The present invention concerns a leakage in¬ dicator for a package. The food industry is increasingly adopting protective gases in packaging food, in the first place meat and fast-food products. The protective gas guards the foodstuffs against deterioration. Non-leaking qual¬ ity of the package is one of the essential properties of gas packages. If the gas package develops a leak, the gas will escape from the package and oxygen, which is harmful to the packed goods, gains access, whereby the added keeping quality afforded by the gas is lost. However, in the case of the majority of products the occurrence of leakage can only be established once the package has left the food processing plant, when the food inside the package has been spoilt (e.g. mould growth) . This means that the consumer may buy a package containing a product which is on the verge of non-ac- ceptability even though there may still be an ample margin of shelf time.

Experience gained in the food industry teaches that leaking of gas packages is often caused by excessive packaging speeds (by sealing failures) or by mechanical damage incurred by the package during hand¬ ling and transports. Such cases of leakage are hard to detect, and they are frequently only revealed by spoil¬ ing of the contents.

In prior art two reagents are known: Ageless Eye by Mitsubishi Gas Chemical Co Inc., and Reagent V by Nippon Kayaku Inc. However, these reagents react to oxygen with undue sensitivity, e.g. to the residual oxygen entrapped in the packaging process. Furthermore, these indicators are not functional in the presence of carbon dioxide, which after all is one of the most important protective gases used in the food industry. It is thus clear that observation of leakage

in food packages poses a problem which has not been solved heretofore.

The object of the present invention is to eliminate the drawbacks mentioned. Specifically, the object of the invention is to disclose a novel leakage indicator which is fit to be used in conjunction with packages, particulary gas-protected packages, and spe¬ cifically food packages. Furthermore, it is an object of the invention to disclose a leakage indicator which does not react to residual oxygen which may be en¬ trapped in the package but which will react to oxygen continuously entering the package e.g. as a result of breakage. The object of the invention is further to disclose a leakage indicator which is appropriate to be used in connection with packages using carbon dioxide for protective gas.

Regarding the features characterizing the in¬ vention, reference is made to the claims.

The invention is based on the inventive idea worked out in an investigation that was made, that the leakage indicator comprises in addition to a colourant reacting by change of colour under influence of oxygen, a reducing agent to reduce said colourant and to keep it in reduced state at the moment of packaging, and a particular oxygen absorbent for eliminating the resid¬ ual oxygen entrapped in the package and for binding it in such manner that said residual oxygen will not react with the colourant. Then, owing to said reducing agent and said oxygen absorbent, the leakage indicator will not react with the residual oxygen entrapped in the package but will react in the event of leakage of the package.

Further, according to an advantageous embodi¬ ment of the invention, the leakage indicator is pack- aged in a double, oxygen-permeable package, e.g. in a double package made of packaging material, such as plastic or paper-based material, oxygen permeability on

the order of 1000 to 10000 ml/m ² day atm, such as a double plastic package, e.g. in double polyethylene film wrapping. The thickness of either film is suitably on the order of about 30 to 70 μm, advantageously about 50 μm. Then, owing to the double packaging material, the oxygen-free interspace between the material courses will at the moment of packaging serve as buffer against atmospheric oxygen. The leakage indicator package, oxy¬ gen permeability of the packaging material 1000 to 10000 ml/m ² day atm, e.g. thickness of polyethylene films on the order of 30 to 70 μm, is oxygen-permeable enough to indicate any increase in oxygen content caused by leakage.

A leakage indicator according to the invention is advantageously packed e.g. inside a transparent food package. Then, owing to the invention, the leakage in¬ dicator will by colour change sensitively indicate any breakage of the food package, whereas it will not react to the residual oxygen entrapped in the package. The leakage indicator reveals e.g. in a retail store to the sales personnel, or in the last instance to the custom¬ er, any leakage and spoiling of the foodstuffs that may have taken place. The leakage indicator may equally be formed so that it reacts to metabolic products gener- ated in bacterial proliferation and/or to oxygen enter¬ ing the package through the leak, and leaking of the package or the microbiological quality of the food can be observed with ease by the colour of the indicator. The leakage indicator also indirectly indicates spoil- ing. Leakage is always followed by faster than normal spoiling of the product, and thus the indicator's change of colour indicates, for instance, spoiling of the foodstuff.

In the leakage indicator of the invention, the colourant is a Redox-type reducible/oxidizable colour¬ ant. Any colourant which is oxidized and changes colour under effect of atmospheric oxygen and which does not

react with the protective gas, e.g. nitrogen and/or carbon dioxide or gas, used in a gas-protected package can be contemplated. The group of such Redox-type colourants includes methylene blue, gallocyanin, methy- lene red, etc.

The reducing agent used in the leakage indica¬ tor of the invention is meant to reduce the colourant and to keep it in reduced state at the moment of pack¬ aging. Any organic or inorganic reducing agent can be used for reducing agent which does not inhibit oxida¬ tion of the colourant by effect of atmospheric oxygen. Usable reducing agents are, for instance, reducing sugars, such as glucose.

The purpose with the oxygen absorbent used in the leakage indicator of the invention is to eliminate the residual oxygen entrapped in the food package in connection with the packaging operation so that the residual oxygen will not react with the colourant. The content of residual oxygen may be e.g. on the order of 0.1 to 0.5 and up to 3%. Usable absorbents are, for instance, sulphates, hydrogen sulphites, thiosulphates, dithionites, hydroquinone, resorcinol, pyrogallol, gal¬ lic acid, rongalite, sodium formaldehyde sulphoxylate, ascorbic and isoascorbic acid and their salts, sorbose, glucose, lignin, dibutyldihydroxytoluene, butylhydroxy- anisol, and iron salts and metal powders such as iron powder; carbon dioxide-producing oxygen absorbents and carbon dioxide-absorbing oxygen absorbents may also be used; most advantageous are ascorbic acid and/or salts thereof, isoascorbic acid and/or salts thereof, unsatu- rated fatty acid components or oxygen absorbents con¬ taining iron powder or strongly reductive enzymes, such as glucose oxidase, etc. In general, the oxygen absorb¬ ents may be inorganic or organic. The reducing agent and the oxygen absorbent may also consist of one and the same, reducing sub¬ stance.

The leakage indicator may in addition comprise a basic component e.g. NaOH, Na_C0 ₃ , NaHCO., etc. The leakage indicator may further include liquids, e.g. water. Furthermore the leakage indicator may include fillers, such as kaoline, silicon oxide, etc. The pur¬ pose with liquid is to make the indicator's structure more homogeneous and, on the other hand, to influence its rate of reaction. The basic components also affect the rate of reaction; the aim is to prevent excessively fast colour reactions, i.e., too rapid oxidation of the colourant.

The invention is described in detail in the following with the aid of embodiment examples, refer¬ ring to the attached drawings, wherein Fig. 1 illustrates the effect of oxygen absorbent use on the colour indicator's colour changes, and Fig. 2 presents the colour changes of the colour indi¬ cator when a foodstuff is becoming spoiled. Example 1. In this study, foodstuffs (ground meat steaks) were packaged in a polyethylene film bag impermeable to oxygen. The studies revealed that the excessively rapid oxidation of the colourants caused problems, i.e., the colourant placed e.g. in a package made of plastic film became oxidized already while it was being handled and inserted in the food package. This problem was elimi¬ nated by enclosing the colourant in a double, oxygen- permeable plastic film, preferably in a double package made of polyethylene film. The oxygen permeability of the packaging material is suitably 1000 to 10000 ml/m ² day atm, film thickness about 30 to 70 μm, advantage¬ ously about 50 μm. The oxygen-free gas space defined between the films will then at the moment of packaging act as buffer against atmospheric oxygen at the packag- ing stage. The polyethylene film is oxygen-permeable enough for indicating the increase in oxygen concentra¬ tion caused by a leak in a leaking package. The leakage

indicator which was used contained, in parts by weight: Methylene blue 0.002375 Gallocyanin 0,000125 Glucose 0,8 Water 0.6

NaHCO. 10.0

Oxygen absorbent 1.0 to 2.0.

Packaging was carried out under protective gas so that the gas mixture contained about 20% CO. and about 80% N ₂ , oxygen less than 1%. There were altogether four groups of packages. A leakage indicator was inserted in the packages of two groups, placed in a double poly¬ ethylene package, film thickness 50 μm. The leakage indicators in two groups contained an oxygen absorbent, while those of the other two groups had no oxygen ab¬ sorbent. Storage temperature was below 5°C. After three days' storage, perforations of 100 μm were made in the bags of two groups so that the bags began to leak. The changes of colour of the leakage indicators, i.e., of the colour indicator, were measured. The results are presented in Fig. 1.

As the results reveal, the oxygen absorbent is adequate to eliminate the residual oxygen present in the package, but not the oxygen introduced by leakage in the package. It should be noted that the quantity of oxygen absorbent in the indicator depends on the size of the package's gas volume. As shown by the results, the colour indicator kept clearly lighter in colour during 10-day storage in the intact packages containing absorbent than the colour packaged without absorbent (the colour darkens with diminishing and grows lighter with increasing L value) . When a hole was made in the package, the colour changed quickly, indicating break¬ age of the package. Example 2.

In this study, foodstuffs were packaged simi¬ larly as in Example 1. The microbe content (log CFU/g)

was determined from the foodstuffs; the change of colour in the packages as a function of storage time was measured as in Example 1. Part of the packages were injured at the moment of packaging (a 1-mm hole), the other part was kept intact. The changes of colour are shown in Fig. 2 in terms of change of the L value, like in Example 1.

According to the results, the colour indicator does not react to spoiling of the foodstuffs, that is, the diagrams indicate hardly any correlation between the colour changes of the indicator and microbe con¬ tents of the foodstuffs. On the other hand, spoiling of the foodstuffs did not interfere with the action of the colour indicator serving as leakage indicator either.

The embodiment examples are meant to illus¬ trate the invention, without restricting it in any way whatsoever.