U.S. patent number 4,432,656 [Application Number 06/253,748] was granted by the patent office on 1984-02-21 for process for monitoring the history of temperature versus time of deep-frozen product, indicator for applying said process and utilization of said process.
Invention is credited to Thomas Allmendinger.
United States Patent |
4,432,656 |
Allmendinger |
February 21, 1984 |
Process for monitoring the history of temperature versus time of
deep-frozen product, indicator for applying said process and
utilization of said process
Abstract
The disclosure concerns an indicator for providing a
time/temperature integrated indication of the temperature history
of a deep-frozen product through the diffusion of water along a
cellulose wick. The cellulose wick is provided on itself with a
water soluble, hydrate forming substance, such as sodium chloride
and citric acid. The wick is covered in a plastic covering with an
opening in the covering exposing the wick to water at the product,
and the advance of the water front along the wick is indicated by a
time/temperature indicator adjacent the wick and/or by an acid/base
indicator on the wick and spaced from the indicator.
Inventors: |
Allmendinger; Thomas (8152
Glattbrugg, CH) |
Family
ID: |
4311122 |
Appl.
No.: |
06/253,748 |
Filed: |
March 12, 1981 |
PCT
Filed: |
July 11, 1980 |
PCT No.: |
PCT/CH80/00085 |
371
Date: |
March 12, 1981 |
102(e)
Date: |
March 12, 1981 |
PCT
Pub. No.: |
WO81/00303 |
PCT
Pub. Date: |
February 05, 1981 |
Foreign Application Priority Data
|
|
|
|
|
Jul 13, 1979 [CH] |
|
|
6535/79 |
|
Current U.S.
Class: |
374/102; 116/206;
116/216; 374/E3.004; 426/88 |
Current CPC
Class: |
G01K
3/04 (20130101); G01N 33/02 (20130101); G01N
31/229 (20130101) |
Current International
Class: |
G01N
31/22 (20060101); G01N 33/02 (20060101); G01K
3/04 (20060101); G01K 3/00 (20060101); G01K
003/04 (); G01K 011/16 () |
Field of
Search: |
;116/216,219,206
;73/356,357 ;426/88 ;374/102,106,162 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Myracle; Jerry W.
Assistant Examiner: Noland; Tom
Attorney, Agent or Firm: Rosen; Lawrence
Claims
I claim:
1. A time temperature integrating indicator for monitoring the
history of a deep-frozen product by the diffusion of water along a
diffusion path wherein the water front advancement along the
diffusion path is a function of elapsed time and of the temperature
of the product, the indicator comprising a wick of a material which
defines a diffusion path along which water will diffuse; the wick
being provided along its diffusion path with citric acid or a
mixture of sodium chloride/citric acid or of sodium chloride/sugar,
as an absorbing and hydrate forming substance; the formation of the
hydrate at least partially providing a visible indication of the
water front location; a moisture tight covering enclosing the wick;
an opening in the covering for permitting intimate contact between
the wick and water on the frozen product, located at the opening in
the covering; and additional indicating means for indicating the
location of the water front along the diffusion path from the
opening in the covering.
2. An indicator according to claim 1, further comprising a viscous
substance on the wick in the vicinity of the opening in the
covering for preventing premature flow of the water front from the
opening along the wick and past the viscous substance.
3. An indicator according to claim 2, wherein the viscous substance
is a sugar.
4. An indicator according to claim 1, wherein the absorbing
substance is impregnated in the wick.
5. An indicator according to claim 1, wherein the wick is comprised
of cellulose material.
6. An indicator according to claim 5, wherein the covering is a
plastic molding.
7. An indicator according to claim 5, wherein the cellulose wick
has a length selected to give an indication related to the maximum
storage life of a product to be monitored by the indicator.
8. An indicator according to claim 7, wherein the wick length is
selected to give its indication for a product that is to be stored
at -20.degree. C.
9. An indicator according to claim 1, wherein the additional
indicating means for indicating the distance of the water front
advance comprises a time scale next to the diffusion path to
indicate the time elapsed as a function of the advance of the water
front, at a predetermined temperature.
10. An indicator according to claim 1, wherein the additional
indicating means for indicating the distance of the water front
advance comprises an acid/base indicator on the diffusion path away
from the opening in the covering for indicating when the water
front reaches the acid/base indicator.
11. An indicator according to claim 10, wherein the acid/base
indicator is litmus paper.
12. An indicator according to claim 10, wherein the absorbing
substance is a sodium chloride/citric acid mixture and the wick
comprises a cellulose wick; on the cellulose wick, between the
opening in the covering and the acid/base indicator, a buffer
substance is provided for absorbing acid gas formed at the wick
inside the covering, in order that the gas not reach the acid/base
indicator.
13. An indicator according to claim 12, wherein the buffer
substance is secondary ammonium citrate.
14. An indicator according to claim 10, wherein the absorbing
substance is a sodium chloride/citric acid mixture where the ratio
of the mixture is adapted to the temperature dependency of the
diffusion rate to be achieved.
15. A method for activating an indicator according to claim 1
comprising the step of contacting the wick at the opening in the
covering thereover with a water or an ice reserve, which water is
one of the water of the product or an additional water reserve at
the product.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a method of monitoring the history
of temperature versus time of a deep-frozen product in the way of a
time integral of its temperature development, with an indicator for
sensing the course of diffusion of water in a path of diffusion;
and relates to an indicator for carrying out said method and to the
utilization of said method.
STATE OF THE ART
There has been a need for a long time to monitor deep-frozen
foodstuffs. Reference is made in this respect to U.S. patent
specification Ser. No. 1,535,536 dating back to the year 1925,
which relates to the monitoring of the storage temperature of ice
cream. This published document was followed by a number of others
relating to overcoming as to how the uncertainty with respect to
the storage temperature of deep-frozen products with the help of a
simple and inexpensive indicator. Attention is called in this
regard to the publication by H. M. Schon and C. H. Byrne in "FOOD
TECHNOLOGY", October 1972, in which relevant publications are
listed in a summarized form. Most conventional indicators are based
on the principle comprising the monitoring of a limit temperature
on the deep-frozen products by indicating any, even though only
brief exceeding of said limit temperature either immediately or
with some delay within the order of magnitude of hours. Such
indicators are known as "defrost indicators". Said indicators
function in most cases on the principle that a liquid, which is in
the frozen state at the required storage temperature, melts if the
limit temperature is exceeded above -18.degree. C., and the liquid
is absorbed by a wick or strip of paper, and subsequently flows to
an indicating field where it causes a color change. Reference is
made in this connection, for example to French patent specification
No. 1,548,424, or U.S. Pat. No. 2,951,764.
However, the durability of deep-frozen products has been
investigated more recently in greater detail. It was found that
specific critical temperatures do not exist, but that the
permissible storage time of deep-frozen products is rather reduced
continuously as the temperature increases, namely with a quality
reduction that takes place approximately logarithmically. It has
become a customary procedure to show graphically by means of
so-called durability curves the periods after which an acceptable
quality limit can be still achieved as a function of the
temperature. FIG. 1 shows a number of durability curves for various
types of product, which are borrowed from the publication
"Recommandations pour la preparation et la distribution des
aliments congoles", 2nd edition, 1972, pages 118/119, published by
the "Institut International du Froid", Paris.
Said durability curves vary for different product categories, and
both the maximum permissible storage duration at -20.degree. C. and
the storability as a function of the temperature may be different.
Normally, the frequency of the temperature fluctuations plays a
negligible role, with the exception of ice cream. More recently, it
has become customary to use for the durability of deep-frozen
products as a function of the temperature the so-called Q.sub.10
-value. This is the quotient which is obtained by dividing the
duration of durability of a specific product at a defined
temperature by the duration of durability of said product at a
temperature that is 10.degree. C. higher. However, since said
Q.sub.10 -value depends on the temperature, which means, for
example, that its value in the range of -2.5.degree. C. and
-12.5.degree. C. is different from the one in the range of
-10.degree. C. and -20.degree. C., the
Q.sub.-20.degree./-2.5.degree. C.-value is introduced in the
present case instead of the Q.sub.10 -value for the
characterization of the function of the temperature, the former
covering the storage temperature range of -20.degree. C. and
-2.5.degree. C. that is of interest to deep-frozen products just
prior to thawing.
According to a rough classification the
Q.sub.-20.degree./-2.5.degree. C.-values are as follows: for
vegetables and fruits about 45; for meat and fish about 20. Other
product categories fall within said extreme cases.
Now, with indicators which have become known more recently, the
reaching of a quality limit characteristic of a deep-frozen product
in dependency on the temperature is indicated in a way
corresponding with the durability curve of said product. This is
achieved by permitting a liquid to diffuse upwardly in a strip,
namely the more rapid the diffusion the higher the temperature rise
within the environment of said strip. If such a strip is
incorporated in a deep-frozen product shortly prior to the freezing
of said product, said strip picks up or senses all changes
occurring in the future, which is reflected by the liquid starting
to flow more rapidly if the temperature should occasionally
increase, and subsequently again more slowly when the temperature
drops. Since this action is irreversible, all reductions in quality
due to temperature rises are integrated. Such indicators are known
under the term "time temperature integrator". Reference is made in
this regard to the following publications:
A. Kramer and J. W. Farquahr: "Testing of Time-Temperature
Indicating and Defrost Devices", FOOD TECHNOLOGY, February 1976,
pp. 5+-56;
C. H. Byrne: "Temperature Indicators-the State of the Art", FOOD
TECHNOLOGY, June 1976, pp. 66-68;
H. Schubert: "Indikatoren zur Kontrolle der
Zeit-Temperatur-Belastung von tiefgefrorenen Lebensmitteln"
(Indicators for Controlling the Time-Temperature Load of
Deep-Frozen Foodstuffs), Zeitschrift fur Lebensmittel Technologie
und Verfahrenstechnik, 31(3): 137-142, 1980.
(this article, however, was published only after the date of our
original application in Switzerland).
In this conjunction, particular reference must be made to U.S. Pat.
No. 3,946,611. Whereas in certain embodiments it is mainly an
aqueous solution that constitutes the supply of diffusion fluid
used for such defrost indicators (for example, French Pat. No. 1
548 424, U.S. Pat. No. 3,414,415, DE Pat. No. 2 130 926), the U.S.
Pat. No. 3,946,611 makes provision for a gas-absorbing substance
along the diffusion path. True, the possibility of utilizing water
vapor as the gas is referred to, but there is no further discussion
as to the substances that might come into consideration as
absorbents in such a case. According to this patent, moreover, the
path of diffusion on the indicator is separated by a semipermeable
foil from the fluid supply provided on the indicator. Along with
the relatively complex configuration necessitated by the
semipermeable foil and the fluid supply to be provided on the
indicator, this indicator has the drawback because of the
semipermeable foil, that the diffusion velocity is reduced at a
time when the fluid supply melts, in other words, in the presence
of water, when the zero-degrees centigrade barrier is surmounted.
Yet, it is precisely at this point that accelerated diffusion is
desirable.
SUMMARY OF THE INVENTION
The present invention proposes a process for monitoring the history
of time versus temperature of deep-frozen products in the way of a
time integral of its temperature development. It uses an indicator
for sensing the development of the diffusion of water in a path of
diffusion. By this process, furthermore, the reaching of a quality
limit characteristic of a deep-frozen product as a function of the
temperature is indicated in a way such that it corresponds with the
durability curve of said product. The process avoids the drawbacks
of providing a semipermeable foil between the water and the path of
diffusion, and also the complicated arrangement of a liquid
reserve, in particular of a water reserve on the indicator.
The process achieves this by activating the indicator by contacting
the path of diffusion of said indicator with water of the product,
which may be the moisture content of the product itself or an
additionally provided water or ice reserve.
An indicator for carrying out said process is characterized by
using as the path of diffusion a body provided with a
water-soluble, hydrate-forming substance, or with a mixture of
substances, said body being wrapped in a moisture-tight cover
except for a point for contacting the water.
Said moisture-tight wrapping is provided because the body, with the
exception of the contacting point, may not absorb any moisture. By
not providing a semipermeable foil that separates the path of
diffusion from the water reserve the rate of diffusion increases
significantly as soon as the water reserve starts to melt, thus as
soon as the 0.degree. C.-limit is exceeded. This additional effect
is practically lost when using a semipermeable membrane. If one
uses as the water or ice reserve not pure water but, instead, for
example, ice cream, this results in additional effects bearing
favorably upon the indication, because the larger the proportion of
dissolved substances in the water reserve the more rapid the
diffusion of the liquid front in the path of diffusion. This effect
correlates positively with the fact that fruit glace, which has
much sugar, is also less durable than the cream glace with low
sugar content.
In the simplest case, for example with meat packaged in a clear,
shrunk-on foil material, or with glace, it is possible to make use
of the moisture content of the product itself that has to be
monitored. In other cases, it is necessary to provide the packaging
material of the product with a small cavity for receiving a small
amount of water. Contrary to providing a water reserve on the
indicator this assures also that no premature, undesired activation
of the indicator can take place.
The indicator is provided in a particularly simple embodiment by a
cellulose body, preferably an absorbent paper.
The treatment of the path of diffusion or body is of decisive
importance for obtaining the desired indication. Normally, it is
felt that it will be necessary to apply to said body a substance
that is known to be hygroscopic, for example such as calcium
chloride, sodium hydroxide, phosphorous pentoxide etc., i.e. a
substance capable of absorbing moisture from the air. However, it
was found that such highly hygroscopic substances result in an only
low temperature dependency of the diffusion rate of water, which
means that said substances permit only the sensing of very low
Q.sub.-20.degree./-2.5.degree. C.-values. Also, most of said
substances are not harmless with respect to foodstuff hygiene,
which is a very important criterion in this connection.
Based on the knowledge that at 100% relative air humidity any
water-soluble substance becomes hygroscopic due to the compensation
in the concentration which occurs, and that the liquid front
becomes clearly visible in the path of diffusion only if the
substance serving for the absorption forms a hydrate, it was found
that the water-soluble, hydrate-forming substance is preferably
sodium chloride, citric acid, a sugar, cobaltous chloride, or a
mixture of two or a plurality of said substances. With said
substances, which are inexpensive and, with the exception of
cobaltous chloride, harmless with respect to foodstuff hygiene, it
is possible to achieve very high Q.sub.-20.degree./-2.5.degree.
C.-values such as, for example of up to 50, using a citric
acid/sodium chloride mixture. Below the cryohydric point, which for
sodium chloride lies at about -22.degree. C. and for citric acid at
approximately -15.degree. C., and for mixtures is slightly lower,
the action of absorption and diffusion comes to a standstill,
because the substances are then no longer water-soluble.
While it was found that a citric acid/sodium chloride mixture is
particularly suitable for monitoring most deep-frozen products, the
use of pure citric acid is recommended for monitoring glace.
Also cobaltous chloride or cobaltous chloride in mixture with
sodium chloride and sugar would be suitable to the extent that in
this case, the absorption of water takes place combined with a
clearly visible color change from blue to red. However, the
Q.sub.-20.degree./-2.5.degree. C.-values achievable with such a
mixture of substances are comparatively low; furthermore, cobaltous
chloride is neither non-toxic nor inexpensive.
It is proposed, furthermore, to provide along the body of the
indicator a time scale for indicating the remaining durability of a
product to be monitored at a specific temperature, preferably at
-20.degree. C. Furthermore, it is proposed to provide on the body
an acid/base indicator, preferably litmus paper, for indicating a
predetermined diffusion limit value, and to apply to the body a
water-soluble acid, preferably citric acid, or a base, which is
dissolved by the diffusing water and carried to the acid/base
indicator. This divides the field of indication into two zones. The
first zone is formed by the logarithmic time scale provided along
the path of diffusion indicating the remaining durability of the
product to be monitored at a specific temperature, preferably at
-20.degree. C. as stated above. Within said zone, indication takes
place in the form of the aqueous front advancing by diffusion. In
the second zone, when a predetermined path limit value is reached
this is indicated by the color change of the acid/base indicator:
an acid or a base, and preferably citric acid originally present on
the body in the solid state, is dissolved by the diffusing water
and carried to the acid/base color indicator. Therefore, the color
change taking place on said indicator tells the consumer that the
monitored product has reached a defined quality limit.
If the path of diffusion is provided with a sodium chloride/citric
acid mixture, the indicator, for example litmus paper, is subject
to a premature, undesirable color change, namely from blue to red,
while the nonactivated indicator is being stored because it is
assumed that the above mixture may form a small amount of
hydrochloric acid which is diffused to the litmus paper.
In order to prevent said premature color change it is proposed to
separate the body--if it is provided in its path of diffusion with
a sodium chloride/citric acid mixture--from the acid/base color
indicator by means of a section of path provided with an acid
ampholyte, preferably with secondary ammonium citrate. The buffer
zone so formed absorbs the undesired hydrochloric gas by virtue of
its basic properties.
Since it is unavoidable in some cases for practical reasons to
already bring the indicator into contact with the water reserve
shortly before the water has frozen, for example if water is filled
first into the cavity provided for this purpose on a package that
is already deep-frozen, and a self-adhesive label is then
immediately applied to the indicator, it is proposed to provide the
body within the zone of the contacting point for the water with a
viscous substance, preferably with a sugar, which upon activation
of the indicator prevents the water from flowing upwardly
prematurely.
An adhering, transparent plastic foil material, preferably an
adhesive chlorotrifluoroethylene foil is used for wrapping the
body, namely for all deep-frozen product with the exception of
glace, or the wrapping or covering is formed by a shaped plastic
object, preferably a deep-drawn part made of transparent PVC, which
is used for glace. The path of diffusion of the body is adjusted to
the maximum durability of the product to be monitored, preferably
at -20.degree. C., and/or the mixing ratio of sodium chloride to
citric acid is adjusted to the temperature-dependency of the
durability of the product to be monitored. The proposed method and
the indicator proposed for carrying out said method are
particularly suitable for monitoring the quality or storage
condition of deep-frozen foodstuffs within the temperature range of
about -25.degree. C. and about +5.degree. C.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is explained in the following with the help of the
drawings of which:
FIG. 1 shows the dependency of the permissible storage duration of
different deep-frozen products as a function of their storage
temperatures;
FIG. 2 shows a perspective view of an indicator without color
change;
FIG. 3 shows the qualitative dependency of the path of diffusion
upon time, on an indicator according to FIG. 2, with the
temperature as parameter;
FIG. 4 illustrates a lateral view of another embodiment of the
indicator according to FIG. 2 with color change;
FIG. 5 shows a general view of an indicator applied to the product
to be monitored;
FIG. 6 is a general view of an indicator applied to the packaging
material of a product to be monitored;
FIG. 7a shows an indicator for glace placed into a recess or cavity
made of plastic; and
FIG. 7b an indicator for glace inserted in a wall of the packaging
material .
DESCRIPTION OF THE PREFERRED EMBODIMENT
The so-called durability curves shown in FIG. 1 were referred to in
the discussion of the state of the art. The indicator shown in FIG.
2 comprises a paper strip 1. Said strip of paper is provided with a
water-soluble, hydrate-forming substance such as sodium chloride,
citric acid, a sugar, cobaltous chloride or with a mixture
comprising two or a plurality of said substances. Reference number
2 in FIG. 2 designates the aqueous front which advances by
diffusion at diffusion rate "v". The strip 1 is encased by a
moisture-tight foil 5 preferably composed of adhesive
chlorotrifluoroethylene foil. Said foil leaves a contact point 1a
of the strip exposed as shown at 5a by the dashed lines, namely at
least on one side. Contact point 1a is thus formed in such a way
that it is capable of directly contacting the water reserve of the
product, which may be the moisture content of the product itself or
an additionally provided water or ice reserve on the product or on
its packaging material.
In order to prevent the water starting to diffuse immediately after
the contact point 1a has been contacted by water, which is the case
if the indicator is applied immediately prior to the freezing of
said water reserve, the contact point 1a is provided with a viscous
substance, for example with a sugar, which is illustrated in FIG. 2
by the sectioning at point 1a.
If a few of the indicators of the type specified based on FIG. 2
are each exposed to a different temperature, we obtain the
diffusion developments as the function of time as qualitatively
shown in FIG. 3, with both the course of diffusion and the time
being marked off logarithmically. The temperature values
-20.degree. C., -10.degree. C. and -2.5.degree. C. were selected as
test temperatures. In the range exceeding 0.degree. C. the rate of
diffusion is still significantly higher, because the contact point
1a is in direct contact with water, the cause being that the ice
contacted first has melted, so that the water is no longer required
to detour by way of the vapor phase. The diagram illustrated in
FIG. 3 clearly shows the time which the front of diffusion requires
at the individual temperatures in order to travel a defined
distance. If the diffusion times so obtained for a predetermined
length of path or distance are plotted against the temperature, we
obtain curves corresponding with the durability curves of
deep-frozen products. Since each deep-frozen product having some
amount of moisture can be approximately compared to a block of ice,
the function of the flow action is practically independent of the
type of deep-frozen product involved, unless the product contains
dissolved substances such as sugar, table salt etc. However, on the
other hand, the storability does depend on the type of product
involved, which means that it is necessary to provide for each type
of deep-frozen product a body in conformity with paper strip 1
having a suitably corresponding length, and to adapt the
temperature dependence of the diffusion rate to the one of the
durability of the product to be monitored by reducing the mixing
ratio of the substances, in particular of sodium chloride and
citric acid, applied to said strip.
The indicator shown in FIG. 4 comprises, in addition to the paper
strip 1, the moisture-tight foil 5 and the viscous substance,
preferably sugar, applied to contact point 1a, an acid/base
indicator disposed at its end opposite contact point 1a, said
indicator preferably being a litmus paper 4. A water-soluble acid,
preferably citric acid, is applied to paper strip 1. Said acid is
dissolved by the water diffusing with front 2 and carried upwardly
to reach acid/base indicator 4. Now, in order to avoid a premature
color change in acid/base indicator 4 caused, for example by the
formation and diffusion of hydrochloric gas, a buffer zone 3 is
interconnected in the path between acid/base indicator 4 and
contact point 1a. Said buffer area is provided with an acid
ampholyte, preferably secondary ammonium citrate and adsorbs
undesirable hydrochloric gas by virtue of its basic property. The
indicator is furthermore provided with a logarithmic time scale 6,
which is related to the remaining durability of the product to be
monitored at a specific temperature, for example at -20.degree.
C.
The strip-shaped indicator shown in FIGS. 2 and 4 is used
particularly in cases in which a mixture of common salt and citric
acid is used as the water-soluble, hydrate-forming substance. This
is the case with all deep-frozen products with the exception of ice
cream. The flow path is relatively long in this case, namely in the
order of magnitude of a few centimeters.
It is shown in FIG. 5 that said strip-shaped indicator may be
attached either directly to the deep-frozen product 8 provided that
the product with the indicator is then packaged or wrapped in a
plastic foil 9, as this is the case, for example with meats, or it
may be attached to a package 11 in the form of a self-adhesive
label 10 as shown in FIG. 6, in which case said package is provided
with a depression 12 for receiving a small water reserve 14.
The cellulose body formed according to FIGS. 3 and 4 by the strip
of paper serving as the diffusion path may, in another embodiment,
have a cylindrical or square shape. This shape is used particularly
if pure citric acid is used as the water-soluble, hydrate-forming
substance, which is the case in the monitoring of ice cream
(glace). In this case, the flow path is relatively short, namely
only in the order of magnitude of a few millimeters, because citric
acid, due to its relatively high viscosity in the liquid state,
causes the rate of diffusion to be low. No reaction takes place at
temperatures of -20.degree. C. and lower. This indicator, which is
shown in FIGS. 7a and 7b, comprises a filter cardboard 18 saturated
with citric acid solution and having a cylindrical or square shape.
The pills 18, which in this case form the body, are contained in
suitable plastic moldings preferably made of polyvinyl chloride
(PVC).
According to FIG. 7a, for example, the package for the ice cream or
glace to be monitored is provided with a depression projecting
outwardly with respect to product 19. Placed into said depression
are first an acid/base indicator 17, namely litmus paper, and then
the pill 18 saturated with citric acid. FIG. 7b shows a plastic
jacket 16 made, for example, of PVC, and attached to the package
for the ice cream or glace to be monitored. Said jacket protrudes
into product 19 and receives also in this case first an acid/base
indicator 17, preferably litmus paper, and subsequently the pill
saturated with citric acid. In both embodiments shown in FIGS. 7a
and 7b the plastic covering is transparent at least in the area of
20, so that the color change of indicator 17 may be seen from the
outside. The product 19, i.e. the ice cream or glace to be
monitored, is contacted by pill 18.
The indicators for monitoring the history of time versus
temperature of a deep-frozen product as herein specified are
particularly suitable for the temperature range of about
-25.degree. C. to about +5.degree. C. Said indicators may be
adapted to the maximum durability of the deep-frozen product at a
specific temperature, preferably at -20.degree. C., by varying the
length of the body, while adjustments to the temperature dependence
of the durability are made by varying the sodium chloride to citric
acid ratio. The indicator may be calibrated or its dependence on
temperature may be determined by preparing a flow diagram, plotting
the logarithm of the flow time against the logarithm of the
diffusion path as shown in FIG. 3. The straight lines so obtained
permit good extrapolation.
BEST PROCEDURE FOR IMPLEMENTING THE INVENTION
The indicator according to FIGS. 2 and 4 is manufactured by first
saturating a strip of paper with a common salt/citric acid solution
and drying said strip subsequently under heat. The one end of said
strip is then briefly dipped into a concentrated sugar solution and
again dried. The buffer zone 3 is prepared analogously. This
component and a piece of litmus paper 4 are then placed in the
proper sequence between two strips of a transparent plastic foil or
sheet with good adhesive power, preferably an adhesive
chlorotrifluoroethylene sheet material. One portion 5a of said foil
may be slightly longer so as to slightly overlap the contact area
in order to protect said area against loss of moisture to the
outside. The embodiment according to FIGS. 7a and 7b is prepared by
punching one or a plurality of pills from a filter cardboard
material, which was first saturated with citric acid solution and
then dried. Said pills are then embedded in cylindrical or square
plastic depressions. The litmus indicator is embedded first.
COMMERCIAL UTILIZATION
The specified process and indicator for applying said process offer
a suitable and simple and thus inexpensive possibility for
monitoring deep-frozen products with respect to their durability,
which may be increasingly important in cases of power failure in
deep-freezing installations.
* * * * *