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 growth13.3.2 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