Combining map with other preservation techniques J.T. Rosnes, M. Sivertsvik and T Skara, NORCONSERV, Norway 14.1 Introduction Modified atmosphere packaging(MAP)is widely used for many food products and is now a commercial and economic reality. MAP is common in markets that have a well established and controlled cold chain and that can sustain a high- priced quality product. However, MAP is a mild preservation method and a major concern is that MAP storage may not provide a sufficient level of safety for the extended storage of fresh chilled food products with regard to pathogenic bacteria. Other preservation steps may be necessary, in addition to MA packaging and low temperatures, in order to delay outgrowth of pathogens or toxin production beyond their point of spoilage. One feasible solution can be to use a combination of different preservation factors or techniques. This approach rovides reliable, yet mild, multi-targeted preservation of foods, thereby facilitating improvements in food safety, quality and economics. The topic of this chapter is to outline the significance of combining MAP with other enervative techni Food environments are generally stressful for bacteria because most nutrients are in the form of complex substrates whereby the conditions for bacterial growth are not optimal. The level of free moisture may be restricted and the presence of acids and other chemicals may be at stressful levels. In addition, there is often competition from other microorganisms which are present Replacing the normal atmosphere with a modified atmosphere, i.e., other concentrations of Oz, CO, and N2, will add additional stress to microorganisms and change the composition of the initial microbial flora. From the early development of MAP(Coyne, 1932; Coyne, 1933), it has been shown that MA can, on its own, inhibit growth of microorganisms. Higher levels of CO2 have a
14.1 Introduction Modified atmosphere packaging (MAP) is widely used for many food products and is now a commercial and economic reality. MAP is common in markets that have a well established and controlled cold chain and that can sustain a highpriced quality product. However, MAP is a mild preservation method and a major concern is that MAP storage may not provide a sufficient level of safety for the extended storage of fresh chilled food products with regard to pathogenic bacteria. Other preservation steps may be necessary, in addition to MA packaging and low temperatures, in order to delay outgrowth of pathogens or toxin production beyond their point of spoilage. One feasible solution can be to use a combination of different preservation factors or techniques. This approach provides reliable, yet mild, multi-targeted preservation of foods, thereby facilitating improvements in food safety, quality and economics. The topic of this chapter is to outline the significance of combining MAP with other preservative techniques. Food environments are generally stressful for bacteria because most nutrients are in the form of complex substrates whereby the conditions for bacterial growth are not optimal. The level of free moisture may be restricted and the presence of acids and other chemicals may be at stressful levels. In addition, there is often competition from other microorganisms which are present. Replacing the normal atmosphere with a modified atmosphere, i.e., other concentrations of O2, CO2 and N2, will add additional stress to microorganisms and change the composition of the initial microbial flora. From the early development of MAP (Coyne, 1932; Coyne, 1933), it has been shown that MA can, on its own, inhibit growth of microorganisms. Higher levels of CO2 have a 14 Combining MAP with other preservation techniques J.T. Rosnes, M. Sivertsvik and T. Ska˚ra, NORCONSERV, Norway
288 Novel food packaging techniques bacteriostatic effect on microorganisms and properly designed MAP can double a product's shelf-life(Davies, 1995). In spite of 70 years of knowledge about CO2 inhibition it is only in the last two decades that MAP has become a widel commercially used technology for storage and distribution of foods. This trend mainly driven by the demands of modern consumers for pre-processed products that have a fresh appearance and are convenient and easy to prepare. The main focus in this chapter will therefore be on chilled Ma packaged products, where pronounced effects of modified atmosphere packaging combined with preserva tion factors can be seen The potential of MAP to extend shelf-life for many foods is well documented g, fish (Dalgaard et al, 1993), sandwiches(Farber, 1991), salads and vegetables(Day, 1990), and meat(Gill, 1996). Several review articles outline the different aspects of Ma packaging(Farber, 1991; Church and Parsons, 1995 Davies, 1995; Phillips, 1996 Sivertsvik et al., 2002). A major concern associated with the use of MAP is that of product safety. The desired suppression of spoilage microorganisms extends the shelf-life if compared to food products stored in a normal air environment, and this may create opportunities for slower growing pathogenic bacteria. In particular the growth of psychrotrophic pathogens in refrigerated ready-to-eat food may create a health risk before the product is overtly spoiled( Farber, 1991) ince some preservation procedures (e.g. chemical additives) used in food products act by inhibiting growth, instead of inactivating their contribution may be most beneficial when used against pathogens that form toxins in foods, or those that need to reach high numbers to cause foodborne illness especially in healthy consumers. However, in order to protect consumers at risk from foodborne illness or against microbes with low infectious doses, there is a need for complete inactivation of pathogens and avoidance of recontamination of foods during processing, distribution, and preparation for consumption. For each specific MA-packaged product this must be done either before packaging or later by adjusting to correct preservation intensity in the product 14.2 Combining MAP with other preservative techniques The preservation of almost all foods in industrialised and developing countries is based on combinations of several factors that secure microbial safety, stability and sensory quality. This is true not only for traditional foods, but also for more novel products. The most important preservative methods in common use for food preservation are high temperature(heat treatment), low temperature, water activity(aw), acidity (pH), redox potential(Eh), some preservatives, and a competitive flora(Leistner, 1992). The application of some processes using the aforementioned preservation methods at low intensity or concentrations is still in the exploratory and developmental stages. Other methods have obtained egulatory approval and are being introduced in haCCP plans and in the marketplace for consumer evaluation and acceptance
bacteriostatic effect on microorganisms and properly designed MAP can double a product’s shelf-life (Davies, 1995). In spite of 70 years of knowledge about CO2 inhibition it is only in the last two decades that MAP has become a widely commercially used technology for storage and distribution of foods. This trend is mainly driven by the demands of modern consumers for pre-processed products that have a fresh appearance and are convenient and easy to prepare. The main focus in this chapter will therefore be on chilled MA packaged products, where pronounced effects of modified atmosphere packaging combined with preservation factors can be seen. The potential of MAP to extend shelf-life for many foods is well documented, e.g., fish (Dalgaard et al., 1993), sandwiches (Farber, 1991), salads and vegetables (Day, 1990), and meat (Gill, 1996). Several review articles outline the different aspects of MA packaging (Farber, 1991; Church and Parsons, 1995; Davies, 1995; Phillips, 1996; Sivertsvik et al., 2002). A major concern associated with the use of MAP is that of product safety. The desired suppression of spoilage microorganisms extends the shelf-life if compared to food products stored in a normal air environment, and this may create opportunities for slower growing pathogenic bacteria. In particular the growth of psychrotrophic pathogens in refrigerated ready-to-eat food may create a health risk before the product is overtly spoiled (Farber, 1991). Since some preservation procedures (e.g. chemical additives) used in food products act by inhibiting growth, instead of inactivating microorganisms, their contribution may be most beneficial when used against pathogens that form toxins in foods, or those that need to reach high numbers to cause foodborne illness, especially in healthy consumers. However, in order to protect consumers at risk from foodborne illness or against microbes with low infectious doses, there is a need for complete inactivation of pathogens and avoidance of recontamination of foods during processing, distribution, and preparation for consumption. For each specific MA-packaged product this must be done either before packaging or later by adjusting to correct preservation intensity in the product. 14.2 Combining MAP with other preservative techniques The preservation of almost all foods in industrialised and developing countries is based on combinations of several factors that secure microbial safety, stability and sensory quality. This is true not only for traditional foods, but also for more novel products. The most important preservative methods in common use for food preservation are high temperature (heat treatment), low temperature, water activity (aw), acidity (pH), redox potential (Eh), some preservatives, and a competitive flora (Leistner, 1992). The application of some processes using the aforementioned preservation methods at low intensity or concentrations is still in the exploratory and developmental stages. Other methods have obtained regulatory approval and are being introduced in HACCP plans and in the marketplace for consumer evaluation and acceptance. 288 Novel food packaging techniques
Combining MAP with other preservation techniques 289 The principle of combined preservation has been well described by Leistner et al, and is often referred to as hurdle technology (Leistner, 1992; Leistner, 1995b, Leistner, 2002). Whilst the hurdle concept is widely accepted as a food preservation strategy, its potential, using MAP, has still to be fully realised. The intelligent selection of hurdles in terms of the number required, the intensity of each and the sequence of applications to achieve a specified outcome are expected to have significant potential for the future(McMeekin and Ross, 2002) Homeostatsis is the tendency towards uniformity and stability in the internal status of living organisms. For instance, the maintenance of a defined ph within narrow limits is a prerequisite and feature of all living cells, and this applies to higher organisms, as well as microorganisms. In food preservation the homeostasis of microorganisms is a key phenomenon because if homeostasis of these organisms is disturbed by some preservation methods in foods, they will not multiply, i.e. they will remain in the lag phase or may even die before their homeostasis is re-established. Therefore, in actual fact, the preservation of food is achieved by disturbing, temporarily or permanently, the homeostasis of microorganisms in the food. In most foods microorganisms are able to operate homeostatically in order to react to the environmental stresses imposed by the applied preservation procedures. Applying additional preservation will inhibit repair of disturbed homeostasis and this requires extra energy from the microorganisms concerned In MA products energy depletion increases as the intensity or concentration of preservation is increased and the restriction of the energy supply will inhibit the repair mechanisms of the microbial cells' factors and leads to growth inhibition or death 14.2.1 Preservation focused on specific groups of microorganisms If the true potential of some of the emerging preservation technologies, combined with MAP is to be realised, it will be important to develop systematic kinetic data describing their efficiency against key target microorganisms. The type and numbers of microorganisms in the raw material have a direct influence on the effectiveness of MAP in inhibiting both spoilage organisms and pathogens. When adding extra preservation to packaged food, it is therefore mportant to understand which part of the bacterial population is inhibited and which is not. The shelf-life extension obtained with ma does not always give the same extension in safety. Pathogenic bacteria may gain advantage when the competing flora is inhibited, e.g., Listeria monocytogenes increased in numbers on raw chicken in 72.5: 22.5: 5(CO2: N2: O2)atmosphere at 4C, irrespective of a decrease in the aerobic spoilage flora(Wimpfheimer et al., 1990). Many MA packaged products of meat, vegetable and sea-food origin have common key target organisms. For chilled products psychrotrophic pathogens are the target. while in heat-treated ready meals spore-forming Clostridium and Bacillus species are the target organisms. There are five food-borne pathogenic bacteria known to be capable of growth below 5C: Bacillus cereus, non-proteolytic Clostridium botulinum type E, B and F(group D), Listeria monocytogenes
The principle of combined preservation has been well described by Leistner et al., and is often referred to as hurdle technology (Leistner, 1992; Leistner, 1995b; Leistner, 2002). Whilst the hurdle concept is widely accepted as a food preservation strategy, its potential, using MAP, has still to be fully realised. The intelligent selection of hurdles in terms of the number required, the intensity of each and the sequence of applications to achieve a specified outcome are expected to have significant potential for the future (McMeekin and Ross, 2002). Homeostatsis is the tendency towards uniformity and stability in the internal status of living organisms. For instance, the maintenance of a defined pH within narrow limits is a prerequisite and feature of all living cells, and this applies to higher organisms, as well as microorganisms. In food preservation the homeostasis of microorganisms is a key phenomenon because if homeostasis of these organisms is disturbed by some preservation methods in foods, they will not multiply, i.e. they will remain in the lag phase or may even die before their homeostasis is re-established. Therefore, in actual fact, the preservation of food is achieved by disturbing, temporarily or permanently, the homeostasis of microorganisms in the food. In most foods microorganisms are able to operate homeostatically in order to react to the environmental stresses imposed by the applied preservation procedures. Applying additional preservation will inhibit repair of disturbed homeostasis and this requires extra energy from the microorganisms concerned. In MA products energy depletion increases as the intensity or concentration of preservation is increased and the restriction of the energy supply will inhibit the repair mechanisms of the microbial cells’ factors and leads to growth inhibition or death. 14.2.1 Preservation focused on specific groups of microorganisms If the true potential of some of the emerging preservation technologies, combined with MAP is to be realised, it will be important to develop systematic, kinetic data describing their efficiency against key target microorganisms. The type and numbers of microorganisms in the raw material have a direct influence on the effectiveness of MAP in inhibiting both spoilage organisms and pathogens. When adding extra preservation to packaged food, it is therefore important to understand which part of the bacterial population is inhibited and which is not. The shelf-life extension obtained with MA does not always give the same extension in safety. Pathogenic bacteria may gain advantage when the competing flora is inhibited, e.g., Listeria monocytogenes increased in numbers on raw chicken in 72.5:22.5:5 (CO2:N2:O2) atmosphere at 4ºC, irrespective of a decrease in the aerobic spoilage flora (Wimpfheimer et al., 1990). Many MA packaged products of meat, vegetable and sea-food origin have common key target organisms. For chilled products psychrotrophic pathogens are the target, while in heat-treated ready meals spore-forming Clostridium and Bacillus species are the target organisms. There are five food-borne pathogenic bacteria known to be capable of growth below 5ºC: Bacillus cereus, non-proteolytic Clostridium botulinum type E, B and F (group II), Listeria monocytogenes, Combining MAP with other preservation techniques 289
290 Novel food packaging techniques Table 14.1 Preservatives used to inhibit specific psychotropic pathogens in combination with map Organism Relevant food reservative References Bacillus cereus Dairy food (Koseki and Itoh, 2002) proteolytic Ready-to-eat food Irradiation (Lambert et al, 1991) Dinner (Lyver et al, 1998) Bacillus species Gibson et al, 2000) Listeria Fish, meat, Competitive (Liserre et al., 2002 monocytogenes vegetables, fresh microbial flora Wimpfheimer et al, 1990 Francis and O Beirne. 1998 Bennik et al, 1999) Nisin (Szabo and Cahill, 1998) Fang and Lin, 1994) Na-lactate ( Devlieghere et al, 2001 Pothuri et al, 1996) Irradiation Thayer and Boyd, 2000 Thayer and Boyd, 1999) Francis and O Beirne, 2001) Oregano essential (Tsigarida et al, 2000) High O2 level (Jacxsens et al., 2001) Yersinia Pork Barakat and Harris, 1999) enterocolitica Lactic acid (Grau,1981) Background flora (Kleinlein and Untermann, 1990) Low temperature (Gill and Reichel, 1989) aeromonas Fish, shellfish Heat Mussels Salmonella Poultry Sorbate (Elliott and Gray, 1981) Yersinia enterocolitica, and Aeromonas hydrophila. Consequently the ability of modified atmospheres to inhibit the growth of these organisms in foods under refrigerated storage is of vital importance and additional preservation factors have therefore been combined with MAP ( Table 14.1). The main cause of concern, however, is the possible growth of non-proteolytic C botulinum because it is both anaerobic and low-temperature tolerant. Of particular concern is the fact that it may grow and produce toxin on the product before spoilage is detectable to the consumer Few non-thermal treatments can currently be relied upon to inactivate bacterial spores. Hence low-temperature storage must be combined with an additional preservation hurdle such as acidic formulation or salt to prevent spore
Yersinia enterocolotica, and Aeromonas hydrophila. Consequently the ability of modified atmospheres to inhibit the growth of these organisms in foods under refrigerated storage is of vital importance and additional preservation factors have therefore been combined with MAP (Table 14.1). The main cause of concern, however, is the possible growth of non-proteolytic C.botulinum, because it is both anaerobic and low-temperature tolerant. Of particular concern is the fact that it may grow and produce toxin on the product before spoilage is detectable to the consumer. Few non-thermal treatments can currently be relied upon to inactivate bacterial spores. Hence low-temperature storage must be combined with an additional preservation hurdle such as acidic formulation or salt to prevent spore Table 14.1 Preservatives used to inhibit specific psychrotropic pathogens in combination with MAP Organism Relevant food Preservative References Bacillus cereus Dairy food Ready-to-eat food (Koseki and Itoh, 2002) Non-proteolytic Clostridium botulinum Ready-to-eat food Dinner Irradiation Microbial inhibition by Bacillus species NaCl (Lambert et al., 1991) (Lyver et al., 1998) (Gibson et al., 2000) Listeria monocytogenes Fish, meat, vegetables, fresh produce Competitive microbial flora Nisin Na-lactate Irradiation pH Oregano essential oils High O2 level (Liserre et al., 2002; Wimpfheimer et al., 1990; Francis and O’Beirne, 1998; Bennik et al., 1999) (Szabo and Cahill, 1998) (Fang and Lin, 1994) (Devlieghere et al., 2001; Pothuri et al., 1996) (Thayer and Boyd, 2000; Thayer and Boyd, 1999) (Francis and O’Beirne, 2001) (Tsigarida et al., 2000) (Jacxsens et al., 2001) Yersinia enterocolitica Pork Poultry Lactate Lactic acid (Barakat and Harris, 1999) (Grau, 1981) Background flora (Kleinlein and Untermann, 1990) Low temperature (Gill and Reichel, 1989) Aeromonas hydrophila Fish, shellfish Mussels Meat Heat pH (Devlieghere et al., 2000b) (Doherty et al., 1996) Salmonella Poultry Sorbate (Elliott and Gray, 1981) 290 Novel food packaging techniques
Combining MAP with other preservation techniques 291 outgrowth. Most food spoilage moulds species have an absolute requirement for oxygen and appear to be sensitive to high levels of CO2. Consequently foods with low aw values, such as bakery products, that are susceptible to spoilage by moulds can have their shelf-lives extended by MAP. Many yeasts are capable of growing in the complete absence of oxygen and most are comparatively resistant to CO2. Although MAP can inhibit the growth of bacterial and fungal spoilage microorganisms, its effect on the survival of enteric viruses, including hepatitis A viruses(HAV), has not been well investigated. Both mussels and lettuce that are packaged in MAP may be a vehicle in the transmission of hav(due to contact with contaminated water) and therefore can contribute to hepatitis A outbreaks(Cliver, 1997). Experiments by Bidawid et al.(2001)indicated that MAP does not influence HAV survival when present on the surface of produce with high CO2 levels. This may have been attributed to the inhibition of spoilage-causing enzymatic activities in the lettuce, which may have reduced exposure of viruses to potential toxic by-produo 14.2.2 Preventative techniques combined with MAP The main preservation techniques currently used act in one of three ways: (i) preventing the access of microorganisms to foods, (ii) inactivating them when they have gained access, or(iii) preventing or slowing down their growth when they have gained access and not been inactivated. During the past few years there has been increasing interest in modifying these approaches or in developing new ones, with the objective of reducing the severity of the more extreme techniques. Many such developments have involved new uses of existing techniques in new combinations to inhibit the growth of micro- organisms. Approaches where preservation techniques are used at lower intensity or at lower concentration, causes inactivation and bacterial growth inhibition to overlap. It is the safety level, the quality level or the outcome of inactivation or growth inhibition of target organisms that determines the final use of the chosen preservation method(s)(Table 14.2) 14.2.3 Hygienic conditions Hygienic production is not a preservation method, but ingredients or raw material used in MAP should always be of superior quality, i.e. low bacterial numbers and preferably without pathogenic bacteria. This is a prerequisite for fresh products with increased shelf-life, and preservation should never be used to compensate for inadequate hygiene or poor raw material quality. A strategy for the control of pathogens and, to a large extent, spoilage microorganisms is basically one of exclusion, which requires reducing or eliminating the initial microbial load or preventing or minimising further contamination. Since MA packaged products are hermetically sealed, recontamination is eliminated and the hygienic pre-packaging conditions are the most important steps. An appropriate design and construction of the pre-packaging premises is necessary
outgrowth. Most food spoilage moulds species have an absolute requirement for oxygen and appear to be sensitive to high levels of CO2. Consequently foods with low aw values, such as bakery products, that are susceptible to spoilage by moulds can have their shelf-lives extended by MAP. Many yeasts are capable of growing in the complete absence of oxygen and most are comparatively resistant to CO2. Although MAP can inhibit the growth of bacterial and fungal spoilage microorganisms, its effect on the survival of enteric viruses, including hepatitis A viruses (HAV), has not been well investigated. Both mussels and lettuce that are packaged in MAP may be a vehicle in the transmission of HAV (due to contact with contaminated water) and therefore can contribute to hepatitis A outbreaks (Cliver, 1997). Experiments by Bidawid et al. (2001) indicated that MAP does not influence HAV survival when present on the surface of produce with high CO2 levels. This may have been attributed to the inhibition of spoilage-causing enzymatic activities in the lettuce, which may have reduced exposure of viruses to potential toxic by-products. 14.2.2 Preventative techniques combined with MAP The main preservation techniques currently used act in one of three ways: (i) preventing the access of microorganisms to foods, (ii) inactivating them when they have gained access, or (iii) preventing or slowing down their growth when they have gained access and not been inactivated. During the past few years there has been increasing interest in modifying these approaches or in developing new ones, with the objective of reducing the severity of the more extreme techniques. Many such developments have involved new uses of existing techniques in new combinations to inhibit the growth of microorganisms. Approaches where preservation techniques are used at lower intensity or at lower concentration, causes inactivation and bacterial growth inhibition to overlap. It is the safety level, the quality level or the outcome of inactivation or growth inhibition of target organisms that determines the final use of the chosen preservation method(s) (Table 14.2). 14.2.3 Hygienic conditions Hygienic production is not a preservation method, but ingredients or raw material used in MAP should always be of superior quality, i.e. low bacterial numbers and preferably without pathogenic bacteria. This is a prerequisite for fresh products with increased shelf-life, and preservation should never be used to compensate for inadequate hygiene or poor raw material quality. A strategy for the control of pathogens and, to a large extent, spoilage microorganisms is basically one of exclusion, which requires reducing or eliminating the initial microbial load or preventing or minimising further contamination. Since MA packaged products are hermetically sealed, recontamination is eliminated and the hygienic pre-packaging conditions are the most important steps. An appropriate design and construction of the pre-packaging premises is necessary Combining MAP with other preservation techniques 291
Table 14.2 Uses and limitations of preservation technologies combined with MAP(Adapted from Leistner, 2002) Preventing assess Inactivation or growth/activity inhibition Effect in MAP Heat ionising NaCl pH Bacterio- Low Essential SGS treatment irradiation Temp' lactate oils Killing spores Killing veg. cells Preventing growth 土++ 士 Solid ++++ +++ treatment Low temperature(super chilling and freezing) may also kill bacteria SGS= soluble
Table 14.2 Uses and limitations of preservation technologies combined with MAP (Adapted from Leistner, 2002) Preventing assess Inactivation or growth/activity inhibition Effect in MAP Heat Ionising NaCl pH Bacterio- Low Preser- Na- Essential SGS2 treatment irradiation cins Temp1 vatives lactate oils Killing spores + + ÿ ÿ ÿ ÿ ÿ ÿ ÿ ÿ Killing veg. cells + + ± ± ± ± + ÿ ÿ ÿ Preventing growth ÿ ÿ + + ÿ ÿ + + + + Solids + + + + + + + + + ÿ Liquids + + + + + + + + + + In-pack treatments + + ÿ ÿ ÿ + ÿ ÿ ÿ + In-line treatment + + ÿ ÿ + ÿ ÿ ÿ ÿ + 1 Low temperature (super chilling and freezing) may also kill bacteria 2 SGS = soluble gas stabilisation
Combining MAP with other preservation techniques 293 to limit entry, multiplication, and spread of microorganisms in the environment where Ma packaged foods are being produced or manufactured, in order to prevent or minimise cross-contamination of the products. New and hygienic design of production facilities, with elements from clean room technology, are now more frequently adopted in the production of high-priced products. These techniques meet the requirements of freeing the products from microorganisms by cross-contamination, decontaminating the packaging material, and sterilising air in contact with the product 14.3 Heat treatment and irradiation Refrigerated ready-to-eat meals and entrees, prepared salads, sandwiches, pizza. fresh pasta, soups, whole meals, and sauces are commonly packaged in MA after heat treatment. These products have received some form of heat treatment, and are for the most part "low acid. They are marketed refrigerated (1 to +4C) and require little preparation before consumption. There has been a recent expansion in the use of the combination of mild heating of vacuum-packaged foods, e.g., sous vide, and cook-and-chill products with controlled chill storage, particularly for catering but also for retail. MA packaging of cook-and-chill foods is now commonly used for processed minimal heat-treated ready meals homes and canteens currently receive heat-treated MA packaged meals prepared in a central kitchen unit. With this method the risk of recontamination of microorganisms after cooking must be taken into account These ready-to-eat meals have a shelf-life of 7-14 days, depending on the amount of heat used The success of heat-treated ready meals results primarily from the inactivation of the vegetative microbial flora by mild heating. Another fact is that the spores of psychrotrophic bacteria, which can grow at low chill temperatures, are generally more heat sensitive than those of mesophiles and thermopiles, which cannot grow at these temperatures. The mild heating therefore destroys the cold-growing fraction of the potential spoilage flora, whilst the minimal thermal damage and conditions of low oxygen tension ensure high product quality. Shelf-lives at temperatures below about 3C can therefore be very long, i.e., in excess of three weeks, with eventual spoilage resulting from the slow growth of psychrothropic strains of Bacillus and Clostridium. In order to ensure safety, heat processes equivalent to 90oC for 10 min.(ACMSF- Advisory Committee on the Microbiological Safety of Food, 1992)are generally regarded as sufficient to ensure inactivation of spores in the coldest-growing pathogenic sporeformers such as psychrotrophic strains of Clostridium botulinum(Notermans et al, 1990, Lund and Peck, 1994). For lower heat treatments. strict limitations of shelf-life. efficient control of stora temperatures below 3.0C or some form of intrinsic preservation is necess During a three-year period, 2168 heat-treated, commercially available made meals with a shelf-life of 3-5 weeks were examined for sporeforming
to limit entry, multiplication, and spread of microorganisms in the environment where MA packaged foods are being produced or manufactured, in order to prevent or minimise cross-contamination of the products. New and hygienic design of production facilities, with elements from clean room technology, are now more frequently adopted in the production of high-priced products. These techniques meet the requirements of freeing the products from microorganisms by cross-contamination, decontaminating the packaging material, and sterilising air in contact with the product. 14.3 Heat treatment and irradiation Refrigerated ready-to-eat meals and entre´es, prepared salads, sandwiches, pizza, fresh pasta, soups, whole meals, and sauces are commonly packaged in MA after heat treatment. These products have received some form of heat treatment, and are for the most part ‘low acid’. They are marketed refrigerated (ÿ1 to 4ºC) and require little preparation before consumption. There has been a recent expansion in the use of the combination of mild heating of vacuum-packaged foods, e.g., sous vide, and cook-and-chill products with controlled chill storage, particularly for catering but also for retail. MA packaging of cook-and-chill foods is now commonly used for processed minimal heat-treated ready meals. Many nursing homes and canteens currently receive heat-treated MA packaged meals prepared in a central kitchen unit. With this method the risk of recontamination of microorganisms after cooking must be taken into account. These ready-to-eat meals have a shelf-life of 7–14 days, depending on the amount of heat used. The success of heat-treated ready meals results primarily from the inactivation of the vegetative microbial flora by mild heating. Another fact is that the spores of psychrotrophic bacteria, which can grow at low chill temperatures, are generally more heat sensitive than those of mesophiles and thermopiles, which cannot grow at these temperatures. The mild heating therefore destroys the cold-growing fraction of the potential spoilage flora, whilst the minimal thermal damage and conditions of low oxygen tension ensure high product quality. Shelf-lives at temperatures below about 3ºC can therefore be very long, i.e., in excess of three weeks, with eventual spoilage resulting from the slow growth of psychrothropic strains of Bacillus and Clostridium. In order to ensure safety, heat processes equivalent to 90ºC for 10 min. (ACMSFAdvisory Committee on the Microbiological Safety of Food, 1992) are generally regarded as sufficient to ensure inactivation of spores in the coldest-growing pathogenic sporeformers such as psychrotrophic strains of Clostridium botulinum (Notermans et al., 1990; Lund and Peck, 1994). For lower heat treatments, strict limitations of shelf-life, efficient control of storage temperatures below 3.0ºC or some form of intrinsic preservation is necessary. During a three-year period, 2168 heat-treated, commercially available readymade meals with a shelf-life of 3–5 weeks were examined for sporeforming Combining MAP with other preservation techniques 293
294 Novel food packaging techniques bacteria(Nissen et al., 2003 ). Three-quarters of the samples had less than ten bacteria/g the day after production, and none had more than 1000. Similar numbers were found at the end of the shelf-life. At abuse temperatures(20oC) the number of bacteria increased to 10-10'cfu/g in seven days. Three hundred and fifty isolates of spore-forming bacteria (aerobic and anaerobic)were collected and characterised as Bacillus licheniformis, B. thuringiensis, B megatherium, B. pumilis, B. subtilis, B. sphaeicus, and B cereus, but no Clostridium strains were detected. Growth experiments of 113 strains from this work showed that only 1 l strains were able to grow at 7C. Furthermore, none of the psychotropic strains were able to produce substantial amounts of toxins These experiments show that spore-formers, especially Bacillus strains, survive mild heat treatments and some of their members may be a health risk in products with long shelf-lives or if stored at high temperatures. Further research on germination, growth and toxin production at chilled temperatures in modified 14.3.1 Low temperature(freezing, partial freezing, super chilling A low and stable temperature is a general prerequisite for many MA products and has a particular importance in fresh storage. Both enzymatic and microbiological activity are greatly influenced by temperature. Many bacteria re unable to grow at temperatures below 10.C and even psychrotrophic organisms grow very slowly, and with extended lag phases, at temperatures that approach 0C. Temperature can, however, be used to achieve special effects in MA products. Guldager et al.(1998)and Boknaes et al. (2000) have found that frozen(-20oC)and thawed cod fillets in MA had longer shelf-life than raw cod in MA. This shelf-life extension was most likely due to the inactivation of the spoilage bacterium Photobacterium phosphoreum during fr zen storage use of frozen fillets as a raw material not only provides a more stable MAP product but also allows much greater flexibility for production and distribution A similar effect was found when frozen and thawed salmon was packaged in MA. Here also the freezing eliminated P. phosphoreum and extended the shelf- life of MAP salmon at 2C by 1-2 weeks(Emborg et al., 2002 Earlier experiments with whole gutted salmon have shown that MAP can be combined with super-chilling to extend further the shelf life and safety of fresh fish(Rosnes et al, 1998; Rosnes et al, 2001; Sivertsvik et al., 1999). In this technique, also known as partial freezing, the temperature of the fish is reduced to between 1 or 2C below the initial freezing point and some ice is formed inside the product( Gould and Peters, 1971). Under normal conditions, the gas atmosphere surrounding a Ma product will insulate the product, leading to a longer time until it is satisfactorily chilled. Partial freezing eliminates this problem by reducing the temperature of the fish before packaging. These experiments showed that super-chilling can decrease the temperature before packaging and increase stored refrigeration capacity during storage, and thereby nificantly decrease microbial growth at temperatures of 2-6.C, which is often
bacteria (Nissen et al., 2003). Three-quarters of the samples had less than ten bacteria/g the day after production, and none had more than 1000. Similar numbers were found at the end of the shelf-life. At abuse temperatures (20ºC), the number of bacteria increased to 106 –107 cfu/g in seven days. Three hundred and fifty isolates of spore-forming bacteria (aerobic and anaerobic) were collected and characterised as Bacillus licheniformis, B. thuringiensis, B. megatherium, B. pumilis, B. subtilis, B. sphaeicus, and B.cereus, but no Clostridium strains were detected. Growth experiments of 113 strains from this work showed that only 11 strains were able to grow at 7ºC. Furthermore, none of the psychotropic strains were able to produce substantial amounts of toxins. These experiments show that spore-formers, especially Bacillus strains, survive mild heat treatments and some of their members may be a health risk in products with long shelf-lives or if stored at high temperatures. Further research on germination, growth and toxin production at chilled temperatures in modified atmosphere is required. 14.3.1 Low temperature (freezing, partial freezing, super chilling) A low and stable temperature is a general prerequisite for many MA products and has a particular importance in fresh storage. Both enzymatic and microbiological activity are greatly influenced by temperature. Many bacteria are unable to grow at temperatures below 10ºC and even psychrotrophic organisms grow very slowly, and with extended lag phases, at temperatures that approach 0ºC. Temperature can, however, be used to achieve special effects in MA products. Guldager et al. (1998) and Bøknæs et al. (2000) have found that frozen (ÿ20ºC) and thawed cod fillets in MA had longer shelf-life than raw cod in MA. This shelf-life extension was most likely due to the inactivation of the spoilage bacterium Photobacterium phosphoreum during frozen storage. The use of frozen fillets as a raw material not only provides a more stable MAP product but also allows much greater flexibility for production and distribution. A similar effect was found when frozen and thawed salmon was packaged in MA. Here also the freezing eliminated P. phosphoreum and extended the shelflife of MAP salmon at 2ºC by 1–2 weeks (Emborg et al., 2002). Earlier experiments with whole gutted salmon have shown that MAP can be combined with super-chilling to extend further the shelf life and safety of fresh fish (Rosnes et al., 1998; Rosnes et al., 2001; Sivertsvik et al., 1999). In this technique, also known as partial freezing, the temperature of the fish is reduced to between 1 or 2ºC below the initial freezing point and some ice is formed inside the product (Gould and Peters, 1971). Under normal conditions, the gas atmosphere surrounding a MA product will insulate the product, leading to a longer time until it is satisfactorily chilled. Partial freezing eliminates this problem by reducing the temperature of the fish before packaging. These experiments showed that super-chilling can decrease the temperature before packaging and increase stored refrigeration capacity during storage, and thereby significantly decrease microbial growth at temperatures of 2–6ºC, which is often 294 Novel food packaging techniques
Combining MAP with other preservation techniques 295 found in chilled retail counters. Sikorski and Sun(1994) found that super- chilling can store enough refrigeration capacity to keep a core temperature <ooC during the first three weeks of chilled storage. a shelf-life extension of seven days has been obtained for super-chilled fish when compared to traditional ice stored fish of the same type (Leblanc and Leblanc, 1992). Untreated salmon steaks in MA, and partial frozen salmon steaks in MA, had an acceptable microbiological quality of 22 days at OoC, but were rejected by odour after 17 days. Salmon steaks in air had and acceptable microbiological quality for only eight days(Rosnes et al., 2001). MAP is also being used to package products for frozen storage. The reasoning behind the use of MAP for ready-to-eat products is that they can be distributed frozen, then thawed and sold as chilled products but with an extended shelf-life(Morris, 1989) 14.3.2 radiation The attraction of combining irradiation with MAP is that the modified atmospheres are not lethal to spoilage organisms and pathogens. The possibility exists, therefore, of using irradiation below the threshold dose, i.e., the level at which spoilage organisms and pathogens are killed and below the level where undesirable organoleptic changes are introduced, in order to enhance the attractiveness of MAP. The effects of MAP/irradiation on sensory properties, and its effect upon depletion of vitamin content during storage, compared to untreated items. have been examined in detail. Studies on the effects of maP/ irradiation methods on nutritional quality showed that the deleterious effects of irradiation on vitamins can be removed by modify ing storage atmospheres (Robins, 1991). For a radiation dose of 0. 25 kGy and in an air atmosphere, 60% of the thiamine content was lost over the storage period, compared to a minimal loss in the non-irradiated control over the same period. The loss of a-tocopherol exposed to l kGy irradiation, was some 50% over this period, compared to a similar minimal loss in the non-irradiated control sample. In both cases there were much reduced loss rates in N2 atmospheres, which demonstrated that the ffects of irradiation on these vitamins could be removed by modifying storage atmospheres The growth rate of surviving microorganisms was measured as a function of atmospheric composition for the irradiated and non-irradiated food samples, and the optimum lethal atmospheres were found to range from CO2/N2: 25/75 to CO,/N2: 50/50. Tests at 10c showed a similar trend, although the effectiveness of high concentrations of CO2 was reduced. The major surviving organisms even in the irradiated packs were lactobacilli, in accordance with general expectations on their resistance to radiation A series of experiments on MAP/irradiation combination, for use with chicken and pork products, with the goal of optimising sensory quality have shown that each particular food item requires careful evaluation and that generalisation can lead to incorrect and inappropriate specifications for optimum storage. However, as one of several different treatment combinations aimed at
found in chilled retail counters. Sikorski and Sun (1994) found that superchilling can store enough refrigeration capacity to keep a core temperature < 0ºC during the first three weeks of chilled storage. A shelf-life extension of seven days has been obtained for super-chilled fish when compared to traditional ice stored fish of the same type (Leblanc and Leblanc, 1992). Untreated salmon steaks in MA, and partial frozen salmon steaks in MA, had an acceptable microbiological quality of 22 days at 0ºC, but were rejected by odour after 17 days. Salmon steaks in air had and acceptable microbiological quality for only eight days (Rosnes et al., 2001). MAP is also being used to package products for frozen storage. The reasoning behind the use of MAP for ready-to-eat products is that they can be distributed frozen, then thawed and sold as chilled products but with an extended shelf-life (Morris, 1989). 14.3.2 Irradiation The attraction of combining irradiation with MAP is that the modified atmospheres are not lethal to spoilage organisms and pathogens. The possibility exists, therefore, of using irradiation below the ‘threshold’ dose, i.e., the level at which spoilage organisms and pathogens are killed and below the level where undesirable organoleptic changes are introduced, in order to enhance the attractiveness of MAP. The effects of MAP/irradiation on sensory properties, and its effect upon depletion of vitamin content during storage, compared to untreated items, have been examined in detail. Studies on the effects of MAP/ irradiation methods on nutritional quality showed that the deleterious effects of irradiation on vitamins can be removed by modifying storage atmospheres (Robins, 1991). For a radiation dose of 0.25 kGy and in an air atmosphere, 60% of the thiamine content was lost over the storage period, compared to a minimal loss in the non-irradiated control over the same period. The loss of -tocopherol, exposed to 1 kGy irradiation, was some 50% over this period, compared to a similar minimal loss in the non-irradiated control sample. In both cases there were much reduced loss rates in N2 atmospheres, which demonstrated that the effects of irradiation on these vitamins could be removed by modifying storage atmospheres. The growth rate of surviving microorganisms was measured as a function of atmospheric composition for the irradiated and non-irradiated food samples, and the optimum lethal atmospheres were found to range from CO2/N2 : 25/75 to CO2/N2 : 50/50. Tests at 10ºC showed a similar trend, although the effectiveness of high concentrations of CO2 was reduced. The major surviving organisms even in the irradiated packs were lactobacilli, in accordance with general expectations on their resistance to radiation. A series of experiments on MAP/irradiation combination, for use with chicken and pork products, with the goal of optimising sensory quality have shown that each particular food item requires careful evaluation and that generalisation can lead to incorrect and inappropriate specifications for optimum storage. However, as one of several different treatment combinations aimed at Combining MAP with other preservation techniques 295
296 Novel food packaging techniques 一 Vacuun MVM +22kG Storage time(days) Fig. 14.1 Effect of irradiation on MAP and vacuum packaged cod fillets reducing mould in strawberries, the MAP/irradiation method gave the best esults. Several studies have been carried out on the use of mapirradiation treatments in fish products, e.g., low dose irradiation extended the shelf-life of haddock fillets and cod fillets (Licciardello et al, 1984)more than either process achieved on its own. Przybylski et al.(1989) examined fresh catfish fillets processed with low dose irradiation in combination with MAP, and demonstrated that irradiation treatments with or without elevated carbon dioxide-modified atmosphere packaging significantly reduced the bacterial load and extended shelf-life from 5-7 days to between 20 and 30 days In an experiment, cod fillets were packaged in MA(80: 20 CO2: N2)and under vacuum before irradiation with 2.2 kGy, and subsequent storage at 4C. The results(Fig. 14. 1)showed a large inhibitory effect of irradiation on micro- organisms. The best results were observed when combining irradiation with MAP. The sensory shelf-life of irradiated MA cod and irradiated vacuum packaged cod was >24 days and 24 days accordingly. For non-irradiated MA cod the shelf-life was <14 days, and for vacuum packaged cod, <9 days. This should indicate a large potential for seafood product shelf-life extensions through the use of maP combined with low-dose irradiation. However. before this method is widely accepted, several issues need to be resolved, such as legislative, scientific (food safety ), and also consumer attitudes towards irradiated foods(Sivertsvik et al, 2001). Nevertheless, all studies have shown that the advantages of maP/irradiation treatment methods must be determined for specific applications with a fair degree of caution and this requires the ascertainment of exact conditions for every product in terms of microbiological safety
reducing mould in strawberries, the MAP/irradiation method gave the best results. Several studies have been carried out on the use of MAP/irradiation treatments in fish products, e.g., low dose irradiation extended the shelf-life of haddock fillets and cod fillets (Licciardello et al., 1984) more than either process achieved on its own. Przybylski et al. (1989) examined fresh catfish fillets, processed with low dose irradiation in combination with MAP, and demonstrated that irradiation treatments with or without elevated carbon dioxide-modified atmosphere packaging significantly reduced the bacterial load and extended shelf-life from 5–7 days to between 20 and 30 days. In an experiment, cod fillets were packaged in MA (80:20 CO2:N2) and under vacuum before irradiation with 2.2 kGy, and subsequent storage at 4ºC. The results (Fig. 14.1) showed a large inhibitory effect of irradiation on microorganisms. The best results were observed when combining irradiation with MAP. The sensory shelf-life of irradiated MA cod and irradiated vacuum packaged cod was >24 days and 24 days accordingly. For non-irradiated MA cod the shelf-life was <14 days, and for vacuum packaged cod, <9 days. This should indicate a large potential for seafood product shelf-life extensions through the use of MAP combined with low-dose irradiation. However, before this method is widely accepted, several issues need to be resolved, such as legislative, scientific (food safety), and also consumer attitudes towards irradiated foods (Sivertsvik et al., 2001). Nevertheless, all studies have shown that the advantages of MAP/irradiation treatment methods must be determined for specific applications with a fair degree of caution and this requires the ascertainment of exact conditions for every product in terms of microbiological safety. Fig. 14.1 Effect of irradiation on MAP and vacuum packaged cod fillets. 296 Novel food packaging techniques