Publication WO 98/21120 discloses a myoglobin indicator reacting to hydrogen sulphide.

Publications US 6,149, 952 and US 5,753, 285 disclose an indicator colour combined with a hydrophilic polymer.

Publication US 5,407, 829 discloses the development of coloured sul- phides from salts of heavy metals. The change in the colour is moni- tored visually or by means of optoelectonic devices.

Publication JP 2001238696 discloses an indicator intended for detect- ing Salmonella bacteria in eggs. The indicator is formed by adsorbing a silver, copper or mercury ion by a fibre substance. The indicator will react to gaseous compounds emitted from the egg by a change in the colour.

The indicator according to the invention is characterized in that the indicator is a layer formed on a substrate comprising metallic silver and/or metallic copper.

The indicator according to the invention has, for example, the following advantages: Because the colour change in the indicator is visible to the eyes, no special equipment will be needed for monitoring it. However, it is pos- sible to use automatic monitoring devices for analysing the change in the colour. In a given gas concentration, an abrupt, visually detectable change takes place in the colour of the indicator. The change in the colour can be adjusted to a given concentration. In tests carried out, it was observed that the change in the colour can be made to take place

in a reproducible way; in other words, the indicator can be made very reliable.

The indicator according to the invention is arranged to react to gaseous substances, particularly hydrogen sulphide. The indicator can be used to indicate a gas concentration which presents odour problems to the environment or a hazard for persons working in the facilities. Further- more, the indicator can be utilized in the maintenance of industrial plants, wherein an increase in gas concentrations can be used to trace, for example, a leak in the process equipment. The indicator can also be used to monitor the concentration of hydrogen sulphide in facilities where it would be harmful to expose equipment, particularly electrical devices, to cold corrosion. Uses of the indicator include, for example, water treatment plants (particularly biological sewage treatment plants), sewer systems, dumping areas, and equipment facilities in the proc- essing industry. The indicator can also be used for controlling the purity of air in houses and places of work.

When the gas to be detected is hydrogen sulphide, the indicator can be set to indicate, for example, maximum hydrogen sulphide concentra- tions given by various guidelines: - 14 glum3 (10 ppm), the concentration known to be harmful after an exposure of 8 h; -21 g/m3 (15 ppm), the concentration known to be harmful after an exposure of 15 min; - 0. 011 g/m3 (0.008 ppm), the concentration detectible by the sense of smell ; - 0. 005 ppm, the WHO guideline not to be exceeded by the concentration in the air, - 0. 1 ppm, maximum temporary health hazard for 60 minutes (ERPG-1); - 30 ppm, without serious health hazards for 60 minutes (ERPG-2); - 100 ppm, without danger to life for 60 minutes (ERPG-3).

The indicator according to the invention can be used, for example, as an indicator of shelf life, wherein in small quantities of substances indi- cating decaying, an abrupt change takes place in the colour of the indi- cator, wherein the indicator can be monitored, for example, in a retail shop or under household conditions. The indicator reacts chemically with substances evaporated from the decaying product in such a way that the colour of the indicator is changed.

The change in the colour of the indicator takes place in a reliable (reproducible) way in relation to the quantity of the substances indicat- ing decaying of the decayable product.

In the following, the invention will be discussed particularly for the use as an indicator of shelf life in food packages, but the indicator accord- ing to the invention can also be applied to other uses, made in the same way and reacting to the same gases. However, indicators made as such by the same principle must be adjusted, in their parameters, to the use; for example, the gas concentration in which the change in the colour of the indicator takes place, must be adjusted in view of the requirements of the use.

The shelf life indicator can be manufactured on a separate substrate to be easily attached to a package, such as a self-adhesive label, or the manufacture of the shelf life indicator can be directly integrated in the manufacture of the food packages; in other words, the indicator can be made in connection with the manufacture of the package material and, for example, any separate steps to attach the shelf life indicator in con- nection with the packing will not be needed.

Metallic silver has been approved for use in connection with foodstuffs, also in direct contact. Moreover, there is a long experience of the use of metallic silver with foodstuffs ; it has been used in cutlery and serving dishes for a long time.

The indicator according to the invention is used particularly in uses requiring an indicator reacting to volatile sulphur compounds. The

indicator according to the invention can be used as a shelf life indicator in food packages intended for the packing of foodstuffs which, when decaying, produce chemical compounds, for example sulphur com- pounds, such as hydrogen sulphide, dimethyl sulphide and/or dimethyl disulphide. The composition of the chemical compounds volatilizing from decaying food depends on the food in question. Sulphur com- pounds are produced particularly in the decaying of poultry meat, such as chicken or turkey. The chemical compound may also be other than a sulphur compound, as long as it reacts with metallic silver and/or copper, causing a change in the colour.

The food may be packed without a shielding gas, the package may contain a shielding gas, such as a nitrogen or carbon dioxide shielding gas or a shielding gas containing both of said shielding gases, or the product may be packed in a vacuum. The shelf life indicator is inside the package, that is, in a space where it can react with the sulphur compound emitted from the decaying food. The shelf life indicator comprises a metal which is selected from the group of transition ele- ments and is preferably of metallic silver and/or copper. When the metal reacts with hydrogen sulphide, sulphides are produced. The chemical reaction with the sulphur compound results in a relatively abrupt change in the colour of the shelf life indicator, which correlates well with the quantity of the sulphur compound emitted into the pack- age, such as hydrogen sulphide, dimethyl sulphide and/or dimethyl disulphide.

The indicator is made either by applying a metal layer on a suitable non-metal substrate or by printing it on a suitable printing base, for example by the screen printing or the flexographic printing technique.

The printing base can be, for example, paper, board, or plastic. The printing paste comprises at least an adhesive and metal particles with an average diameter of preferably less than 10 um. Advantageously, the printing paste contains metal particles 70 to 90 percent of the dry substance of the printing paste. Suitable adhesives to be used in flex- ographic printing include, for example, shellac, cellulose nitrate, ethyl cellulose, cellulose acetate propionate (CAP), polyamide resins, acrylic

and methacrylic resins, ketone resins, polyvinyl acetate, and polyvinyl chloride. Suitable adhesives to be used in screen printing include, for example, acrylic, polyurethane and vinyl resins.

When a metal coating is applied on a non-metallic substrate, for exam- ple by vapour deposition or sputtering, it can be made very thin, nor- mally with a layer thickness of 1 to 10nm. An advantageous layer thickness is 1 to 5 nm.

The metal layer to be applied either by printing, sputtering or vapour deposition can be made, for example, on a transparent plastic film to be used as the top of a food package, wherein the shelf life indicator remains inside the food package but can be observed from the outside of the package. Advantageously, the metal layer covers the plastic film for only a given part; that is, the indicator may be relatively small in size when compared with a plastic film top. The colour of the thin metal layer is changed throughout, wherein said embodiment is very handy.

For example, in a silver layer with the thickness of 1 nm, a distinct change was observed in the colour by the effect of hydrogen sulphide in the concentration of 0.016 mg/i (silver film applied on a polyester or polycarbonate film).

The sensitivity of the shelf life indicator can be adjusted e. g. by chang- ing the thickness of the metal layer. As the layer thickness is increased, the change in the colour takes place in a higher gas concentration. The sensitivity of the indicator is also affected by the quality of the material onto whose surface the indicator layer is applied.

In the following, the invention will be described with reference to the examples and Fig. 1. Figure 1 is a graphic representation of the values of table 3.

Example 1.

Chicken fillet slices (about 115 + 5 g) were packed in 210 ml box pack- ages (material HDPE) with a shielding gas (80 % C02/20 % N2) and stored at a constant temperature of +5. 5°C and +8°C.

For the analysis of volatile metabolites, a 5 ml gas sample was taken with a gas-tight syringe from the gas space of the packages and injected into a head-space flask (volume 22 ml) sealed in a gas-tight manner in a clean room. The nitrogen sulphide concentrations in the gas space were determined by gas chromatography by using a sulphur selective detector.

The nitrogen sulphide concentration of the gas space increased as a function of the storage time and temperature (table 1) and represented the shelf life of the chicken slices.

Table 1. Nitrogen sulphide concentration (mg/l) in the gas space of chicken fillet slices stored at different temperatures 1) Storage time (days) H2S concentration H2S concentration (mg/l) in gas space, (mg/l) in gas space, storage temperature storage temperature +5. 5°C +8°C 0 no observation no observation 5 no observation no observation 7 no observation + 9 + 0. 2 12 0. 15 0. 75 1) The quantizability limit in the test is 0.1 mali ; no observation = no observable peak in the chromatograph, + = a detectable peak below the quantizability limit in the chromatograph. The number of parallel tests was 2.

Example 2.

Table 2. Change in the colour of a silver indicator made by flexographic printing in a reaction with nitrogen sulphide (measured by X-rite densi- tometer). Nitrogen sulphide Darkness in colour of Darkness in colour of concentration (mgA) indicator (in Os) indicator (in N2) 0 0. 32 0. 31 0. 03 0. 51 0. 43 0. 08 0. 57 0. 45 0. 16 0. 61 0. 48 1. 6 0. 63 0. 48 3. 2 0. 63 0. 52

Example 3.

Table 3 shows, as a function of time, the change in the colour of shelf life indicators made by flexographic printing and placed in the gas spaces of turkey packages stored at different temperatures. A E = total index of the change in the colour of the indicators, measured in a scale of L, a, b and calculated from the formula A E = (AL2 + Aa2 + Ab2) % The shielding gas used in the food packages was carbon dioxide. The values of table 3 are shown graphically in Fig. 1.

Table 3. Change in the colour of shelf life indicators as a function of time. Colour change (#E) Storage 0 7 11 14 18 19 27 time/d Tempera- ture/ sample 2. 3°C / 0 0.8 1.4 2. 1 1.9 sample A 2. 3°C / 0 1.2 2.2 4.2 10.6 sample B 5. 5°C/0 1. 8 3. 8 8.2 20.6 sample C 5. 5°C/0 1. 7 4.6 7. 8 9.6 sample D 8. 3°C / 0 1.9 6.9 19.2 22.3 sample E 8. 3°C / 0 1.4 20.2 23.3 27.5 sample F Example 4.

Table 4. Change in the colour of silver indicators made by sputtering, in a reaction with nitrogen sulphide (measured by X-rite densitometer).

Nitrogen sulphide Indicator colour/darkness (reaction in air) concentration (mg/ !) 1 nm film on 3 nm film on pigment- polycarbonate coated polycarbonate 0 0. 41 (pink) 0.80 (dark grey) 0. 016 0. 33 (colourless) 0. 54 (light grey) 0. 03 0. 33 (colourless) 0.52 (light grey) 0. 08 0. 33 (colourless) 0 39 (yellow) 0. 16 0. 32 (colourless) 0.36 (yellow) 3. 2 0. 31 (colourless) 0.41 (yellow)

Example 5.

Table 5. Nitrogen sulphide concentration in the gas spaces of pack- ages of chicken slices packed in shielding gas and stored at various temperature conditions, and the perceivable acceptability of the slices Storage Storage Nitrogen sulphide Perceivable temperature time concentration in gas acceptability of (mean) (°C) (days) space/ product 3.4 9 5* Acceptable 6.5 9 1300 Unacceptable 8.3 9 2000 Clearly unacceptable 3.4 12 # 5* Acceptable 6.5 12 5500 Clearly unacceptable 8.3 12 1000Q Clearly unacceptable * Detection limit of the GC method

Table 6. Nitrogen sulphide concentration and sulphurous odour in the gas space of the package of turkey slices packed in shielding gas and stored at various temperature conditions, as well as change in the colour of indicators based on a silver film (sputtered silver film of about 1 nm) attached to the gas space of the package. Storage Storage Nitrogen Sulphurous Change in temperature time sulphide odour indicator (mean) (°C) (days) concentration colour in gas space (aga) 3.0 11 < 1 * No No 3.0 14 <1* No No 3.0 18 < 1* No No (package A) (package A) Yes Yes (package B) (package B) 7.9 11 2.5 Yes Yes (package A) 5.5 (package B) 7.9 12 4 Yes Yes (package A)

* Detection limit of the GC method Example 6.

Various nitrogen sulphide indicators based on a thin silver film were compared for their sensitivity in the change of colour to low concentra- tions of nitrogen sulphide. The indicators which contained silver were applied on two different films (PET, PP), and two different thicknesses of the indicator layer were used (1 nm, 2 nm).

The indicator samples were: 1. PET 1 nm, 2. PET 2 nm, 3. PP 1 nm, and 4. PP 2 nm.

The indicators were attached to the inside of the top film in 600 ml packages, and the boxes were packed by rinsing with a shielding gas.

The composition of the shielding gas was 9.5 % of 02, 90. 5 % of C02, 100 ppm nitrogen sulphide was added into the packages, according to table 7. Next, the changes in the colour of the different indicators were compared. The colour of the indicators was evaluated visually. In par- ticular, the aim was to compare different indicator materials with each other.

The results from the comparison in the sensitivity of the indicators are shown in table 8. Distinct differences were observed in the rate and completeness of the change in the colour of the indicators. The indica- tor was printed in the shape of a cross. When the indicator reacted with the gas, the cross was highlighted from the rest of the printing.

With the lowest examined H2S concentration (0. 1 ug/I), an incipient change in the colour was observed in the indicators 1 and 2 made on a PET film in only less than an hour after the addition of nitrogen sul- phide. Also, in the indicator 3 made of a PP film, a change in the colour was observed relatively soon (1.3 h from the starting of the reaction).

The 2 nm indicator 4 made of another PP film did not react until 1 day from the addition of nitrogen sulphide. In the lowest concentration, the final change in the colour was most distinct in the indicators 1,2 and 3.

In the lowest H2S concentration of 0. 5 ugA, an incipient change in the colour was observed in the indicators 1 and 2 made of a PET film in only 15 minutes after the addition of nitrogen sulphide. In the indica- tor 3 made of a PP film, a change in the colour was observed relatively soon (35 min after starting the reaction). Also, the 2 nm indicator 4 made by using a thicker PP film reacted in 35 minutes but to a clearly lesser extent than the indicator 3.

In the H2S concentration of 0. 9ugA, an incipient change in the colour was observed in the indicators 1 and 2 made of a PET film and in the indicator 3 made of a PP film in only 10 minutes after the addition of

nitrogen sulphide. The 2 nm indicator 4 made by using the thicker PP film reacted in about 30 minutes in such a way that the cross in the indicator became distinguishable.

In the highest tested H2S concentration (4. 7 ugA), a complete change in the colour was observed in all the tested indicators in only about 2.5 hours after the addition of nitrogen sulphide. In the indicators 1 and 2 made of a PET film, the colour change started in only 0.5 minutes after the addition of nitrogen sulphide. For example, in 3 minutes after the addition of nitrogen sulphide, the 2 nm indicator 4 made by using a PP film had reacted by becoming clearly lighter at the edges.

On the basis of the results, it could be concluded that the indicators 1 and 2 made of a PET film were the fastest ones to react of the indica- tors tested. The 1 nm indicator 3 made of a PP film was slightly slower than these to react. The indicator 4 (PP/2 nm) had the slowest reac- tion. In the final colour change of the indicators, differences could be found primarily in the lowest test concentration (0. 1 ug/l) of nitrogen sulphide. The difference was manifested in the size of the reaction area in the indicator. The largest reaction areas were found for the indicators 1,2 and 3. The reaction area of indicator 4 was clearly smaller. In the other tested concentrations of nitrogen sulphide, the reaction of the indicators was virtually complete after the reaction of 2 days.

Table 7. Additions of nitrogen sulphide Indica-Sample Final H2Sconcentration Addition of tor package in package 100 ppm H2S (µg/l) 1. 1, 2 0- 2.3, 4 0- 3.5, 6 0- 4.7, 8 0- 1.11, 12 0.093 400 µl 2.13, 14 0.093 400 µl 3.15, 16 0. 093 400 I 4.17, 18 0.093 400 µl 1. 21, 22 0.47 2 ml 2. 23, 24 0.47 2 ml 3.25, 26 0.47 2 ml 4.27, 28 0. 47 2 ml 1.31, 32 0.93 4 ml 2.33, 34 0.93 4 ml 3.35, 36 0.93 4 ml 4.37, 38 0. 93 4 ml 1.41, 42 4. 7 20ml 2.43, 44 4.7 20 ml 3,45, 46 4.7 20 ml 4.47, 48 4.7 20 ml Table 8. Visual evaluation of the colour of the indicators. Indica-Samples Evaluation of the colour change tor H2S 0 119/1 1.1, 2 No change in the colour of the indicators within two days. 2.3, 4 3.5, 6 4.7, 8 H2S 0.1 ftg/l 20 min after the 40 min after the 1 d after the addition addition of H2S addition of HaS of H2S 1.11, 12 no change indicator edges light edges in the slightly lighter cross in the indicator 2.13, 14 no change indicator edges light edges in the slightly lighter cross in the indicator 3.15, 16 no change no detectable change light edges in the cross in the indicator (less than 11 to 14) 4.17, 18 no change no detectable change light edges in the cross in the indicator (less than 15 to 16) Table 8. continued H2S 0. 5 pg/l 15 min after the 35 min after the 1 d after the addition addition of H2S addition of HzS of HzS 1.21, 22 a sliver of light in the a clear change in the changeable areas edges of the indicator indicator completely changed 2.23, 24 a sliver of light in the a clear change in the changeable areas edges of the indicator indicator completely changed 3.25, 26 no change a sliver of light in the changeable areas are edges of the indicator, light, also part of the change slightly smaller cross is lighter than in 21 to 24 4.27, 28 no change a sliver of light in the changeable areas of edges of the indicator, the indicator are light, change clearly smaller a small dark point at than in 25 to 26 the upper edge H2S 0. 9 p911 10 min after the 30 min after the 1 d after the addition addition of HzS addition of HzS of HaS 1.31, 32 edges of the indicator indicator almost indicator completely are slightly changed completely changed changed 2.33, 34 edges of the indicator indicator almost indicator completely are slightly changed completely changed changed 3.35, 36 edges of the indicator edges of the indicator indicator is almost are slightly changed are changed, also the completely light, also cross is lighter the cross is lighter 4.37, 38 no change edges of the indicator one of two specimens are changed, cross is completely changed, distinguishable in one one having a small of two specimens dark point Table 2. continued H2S 4.7 ll9/l 1 to 2 min after the 4 to 5 min after the about 2 to 2.5 h after addition of H2S addition of HzS the addition of H2S 1. 41, 42 sides of the indicator indicator completely indicator completely are light, also upper light but not yet changed (bright white edge becoming lighter completely changed edge) 2.43, 44 sides of the indicator indicator completely indicator completely are light light but not yet changed (bright white completely changed edge) 3.45, 46 about 1 mm from the half of the indicator is indicator completely side edge is lighter lighter (also cross changed (also cross of becoming lighter) indicator changed) (7 min) 4.47, 48 light areas at the sides sides of the indicator indicator completely of the indicator are lighter (6 min) changed

The invention is not restricted to the description above, but it may vary within the scope of the claims. The examples presented only discuss metallic silver which is a particularly advantageous choice of metal, but the metal can also be copper or a mixture of silver and copper, or a layered structure.