M. Sivertsvik, J.T. Rosnes and H. Bergslien, Institute of Fish
4.6 MAP of non-respiring foods
4.6.1 Fish and other seafood
Fish and shellfish are highly perishable, due to their high aW, neutral pH and the presence of autolytic enzymes which cause the rapid development of undesirable odours and flavours. The quality deterioration of chilled fish is usually dominated by microbial activity, but oxidative activities also play an important role for some fish species. The deterioration is highly temperature dependent (Rosnes et al., 1994) and can be inhibited with the use of low storage temperatures (e.g. fish stored on ice). A further inhibition and an increased shelf-life could be obtained through the use of low storage temperatures combined with high CO2contents in the atmosphere surrounding the product (i.e. MAP). It is evident that MAP can extend the shelf-life of fish and shellfish products. For raw fish an increase of 50–100% in storage life is usually observed, and for cooked shellfish a shelf-life extension of 100–200% can be obtained under ideal storage conditions (Sivertsvik et al., 2002; Stammen et al., 1990).
Fish normally have a particularly heavy microbial load owing to the method of capture and transport to shore, slaughtering methods, evisceration and the retention of skin in retail portions. Microbial activity causes the breakdown of fish proteins and trimethylamin oxide (TMAO), with a resulting release of undesirable fishy odours such as trimethylamine (TMA). The main spoilage bacteria in cod packaged in high levels of CO2 has been assumed to be Photobacterium phosphoreum (Dalgaard et al., 1993), which is different from the main spoilage bacteria of traditionally ice-stored fish: Shewanella putrefacies (Gram et al., 1989). Thus a different bacterial flora will develop in MAP conditions.
A gas mixture containing 40–60% CO2, 40–60% N2 and no O2 is re-commended for fatty fish products, since the oxidative rancidity of unsaturated fat in fatty fish also results in other additional offensive odours and flavours, apart from microbial spoilage. Vacuum packaging could also be an alternative to MAP for fatty fish such as salmon, providing similar sensory shelf-lives when Modified atmosphere packaging 67
the primary sensory spoilage parameter is oxidative rancidity (Rosnes et al., 1997; Randell et al., 1999). But the microbial quality is still better under MAP conditions compared to vacuum packaging.
For white fish, crustaceans and molluscs, a gas mixture containing either 40%
CO2/30% O2/30% N2or 40% CO2/60% N2is recommended. The level of CO2 and gas/product volume ratio is the decisive factor for determining the observed shelf-life extension. The use of 30% O2in the package reduces the drip. The drip from MAP lean fish could also be reduced significantly by dipping fillets in 20%
NaCl solution for about 20 seconds prior to packaging (Bjerkeng et al., 1995), hence oxygen-free gas mixtures could be used for all types of raw fish. The pre-treatment with salt solution does not negatively influence sensory parameters, the dipping resulting in a salt content of around 1% in the fish fillet. A gas/
product ratio of 3:1 is usually recommended for MAP of raw fish.
The inclusion of CO2is necessary for inhibiting common aerobic spoilage bacteria, such as the Pseudomonas species and Acinetobacter/Moraxella species.
However, for retail packages of fish and other seafood products, too high a proportion of CO2 in the gas mixture can induce packaging collapse and excessive drip. In fishery products eaten without prior heating, such as crab and cooked fish, an acidic, sherbet-like flavour can be noted when high partial pressures of CO2are used.
MAP can be combined with superchilling processes to further extend the shelf-life and safety of fresh fish. In this technique, also known as partial freezing, the temperature of the fish is reduced to 1–2ºC below the initial freezing point and some ice is formed inside the product (Sikorski and Pan, 1994). A shelf-life extension of about seven days is obtained for superchilled fish compared to traditionally ice-stored fish of the same type (LeBlanc and LeBlanc, 1992). The superchilling process will store refrigeration capacity inside the product to help keep the core temperature low during chilled storage.
Superchilling combined with MA packaging is a mild preservation system that can maintain a high microbiological and sensory quality of whole salmon (Sivertsvik et al., 2000) and salmon fillets (Fig. 4.1) for more than three weeks.
When combining MAP with superchilling, a synergistic hurdle effect of the very low storage temperature, CO2, and the increased CO2solubility into the food is achieved, which MAP or superchilling used on their own cannot achieve.
CO2/N2 mixtures with up to 100% CO2 are recommended for the bulk transportation of fresh fish. Freshness of chilled fish is often evaluated on the red colour of the gills, turning grey or brown during storage. Absence of O2 will cause discoloration of the gills, but can be avoided by using small amounts of carbon monoxide (CO) in the atmosphere (Rosnes et al., 1998; see also section 4.6.2 about red meat and discoloration). Bulk storage of salmon fillets (Oncorhynchus kisutch and O. keta) in air-tight bulk containers has been reported to have an acceptable sensory shelf-life of 21 days in 90% CO2 atmospheres at 0ºC (Barnett et al., 1982). In bulk storage of whole cod (Gadus morhua) in CO2, a shelf-life increase of at least four days was observed when compared to cod stored in air (Einarsson and Valdimarsson, 1990).
A further use of MAP is for the packaging and distribution of live blue mussels.
The mussels are packaged in plastic barrier bags with a 50 : 50 mixture of O2and CO2together with some seawater. The O2keeps the mussels alive while the CO2 has the dual effect of inhibiting microbial growth and creating under-pressure inside the package (because of the solubility), and thus keeps the mussels closed until the packages are opened (European Patent Application, 2001).
Fig. 4.1 Modified atmosphere packaging (60% CO2: 40% N2) versus over-wrap air packaging on salmon fillets stored at chill temperature (4ºC) and under superchilling conditions (ÿ2ºC). Effect on (a) psychrotrophic plate count, (b) cooked flavour score,
sensory evaluation (Sivertsvik et al., unpublished results).
Modified atmosphere packaging 69
Only the highest quality fish and seafood products should be used to benefit from the extended shelf-life advantages of MAP. The achievable shelf-life will depend on the species, fat content, initial microbial load, gas mixture and the temperature of storage. The maintenance of recommended chilled temperatures and good hygiene and handling practices throughout the entire capture-to-consumption chain is essential for ensuring the safety and extended shelf-life of fish and seafood products.
4.6.2 Meat
The two principal spoilage mechanisms affecting the shelf-life of raw red meat are microbial growth and colour changes (oxidation of the red oxymyoglobin pigment) (Gill, 1996). When red meat is kept under proper chilled conditions, the controlling factor influencing the shelf-life of the product is the rate of oxidation of the red oxymyoglobin pigment into its brown oxidated form, metmyoglobin (Stiles, 1991). For this reason, high concentrations of O2 are necessary for the MAP of red meats in order to maintain the desirable bright red colour for a longer period. Highly pigmented red meats, such as venison and wild boar, require higher concentrations of O2.
Aerobic spoilage bacteria, such as Pseudomonas species, which are normally predominant on red meats, are inhibited by CO2 (Gill and Molin, 1991).
Consequently, to create the dual effect of red colour stability and microbial inhibition, gas mixtures containing 20–30% CO2 and 70–80% O2 and a gas/
product ratio of 2:1 are recommended for extending the chilled shelf-life of red meats from two to four days up to five to eight days. In Norway an alternative gas mixture is used for red meat: 60–70% CO2, 30–40% N2 and < 0.5% carbon monoxide (CO). This mixture provides a unique combination of long microbiological shelf-life and a stable, cherry-red colour of the meat (Sørheim et al., 1997). The shelf-life of meat packaged in the CO mixture is longer than that of meat packaged in the commonly used atmospheres with high O2, because of higher CO2 levels and the lack of O2 in the package. The consumption of meat packaged in atmospheres containing< 0.5% CO only results in negligible levels of carboxyhaemoglobin entering into the bloodstream, and it is highly improbable that the CO will present a toxic threat to consumers (Sørheim et al., 1997).
For non-red meats, microbiological growth and oxidative rancidity limit the shelf-life, and gas mixtures of CO2and N2should be used. Meat with higher levels of unsaturated fat (like pork) are more prone to oxidative rancidity, hence the complete removal of O2is more important. The microbial spoilage is usually dependent on the pH in the meat; a lower pH level provides for a longer shelf-life. Beef, for example, can therefore be vacuum packaged because of its low muscle pH, whereas lamb, which has higher muscle pH, must be packaged in a CO2enriched atmosphere in order to achieve a comparable shelf-life (Church and Parsons, 1995).
The maintenance of the recommended chilled temperatures and the ‘good hygiene and handling’ procedures throughout the slaughtering, processing,
packaging, distribution and retail chain are also of vital importance in ensuring the safety and extended shelf-life of all meat products.
4.6.3 Poultry
In contrast to red meats, poultry does not undergo irreversible discoloration of the meat surface in the presence of O2(Stiles, 1991). The spoilage of raw poultry is mainly caused by microbial growth, particularly growth of Pseudomonas species and Achromobacter species. These aerobic spoilage bacteria are effectively inhibited by the use of CO2in MAP. Levels of CO2 in excess of 20% are required to extend the shelf-life of poultry significantly. Package collapse and excessive drip can be a problem for raw poultry, so if higher levels of CO2are being used the gas/product ratio also should be increased. In cases where package collapse is not a problem (e.g. bulk or master bags), 100% CO2is recommended. In both retail and bulk MA packages, N2is used as an inert filler gas.
The achievable shelf-life of MA packed raw poultry and game bird products will depend on the species, fat content, initial microbial load, packaging type, gas mixture and temperature of storage (Stiles, 1991).
4.6.4 Cooked, cured and processed foods and ready meals
For cooked, cured and processed foods the main spoilages are microbial growth, colour changes and oxidative rancidity. For cooked products the heating process should be sufficient to kill vegetative bacterial cells, to inactivate degradative enzymes and to stabilise colour. Consequently, spoilage of cooked meat products is primarily due to post-cooking contamination by microorganisms. MAP with mixtures of CO2/N2should be used without the use of O2, combined with good hygiene and handling procedures. Cooked shrimp and crab can benefit greatly from such a gas mixture (Sivertsvik, 1995). Whole cooked shrimp stored in 60% CO2/40% N2atmospheres had the same sensory and microbiological quality as freshly cooked shrimp even after 14 days of chilled storage, (Sivertsvik et al., 1997). The gas/product ratio of cooked products would depend on the level of CO2and the food’s aW, but a ratio of 2:1 is usual.
For processed food products with relatively high levels of salt and/or other preservatives (e.g. smoked salmon), which effectively inhibit a wide range of spoilage microorganisms, oxygen-free MAP or vacuum packaging could be used to inhibit oxidative rancidity. Cooked, cured and processed meat products containing high levels of unsaturated fat are liable to be spoiled by oxidative rancidity, but MAP with CO2/N2mixtures inhibits this undesirable reaction, and delays the development of oxidative warmed-over flavour (Church and Parsons, 1995). Sliced, cooked, cured and processed meat products are often vacuum packed for retail sale, but the same shelf-life could be achieved using MA and with the additional benefit of an easier separation of meat slices.
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The principal spoilage mechanism for ready meals and other cook-chill products is microbial growth, which is primarily due to post-cooking contamination and/or poor temperature control. The pasteurisation process should kill vegetative bacterial cells, inactivate degradative enzymes, and restore the colour. However, heat-resistant spores, such as those from Clostridium species and Bacillus species, will survive the cooking process and may germinate if the recommended chilled temperature is not maintained. Other possible food poisoning hazards can arise from post-cooking contamination as a result of poor hygiene and handling practices and/or faulty seal integrity. Poor temperature control will exacerbate the problem of microbial growth. Therefore it is recommended that strict control of temperature, hygiene and handling should be maintained throughout the shelf-life. The use of additional barriers to microbial growth (such as acidification, use of preservatives and/or reduction in aW), as and when appropriate, is strongly recommended.
Ahvenainen et al. (1990) studied the influences of MAP on selected ready-to-eat foods. MAP with >20% CO2retarded the growth of mould and discoloration of ham pizzas. MAP also created benefits for the packaging of vegetable salads with herring, increasing the shelf-life by a few days. Mayonnaise-based potato salad could not be improved by gas packaging, as atmospheres with >20% CO2 caused a strong, objectionable odour and taste in the salad.
4.6.5 Dairy products
The principal spoilage mechanisms affecting dairy products are microbial growth and oxidative rancidity. The type of spoilage affecting dairy products will depend on the intrinsic properties of the different products. For example, low aWproducts such as hard cheeses are generally spoiled by mould, whereas higher aWproducts such as creams and soft cheeses are susceptible to yeast and bacterial spoilage, oxidative rancidity and physical separation.
MAP can extend the shelf-lives of dairy products, but often similar shelf-lives to those with vacuum packaging are achieved. Hard cheeses are generally packed in CO2/N2 gas mixtures, which are very effective at inhibiting mould growth. CO2levels should be at least 30%. Similarly, soft cheeses are packed in CO2/N2 gas mixtures, which can also inhibit bacterial spoilage and oxidative rancidity. MAP is particularly effective for crumbly cheeses such as Cheshire and grated cheeses, where vacuum packaging would cause undesirable compression (Fierheller, 1991). MAP is not suitable for mould-ripened cheeses since the gas mixtures would inhibit the desirable mould growth. Some dairies flush CO2into the head-space of yoghurt and sour cream packages to increase the shelf-life (Fierheller, 1991).
4.6.6 Bakery products
The principal spoilage mechanisms for non-dairy bakery products are mould growth, staling and moisture migration. Water activity (aW) and storage
temperature are the two most important parameters influencing the shelf-life (Ooraikul, 1991). Yeasts may cause a problem in certain filled (cakes) or chilled (pizza crust) products. Bacterial growth in bakery products is restricted and rarely a problem, because of the low aW. However, it is possible that Staphylococcus aureus and Bacillus species may be able to grow in certain products and hence pose a potential food poisoning hazard. Consequently, good hygiene and handling practices must be observed throughout processing, packaging and storage.
The use of MAP can significantly extend the shelf-lives of non-dairy bakery products. Since moulds are aerobic microorganisms, CO2/N2gas mixtures very effectively inhibit them. A gas/product ratio of 2:1 is recommended. Materials with high water vapour barriers are used to prevent moisture migration.
MAP appears to have little effect on the rate of staling. Staling rates increase at chilled temperatures and hence most cold-eating bakery products are normally stored at ambient temperatures. For hot-eating bakery products, such as pizza bases, the staling process, which is caused by starch retro gradation, is reversed during the reheating cycle.
For bakery products containing fruit as an ingredient (e.g. strawberry layer cake, apple pie, cherry cream cheese cake) yeast is a major cause of spoilage.
Yeasts being facultative microorganisms will grow under anaerobic conditions and oxygen-free MAP has no effect on their growth. A possible method to control yeast growth in these products is to modify the head-space atmosphere using ethanol vapour. The ethanol could be sprayed on the product prior to packaging or be released into the atmosphere after packaging using ethanol emitters (Ethicap, Freund Industrial Co., Tokyo, Japan). Ethanol emitters can also delay the staling of bakery products (Hurme et al., 1998).
4.6.7 Dry foods
Because of low aW, bacterial growth is not an important parameter for dry foods (Fierheller, 1991). The principal spoilage mechanism affecting dried foods containing a high proportion of unsaturated fatty acids is oxidative rancidity.
MAP using N2very effectively inhibits this deleterious reaction. Due to the very long achievable shelf-lives in MAP for dried foods, MAP materials must have very high moisture and gas barriers. Some products also benefit from using packaging materials with light barriers such as colour printed, pigmented or metallised films. Certain dried foods, such as dried baby milk, are particularly susceptible to oxidative rancidity and residual O2 levels should be below
< 0.2%. In order to achieve very low residual O2levels, O2scavengers may be incorporated into MA packages. These O2scavengers may also be used for other low aWfoods such as bakery products or other very O2sensitive foods (more information about active packaging technologies is found in Chapter 5).
Fresh pasta is often packaged in MAP. There are numerous products of this type with ground meat (lasagne, ravioli) or without (pasta, gnocchi) (Coulon and Louis, 1989). For pasta without meat, an atmosphere of > 80% CO2 is Modified atmosphere packaging 73
recommended in order to achieve a shelf-life of two weeks at 4ºC. Less CO2in the atmosphere has to be used for pasta with meat, because of the problems associated with package collapse.