Modified atmosphere packaging MAP) F. Devlieghere, Ghent University; M.I. Gil, CEBAS-CSIC, Spain; and J. Debevere, Ghent University 16.1 Introduction Modified atmosphere packaging(MAP) may be defined as"the enclosure of food products in gas-barrier materials, in which the gaseous environment has been changed(Young et al, 1988). Because of its substantial shelf-life extending effect, MAP has been one of the most significant and innovative growth areas in retail food packaging over the past two decades. The potential advantages and disadvantages of MAP have been presented by both Farber(1991) and Parry (1993), and summarised by Davies(1995)in Table 16.1 There is considerable information available regarding suitable gas mixtures for different food products. However, there is still a lack of scientific detail regarding many aspects relating to MAP. These include: Mechanism of action of carbon dioxide(COz on microorganisms Safety of MAP packaged food products. Interactive effects of MAP and other preservation methods The influence of CO, on the microbial ecology of a food product The effect of MAP on the nutrional quality of packaged food products 16.2 Principles of MAP 16.2.1 General principles Modified atmosphere packaging can be defined as packaging a product in an atmosphere that is different from air. This atmosphere can be altered in four different ways:
16 Modified atmosphere packaging (MAP) F. Devlieghere, Ghent University; M. I. Gil, CEBAS-CSIC, Spain; and J. Debevere, Ghent University 16.1 Introduction Modified atmosphere packaging (MAP) may be defined as ‘the enclosure of food products in gas-barrier materials, in which the gaseous environment has been changed’ (Young et al, 1988). Because of its substantial shelf-life extending effect, MAP has been one of the most significant and innovative growth areas in retail food packaging over the past two decades. The potential advantages and disadvantages of MAP have been presented by both Farber (1991) and Parry (1993), and summarised by Davies (1995) in Table 16.1. There is considerable information available regarding suitable gas mixtures for different food products. However, there is still a lack of scientific detail regarding many aspects relating to MAP. These include: • Mechanism of action of carbon dioxide (CO2) on microorganisms. • Safety of MAP packaged food products. • Interactive effects of MAP and other preservation methods. • The influence of CO2 on the microbial ecology of a food product. • The effect of MAP on the nutrional quality of packaged food products. 16.2 Principles of MAP 16.2.1 General principles Modified atmosphere packaging can be defined as packaging a product in an atmosphere that is different from air. This atmosphere can be altered in four different ways:
Modified atmosphere packaging(MAP) 343 Table 16.1 The potential positive and negative effects MAP has on the food industry Benefits Disadvantages 1. Product A centralised packaging system Increased package volume, adds ortion contro Clear, all-round visibility of the area required for retail display roduct, improving its Benefits are lost when the presentation characteristics package leaks or is opened Product Overall product quality is high Product safety has not yet been quality Sliced products are much easier fully established to separate Shelf life increases by 50-400% 3. Special Use of chemical preservatives Temperature control is essential Different products require their Speciality equipment and 4. Economics Improved shelf life decreases Increased costs financial losses Distribution costs are reduced due to fewer deliveries being necessary over long distances after Davies. 1995 1. Vacuum packaging 2. Passive mAp 3. Introduction of a gas at the moment of packaging 4. Active packaging. In passive MAP, the modified atmosphere is created by the packaged commodity that continues its respiration after packaging Active packaging systems alter the atmosphere using packaging materials or inserts absorbing and/or generating gases. Typical examples are oxygen absorbers and co, emitting films or sachets. The gases that are applied in MAP today are basically O2, CO2 and N2. The last has no specific preservative effect but functions mainly as a filler gas to avoid the collapse that takes place when CO2 dissolves in the food product. The func- tions of co, and o will be discussed in more detail 16.2.2 Carbon dioxide as anti- microbial gas CO2, because of its antimicrobial activity, is the most important component in applied gas mixtures. When CO2 is introduced into the package, it is partly dis olved in the water phase and the fat phase of the food. This results, after equi- librium, in a certain concentration of dissolved CO2([CO2]diss)in the water phase of the product. Devlieghere et al(1998)have demonstrated that the growth
1. Vacuum packaging. 2. Passive MAP. 3. Introduction of a gas at the moment of packaging. 4. Active packaging. In passive MAP, the modified atmosphere is created by the packaged commodity that continues its respiration after packaging. Active packaging systems alter the atmosphere using packaging materials or inserts absorbing and/or generating gases. Typical examples are oxygen absorbers and CO2 emitting films or sachets. The gases that are applied in MAP today are basically O2, CO2 and N2. The last has no specific preservative effect but functions mainly as a filler gas to avoid the collapse that takes place when CO2 dissolves in the food product. The functions of CO2 and O2 will be discussed in more detail. 16.2.2 Carbon dioxide as anti-microbial gas CO2, because of its antimicrobial activity, is the most important component in applied gas mixtures. When CO2 is introduced into the package, it is partly dissolved in the water phase and the fat phase of the food. This results, after equilibrium, in a certain concentration of dissolved CO2 ([CO2]diss) in the water phase of the product. Devlieghere et al (1998) have demonstrated that the growth Modified atmosphere packaging (MAP) 343 Table 16.1 The potential positive and negative effects MAP has on the food industry Benefits Disadvantages 1. Product A centralised packaging system Increased package volume, adds to packaging incorporating portion control the transport costs and affects Clear, all-round visibility of the area required for retail display product, improving its Benefits are lost when the presentation characteristics package leaks or is opened 2. Product Overall product quality is high Product safety has not yet been quality Sliced products are much easier fully established to separate Shelf life increases by 50–400% 3. Special Use of chemical preservatives Temperature control is essential features can be reduced or Different products require their discontinued own specific gas formulation Speciality equipment and associated training is required 4. Economics Improved shelf life decreases Increased costs financial losses Distribution costs are reduced due to fewer deliveries being necessary over long distances after Davies, 1995
344 The nutrition handbook for food processors inhibition of microorganisms in modified atmospheres is determined by the con- centration of dissolved CO2 in the water phase. The effect of the gaseous environment on microorganisms in foods is not as well understood by microbiologists and food technologists as are other external factors, such as pH and aw. Despite numerous reports of the effects of CO2 on microbial growth and metabolism, the 'mechanismof COz inhibition still remains unclear (Dixon and Kell, 1989: Day, 2000). The question of whether any specific metabolic pathway or cellular activity is critically sensitive to CO2 inhi- bition has been examined by several workers. The different proposed mechanisms of action are. 1. Lowering the ph of the food. 2. Cellular penetration followed by a decrease in the cytoplasmic pH of the cell. 3. Specific actions on cytoplasmic enzymes 4. Specific actions on biological membranes. When gaseous CO2 is applied to a biological tissue, it first dissolves in the liquid phase, where hydration and dissociation lead to a rapid pH decrease in the tissue. This drop in pH, which depends on the buffering capacity of the medium (Dixon and Kell, 1989), is not large in food products. In fact, the ph drop in cooked meat products only amounted to 0.3 pH units when 80% of CO, was applied in the gas phase with a gas/product volume ratio of 4: 1 (Devlieghere et al, 2000b). Several studies have proved that the observed inhibitory effects of CO2 could not solely be explained by the acidification of the substrate(Becker, 1933; Coyne,1933 Many researchers have documented the rapidity with which CO2 in solution enetrates into the cell. Krogh(1919)discovered that this rate is 30 times faster than for oxygen(O2), under most circumstances. Wolfe(1980) suggested the inhibitory effects of CO2 are the result of internal acidification of the cytoplasm. Eklund(1984)supported this idea by pointing out that the growth inhibition of four bacteria obtained with CO, had the same general form as that obtained with eak organic acids(chemical preservatives), such as sorbic and benzoic acid. Tan and Gill(1982) also found that the intracellular pH of Pseudomonas fluorescens fell by approximately 0.03 units for each I mM rise in extracellular CO concentration CO2 may also exert its influence upon a cell by affecting the rate at which particular enzymatic reactions proceed. One way this may be brought about is to cause an alteration in the production of a specific enzyme, or enzymes, via induc tion or repression of enzyme synthesis (Dixon, 1988; Dixon and Kell, 1989 Jones, 1989). It was also suggested (ones and greenfield, 1982; Dixon and Kell, 1989)that the primary sites where COz exerts its effects are the enzymatic car- boxylation and decarboxylation reactions, although inhibition of other enzymes has also been reported (ones and Greenfield, 1982 nother possible factor contributing to the growth-inhibitory effect of CO could be an alteration of the membrane properties(Daniels et al, 1985; Dixon and Kell, 1989). It was suggested that CO2 interacts with lipids in the cell mem-
inhibition of microorganisms in modified atmospheres is determined by the concentration of dissolved CO2 in the water phase. The effect of the gaseous environment on microorganisms in foods is not as well understood by microbiologists and food technologists as are other external factors, such as pH and aw. Despite numerous reports of the effects of CO2 on microbial growth and metabolism, the ‘mechanism’ of CO2 inhibition still remains unclear (Dixon and Kell, 1989; Day, 2000). The question of whether any specific metabolic pathway or cellular activity is critically sensitive to CO2 inhibition has been examined by several workers. The different proposed mechanisms of action are: 1. Lowering the pH of the food. 2. Cellular penetration followed by a decrease in the cytoplasmic pH of the cell. 3. Specific actions on cytoplasmic enzymes. 4. Specific actions on biological membranes. When gaseous CO2 is applied to a biological tissue, it first dissolves in the liquid phase, where hydration and dissociation lead to a rapid pH decrease in the tissue. This drop in pH, which depends on the buffering capacity of the medium (Dixon and Kell, 1989), is not large in food products. In fact, the pH drop in cooked meat products only amounted to 0.3 pH units when 80% of CO2 was applied in the gas phase with a gas/product volume ratio of 4 :1 (Devlieghere et al, 2000b). Several studies have proved that the observed inhibitory effects of CO2 could not solely be explained by the acidification of the substrate (Becker, 1933; Coyne, 1933). Many researchers have documented the rapidity with which CO2 in solution penetrates into the cell. Krogh (1919) discovered that this rate is 30 times faster than for oxygen (O2), under most circumstances. Wolfe (1980) suggested the inhibitory effects of CO2 are the result of internal acidification of the cytoplasm. Eklund (1984) supported this idea by pointing out that the growth inhibition of four bacteria obtained with CO2 had the same general form as that obtained with weak organic acids (chemical preservatives), such as sorbic and benzoic acid. Tan and Gill (1982) also found that the intracellular pH of Pseudomonas fluorescens fell by approximately 0.03 units for each 1 mM rise in extracellular CO2 concentration. CO2 may also exert its influence upon a cell by affecting the rate at which particular enzymatic reactions proceed. One way this may be brought about is to cause an alteration in the production of a specific enzyme, or enzymes, via induction or repression of enzyme synthesis (Dixon, 1988; Dixon and Kell, 1989; Jones, 1989). It was also suggested (Jones and Greenfield, 1982; Dixon and Kell, 1989) that the primary sites where CO2 exerts its effects are the enzymatic carboxylation and decarboxylation reactions, although inhibition of other enzymes has also been reported (Jones and Greenfield, 1982). Another possible factor contributing to the growth-inhibitory effect of CO2 could be an alteration of the membrane properties (Daniels et al, 1985; Dixon and Kell, 1989). It was suggested that CO2 interacts with lipids in the cell mem- 344 The nutrition handbook for food processors
Modified atmosphere packaging(MAP) 345 brane, decreasing the ability of the cell wall to uptake various ions. Moreover, perturbations in membrane fluidity, caused by the disordering of the lipid bilayer, are postulated to alter the function of membrane proteins( Chin et al, 1976: Roth 1980) Studies examining the effect of a CO2 enriched atmosphere on the growth of microorganisms are often difficult to compare because of the lack of information regarding the packaging configurations applied. The gas/product volume ratio and the permeability of the applied film for O2 and CO2 will influence the amount of dissolved CO2 and thus the microbial inhibition of the atmosphere. For this reason, the concentration of dissolved CO2 in the aqueous phase of the food should always be measured and mentioned in publications cone (Devlieghere et al, 1998) me nly a few publications deal with the effect of MAP on specific spoilage roorganisms. Gill and Tan(1980) compared the effect of CO2 on the growth of some fresh meat spoilage bacteria at 30C. Molin (1983)determined the resis- tance to CO2 of several food spoilage bacteria. Boskou and Debevere(1997: 1998) investigated the effect of COz on the growth and trimethylamine production of Shewanella putrifaciens in marine fish, and Devlieghere and Debevere(2000) compared the sensitivity for dissolved CO2 of different spoilage bacteria at 7C In general, Gram-negative microorganisms such as Pseudomonas, Shewanella and Aeromonas are very sensitive to CO2. Gram-positive bacteria show less sen sitivity and lactic acid bacteria are the most resistant. Most yeasts and moulds are also sensitive to COz. The effect of CO2 on psychrotrophic food pathogens is discussed in section 16.5 16.3 The use of oxygen in MAP 16.3.1 Colour retention in fresh meat products The colour of fresh meat is determined by the condition of myoglobin in the meat When an anaerobic atmosphere is applied, myoglobin(purplish-red) will be trans formed to metmyoglobin, producing a brown colour, which is an undesirable trait for European consumers. It is therefore essential that O2 is included(e.g. 40%o) into the applied gas atmosphere when fresh meat, destined for the consumer, is packaged. This will ensure the myoglobin is oxygenated, resulting in an attrac tive bright red colour. However, by doing this, the microbial shelf life of the pack aged meat is decreased compared with meat that is packaged in an O2 free 16.3.2 Inhibition of the reduction of trimethylamineoxide (TmAo) in marine fish Marine fish contain TMAO, which is an osmo-regulator In O2 poor conditions (e.g. when stored in ice), TMAO is used by spoilage organisms(e.g Shewanella putrifaciens)as a terminal electron-acceptor, and is reduced to trimethylamine
brane, decreasing the ability of the cell wall to uptake various ions. Moreover, perturbations in membrane fluidity, caused by the disordering of the lipid bilayer, are postulated to alter the function of membrane proteins (Chin et al, 1976; Roth, 1980). Studies examining the effect of a CO2 enriched atmosphere on the growth of microorganisms are often difficult to compare because of the lack of information regarding the packaging configurations applied. The gas/product volume ratio and the permeability of the applied film for O2 and CO2 will influence the amount of dissolved CO2 and thus the microbial inhibition of the atmosphere. For this reason, the concentration of dissolved CO2 in the aqueous phase of the food should always be measured and mentioned in publications concerning MAP (Devlieghere et al, 1998). Only a few publications deal with the effect of MAP on specific spoilage microorganisms. Gill and Tan (1980) compared the effect of CO2 on the growth of some fresh meat spoilage bacteria at 30 °C. Molin (1983) determined the resistance to CO2 of several food spoilage bacteria. Boskou and Debevere (1997;1998) investigated the effect of CO2 on the growth and trimethylamine production of Shewanella putrifaciens in marine fish, and Devlieghere and Debevere (2000) compared the sensitivity for dissolved CO2 of different spoilage bacteria at 7 °C. In general, Gram-negative microorganisms such as Pseudomonas, Shewanella and Aeromonas are very sensitive to CO2. Gram-positive bacteria show less sensitivity and lactic acid bacteria are the most resistant. Most yeasts and moulds are also sensitive to CO2. The effect of CO2 on psychrotrophic food pathogens is discussed in section 16.5. 16.3 The use of oxygen in MAP 16.3.1 Colour retention in fresh meat products The colour of fresh meat is determined by the condition of myoglobin in the meat. When an anaerobic atmosphere is applied, myoglobin (purplish-red) will be transformed to metmyoglobin, producing a brown colour, which is an undesirable trait for European consumers. It is therefore essential that O2 is included (e.g. 40%) into the applied gas atmosphere when fresh meat, destined for the consumer, is packaged. This will ensure the myoglobin is oxygenated, resulting in an attractive bright red colour. However, by doing this, the microbial shelf life of the packaged meat is decreased compared with meat that is packaged in an O2 free atmosphere. 16.3.2 Inhibition of the reduction of trimethylamineoxide (TMAO) in marine fish Marine fish contain TMAO, which is an osmo-regulator. In O2 poor conditions (e.g. when stored in ice), TMAO is used by spoilage organisms (e.g. Shewanella putrifaciens) as a terminal electron-acceptor, and is reduced to trimethylamine Modified atmosphere packaging (MAP) 345
46 The nutrition handbook for food processors TMA). TMA is the main active component responsible for the unpleasant'fishy dour. However, by introducing high levels of O2 in the gas atmosphere, the TMAO-reduction can be retarded, and consequently the shelf-life of the fish is increased. This was clearly demonstrated by Boskou and Debevere(1997, 1998) Therefore, packaging atmospheres for lean marine fish should contain oxygen levels of at least 30%o 16.3.3 Avoiding anaerobic respiration of fresh produce respire. It is of great importance to avoid anaerobic conditions in the package of fresh produce because anaerobic respiration of the plant tissue will result in the production of off-odour compounds such as ethanol and acetaldehyde. The tech- niques applied to maintain an aerobic atmosphere in the packaging of fresh produce are discussed in detail in section 16.4.2. 16.4 Applications of MAP in the food industry 16.4.1 Non-respiring products Non-respiring food products do not consume any oxygen during further storage When such food products are packaged in a modified atmosphere, the aim is to retain the introduced atmosphere during the storage period. Therefore, high barrier films are used which are most often composed out of different layers of materials. Typical O, and CO2 barrier materials are PA(polyamide), PVDC (polyvinylidenechloride) and EVOH (ethylenevinyl alcohol). Depending on the intended storage time, the O2-permeability of the applied films should be <2 ml O/m: 24h atm determined at 75%o relative humidity at 23C for products with a long shelf life and <10ml O,/m2. 24h atm determined at the same conditions fo products with a limited shelf life(<I week) One of the bottlenecks in modified atmosphere packaging lies in defining the optimal gas atmosphere for a food product in a specific packaging design. This optimal atmosphere depends on the intrinsic parameters of the food product(pH, water activity, fat content, type of fat) and the gas/product volume ratio in the chosen package type. The intrinsic parameters determine the sensitivity of the product for specific microbial, chemical and enzymatic degradation reactions. Products that are susceptible to microbial spoilage due to the development of Gram-negative bacteria(e.g. fresh meat and fish) and yeasts(salads) should be packaged in a CO2 enriched atmosphere because the growth of those micro- organisms is significantly retarded by COz. In general, oxygen is excluded from the gas mixture For prolonging the shelf life of products which are spoiled by mould growth(e. g. hard cheeses)or by oxidation, it is essential to package in O free atmospheres. In some cases, O2 will be included for the reasons previously mentioned in section 16.3 The use of CO, is however limited due to its solubility in water and fat. This
(TMA). TMA is the main active component responsible for the unpleasant ‘fishy’ odour. However, by introducing high levels of O2 in the gas atmosphere, the TMAO-reduction can be retarded, and consequently the shelf-life of the fish is increased . This was clearly demonstrated by Boskou and Debevere (1997, 1998). Therefore, packaging atmospheres for lean marine fish should contain oxygen levels of at least 30%. 16.3.3 Avoiding anaerobic respiration of fresh produce When fresh produce is packaged in a closed packaging system, it continues to respire. It is of great importance to avoid anaerobic conditions in the package of fresh produce because anaerobic respiration of the plant tissue will result in the production of off-odour compounds such as ethanol and acetaldehyde. The techniques applied to maintain an aerobic atmosphere in the packaging of fresh produce are discussed in detail in section 16.4.2. 16.4 Applications of MAP in the food industry 16.4.1 Non-respiring products Non-respiring food products do not consume any oxygen during further storage. When such food products are packaged in a modified atmosphere, the aim is to retain the introduced atmosphere during the storage period. Therefore, high barrier films are used which are most often composed out of different layers of materials. Typical O2 and CO2 barrier materials are PA (polyamide), PVDC (polyvinylidenechloride) and EVOH (ethylenevinyl alcohol). Depending on the intended storage time, the O2-permeability of the applied films should be <2 ml O2/m2 .24h.atm determined at 75% relative humidity at 23 °C for products with a long shelf life and <10 ml O2/m2 .24h.atm determined at the same conditions for products with a limited shelf life (<1 week). One of the bottlenecks in modified atmosphere packaging lies in defining the optimal gas atmosphere for a food product in a specific packaging design. This optimal atmosphere depends on the intrinsic parameters of the food product (pH, water activity, fat content, type of fat) and the gas/product volume ratio in the chosen package type. The intrinsic parameters determine the sensitivity of the product for specific microbial, chemical and enzymatic degradation reactions. Products that are susceptible to microbial spoilage due to the development of Gram-negative bacteria (e.g. fresh meat and fish) and yeasts (salads) should be packaged in a CO2 enriched atmosphere because the growth of those microorganisms is significantly retarded by CO2. In general, oxygen is excluded from the gas mixture. For prolonging the shelf life of products which are spoiled by mould growth (e.g. hard cheeses) or by oxidation, it is essential to package in O2 free atmospheres. In some cases, O2 will be included for the reasons previously mentioned in section 16.3. The use of CO2 is however limited due to its solubility in water and fat. This 346 The nutrition handbook for food processors