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

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 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 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 respira￾tion of the living plant tissue also results in the production of CO2, which dif￾fuses 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

48 The nutrition handbook for food processors (COTR). The type of packaging film selected is based on the film OTR and CO TR, which is required to obtain a desirable equilibrium modified atmosphere For packaging fruits, the film also needs to have a certain permeability for ethylene(C2H4), which prevents an accumulation of the ripening hormone and prolongs fruit shelf life(Kader et al, 1989) The modified atmosphere not only reduces the respiration rate and the ripen ing behaviour of fruit, but it also maintains the general structure and turgidity of the plant tissue for a much longer period, which results in better protection against microbial invasion. This atmosphere is also thought to inhibit the growth of spoilage microorganisms(Farber, 1991), which is mostly due to the low O2 con- centration, because the elevated COz concentration(<10%)inside the package is not sufficiently high enough to act as an antimicrobial (Bennik et al, 1998). The shelf life is also prolonged by the suppression of the enzymatic browning reac- tions on cut surfaces(Kader et al, 1989, Jacxsens et al, 1999a) egarding the relatively short shelf life of fruits, raw vegetables, and fresh cut vegetables, an active modification of the atmosphere is preferred, compared to a passive modification, which is caused by the produce respiring. Form-Fill- Seal (FFS)machines are used with a flushing system to obtain the optimal mod- ed atmosphere for packaging this type of product The attained EMAs are influenced by produce respiration(which in turn infected by product type, temperature, variety, size, maturity, and processing method), packaging film permeability (OTR, CO2TR, and C2HiTR), package dimensions, and fill weight. Consequently, it is a very complex procedure to establish an optimal EMA for different items of produce. The current knowl- edge of EMAP of fruits and vegetables is mainly empirical, but a systematic ap- proach for designing optimal EMA packages for minimally processed fruits and vegetables is proposed by a number of different authors(Exama et al, 1993 Peppelenbos, 1996: Jacxsens et al, 1999b; Jacxsens et al, 2000). Several mathe matical models have been published that predict the OTR and CO2TR of the aging film, which is necessary to obtain the desired equilibrium gas atmo h. 1994: Solomos. 1994: and Talasila et However, in these models an unrealistic constant storage temperature is assumed Two important parameters in EMAP of fresh-cut produce, respiration rate and permeability of the packaging film are temperature dependent. The respiration rate is less affected by the temperature change(Q10=2-3)than is the perme- ability of the packaging film(Q10= 1-2)(Exama et al, 1993; Jacxsens et al, 2000), as is illustrated in Fig. 16.1 When temperature increases, a larger volume of O2 will be consumed by the fresh-cut produce than is diffused through the packaging film, resulting in a shift of the EMA towards an anaerobic atmosphere(<%O2 and >10% CO2).Anaer obic atmospheres must be avoided in EMAP of respiring products because the shift towards anaerobic respiration will cause the formation of ethanol, acetal- dehyde, off-flavours, and off-odours. At lower temperatures, the O2 level will increase(>5%)in the EMA package and the benefits of EMA are lost. Changing temperatures during the transport, distribution, or storage of EMa packages will

(CO2TR). The type of packaging film selected is based on the film OTR and CO2TR, which is required to obtain a desirable equilibrium modified atmosphere. For packaging fruits, the film also needs to have a certain permeability for ethylene (C2H4), which prevents an accumulation of the ripening hormone and prolongs fruit shelf life (Kader et al, 1989). The modified atmosphere not only reduces the respiration rate and the ripen￾ing behaviour of fruit, but it also maintains the general structure and turgidity of the plant tissue for a much longer period, which results in better protection against microbial invasion. This atmosphere is also thought to inhibit the growth of spoilage microorganisms (Farber, 1991), which is mostly due to the low O2 con￾centration, because the elevated CO2 concentration (<10%) inside the package is not sufficiently high enough to act as an antimicrobial (Bennik et al, 1998). The shelf life is also prolonged by the suppression of the enzymatic browning reac￾tions on cut surfaces (Kader et al, 1989, Jacxsens et al, 1999a). Regarding the relatively short shelf life of fruits, raw vegetables, and fresh￾cut vegetables, an active modification of the atmosphere is preferred, compared to a passive modification, which is caused by the produce respiring. Form-Fill￾Seal (FFS) machines are used with a flushing system to obtain the optimal mod￾ified atmosphere for packaging this type of product. The attained EMAs are influenced by produce respiration (which in turn is affected by product type, temperature, variety, size, maturity, and processing method), packaging film permeability (OTR, CO2TR, and C2H4TR), package dimensions, and fill weight. Consequently, it is a very complex procedure to establish an optimal EMA for different items of produce. The current knowl￾edge of EMAP of fruits and vegetables is mainly empirical, but a systematic ap￾proach for designing optimal EMA packages for minimally processed fruits and vegetables is proposed by a number of different authors (Exama et al, 1993; Peppelenbos, 1996; Jacxsens et al, 1999b; Jacxsens et al, 2000). Several mathe￾matical models have been published that predict the OTR and CO2TR of the pack￾aging film, which is necessary to obtain the desired equilibrium gas atmosphere (Mannaperuma and Singh, 1994; Solomos, 1994; and Talasila et al, 1995). However, in these models an unrealistic constant storage temperature is assumed. Two important parameters in EMAP of fresh-cut produce, respiration rate and permeability of the packaging film are temperature dependent. The respiration rate is less affected by the temperature change (Q10R = 2–3) than is the perme￾ability of the packaging film (Q10P = 1–2) (Exama et al, 1993; Jacxsens et al, 2000), as is illustrated in Fig. 16.1. When temperature increases, a larger volume of O2 will be consumed by the fresh-cut produce than is diffused through the packaging film, resulting in a shift of the EMA towards an anaerobic atmosphere (<1% O2 and >10% CO2). Anaer￾obic atmospheres must be avoided in EMAP of respiring products because the shift towards anaerobic respiration will cause the formation of ethanol, acetal￾dehyde, off-flavours, and off-odours. At lower temperatures, the O2 level will increase (>5%) in the EMA package and the benefits of EMA are lost. Changing temperatures during the transport, distribution, or storage of EMA packages will 348 The nutrition handbook for food processors

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 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 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 (biaxi￾ally oriented 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 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 concen￾trations 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 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 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 con￾centrations. Amanatidou et al, (1999) as well as Kader and Ben-Yehoshua (2000) suggested that these high O2-levels could lead to intracellular generation of reac￾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% O2 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% 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, 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 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% O2 in the head￾space 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 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 CO2, as applied in EMA pack￾aging, 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 nor￾mal 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 psy￾chrotrophic 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, 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% 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. 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 Modified atmosphere packaging (MAP) 351