《食品加工营养手册》参考资料（英文版）16 Modified atmosphere packaging（MAP）.pdf_大学文库

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

Modified atmosphere packaging(MAP) 347 Table 16.2 Recommended gas regimes for MAP of various non-respiring foods as composition(%) Food type Purpose O, Fresh meat 0 60-85 <>" organisms(CO2)& Colour(O2) industrial packages ∈ Gram organisms 2010←Gram. colour fatty or fresh water 063530 30-40 6 Gram, TMA production 台Gram oxIdaton Meat and fish products ←Gram an<0.94 0台 Yeasts and moulds 台Gram-&Gram Cheese 0-700-300 台 Moulds,台 oxidation Bakery products 台 Yeasts& moulds Dry products(aw < 0.60) 0 100 0 Oxidation high solubility can cause collapsing of the package when the concentrations of CO2 are too high. This will especially be the case for food products containing high amounts of unsaturated fat such as smoked salmon and salads that contain mayonnaise. The influence of pH, temperature, fat content, water activity and gas/product ratio on the CO2 solubility has been quantified by Devlieghere et al (1998). Moreover, too high CO2 concentrations in the atmosphere can lead to an increased drip loss during storage. This can be explained by the pH drop induced by CO2 dissolving in the water phase of the product, causing a decrease in the water binding capacity of the proteins. Table 16.2 gives an overview of the rec ommended gas regimes for different non-respiring food products and the specific purpose of the gas mixture 16.4.2 Respiring products(Equilibrium Modified Atmosphere Packaging) In contrast to other types of food, fruits and vegetables continue to respire actively after harvesting. A packaging technology, used for prolonging the shelf life of respiring products, is Equilibrium Modified Atmosphere Packaging (EMAP The air around the commodity is replaced by a gas combination of 1-5% O2 and 10% CO2 with the balance made up of N2. Inside the package, an equilibrium becomes established, when the O2 transmission rate(OTR)of the packaging film is matched by the O2 consumption rate of the packaged commodity. The respira tion of the living plant tissue also results in the production of CO2, which dif- fuses through the packaging film, depending on the films CO2 transmission rate

high solubility can cause collapsing of the package when the concentrations of CO2 are too high. This will especially be the case for food products containing high amounts of unsaturated fat such as smoked salmon and salads that contain mayonnaise. The influence of pH, temperature, fat content, water activity and gas/product ratio on the CO2 solubility has been quantified by Devlieghere et al (1998). Moreover, too high CO2 concentrations in the atmosphere can lead to an increased drip loss during storage. This can be explained by the pH drop induced by CO2 dissolving in the water phase of the product, causing a decrease in the water binding capacity of the proteins. Table 16.2 gives an overview of the recommended gas regimes for different non-respiring food products and the specific purpose of the gas mixture. 16.4.2 Respiring products (Equilibrium Modified Atmosphere Packaging) In contrast to other types of food, fruits and vegetables continue to respire actively after harvesting. A packaging technology, used for prolonging the shelf life of respiring products, is Equilibrium Modified Atmosphere Packaging (EMAP). The air around the commodity is replaced by a gas combination of 1–5% O2 and 3–10% CO2 with the balance made up of N2. Inside the package, an equilibrium becomes established, when the O2 transmission rate (OTR) of the packaging film is matched by the O2 consumption rate of the packaged commodity. The respiration of the living plant tissue also results in the production of CO2, which diffuses through the packaging film, depending on the film’s CO2 transmission rate Modified atmosphere packaging (MAP) 347 Table 16.2 Recommended gas regimes for MAP of various non-respiring foods Food type Gas composition (%) Purpose CO2 N2 O2 Fresh meat retail 15–40 0 60–85 ´ Gram- organisms (CO2) & 20 10 70 Colour (O2) industrial packages 50–100 0–50 0 ´ Gram- organisms Poultry 70 20 10 ´ Gram- , colour Fish lean, marine 50–60 0–20 30–40 ´ Gram- , ´ TMA production fatty or fresh water 40–65 35–60 0 ´ Gram- , ´ oxidation Meat and fish products aw > 0.94 50–70 30–50 0 ´ Gram+ aw < 0.94 10–20 80–90 0 ´ Yeasts and moulds Shrimps 35 65 ´ Gram- & Gram+ Cheese hard 0–70 0–30 0 0 100 0 ´ Moulds, ´ oxidation soft 0 100 0 Bakery products 20–70 30–80 0 ´ Yeasts & moulds Dry products (aw < 0.60) 0 100 0 ´ Oxidation

Modified atmosphere packaging(MAP) 349 25t Film permeability 4000手 oEoE E Respiration rate 2000 10 1500 Fig 16.1 Temperature dependence of the oxygen permeability and the respiration rate of shredded chicory(Devlieghere et al, 2000c) result in an equilibrium O2 level inside the packages that differs from the optimal 3%0. A lack of OTR and CO,TR of commercial films adapted to the needs of middle and high respiring products can result in undesirable anaerobic atmos pheres. When both gas fluxes cannot be matched, the O2 flux should take prior ity because it is the limiting factor in EMA packaging. A decreased O2 content is more effective in inhibiting respiration rate and decay than is a decreased CO2 concentration(Kader et al, 1989; Bennik et al, 1995). New types of packagin films, with an OtR that is adaptable to the needs of fresh cut packaged produce, offer new possibilities in replacing OPP (oriented polypropylene), BOPP(bixi- tly used polypropylene), or LDPE (low density polyethylene)that are cur rently used in the industry and from which the OTR is not high enough for packaging products with medium or high respiration rates(Exama et al, 1993) Jacxsens et al (2000) proposed an integrated model in which the design of an ptimal EMa package for fresh-cut produce and fruits is possible, taking into consideration the changing temperatures and Oy/CO2 concentrations inside the package. A prediction of the equilibrium O2 concentration inside the packages designed to obtain 3%O2 at 7C, could be conducted between a temperature range of 2 to 15C. These packages(3%O2 at 7C)had acceptable O2 concen- trations between 2 and 10C. However, above 10C an increase in the growth of spoilage microorganisms and a sharp decrease in sensorial quality were noticed The application of high O2 concentrations (i.e. >70% O2)could overcome the disadvantages of low O, modified atmosphere packaging(EMA) for some ready to-eat vegetables. High O2 was found to be particularly effective in inhibit- ing enzymatic discolouration, preventing anaerobic fermentation reactions and inhibiting microbial growth(Day, 1996; Day, 2000: Day, 2001). Amanatidou et

result in an equilibrium O2 level inside the packages that differs from the optimal 3%. A lack of OTR and CO2TR of commercial films adapted to the needs of middle and high respiring products can result in undesirable anaerobic atmospheres. When both gas fluxes cannot be matched, the O2 flux should take priority because it is the limiting factor in EMA packaging. A decreased O2 content is more effective in inhibiting respiration rate and decay than is a decreased CO2 concentration (Kader et al, 1989; Bennik et al, 1995). New types of packaging films, with an OTR that is adaptable to the needs of fresh cut packaged produce, offer new possibilities in replacing OPP (oriented polypropylene), BOPP (biaxially oriented polypropylene), or LDPE (low density polyethylene) that are currently used in the industry and from which the OTR is not high enough for packaging products with medium or high respiration rates (Exama et al, 1993). Jacxsens et al (2000) proposed an integrated model in which the design of an optimal EMA package for fresh-cut produce and fruits is possible, taking into consideration the changing temperatures and O2/CO2 concentrations inside the package. A prediction of the equilibrium O2 concentration inside the packages, designed to obtain 3% O2 at 7 °C, could be conducted between a temperature range of 2 to 15 °C. These packages (3% O2 at 7 °C) had acceptable O2 concentrations between 2 and 10 °C. However, above 10 °C an increase in the growth of spoilage microorganisms and a sharp decrease in sensorial quality were noticed. The application of high O2 concentrations (i.e. >70% O2) could overcome the disadvantages of low O2 modified atmosphere packaging (EMA) for some readyto-eat vegetables. High O2 was found to be particularly effective in inhibiting enzymatic discolouration, preventing anaerobic fermentation reactions and inhibiting microbial growth (Day, 1996; Day, 2000; Day, 2001). Amanatidou et Modified atmosphere packaging (MAP) 349 Film permeability 0 5 10 15 20 25 30 2 4 7 10 12 15 0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 Temperature (°C) Respiration rate (ml O2/kg.h) Film permeability (ml O2/m.24h.atm) Respiration rate Fig. 16.1 Temperature dependence of the oxygen permeability and the respiration rate of shredded chicory. (Devlieghere et al, 2000c)

350 The nutrition handbook for food processors al (1999) screened microorganisms associated with the spoilage and safety of minimally processed vegetables. In general, exposure to high oxygen alone(80 to 90%O2, balance N2) did not inhibit microbial growth strongly and was highly variable. A prolongation of the lag phase was more pronounced at higher O2 con- centrations. Amanatidou et al, (1999)as well as Kader and Ben-Yehoshua(2000) uggested that these high O2-levels could lead to intracellular generation of read tive oxygen species(ROS, O2, H2O2, OH*), damaging vital cell components and thereby reducing cell viability when oxidative stresses overwhelm cellular pro- tection systems. Combined with an increased CO2 concentration (10 to 20%),a more effective inhibitory effect on the growth of all microorganisms was noticed in comparison with the individual gases alone( Gonzalez Roncero and Day, 1998 Amanatidou et al, 1999; Amanatidou et al, 2000). Wszelaki and Mitcham(1999) found that 80-100%O, inhibited the in vivo growth of Botrytis cinerea on straw berries. Based on practical trials(best benefits on sensory quality and anti- microbial effects), the recommended gas levels immediately after packaging are 80-95% O2 and 5-20%o N2. Carbon dioxide level increases naturally due to product respiration(Day, 2001; Jacxsens et al, 2001a). Exposure to high O2 levels may stimulate, have no effect on or reduce rates of respiration of produce depend- ing on the commodity, maturity and ripeness stage, concentrations of O2, CO and C2 Ha and time and temperature of storage(Kader and Ben-Yehoshua, 2000) Respiration intensity is directly correlated to the shelf life of produce(Kader et al, 1989). Therefore, the quantification of the effect of high O2 levels on the res- piratory activity is necessary (Jacxsens et al, 2001a). To maximise the benefits of a high O2 atmosphere, it is desirable to maintain levels of >40%0O2 in the head space and to build up CO2 levels to 10-25%b, depending on the type of packaged produce. These conditions can be obtained by altering packaging parameters such Ls storage temperature, selected permeability for O2 and CO2 of the packaging film and reducing or increasing gas/product ratio(Day, 2001) High O2 MAP of vegetables is only commercialised in some specific cases, probably because of the lack of understanding of the basic biological mechanisms involved in inhibiting microbial growth, enzymatic browning and concerns about possible safety implications. Concentrations higher than 25% O2 are consid- ered to be explosive and special precautions have to be taken on the work floor BCGA, 1998). In order to keep the high oxygen inside the package, it is advised to apply barrier films or low permeable OPP films(Day, 2001). However, for high respiring products, such as strawberries or raspberries, it is better to combine high O2 atmospheres with a permeable film for O2 and COz, as applied in EMA pack aging, in order to prevent a too high accumulation of coz ( Jacxsens et al, 2001b) 16.5 The microbial safety of MAP Modified atmospheres containing CO2 are effective in extending the shelf life of many food products. However, one major concern is the inhibition of nor mal aerobic spoilage bacteria and the possible growth of psychrotrophic food

al (1999) screened microorganisms associated with the spoilage and safety of minimally processed vegetables. In general, exposure to high oxygen alone (80 to 90% O2, balance N2) did not inhibit microbial growth strongly and was highly variable. A prolongation of the lag phase was more pronounced at higher O2 concentrations. Amanatidou et al, (1999) as well as Kader and Ben-Yehoshua (2000) suggested that these high O2-levels could lead to intracellular generation of reactive oxygen species (ROS, O2 - , H2O2, OH*), damaging vital cell components and thereby reducing cell viability when oxidative stresses overwhelm cellular protection systems. Combined with an increased CO2 concentration (10 to 20%), a more effective inhibitory effect on the growth of all microorganisms was noticed in comparison with the individual gases alone (Gonzalez Roncero and Day, 1998; Amanatidou et al, 1999; Amanatidou et al, 2000). Wszelaki and Mitcham (1999) found that 80–100% O2 inhibited the in vivo growth of Botrytis cinerea on strawberries. Based on practical trials (best benefits on sensory quality and antimicrobial effects), the recommended gas levels immediately after packaging are 80–95% O2 and 5–20% N2. Carbon dioxide level increases naturally due to product respiration (Day, 2001; Jacxsens et al, 2001a). Exposure to high O2 levels may stimulate, have no effect on or reduce rates of respiration of produce depending on the commodity, maturity and ripeness stage, concentrations of O2, CO2 and C2 H4 and time and temperature of storage (Kader and Ben-Yehoshua, 2000). Respiration intensity is directly correlated to the shelf life of produce (Kader et al, 1989). Therefore, the quantification of the effect of high O2 levels on the respiratory activity is necessary (Jacxsens et al, 2001a). To maximise the benefits of a high O2 atmosphere, it is desirable to maintain levels of >40% O2 in the headspace and to build up CO2 levels to 10–25%, depending on the type of packaged produce. These conditions can be obtained by altering packaging parameters such as storage temperature, selected permeability for O2 and CO2 of the packaging film and reducing or increasing gas/product ratio (Day, 2001). High O2 MAP of vegetables is only commercialised in some specific cases, probably because of the lack of understanding of the basic biological mechanisms involved in inhibiting microbial growth, enzymatic browning and concerns about possible safety implications. Concentrations higher than 25% O2 are considered to be explosive and special precautions have to be taken on the work floor (BCGA, 1998). In order to keep the high oxygen inside the package, it is advised to apply barrier films or low permeable OPP films (Day, 2001). However, for high respiring products, such as strawberries or raspberries, it is better to combine high O2 atmospheres with a permeable film for O2 and CO2, as applied in EMA packaging, in order to prevent a too high accumulation of CO2 (Jacxsens et al, 2001b). 16.5 The microbial safety of MAP Modified atmospheres containing CO2 are effective in extending the shelf life of many food products. However, one major concern is the inhibition of normal aerobic spoilage bacteria and the possible growth of psychrotrophic food 350 The nutrition handbook for food processors

Modified atmosphere packaging (MAP) 351 pathogens, which may result in the food becoming unsafe for consumption before it appears to be organoleptically unacceptable. Most of the pathogenic bacteria can be inhibited by low temperatures(<7C). At these conditions, only psy- chrotrophic pathogens can proliferate. The effect of CO2 on the different psychrotrophic foodborne pathogens is described belo 16.5.1 Clostridium botulinum Dne major concern is the suitability of MAP in the food industry. This is mainly due to the possibility that psychrotrophic, non-proteolytic strains of C. botulinum types B, E, and F are able to grow and produce toxins under MAP conditions Little is known about the effects of modified atmosphere storage conditions on toxin production by C. botulinum. The possibility of inhibiting C botulinum by incorporating low levels of O2 in the package does not appear to be feasible. Miller(1988, cited by Connor et al, 1989)reported that psychrotrophic strains of C. botulinum are able to produce toxins in an environment with up to 10%0 O2 Toxin production by C. botulinum type E, prior to spoilage, has been described in 3 types of fish, at O2 levels of 2% and 4%(O'Connor-Shaw and Reyes, 2000) Dufresne et al (2000)also proposed that additional barriers, other than headspace O2 and film, need to be considered to ensure the safety of MAP trout fillets, par ticularly at moderate temperature abuse conditions The probability of one spore of non-proteolytic C. botulinum( types B, E, and F)being toxicogenic in rock fish was outlined in a report by Ikawa and Genigeorgis (1987). The results showed that the toxigenicity was significantly affected(P<0.005) by temperature and storage time, but not by the used modi fied atmosphere(vacuum, 100% CO2, or 70% CO2 30%0 air). In Tilapia fillets, a modified atmosphere(75% CO2/25% N2), at 8C, delayed toxin formation by C botulinum type E, from 17 to 40 days, when compared to vacuum packaged fillets (Reddy et al, 1996). Similar inhibiting effects were recorded for salmon fillet and catfish fillets, at 4C (Reddy et al, 1997a and 1997b). Toxin production from non-proteolytic C. botulinum type B spores was also retarded by a CO2 enriched atmosphere(30% CO/70% N2)in cooked turkey at 4C but not at 10 C nor at 15C (Lawlor et al, 2000). Recent results in a study by Gibson et al(2000)also ndicated that 100% CO2 slows the growth rate of C. botulinum, and that this inhibitory effect is further enhanced with appropriate Nacl concentrations and chilled temperatures 16.5.2 Listeria monocytogenes Listeria monocytogenes is considered a psychrotrophic foodborne pathogen Growth is possible at 1C( Varnam and Evans, 1991)and has even been reported at temperatures as low as -15C(Hudson et al, 1994). The growth of L. mono- cytogenes in food products, packaged under modified atmospheres, has been the focus of several, although in some cases contradicting, studies ( Garcia de Fernando et al, 1995). In general, L. monocytogenes is not greatly inhibited by

pathogens, which may result in the food becoming unsafe for consumption before it appears to be organoleptically unacceptable. Most of the pathogenic bacteria can be inhibited by low temperatures (<7 °C). At these conditions, only psychrotrophic pathogens can proliferate. The effect of CO2 on the different psychrotrophic foodborne pathogens is described below. 16.5.1 Clostridium botulinum One major concern is the suitability of MAP in the food industry. This is mainly due to the possibility that psychrotrophic, non-proteolytic strains of C. botulinum types B, E, and F are able to grow and produce toxins under MAP conditions. Little is known about the effects of modified atmosphere storage conditions on toxin production by C. botulinum. The possibility of inhibiting C. botulinum by incorporating low levels of O2 in the package does not appear to be feasible. Miller (1988, cited by Connor et al, 1989) reported that psychrotrophic strains of C. botulinum are able to produce toxins in an environment with up to 10% O2. Toxin production by C. botulinum type E, prior to spoilage, has been described in 3 types of fish, at O2 levels of 2% and 4% (O’Connor-Shaw and Reyes, 2000). Dufresne et al (2000) also proposed that additional barriers, other than headspace O2 and film, need to be considered to ensure the safety of MAP trout fillets, particularly at moderate temperature abuse conditions. The probability of one spore of non-proteolytic C. botulinum (types B, E, and F) being toxicogenic in rock fish was outlined in a report by Ikawa and Genigeorgis (1987). The results showed that the toxigenicity was significantly affected (P < 0.005) by temperature and storage time, but not by the used modi- fied atmosphere (vacuum, 100% CO2, or 70% CO2/30% air). In Tilapia fillets, a modified atmosphere (75% CO2/25% N2), at 8 °C, delayed toxin formation by C. botulinum type E, from 17 to 40 days, when compared to vacuum packaged fillets (Reddy et al, 1996). Similar inhibiting effects were recorded for salmon fillets and catfish fillets, at 4 °C (Reddy et al, 1997a and 1997b). Toxin production from non-proteolytic C. botulinum type B spores was also retarded by a CO2 enriched atmosphere (30% CO2/70% N2) in cooked turkey at 4 °C but not at 10 °C nor at 15 °C (Lawlor et al, 2000). Recent results in a study by Gibson et al (2000) also indicated that 100% CO2 slows the growth rate of C. botulinum, and that this inhibitory effect is further enhanced with appropriate NaCl concentrations and chilled temperatures. 16.5.2 Listeria monocytogenes Listeria monocytogenes is considered a psychrotrophic foodborne pathogen. Growth is possible at 1 °C (Varnam and Evans, 1991) and has even been reported at temperatures as low as -1.5 °C (Hudson et al, 1994). The growth of L. monocytogenes in food products, packaged under modified atmospheres, has been the focus of several, although in some cases contradicting, studies (Garcia de Fernando et al, 1995). In general, L. monocytogenes is not greatly inhibited by Modified atmosphere packaging (MAP) 351