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 sporeformingto 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. (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 storage temperatures below 3.0ºC or some form of intrinsic preservation is necessary. During a three-year period, 2168 heat-treated, commercially available ready￾made meals with a shelf-life of 3–5 weeks were examined for sporeforming Combining MAP with other preservation techniques 293