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《食品包装技术》（英文版）Chapter 13 Detecting leaks in modified atmosphere packaging

Package integrity is an essential requirement for maintaining the high quality of, for example, sterilised foods and modified atmosphere packaged foods. The increasing focus on quality assurance is putting demands on verification of food package integrity. The foremost noticeable package integrity problem is probably leaking seals, particularly with flexible plastic packages which are more prone to mechanical damage than traditional rigid metal packages. A non- destructive leak test device allowing evaluation of every container produced is. therefore, of interest to food manufacturers.

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Detecting leaks in modified atmosphere packaging 277 Table 13.1 Deterioration factors related to critical channel leakages in different packaged foods epic and Ready-to-eat Baked goods Dried goods sterilised meal microbial on, oxidation oxidation moisture 1,3591230-50m chang >13 diameter in package package types and products. That is, how big a leakage can there be without the packed product deteriorating microbiologically or chemically before the use-by date, and how small a leakage should the leak testing method detect (Table In many studies leakages of around 10 um in diameter have been demonstrated. under strict conditions. to cause microbial contamination in model packages- and in commercially processed and packaged aseptic packages ,", The critical leakage size causing accelerated quality deterioration in gas-flushed modified atmosphere packages (MAP)may vary considerabl ly between different products and packaging methods. Small leakages(hole diameter 169um, hole length 3mm)in gas packages have even been reported to retain the quality of packed minced meat steaks better than in intact packages. Other recent studies have, on the other hand, revealed accelerated quality deterioration of raw marinated chicken breast and raw rainbow trout and pizza in gas packages with leakages as small as 30um and 55um(hole length 3mm), respectively. Table 13.1 summarises the most important deterioration factors of different packaged foods and studies concerning critical leakages 13.3 Package leak detection during processing 13.3.1 Methods in use package and seal integrity is widely verified using destructive manual ods, such as a biotest, electrolytic test, dye penetration test and bubble test The major drawbacks of destructive test methods are that it is not possible to check every package produced, and the tests are often laborious. An automated, iable, 100% in-line non-destructive leak test machine allowing testing of every container produced would, therefore, be of interest to companies. This kind of package testing would serve as an immediate process control tool resulting in an overall cost reduction in terms of a reduced number of packages

package types and products. That is, how big a leakage can there be without the packed product deteriorating microbiologically or chemically before the use-by date, and how small a leakage should the leak testing method detect (Table 13.1). In many studies leakages of around 10 m in diameter have been demonstrated, under strict conditions, to cause microbial contamination in model packages1,2 and in commercially processed and packaged aseptic packages. 3,4,5 The critical leakage size causing accelerated quality deterioration in gas-flushed modified atmosphere packages (MAP) may, however, vary considerably between different products and packaging methods. Small leakages (hole diameter < 169m, hole length 3mm) in gas packages have even been reported to retain the quality of packed minced meat steaks better than in intact packages.6 Other recent studies have, on the other hand, revealed accelerated quality deterioration of raw marinated chicken breast and raw rainbow trout7 and pizza8 in gas packages with leakages as small as 30m and 55m (hole length 3mm), respectively. Table 13.1 summarises the most important deterioration factors of different packaged foods and studies concerning critical leakages. 13.3 Package leak detection during processing 13.3.1 Methods in use Food package and seal integrity is widely verified using destructive manual methods, such as a biotest, electrolytic test, dye penetration test and bubble test. The major drawbacks of destructive test methods are that it is not possible to check every package produced, and the tests are often laborious. An automated, reliable, 100% in-line non-destructive leak test machine allowing testing of every container produced would, therefore, be of interest to companies. This kind of package testing would serve as an immediate process control tool, resulting in an overall cost reduction in terms of a reduced number of packages Table 13.1 Deterioration factors related to critical channel leakages in different packaged foods Aseptic and Ready-to-eat- Baked goods Dried goods sterilised meals foods Most important microbial microbial oxidation, oxidation, deteriorative changes, changes, mould moisture factors oxidation oxidation growth, changes moisture changes Critical leakage 5–25 m1,3,5,9–12 30-50 m 6,12–13 50 m13 > 130 m13 diameter in package Detecting leaks in modified atmosphere packaging 277

278 Novel food packaging techniques Table 13.2 Commercial methods for non-destructive food package leakage detection Test stimulus Test response Application External pressure Package movement Packages with headspace Pressure decay* External vacuum Package movement Pressure decay* Tracer gas(H2. CO2, He, SF6 Internal pressure queerer movement Packages with headspace Package movement* Internal vacuum kage moveme All packages Machine vision Image change Foil packages On-line application available. ost both in production and in destructive testing. Also, complaints and returns from retailers and consumers relating to leaky packages and deteriorated products would be diminished Much interest has concentrated on plastic food packages to define thei integrity requirements, .2.4, 8. 4, I5 to research and develop new non-destructive test methods, 6-9 and to evaluate the reliability of commercial non-destructive test methods. 4 In-line non-destructive test equipment should meet demands such as: reliable identification and rejection of all the defective packages produced; fast leak detection; non-damaging to the product; easy to use and maintain; and reasonable supply and operating costs Most non-destructive leak inspection systems for flexible and semi-rigid packages are based on a stimulus response technique: the stimulus to the package can be, for example, ultrasound,pressure, tracer gas like helium, carbon dioxide or hydrogen- and the response can be, for example, sound/beam reflection, package movement, pressure change, or tracer gas detection(Table 13.2). In recent years, numerous new patents suitable for non-destructive food or medical package integrity testing have been published Although tracer gas detection is a very sensitive method, detection of pressure differential is perhaps currently the most popular method employed for flexible and semi-rigid packages with a headspace. Commercial pressure differential methods are typically based either(i)on detection of an external rise or fall in pressure in a test chamber created outside the package with compressed air or a vacuum pump, respectively, or(ii) on detection of an internal fall in pressure created inside the package either mechanically or by heat. Evaluation studies of commercial automated non-destructive leak detectors based on detection pressure differentials revealed that these test methods-although used in industry were not capable of reliably detecting leakages that proven to be penetrable by harmful microbes

lost both in production and in destructive testing. Also, complaints and returns from retailers and consumers relating to leaky packages and deteriorated products would be diminished. Much interest has concentrated on plastic food packages to define their integrity requirements,1,2,4,8,14,15 to research and develop new non-destructive test methods, 16-19 and to evaluate the reliability of commercial non-destructive test methods. 15,20 In-line non-destructive test equipment should meet demands such as: reliable identification and rejection of all the defective packages produced; fast leak detection; non-damaging to the product; easy to use and maintain; and reasonable supply and operating costs. Most non-destructive leak inspection systems for flexible and semi-rigid packages are based on a stimulus response technique: the stimulus to the package can be, for example, ultrasound,18 pressure,22 tracer gas like helium,23 carbon dioxide16 or hydrogen21 and the response can be, for example, sound/beam reflection, package movement, pressure change, or tracer gas detection (Table 13.2). In recent years, numerous new patents suitable for non-destructive food or medical package integrity testing have been published. Although tracer gas detection is a very sensitive method, detection of pressure differential is perhaps currently the most popular method employed for flexible and semi-rigid packages with a headspace. Commercial pressure differential methods are typically based either (i) on detection of an external rise or fall in pressure in a test chamber created outside the package with compressed air or a vacuum pump, respectively, or (ii) on detection of an internal fall in pressure created inside the package either mechanically or by heat. Evaluation studies of commercial automated non-destructive leak detectors based on detection pressure differentials revealed that these test methods – although much used in industry – were not capable of reliably detecting leakages that were proven to be penetrable by harmful microbes.20,22,23 Table 13.2 Commercial methods for non-destructive food package leakage detection Test stimulus Test response Application External pressure Package movement* Packages with headspace Pressure decay* External vacuum Package movement* Packages with headspace Pressure decay* Tracer gas (H2, CO2, He, SF6) Internal pressure Squeezer movement* Packages with headspace Package movement* Internal vacuum Package movement* All packages Machine vision Image change* Foil packages * On-line application available. 278 Novel food packaging techniques

Detecting leaks in modified atmosphere packaging 279 13.3.2 Novel tracer gas system for in-line application Tracer ak detection methods are very sensitive, and the most commonly used gas has been helium. Another possibility is to use the more economical carbon dioxide as a tracer gas. Carbon dioxide is often routinely used as protective packaging gas in food packages, which eliminates the need for the special addition of tracer gas in the package. However, introduction of automatic in-line leak detectors based on helium or carbon dioxide tracer gases has not been successful. The reasons for this have possibly been the relatively high operating and supply costs of the helium method, or the unfavourable physical characteristics of the carbon dioxide method A novel leak-detection system has recently developed at VTT using hydrogen (H2)as a tracer gas. The leak tester utilising H2 and a very sensitive hydrogen detector is very effective and fast and is especially suitable for MAP For example, at least 30um diameter holes in a gas-flushed package have been demonstrated to be reliably detected within one second. Using this method, a package containing H2 tracer gas is positioned in a specially designed test chamber. A vacuum pressure is then drawn into the test chamber and the package expands due to the increased pressure differential between the package walls. Trace amounts of H2 are then forced out of leaking packages through a pipe in which a H2 sensor is positioned towards the gas flow. The sensor connected to the H2 detector reacts to the H2, and immediately gives an electrical signal to the H, detector H2 has many characteristics advantageous to its use as a tracer gas in leak detection. First of all, it is a colourless, odourless, tasteless and non-toxic gas at atmospheric temperatures and pressures. A non-flammable concentration (5% in air) of hydrogen is sufficient for sensitive leak detection evertheless, the tracer gas concentration in the package heads background concentration of H2 in air, only 0. 5ppm, enables sensitive leak detection. That is, the minimum detection limit of H2 escaping from a defective package is very low. In comparison, the carbon dioxide and helium concentrations in air are 300 and 5 ppm, respectively. Hydrogen is also the lightest of all gases(molecular weight: H2 2.0, He 4.0, CO2 44.0, air 29. 0g/ mol) thus reducing the risk of background gas contamination in the leak test area. For example, carbon dioxide as a heavier gas than air may accumulate in the leak test area creating a risk of false readings 13.4 Package leak indicators during distribution The modified atmosphere package for non-respiring food typically has a low(0 2%)oxygen(O2)concentration and a high(20-80%)carbon dioxide(co2) concentration. Hence a leak means a considerable increase in O, concentration and a decrease in CO2 concentration. If the package leaks, microbial growth

13.3.2 Novel tracer gas system for in-line application Tracer gas leak detection methods are very sensitive, and the most commonly used gas has been helium. Another possibility is to use the more economical carbon dioxide as a tracer gas. Carbon dioxide is often routinely used as a protective packaging gas in food packages, which eliminates the need for the special addition of tracer gas in the package. However, introduction of automatic in-line leak detectors based on helium or carbon dioxide tracer gases has not been successful. The reasons for this have possibly been the relatively high operating and supply costs of the helium method, or the unfavourable physical characteristics of the carbon dioxide method. A novel leak-detection system has recently developed at VTT using hydrogen (H2) as a tracer gas.21,24 The leak tester utilising H2 and a very sensitive hydrogen detector is very effective and fast and is especially suitable for MAP. For example, at least 30m diameter holes in a gas-flushed package have been demonstrated to be reliably detected within one second.21 Using this method, a package containing H2 tracer gas is positioned in a specially designed test chamber. A vacuum pressure is then drawn into the test chamber and the package expands due to the increased pressure differential between the package walls. Trace amounts of H2 are then forced out of leaking packages through a pipe in which a H2 sensor is positioned towards the gas flow. The sensor connected to the H2 detector reacts to the H2, and immediately gives an electrical signal to the H2 detector. H2 has many characteristics advantageous to its use as a tracer gas in leak detection. First of all, it is a colourless, odourless, tasteless and non-toxic gas at atmospheric temperatures and pressures. A non-flammable concentration (<5% in air) of hydrogen is sufficient for sensitive leak detection. Nevertheless, the tracer gas concentration in the package headspace is proportional to the leak detection sensitivity and speed; even concentrations as low as 0.5% can be used to detect relatively small leakages. The low background concentration of H2 in air, only 0.5ppm, enables sensitive leak detection. That is, the minimum detection limit of H2 escaping from a defective package is very low. In comparison, the carbon dioxide and helium concentrations in air are 300 and 5 ppm, respectively. Hydrogen is also the lightest of all gases (molecular weight: H2 2.0, He 4.0, CO2 44.0, air 29.0g/ mol) thus reducing the risk of background gas contamination in the leak test area. For example, carbon dioxide as a heavier gas than air may accumulate in the leak test area creating a risk of false readings. 13.4 Package leak indicators during distribution The modified atmosphere package for non-respiring food typically has a low (0– 2%) oxygen (O2) concentration and a high (20–80%) carbon dioxide (CO2) concentration. Hence, a leak means a considerable increase in O2 concentration and a decrease in CO2 concentration. If the package leaks, microbial growth is Detecting leaks in modified atmosphere packaging 279

280 Novel food packaging techniques likely to take place. This means that CO2 may in some cases accumulate in package. In the worst case, the CO2 concentration will remain high despite leakage and microbial growth. Thus, the leak indicators for modified atmosphere packages should rely on the detection of oxygen rather than on the detection of 13. 4.1 Visual oxygen indicators At present, the main application of commercially available O2-sensitive package indicators is to ensure the proper functioning of oxygen absorption; companies that also deal with O2 absorbers have developed the indicators. For example, Mitsubishi Gas Chemical Company in Japan has greatly contributed to the development of O2 absorbers and was the first to commercialise O2-absorbing sachets under the trade name 'Ageless.The"Ageless-Eyesachets containing an O, indicator tablet have been designed to confirm that the 'Ageless absorbers are functioning properly. The manufacturer claims that indicator tablet turns from blue into pink within 2-3 hours after Oz has reached a zero concentration at 25C and into blue again in about five minutes when it is in contact with O2. Also some other Japanese companies like Toppan Printing have been active in developing oxygen indicators A typical visual O2 indicator consists of a redox dye, i. e, a reducing compound and an alkaline compound. In addition to these main component compounds such as a solvent(typically water and/or alcohol)and bulking agent (e.g. zeolite, silica gel, cellulose materials, polymers)are added to the indicator The indicator can be formulated as a tablet26-27 or a printed layer28-30 or it can be laminated in a polymer film. The redox dyes of the indicators are oxidised by O2 and a colour change can be observed. The most common dye used in the indicators is methylene blue, which is typically white in the reduced state and blue in the oxidised state. Other redox dyes used in O2 indicators are 2,6- dichloroindophenol and N, N, N, N-tetramethyl-p-phenylenediamine 3A reducing compound is added to the O2 indicator to reduce the dye and to keep it in the reduced state during the packaging process. Common reducing compounds for O2 indicators are reducing sugars, but inorganic salts as well as reduction by irradiation have also been used. An alkaline compound is added to the indicator to maintain the ph on the alkaline side and thus prevent too rapid an oxidation reaction of the dye 34-35 Inorganic compounds, such as sodium have typically been used for this purpose. ,30-ide and magnesium hydroxide hydroxide, potassium hydroxide, calcium hydrox a different approach to constructing a visual O2 indicator was introduced by Krumhar Karel- who developed a two-step colour reaction. In the first reaction step O2-sensitive material is oxidised and the formation of an acid or peroxide occurs. These components will cause a colour change in the specific colorant included in the system. Oxidative enzyme-based oxygen indicators ave been described by ahvenainen et al. and gardiol et a

likely to take place. This means that CO2 may in some cases accumulate in package. In the worst case, the CO2 concentration will remain high despite leakage and microbial growth. Thus, the leak indicators for modified atmosphere packages should rely on the detection of oxygen rather than on the detection of CO2. 13.4.1 Visual oxygen indicators At present, the main application of commercially available O2-sensitive package indicators is to ensure the proper functioning of oxygen absorption; companies that also deal with O2 absorbers have developed the indicators. For example, Mitsubishi Gas Chemical Company in Japan has greatly contributed to the development of O2 absorbers and was the first to commercialise O2-absorbing sachets under the trade name ‘Ageless’. 25 The ‘Ageless-Eye’ sachets containing an O2 indicator tablet have been designed to confirm that the ‘Ageless’ absorbers are functioning properly. The manufacturer claims that indicator tablet turns from blue into pink within 2–3 hours after O2 has reached a zero concentration at 25ºC and into blue again in about five minutes when it is in contact with O2. Also some other Japanese companies like Toppan Printing have been active in developing oxygen indicators. A typical visual O2 indicator consists of a redox dye, i.e., a reducing compound and an alkaline compound. In addition to these main components, compounds such as a solvent (typically water and/or alcohol) and bulking agent (e.g. zeolite, silica gel, cellulose materials, polymers) are added to the indicator. The indicator can be formulated as a tablet26–27 or a printed layer28–30 or it can be laminated in a polymer film.31 The redox dyes of the indicators are oxidised by O2 and a colour change can be observed. The most common dye used in the indicators is methylene blue, which is typically white in the reduced state and blue in the oxidised state. Other redox dyes used in O2 indicators are 2,6- dichloroindophenol 32 and N,N,N0 ,N0 -tetramethyl-p-phenylenediamine. 33 A reducing compound is added to the O2 indicator to reduce the dye and to keep it in the reduced state during the packaging process. Common reducing compounds for O2 indicators are reducing sugars, but inorganic salts as well as reduction by irradiation have also been used. An alkaline compound is added to the indicator to maintain the pH on the alkaline side and thus prevent too rapid an oxidation reaction of the dye.34–35 Inorganic compounds, such as sodium hydroxide, potassium hydroxide, calcium hydroxide and magnesium hydroxide, have typically been used for this purpose.27,30 A different approach to constructing a visual O2 indicator was introduced by Krumhar & Karel29 who developed a two-step colour reaction. In the first reaction step O2-sensitive material is oxidised and the formation of an acid or peroxide occurs. These components will cause a colour change in the specific colorant included in the system. Oxidative enzyme-based oxygen indicators have been described by Ahvenainen et al.36 and Gardiol et al.37–38 280 Novel food packaging techniques

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