16 modified atmosphere packaging MAP

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

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 foodproducts in gas-barrier materials, in which the gaseous environment has beenchanged’ (Young et al, 1988) Because of its substantial shelf-life extendingeffect, MAP has been one of the most significant and innovative growth areas inretail food packaging over the past two decades The potential advantages anddisadvantages 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 mixturesfor different food products However, there is still a lack of scientific detailregarding 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 CO2on 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 anatmosphere that is different from air This atmosphere can be altered in four different ways:

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 oxygenabsorbers and CO2emitting films or sachets

The gases that are applied in MAP today are basically O2, CO2and N2 Thelast has no specific preservative effect but functions mainly as a filler gas to avoidthe collapse that takes place when CO2dissolves in the food product The func-tions of CO2 and O2will 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 inapplied gas mixtures When CO2is introduced into the package, it is partly dis-solved 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

Table 16.1 The potential positive and negative effects MAP has on the food industry

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

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.

inhibition of microorganisms in modified atmospheres is determined by the centration of dissolved CO2in the water phase.

con-The effect of the gaseous environment on microorganisms in foods is not aswell understood by microbiologists and food technologists as are other externalfactors, such as pH and aw Despite numerous reports of the effects of CO2

on microbial growth and metabolism, the ‘mechanism’ of CO2inhibition stillremains unclear (Dixon and Kell, 1989; Day, 2000) The question of whether anyspecific 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 CO2is applied to a biological tissue, it first dissolves in theliquid phase, where hydration and dissociation lead to a rapid pH decrease in thetissue 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 incooked meat products only amounted to 0.3 pH units when 80% of CO2wasapplied 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

CO2could not solely be explained by the acidification of the substrate (Becker,1933; Coyne, 1933)

Many researchers have documented the rapidity with which CO2in solutionpenetrates into the cell Krogh (1919) discovered that this rate is 30 times fasterthan for oxygen (O2), under most circumstances Wolfe (1980) suggested theinhibitory effects of CO2are the result of internal acidification of the cytoplasm.Eklund (1984) supported this idea by pointing out that the growth inhibition offour bacteria obtained with CO2had the same general form as that obtained withweak 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 CO2concentration

CO2may 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 tocause 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 (Jones and Greenfield, 1982; Dixon and Kell,1989) that the primary sites where CO2exerts its effects are the enzymatic car-boxylation and decarboxylation reactions, although inhibition of other enzymeshas also been reported (Jones and Greenfield, 1982)

Another possible factor contributing to the growth-inhibitory effect of CO2could be an alteration of the membrane properties (Daniels et al, 1985; Dixonand Kell, 1989) It was suggested that CO interacts with lipids in the cell mem-

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 CO2enriched atmosphere on the growth ofmicroorganisms are often difficult to compare because of the lack of informationregarding the packaging configurations applied The gas/product volume ratio andthe permeability of the applied film for O2and CO2will influence the amount

of dissolved CO2and thus the microbial inhibition of the atmosphere For thisreason, the concentration of dissolved CO2 in the aqueous phase of the foodshould 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 spoilagemicroorganisms Gill and Tan (1980) compared the effect of CO2on the growth

of some fresh meat spoilage bacteria at 30 °C Molin (1983) determined the tance to CO2 of several food spoilage bacteria Boskou and Debevere (1997;1998)investigated the effect of CO2 on the growth and trimethylamine production

resis-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 sen-sitivity and lactic acid bacteria are the most resistant Most yeasts and mouldsare also sensitive to CO2 The effect of CO2on psychrotrophic food pathogens isdiscussed 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 traitfor European consumers It is therefore essential that O2is included (e.g 40%)into the applied gas atmosphere when fresh meat, destined for the consumer, ispackaged 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 freeatmosphere

16.3.2 Inhibition of the reduction of trimethylamineoxide (TMAO)

in marine fish

Marine fish contain TMAO, which is an osmo-regulator In O2poor 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

(TMA) TMA is the main active component responsible for the unpleasant ‘fishy’odour However, by introducing high levels of O2 in the gas atmosphere, theTMAO-reduction can be retarded, and consequently the shelf-life of the fish isincreased This was clearly demonstrated by Boskou and Debevere (1997, 1998).Therefore, packaging atmospheres for lean marine fish should contain oxygenlevels 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 torespire It is of great importance to avoid anaerobic conditions in the package offresh produce because anaerobic respiration of the plant tissue will result in theproduction of off-odour compounds such as ethanol and acetaldehyde The tech-niques applied to maintain an aerobic atmosphere in the packaging of freshproduce are discussed in detail in section 16.4.2

16.4 Applications of MAP in the food industry

O2/m2.24h.atm determined at 75% relative humidity at 23 °C for products with along shelf life and <10 ml O2/m2.24h.atm determined at the same conditions forproducts with a limited shelf life (<1 week)

One of the bottlenecks in modified atmosphere packaging lies in defining theoptimal gas atmosphere for a food product in a specific packaging design Thisoptimal 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 thechosen package type The intrinsic parameters determine the sensitivity of theproduct for specific microbial, chemical and enzymatic degradation reactions.Products that are susceptible to microbial spoilage due to the development ofGram-negative bacteria (e.g fresh meat and fish) and yeasts (salads) should

be packaged in a CO2enriched atmosphere because the growth of those organisms is significantly retarded by CO2 In general, oxygen is excluded fromthe gas mixture For prolonging the shelf life of products which are spoiled bymould growth (e.g hard cheeses) or by oxidation, it is essential to package in O2free atmospheres In some cases, O2will be included for the reasons previouslymentioned in section 16.3

micro-The use of CO is however limited due to its solubility in water and fat This

high solubility can cause collapsing of the package when the concentrations of

CO2are too high This will especially be the case for food products containinghigh amounts of unsaturated fat such as smoked salmon and salads that containmayonnaise The influence of pH, temperature, fat content, water activity andgas/product ratio on the CO2solubility has been quantified by Devlieghere et al(1998) Moreover, too high CO2concentrations in the atmosphere can lead to anincreased drip loss during storage This can be explained by the pH drop induced

by CO2dissolving in the water phase of the product, causing a decrease in thewater 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 specificpurpose 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 activelyafter harvesting A packaging technology, used for prolonging the shelf life ofrespiring products, is Equilibrium Modified Atmosphere Packaging (EMAP) The air around the commodity is replaced by a gas combination of 1–5% O2and3–10% CO2with the balance made up of N2 Inside the package, an equilibriumbecomes established, when the O2transmission rate (OTR) of the packaging film

is matched by the O2consumption rate of the packaged commodity The 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 CO transmission rate

respira-Table 16.2 Recommended gas regimes for MAP of various non-respiring foods

Fish

Meat and fish products

a w > 0.94 50–70 30–50 0 ´ Gram +

a w < 0.94 10–20 80–90 0 ´ Yeasts and moulds

Cheese

0 100 0 ´ Moulds, ´ oxidation

(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 andprolongs fruit shelf life (Kader et al, 1989)

The modified atmosphere not only reduces the respiration rate and the ing behaviour of fruit, but it also maintains the general structure and turgidity ofthe plant tissue for a much longer period, which results in better protection againstmicrobial invasion This atmosphere is also thought to inhibit the growth ofspoilage microorganisms (Farber, 1991), which is mostly due to the low O2con-centration, because the elevated CO2concentration (<10%) inside the package isnot sufficiently high enough to act as an antimicrobial (Bennik et al, 1998) Theshelf life is also prolonged by the suppression of the enzymatic browning reac-tions on cut surfaces (Kader et al, 1989, Jacxsens et al, 1999a)

ripen-Regarding the relatively short shelf life of fruits, raw vegetables, and cut vegetables, an active modification of the atmosphere is preferred, compared

fresh-to a passive modification, which is caused by the produce respiring Seal (FFS) machines are used with a flushing system to obtain the optimal mod-ified atmosphere for packaging this type of product

Form-Fill-The attained EMAs are influenced by produce respiration (which in turn isaffected by product type, temperature, variety, size, maturity, and processingmethod), packaging film permeability (OTR, CO2TR, and C2H4TR), packagedimensions, and fill weight Consequently, it is a very complex procedure

to establish an optimal EMA for different items of produce The current edge of EMAP of fruits and vegetables is mainly empirical, but a systematic ap-proach for designing optimal EMA packages for minimally processed fruits andvegetables 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 andpermeability of the packaging film are temperature dependent The respirationrate 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

knowl-When temperature increases, a larger volume of O2 will be consumed by thefresh-cut produce than is diffused through the packaging film, resulting in a shift

of the EMA towards an anaerobic atmosphere (<1% O2and >10% CO2) obic atmospheres must be avoided in EMAP of respiring products because theshift towards anaerobic respiration will cause the formation of ethanol, acetal-dehyde, off-flavours, and off-odours At lower temperatures, the O2 level willincrease (>5%) in the EMA package and the benefits of EMA are lost Changingtemperatures during the transport, distribution, or storage of EMA packages will

Anaer-result in an equilibrium O2level inside the packages that differs from the optimal3% A lack of OTR and CO2TR of commercial films adapted to the needs ofmiddle and high respiring products can result in undesirable anaerobic atmos-pheres When both gas fluxes cannot be matched, the O2flux should take prior-ity because it is the limiting factor in EMA packaging A decreased O2content ismore effective in inhibiting respiration rate and decay than is a decreased CO2concentration (Kader et al, 1989; Bennik et al, 1995) New types of packagingfilms, 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 forpackaging products with medium or high respiration rates (Exama et al, 1993).Jacxsens et al (2000) proposed an integrated model in which the design of anoptimal EMA package for fresh-cut produce and fruits is possible, taking intoconsideration the changing temperatures and O2/CO2concentrations inside thepackage A prediction of the equilibrium O2concentration inside the packages,designed to obtain 3% O2 at 7 °C, could be conducted between a temperaturerange of 2 to 15 °C These packages (3% O2at 7 °C) had acceptable O2concen-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 werenoticed

The application of high O2concentrations (i.e >70% O2) could overcome thedisadvantages of low O2modified 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 andinhibiting microbial growth (Day, 1996; Day, 2000; Day, 2001) Amanatidou et

Temperature ( ° C)

Respiration rate

Fig 16.1 Temperature dependence of the oxygen permeability and the respiration rate

of shredded chicory (Devlieghere et al, 2000c)

al (1999) screened microorganisms associated with the spoilage and safety ofminimally processed vegetables In general, exposure to high oxygen alone (80

to 90% O2, balance N2) did not inhibit microbial growth strongly and was highlyvariable A prolongation of the lag phase was more pronounced at higher O2con-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 andthereby reducing cell viability when oxidative stresses overwhelm cellular pro-tection systems Combined with an increased CO2concentration (10 to 20%), amore 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% O2inhibited the in vivo growth of Botrytis cinerea on straw-

berries Based on practical trials (best benefits on sensory quality and microbial effects), the recommended gas levels immediately after packaging are 80–95% O2and 5–20% N2 Carbon dioxide level increases naturally due toproduct respiration (Day, 2001; Jacxsens et al, 2001a) Exposure to high O2levelsmay stimulate, have no effect on or reduce rates of respiration of produce depend-ing on the commodity, maturity and ripeness stage, concentrations of O2, CO2and C2H4and time and temperature of storage (Kader and Ben-Yehoshua, 2000).Respiration intensity is directly correlated to the shelf life of produce (Kader et

anti-al, 1989) Therefore, the quantification of the effect of high O2levels on the piratory activity is necessary (Jacxsens et al, 2001a) To maximise the benefits of

res-a high O2atmosphere, it is desirable to maintain levels of >40% O2in the space and to build up CO2levels to 10–25%, depending on the type of packagedproduce These conditions can be obtained by altering packaging parameters such

head-as storage temperature, selected permeability for O2and CO2of the packagingfilm and reducing or increasing gas/product ratio (Day, 2001)

High O2MAP of vegetables is only commercialised in some specific cases,probably because of the lack of understanding of the basic biological mechanismsinvolved in inhibiting microbial growth, enzymatic browning and concerns aboutpossible 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 highrespiring products, such as strawberries or raspberries, it is better to combine high

O2atmospheres with a permeable film for O2and CO2, as applied in EMA aging, in order to prevent a too high accumulation of CO2(Jacxsens et al, 2001b)

pack-16.5 The microbial safety of MAP

Modified atmospheres containing CO2are effective in extending the shelf life

of many food products However, one major concern is the inhibition of mal aerobic spoilage bacteria and the possible growth of psychrotrophic food

nor-pathogens, which may result in the food becoming unsafe for consumption before

it appears to be organoleptically unacceptable Most of the pathogenic bacteriacan 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 O2in 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 O2levels of 2% and 4% (O’Connor-Shaw and Reyes, 2000).Dufresne et al (2000) also proposed that additional barriers, other than headspace

O2and film, need to be considered to ensure the safety of MAP trout fillets, ticularly at moderate temperature abuse conditions

par-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 significantlyaffected (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, amodified 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 filletsand 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 CO2enrichedatmosphere (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) alsoindicated that 100% CO2slows the growth rate of C botulinum, and that this

inhibitory effect is further enhanced with appropriate NaCl concentrations andchilled 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 cytogenes in food products, packaged under modified atmospheres, has been the

mono-focus of several, although in some cases contradicting, studies (Garcia de

Fernando et al, 1995) In general, L monocytogenes is not greatly inhibited by

CO2enriched atmospheres (Zhao et al, 1992) although when combined with otherfactors such as low temperature, decreased water activity and the addition of Nalactate the inhibiting effect of CO2is significant (Devlieghere et al, 2001) Liste-

ria growth in anaerobic CO2enriched atmospheres has been demonstrated in lamb

in an atmosphere of 50:50 CO2/N2, at 5 °C (Nychas, 1994); in frankfurter typesausages in atmospheres of distinct proportions of CO2/N2, at 4, 7, and 10 °C(Krämer and Baumgart, 1992) and in pork in an atmosphere of 40:60 CO2/N2, at

4 °C (Manu-Tawiah et al, 1993) However, other authors have not detected growth

in chicken anaerobically packaged in 30:70 CO2/N2, at 6 °C (Hart et al, 1991); in

75 : 25 CO2/N2at 4 °C (Wimpfheimer et al, 1990) and at 4 °C in 100% CO2in rawminced meat (Franco-Abuin et al, 1997) or in buffered tryptose broth (Szabo and

Cahill, 1998) Several investigations demonstrated possible growth of L

mono-cytogenes on modified atmosphere packaged fresh-cut vegetables, although the

results depended very much on the type of vegetables and the storage ture (Berrang et al, 1989a; Beuchat and Brackett, 1990; Omary et al, 1993; Carlin

tempera-et al, 1995; Carlin tempera-et al, 1996a and 1996b; Zhang and Farber, 1996; Juneja tempera-et al,1998; Bennik et al, 1999; Jacxsens et al, 1999a; Liao and Sapers, 1999; Thomas

et al, 1999; Castillejo-Rodriguez et al, 2000)

There is no agreement about the effect of incorporating O2in the atmosphere

on the antimicrobial activity of CO2on L monocytogenes (Garcia de Fernando

et al, 1995) However, this effect could be very important in practice, as the tence of residual O2levels after packaging, and the diffusion of O2through thepackaging film, can result in substantial O2levels during the storage of industri-ally ‘anaerobically’ modified atmosphere packaged food products Most publica-tions suggest there is a decrease in the inhibitory effect of CO2 on L.

exis-monocytogenes when O2is incorporated into the atmosphere Experiments on raw

chicken showed L monocytogenes failed to grow at 4, 10, and 27 °C, in an

anaer-obic atmosphere containing 75% CO2and 25% N2(Wimpfheimer et al, 1990).However, an aerobic atmosphere containing 72.5% CO2, 22.5% N2, and 5% O2

did not inhibit the growth of L monocytogenes, even at 4 °C L monocytogenes

was also only minimally inhibited on chicken legs, in an atmosphere containing10% O2and 90% CO2(Zeitoun and Debevere, 1991) There was no significantdifference in the inhibitory effect of CO2, between the range of 0% and 50%,when 1.5% O2, or 21% O2was present in the atmosphere of gas packaged brain

heart infusion agar plates (Bennik et al, 1995) When L monocytogenes was

cul-tured in buffered nutrient broth, at 7.5 °C, in atmospheres containing 30% CO2,with four different O2concentrations (0, 10, 20, and 40%), the results showedthat bacterial growth increased with the increasing O2 concentrations (Hendricksand Hotchkiss, 1997)

16.5.3 Yersinia enterocolitica

Yersinia enterocolitica is generally regarded as one of the most psychrotrophic

foodborne pathogens Growth of Y enterocolitica was reported in vacuum

pack-aged lamb at 0 °C (Doherty et al, 1995; Sheridan and Doherty, 1994; Sheridan

et al, 1992), beef at -2 °C (Gill and Reichel, 1989), pork at 4 °C (Bodnaruk andDraughon, 1998; Manu-Tawiah et al, 1993), fresh chicken breasts (Özbas et al,1997) and roast beef at 3 °C but not at -1.5 °C (Hudson et al, 1994).

CO2retards the growth of Y enterocolitica at refrigerated temperatures The

effect of CO2on the growth of Y enterocolitica has been described by several

authors Some of the results are shown in Table 16.3 Oxygen also seems to play

an inhibiting role on the growth of Y enterocolitica (Garcia de Fernando et al, 1995) To ensure total inhibition of Y enterocolitica in O2poor atmospheres and

at realistic temperatures throughout the cooling chain, high CO2concentrations

in the headspace are necessary

Table 16.3 Growth of Yersina enterocolitica in different atmospheres

16.5.4 Aeromonas spp.

Aeromonas species are able to multiply in food products stored in refrigerated

conditions Growth of A hydrophila has been detected at low temperatures in a

variety of vacuum packaged fresh products, such as chicken breasts at 3 °C(Özbas et al, 1996), lamb at 0 °C under high pH conditions (Doherty et al, 1996),and at -2 °C (Gill and Reichel, 1989), and in sliced roast beef at -1.5 °C (Hudson

et al, 1994) Devlieghere et al (2000a) developed a model, predicting the ence of temperature and CO2on the growth of A hydrophila Proliferation of A.

influ-hydrophila is greatly affected by CO2enriched atmospheres Some reports ing the effect of CO2on the growth of A hydrophila on meat are summarised in

regard-Table 16.4

In a study by Berrang et al (1989b), regarding controlled atmosphere storage

of broccoli, cauliflower and asparagus stored at 4 °C and 15 °C, fast proliferation

of A hydrophila was observed at both temperatures, but growth was not

signifi-cantly affected by gas atmosphere Garcia-Gimeno et al (1996) published the

sur-vival of A hydrophila on mixed vegetable salads (lettuce, red cabbage and

carrots) packaged under MA (initial 10% of O2–10% CO2, after 48h 0% O2–18%

CO2) and stored at 4 °C while at 15 °C a fast growth was noticed (5 log units

in 24 h) The combination of high CO2concentration and low temperature wererevealed as responsible for the inhibition of growth Bennik et al (1995) con-cluded from their solid-surface model that at MA-conditions, generally appliedfor minimally processed vegetables (1–5% O2 and 5–10% CO2), growth of A.

hydrophila is possible Growth was virtually the same under 1.5% and 21% O2

The behaviour of a cocktail of A caviae (HG4) and A bestiarum (HG2) in air or

in low O2–low CO2atmosphere was investigated in fresh-cut vegetables: no ference between both atmospheres was observed on grated carrots, a decreasedgrowth on shredded Belgian endive and Brussels sprouts in MA but an increasedgrowth on shredded iceberg lettuce in MA storage (Jacxsens et al, 1999a)

dif-16.6 The effect of MAP on the nutritional quality of

non-respiring food products

Because by using modified atmosphere packaging, the shelf-life of the packagedproducts can be extended by 50–200% questions could arise regarding the nutri-tional consequences of MAP on the packaged food products This section willdiscuss the effect of MAP on the nutritional quality of non-respiring food prod-ucts while the effect of MAP on the nutritional value of respiring products, such

as fresh fruits and vegetables, will be discussed in detail in section 16.7

Very little information is available about the influence of MAP on the tional quality of non-respiring food products In most cases, for packaging non-respiring food products, oxygen is excluded from the atmosphere and thereforeone should expect a retardation of oxidative degradation reactions Moreover,modified atmosphere packaged food products should be stored under refrigera-

nutri-tion to allow CO2to dissolve and perform its antimicrobial action At these chilledconditions, chemical degradation reactions have only a limited importance.

No information is available regarding the nutritional consequences of enrichedoxygen concentrations in modified atmospheres which can be applied for pack-aging fresh meat and marine fish Some oxidative reactions can occur with nutri-tionally important compounds such as vitamins and polyunsaturated fatty acids.However, no quantitative information is available about these degradation reac-tions in products packaged in O enriched atmospheres

Table 16.4 Growth of Aeromonas hydrophila in different atmospheres