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