18 Active packaging in practice: fish M. Sivertsvik, NORCONSERV, Norway 18.1 Introduction Fresh fish products are usually more perishable than most other foodstuffs due to their high aw, neutral pH, and presence of autolytic enzymes. The spoilage of fish and shellfish results from changes caused by oxidation of lipids, reactions due to activities of the fishes'own enzymes, and the metabolic activities of microorganisms(Ashie et al, 1996). The rate of deterioration is highly temperature dependent and can be inhibited by the use of low storage temperature (e.g. fish stored on ice). The spoilage of fresh fish is usually such as autoxidation or enzymatic hydrolysis of the lipid fraction may resul,: g dominated by microbial activities, however, in some cases chemical change off-odours and flavours and, in other cases, tissue enzyme activity can lead to unacceptable softening of the fish(Huss et al., 1997). The degree of processing and preservation together with product composition and storage temperature will decide whether fish undergoes microbial spoilage, biochemical spoilage, or a ombination of both. These factors contributed to difficulties when using different technologies to extend the storage of fresh fish above that obtained by traditional ice storage The term fish product covers a wide range, and includes fish that differ widely in composition, origin, shelf-life and applicability to novel packaging technologies. The range covers fish from temperate waters with a microbial flora dapted to psychrotrophic conditions to fish from tropical waters with different microflora, just as freshwater fish differ from seafish. There is also a wide distribution in the chemical composition of fish, for example, lean fish almost without fat, such as cod, and fatty fish, such as salmon; which often contains 20% fat. Some fish have a long natural shelf-life on ice(e.g. halibut) but othe
18.1 Introduction Fresh fish products are usually more perishable than most other foodstuffs due to their high aW, neutral pH, and presence of autolytic enzymes. The spoilage of fish and shellfish results from changes caused by oxidation of lipids, reactions due to activities of the fishes’ own enzymes, and the metabolic activities of microorganisms (Ashie et al., 1996). The rate of deterioration is highly temperature dependent and can be inhibited by the use of low storage temperature (e.g. fish stored on ice). The spoilage of fresh fish is usually dominated by microbial activities, however, in some cases chemical changes, such as autoxidation or enzymatic hydrolysis of the lipid fraction may result in off-odours and flavours and, in other cases, tissue enzyme activity can lead to unacceptable softening of the fish (Huss et al., 1997). The degree of processing and preservation together with product composition and storage temperature will decide whether fish undergoes microbial spoilage, biochemical spoilage, or a combination of both. These factors contributed to difficulties when using different technologies to extend the storage of fresh fish above that obtained by traditional ice storage. The term fish product covers a wide range, and includes fish that differ widely in composition, origin, shelf-life and applicability to novel packaging technologies. The range covers fish from temperate waters with a microbial flora adapted to psychrotrophic conditions to fish from tropical waters with different microflora, just as freshwater fish differ from seafish. There is also a wide distribution in the chemical composition of fish, for example, lean fish almost without fat, such as cod, and fatty fish, such as salmon; which often contains 20% fat. Some fish have a long natural shelf-life on ice (e.g. halibut) but others, 18 Active packaging in practice: fish M. Sivertsvik, NORCONSERV, Norway
Active packaging in practice: fish 385 like the pelagic species(e.g. mackerel and herring) have a very short shelf-life Fresh fish is very different from the various processed fish products that need packaging: heat-treated fish products(ready meals, fish pudding/balls), smoked, dried. or salted fish. We still have not taken into account the numerous different pecies of crustaceans and molluscs This chapter will cover active packaging of fish products including the use of atmosphere modifiers such as oxygen scavengers and carbon dioxide emitters, packaging that controls water or with anti-microbial and anti-oxidative properties, and indicator mechanisms. Modified atmosphere packaging(MAP) is regarded by some as an active packaging technology. This is by now a well established method to extend the shelf-life of foods, including fish products, and will not be covered in this chapter except for the MA methods different from traditional MAP using gas flushing Obviously, having such a broad spectrum of products, it is unlikely that one pecific novel or active packaging technology will be a success for all, just as not all fishery products benefit from MAP when compared to vacuum packaging (Sivertsvik et al., 2002a). So, the potential for an active packaging technology to be successful for a product would depend on the technology's ability to control and inhibit the shelf-life deteriorating spoilage reactions(e.g. bacterial growth of pecific bacteria, oxidative rancidity, colour changes) in the specific product 18.2 The microbiology of fish products As mentioned, the deterioration of fresh fish is usually microbial so controlling the microbial growth is usually the most important parameter for an active packaging technology to be successful. Fish normally have a particularly heavy microbial load owing to the method of capture and transport to shore slaughtering method, evisceration and retention of skin in retail portions. The microorganisms associated with most seafood reflect the microbial population in their aquatic environment(Colby et al, 1993; Liston 1980; Gram and Huss 1996). Microorganisms are found on all the outer surfaces(skin and gills)and in the intestines of live and newly caught fish. The total numbers of organisms vary enormously and Liston(1980) states a normal range of 10-10 cfu(colony forming units)/cm on the skin surface. The gills and the intestines both contain between 10 and 10 cfu/g(Huss 1995). The fish muscle is sterile at the time of slaughtering/catch, but quickly becomes contaminated by surface and intestinal bacteria, equipment, and humans during handling and processing. The microflora of temperate-water fish is dominated by psychrotrophic, aerobic or facultative anaerobic Gram-negative, rod-shaped bacteria, but Gram-positive organisms can also be found in varying proportions. The flora on tropical fish often carries a slightly higher load of Gram-positive and enteric bacteria. The composition of fresh fish flesh makes it favourable to microbial growth he muscle is composed of low collagen, low lipid, and high levels of soluble non-protein-nitrogen(NPN) compounds. Trimethylamine-oxide(TMAO), a part
like the pelagic species (e.g. mackerel and herring) have a very short shelf-life. Fresh fish is very different from the various processed fish products that need packaging: heat-treated fish products (ready meals, fish pudding/balls), smoked, dried, or salted fish. We still have not taken into account the numerous different species of crustaceans and molluscs. This chapter will cover active packaging of fish products including the use of atmosphere modifiers such as oxygen scavengers and carbon dioxide emitters, packaging that controls water or with anti-microbial and anti-oxidative properties, and indicator mechanisms. Modified atmosphere packaging (MAP) is regarded by some as an active packaging technology. This is by now a well established method to extend the shelf-life of foods, including fish products, and will not be covered in this chapter except for the MA methods different from traditional MAP using gas flushing. Obviously, having such a broad spectrum of products, it is unlikely that one specific novel or active packaging technology will be a success for all, just as not all fishery products benefit from MAP when compared to vacuum packaging (Sivertsvik et al., 2002a). So, the potential for an active packaging technology to be successful for a product would depend on the technology’s ability to control and inhibit the shelf-life deteriorating spoilage reactions (e.g. bacterial growth of specific bacteria, oxidative rancidity, colour changes) in the specific product. 18.2 The microbiology of fish products As mentioned, the deterioration of fresh fish is usually microbial so controlling the microbial growth is usually the most important parameter for an active packaging technology to be successful. Fish normally have a particularly heavy microbial load owing to the method of capture and transport to shore, slaughtering method, evisceration and retention of skin in retail portions. The microorganisms associated with most seafood reflect the microbial population in their aquatic environment (Colby et al., 1993; Liston 1980; Gram and Huss 1996). Microorganisms are found on all the outer surfaces (skin and gills) and in the intestines of live and newly caught fish. The total numbers of organisms vary enormously and Liston (1980) states a normal range of 102 –107 cfu (colony forming units)/cm2 on the skin surface. The gills and the intestines both contain between 103 and 109 cfu/g (Huss 1995). The fish muscle is sterile at the time of slaughtering/catch, but quickly becomes contaminated by surface and intestinal bacteria, equipment, and humans during handling and processing. The microflora of temperate-water fish is dominated by psychrotrophic, aerobic or facultative anaerobic Gram-negative, rod-shaped bacteria, but Gram-positive organisms can also be found in varying proportions. The flora on tropical fish often carries a slightly higher load of Gram-positive and enteric bacteria. The composition of fresh fish flesh makes it favourable to microbial growth. The muscle is composed of low collagen, low lipid, and high levels of soluble non-protein-nitrogen (NPN) compounds. Trimethylamine-oxide (TMAO), a part Active packaging in practice: fish 385
386 Novel food packaging techniques of the NPN compounds, can be broken down to trimethylamine (TMa) by endogenous enzymes. However, at chilled temperatures TMA is produced by the bacterial enzyme TMA oxidase. TMA is recognised as the characteristic fishy odour of spoiled fish. When the oxygen level is depleted, many of the spoilage bacteria can utilise TMAO as a terminal hydrogen acceptor, thus allowing them to grow under anoxic conditions. Towards the end of shelf-life. various malodorous low molecular-weight sulphur-compounds such as H2S and CH3 SH, ogether with volatile fatty acids and ammonia are produced because of bacterial During chilled storage, there is a shift in bacterial types. The part of the microflora, which will ultimately grow on the products, is determined by the intrinsic(e.g. post mortem pH in the flesh, the poikilothermic nature of fish, and presence of TMAO and other NPN components)and extrinsic parameters(e.g temperature, processing, and packaging atmosphere). When a product is microbial spoiled, the spoilage microflora will usually consist of a mixture of species, many of which can be completely harmless both in terms of health hazards and in terms of ability to produce off-odours and off-flavours. The bacterial group causing the important chemical changes during fish spoilage often consists of a single species; the specific spoilage organisms(SSO). Little is known of the SSOs of different fish from various aquatic environments under different packaging conditions. However, for many fish stored under aerobic conditions in ice, Shewanella putrefaciens has been identified as the main spoilage bacteria( Gram et al., 1987). S. putrefaciens produce very intense and unpleasant off-odours, reduce TMAO to TMA and produce H2S Under anaerobic conditions(MAP, vacuum packaging, active packaging technologies)the spoilage bacteria differ from aerobic spoilage. The Gram- negative organism Photobacterium phosphoreum has been identified as the organism responsible for spoilage in VP and in MA packs(Dalgaard 1995). The growth rate of this organism is increased under anaerobic conditions and in contrast to S. putrefaciens, P. phosphoreum is shown to be highly resistant to CO2. It was also shown that the growth of this bacteria corresponds very well with the shelf-life of packed fresh cod. P phosphoreum reduces TMAO to TMA at 10-100 times the amount per cell than S. putrefaciens probably due to the large size of the former( diameter 5um) while very little HS is produced during growth in fish substrates(Dalgaard et al, 1996). Spoiled MAP cod is characterised by high levels of TMA, but little or no development of the putrid or HiS odours typical for some aerobically stored spoiled fish. P. phosphoreum is widespread in the marine environment and it seems likely that this organism or other highly CO2 resistant microorganisms are responsible for spoilage of packed seafood products. Lactic acid bacteria and Brochothrix thermosphacta have been identified as the typical SSOs of freshwater fish and fish from warmer waters To obtain a longer shelf-life for fresh fish than obtained by ice or chilled MAP/vacuum, the approach is to inhibit the SSo limiting shelf-life for the technology chosen. For example P. phosphoreum is sensitive to freezing and is
of the NPN compounds, can be broken down to trimethylamine (TMA) by endogenous enzymes. However, at chilled temperatures TMA is produced by the bacterial enzyme TMA oxidase. TMA is recognised as the characteristic ‘fishy’ odour of spoiled fish. When the oxygen level is depleted, many of the spoilage bacteria can utilise TMAO as a terminal hydrogen acceptor, thus allowing them to grow under anoxic conditions. Towards the end of shelf-life, various malodorous low molecular-weight sulphur-compounds such as H2S and CH3SH, together with volatile fatty acids and ammonia are produced because of bacterial growth. During chilled storage, there is a shift in bacterial types. The part of the microflora, which will ultimately grow on the products, is determined by the intrinsic (e.g. post mortem pH in the flesh, the poikilothermic nature of fish, and presence of TMAO and other NPN components) and extrinsic parameters (e.g. temperature, processing, and packaging atmosphere). When a product is microbial spoiled, the spoilage microflora will usually consist of a mixture of species, many of which can be completely harmless both in terms of health hazards and in terms of ability to produce off-odours and off-flavours. The bacterial group causing the important chemical changes during fish spoilage often consists of a single species; the specific spoilage organisms (SSO). Little is known of the SSOs of different fish from various aquatic environments under different packaging conditions. However, for many fish stored under aerobic conditions in ice, Shewanella putrefaciens has been identified as the main spoilage bacteria (Gram et al., 1987). S. putrefaciens produce very intense and unpleasant off-odours, reduce TMAO to TMA and produce H2S. Under anaerobic conditions (MAP, vacuum packaging, active packaging technologies) the spoilage bacteria differ from aerobic spoilage. The Gramnegative organism Photobacterium phosphoreum has been identified as the organism responsible for spoilage in VP and in MA packs (Dalgaard 1995). The growth rate of this organism is increased under anaerobic conditions and in contrast to S. putrefaciens, P. phosphoreum is shown to be highly resistant to CO2. It was also shown that the growth of this bacteria corresponds very well with the shelf-life of packed fresh cod. P. phosphoreum reduces TMAO to TMA at 10–100 times the amount per cell than S. putrefaciens probably due to the large size of the former (diameter 5m) while very little H2S is produced during growth in fish substrates (Dalgaard et al., 1996). Spoiled MAP cod is characterised by high levels of TMA, but little or no development of the putrid or H2S odours typical for some aerobically stored spoiled fish. P. phosphoreum is widespread in the marine environment and it seems likely that this organism or other highly CO2 resistant microorganisms are responsible for spoilage of packed seafood products. Lactic acid bacteria and Brochothrix thermosphacta have been identified as the typical SSOs of freshwater fish and fish from warmer waters. To obtain a longer shelf-life for fresh fish than obtained by ice or chilled MAP/vacuum, the approach is to inhibit the SSO limiting shelf-life for the technology chosen. For example P. phosphoreum is sensitive to freezing and is 386 Novel food packaging techniques
Active packaging in practice: fish 387 totally inactivated in thawed chilled MAP cod fillets after frozen storage at 20C and -30.C for 6-8 weeks. This approach has been used to further extend the shelf-life of MAP cod(Guldager et al, 1998)and salmon(Emborg et al 2002)at 2C, and should also be the approach for successful use of active ackaging technologies to control microbial spoilage 18.3 Active packaging: atmosphere modifiers Many of the most used active packaging technologies are closely modified atmosphere packaging. Together with anaerobic condition dioxide is the active gas of MaP because it inhibits growth of man normal spoilage bacteria (Sivertsvik et al, 2002b). The effect of CO2 on bacterial growth is complex and four activity mechanisms of CO2on microorganisms has been identified(Farber 1991; Daniels et al., 1985; Dixon and Kell 1989; Parkin and Brown 1982): Alteration of cell membrane function includes effects on nutrient uptake and absorption; direct inhibition of enzymes or decreases in the rate of enzyme reactions; penetration of bacterial membranes leading to intracellular pH changes, and direct changes in the physico-chemical properties of proteins. Probably a combination of all these activities accounts for the bacteriostatic effect(Sivertsvik et al, 2002a). The CO2 is usually introduced into the MA-package by evacuating the air and flushing the appropriate gas mixture into the package prior to sealing, typically using automatic form-fill- seal or flow-packaging machines. Two other approaches to create a modified atmosphere for a product are either to generate the CO2 and/or remove O2 inside the package after packaging or to dissolve the CO2 into the product prior to packaging. Both methods can give appropriate packages with smaller gas/ roduct ratio, and thus decrease the package size that has been a disadvantage of MAP from the start The first approach involves the most commercialised active packaging technology, namely oxygen scavengers, that by now are available from several manufacturers (Mitsubishi Gas Chemical Co., ATCO, Bioka, Sealed Air/ Cryovac, Multisorb a.o. ), in different forms(sachets, packaging film, closures) with different active ingredients (iron, enzymes, dye). Some of the same companies have also developed COz emitters, using the O2 in the package headspace to produce CO2 and to develop a CO2/N2 atmosphere inside a kage without the use of gas flushing. Other methods for generating the cO2 gas inside the packages after closure include the use of dry ice(solid CO2) (Sivertsvik et al., 1999)or carbonate possibly mixed with weak acids(Bjerkeng etal.,1995) The second approach is to dissolve the CO2 into the product prior to packaging Since the solubility increases at lower temperatures and at higher CO, pressures, a sufficient amount of CO2 can be dissolved into the product during 1-2 hours prior to packaging using elevated pressures. This method is called soluble gas stabilisation(SGS)(Sivertsvik 2000). This is not an active packaging technology
totally inactivated in thawed chilled MAP cod fillets after frozen storage at ÿ20ºC and ÿ30ºC for 6–8 weeks. This approach has been used to further extend the shelf-life of MAP cod (Guldager et al., 1998) and salmon (Emborg et al., 2002) at 2ºC, and should also be the approach for successful use of active packaging technologies to control microbial spoilage. 18.3 Active packaging: atmosphere modifiers Many of the most used active packaging technologies are closely related to modified atmosphere packaging. Together with anaerobic conditions, carbon dioxide is the active gas of MAP because it inhibits growth of many of the normal spoilage bacteria (Sivertsvik et al., 2002b). The effect of CO2 on bacterial growth is complex and four activity mechanisms of CO2 on microorganisms has been identified (Farber 1991; Daniels et al., 1985; Dixon and Kell 1989; Parkin and Brown 1982): Alteration of cell membrane function includes effects on nutrient uptake and absorption; direct inhibition of enzymes or decreases in the rate of enzyme reactions; penetration of bacterial membranes, leading to intracellular pH changes; and direct changes in the physico-chemical properties of proteins. Probably a combination of all these activities accounts for the bacteriostatic effect (Sivertsvik et al., 2002a). The CO2 is usually introduced into the MA-package by evacuating the air and flushing the appropriate gas mixture into the package prior to sealing, typically using automatic form-fillseal or flow-packaging machines. Two other approaches to create a modified atmosphere for a product are either to generate the CO2 and/or remove O2 inside the package after packaging or to dissolve the CO2 into the product prior to packaging. Both methods can give appropriate packages with smaller gas/ product ratio, and thus decrease the package size that has been a disadvantage of MAP from the start. The first approach involves the most commercialised active packaging technology, namely oxygen scavengers, that by now are available from several manufacturers (Mitsubishi Gas Chemical Co., ATCO, Bioka, Sealed Air/ Cryovac, Multisorb a.o.), in different forms (sachets, packaging film, closures) with different active ingredients (iron, enzymes, dye). Some of the same companies have also developed CO2 emitters, using the O2 in the package headspace to produce CO2 and to develop a CO2/N2 atmosphere inside a package without the use of gas flushing. Other methods for generating the CO2 gas inside the packages after closure include the use of dry ice (solid CO2) (Sivertsvik et al., 1999) or carbonate possibly mixed with weak acids (Bjerkeng et al., 1995). The second approach is to dissolve the CO2 into the product prior to packaging. Since the solubility increases at lower temperatures and at higher CO2 pressures, a sufficient amount of CO2 can be dissolved into the product during 1–2 hours prior to packaging using elevated pressures. This method is called soluble gas stabilisation (SGS) (Sivertsvik 2000). This is not an active packaging technology Active packaging in practice: fish 387
388 Novel food packaging by definition, but it is a novel alternative to MA and it has been used successfully alone and in combination with O2 scavengers(see below) The commercial use of atmosphere modifiers, and O scavengers in particular, with fish products has been mostly limited to the Japanese market and to dried (seaweed, salmon jerky, sardines, shark's fin, rose mackerel, cod, squid)or smoked(salmon) products(Ashie et al., 1996). These ambient stored products have low aw (<0.85)so the microbial deterioration is not shelf-life limiting, therefore the effect of the O2 scavengers is to prevent oxidative reactions discolouration, and mould growth. Other commercial products are fresh yellow- tail, salmon roe, and sea urchin all stored at superchilling conditions packaged with O2 scavenger primarily to prevent oxidation and discolouration, but also to inhibit bacterial growth to a lesser degree(Ashie et al., 1996). Different O scavengers are chosen dependent on the amount of O2 to scavenge(pack size and material)and product aw. O2 scavengers for high aw foods react faster compared to scavengers for dry foods but in general the absorption is slow and exothermic Removal of oxygen from package interiors improves shelf-life by sub- ptimising the environment for aerobic microbiological growth and for adverse oxidative reactions such as rancidity. Ferrous ironbased oxygen scavengers rely on the presence of moisture for activation, with a water activity of at least 0.7 required, and 0.85-0.9 being preferred(Brody 2001). Oxygen absorbers are designed to reduce oxygen levels to less than 100 ppm in package head-space. In on-based en scavengers the oxygen is removed by oxidation(rusting) of powdered iron forming non-toxic iron oxide( Ashie et al., 1996). Oxygen absorbers could be used to create oxygen-free conditions in head-space of packages of medium barrier properties. The sachet will absorb residual oxygen and oxygen permeated through the packaging material during storage. More environmentally 'friendly' packaging materials with lower oxygen barriers could be used in combination with an oxygen absorber instead of high-cost barrier materials (Sivertsvik, 1997). However, not all oxygen absorbers can be combined with MAP. Some of them. like the iron-based Ageless SS-type from Mitsubishi meant for use in high aw foods, will unintentionally absorb some of the carbon dioxide present, and decrease some of the inhibitory effects of CO2 on bacterial growth. This is caused by a reaction of iron with CO2 to form ferrous carbonate, and secondarily this ferrous carbonate reacts with O This reaction will also slow down the O2 absorption(Sivertsvik, 1997 and 1999) The use of O2 absorbers(Ageless SS-100) had only a marginal effect on microbial growth in packages of fishcakes, fish pudding and mackerel fillets, and far less than the significant effect obtained by MAP(Sivertsvik 1997) However, a signficant effect of the O2 absorber was observed in packages with salmon fillets. The use of O2 absorbers inhibited development of rancidity TBARS)in both mackerel and salmon fillets (Sivertsvik, 1997), but in no higher degree than O2-free MAP. The effect of Sgs treatment. different O -absorbers/CO-emitters and combinations of these on growth of psychrotrophic bacteria in salmon fillets shown in Fig. 18.1(Sivertsvik, 1999). The best microbial quality was
by definition, but it is a novel alternative to MA and it has been used successfully alone and in combination with O2 scavengers (see below). The commercial use of atmosphere modifiers, and O2 scavengers in particular, with fish products has been mostly limited to the Japanese market and to dried (seaweed, salmon jerky, sardines, shark’s fin, rose mackerel, cod, squid) or smoked (salmon) products (Ashie et al., 1996). These ambient stored products have low aW (<0.85) so the microbial deterioration is not shelf-life limiting, therefore the effect of the O2 scavengers is to prevent oxidative reactions, discolouration, and mould growth. Other commercial products are fresh yellowtail, salmon roe, and sea urchin all stored at superchilling conditions packaged with O2 scavenger primarily to prevent oxidation and discolouration, but also to inhibit bacterial growth to a lesser degree (Ashie et al., 1996). Different O2 scavengers are chosen dependent on the amount of O2 to scavenge (pack size and material) and product aW. O2 scavengers for high aW foods react faster compared to scavengers for dry foods but in general the absorption is slow and exothermic. Removal of oxygen from package interiors improves shelf-life by suboptimising the environment for aerobic microbiological growth and for adverse oxidative reactions such as rancidity. Ferrous ironbased oxygen scavengers rely on the presence of moisture for activation, with a water activity of at least 0.7 required, and 0.85–0.9 being preferred (Brody 2001). Oxygen absorbers are designed to reduce oxygen levels to less than 100 ppm in package head-space. In iron-based oxygen scavengers the oxygen is removed by oxidation (rusting) of powdered iron forming non-toxic iron oxide (Ashie et al., 1996). Oxygen absorbers could be used to create oxygen-free conditions in head-space of packages of medium barrier properties. The sachet will absorb residual oxygen and oxygen permeated through the packaging material during storage. More inexpensive or environmentally ‘friendly’ packaging materials with lower oxygen barriers could be used in combination with an oxygen absorber instead of high-cost barrier materials (Sivertsvik, 1997). However, not all oxygen absorbers can be combined with MAP. Some of them, like the iron-based Ageless SS-type from Mitsubishi meant for use in high aW foods, will unintentionally absorb some of the carbon dioxide present, and decrease some of the inhibitory effects of CO2 on bacterial growth. This is caused by a reaction of iron with CO2 to form ferrous carbonate, and secondarily this ferrous carbonate reacts with O2. This reaction will also slow down the O2 absorption (Sivertsvik, 1997 and 1999). The use of O2 absorbers (Ageless SS-100) had only a marginal effect on microbial growth in packages of fishcakes, fish pudding and mackerel fillets, and far less than the significant effect obtained by MAP (Sivertsvik 1997). However, a signficant effect of the O2 absorber was observed in packages with salmon fillets. The use of O2 absorbers inhibited development of rancidity (TBARS) in both mackerel and salmon fillets (Sivertsvik, 1997), but in no higher degree than O2-free MAP. The effect of SGS treatment, different O2-absorbers/CO2-emitters and combinations of these on growth of psychrotrophic bacteria in salmon fillets is shown in Fig. 18.1 (Sivertsvik, 1999). The best microbial quality was 388 Novel food packaging techniques