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