support pigment or latex (9) particles which act as spacers and 
prevent touching of the two film sides may be added to the dispersion. 
FIG. 2 shows the magnetic strip (8) applied to the front wall (7), and the 
particles (9) separating the front wall from the back wall (13). Another 
solution comprises (see FIG. 3) structuring the inside of one or both film 
surfaces (12), for example by means of nubs (14, shown attached to the 
back wall), so that the interior of the bag is divided into smaller 
regions to prevent merging of the dispersion (20). FIG. 3 shows the 
magnetic stripe (8) applied to the front wall (7), with nubs projecting 
from the back wall. 
Suitable wall material for the film bag (5, 12) is, for example, polyester, 
polyethylene or polypropylene. After introduction of the abovementioned 
dispersion liquid, the bag is sealed, preferably by welding, to prevent 
the liquid from running out. 
A magnetic foil, as found, for example, in credit and ATM cards, telephone 
cards, tickets or savings books and on which the abovementioned 
information has been recorded can be used for the magnetic inscription of 
the described temperature indicator. The magnetic foil is stuck to the bag 
and is placed, with a heat-conducting connection, on the material to be 
frozen, as shown in FIG. 1. After information transfer and cooling below 
the limiting temperature, the magnetic stripe (8) is simply removed and 
the novel temperature indicator is ready to use. 
By suitable combination, the novel apparatus may contain even more 
advantageous embodiments. Thus, an outer or inner surface of the bag may 
contain a reflective or colored layer to enhance the contrast of the 
indication. 
In a variant (see FIG. 4), it is furthermore possible to transform a 
visible pattern into another, likewise visible pattern on exceeding the 
critical temperature. For this purpose, in addition to the magnetic stripe 
(17) stuck to the upper surface, a second magnetic stripe (19) whose 
pattern is magnetically recorded only after freezing of the bag and which 
is applied to the bag only after freezing is mounted on the opposite side, 
ie. the lower side, of the bag. If, after removal of the stripe (17) stuck 
to the upper side, the critical temperature is exceeded, a pattern becomes 
visible in the dispersion as a result of the magnetic marking recorded on 
the second magnetic stripe and can be detected. In this way it is possible 
to read unambiguous yes/no information by a method in which, for example, 
the first magnetic stripe (17) visualizes the text deep-frozen and the 
second magnetic stripe visualizes (19) the text thawed in the medium. FIG. 
4 illustrates this application, with the first, upper magnetic stripe (17) 
patterned to read "DEEP FROZEN" and the second, lower magnetic stripe 
patterned to read "THAWED". 
However, it is also possible merely to use this second magnetic stripe (19) 
for detection in the manner described. In this case, a pattern only 
appears when the critical temperature is exceeded. 
In a further variant (see FIG. 5), a magnetic stripe (29) having a magnetic 
pattern (30) as described above may be placed on the lower side of the 
bag, in addition or alternatively to the magnetic stripe (27) applied to 
the upper side, the pattern (8) of said magnetic stripe (27) being deleted 
by an external magnetic field (10, 11) when the freezing process is 
complete. 
This permits many combinations or transitions from one visible or 
detectable pattern into another.