Technologies and processes
Part II Technologies and processes
The refrigeration of chilled foods R D. Heap, Cambridge Refrigeration Technology 4.1 Introduction Chilled foods are foods which are cooled to a temperature above their freezing point and which need to be maintained at that temperature to preserve quality Generally such foods will lose value if frozen, and in many cases freezing will destroy them. From the refrigeration viewpoint, the range of foods regarded as chilled is very wide. In this chapter they are taken to include fresh fruits and vegetables, both temperate and tropical in origin, the whole range of meat, fish and dairy products, and prepared complete meals. Frequently a narrower definition covering only prepared foods is used(Anon. 1997) It is immediately obvious that refrigeration is essential for the production, storage and distribution of chilled foods. However, the range and variety of refrigeration equipment required is less readily apparent. Consider, for example the operation of a cook-chill catering facility. Raw materials from around the world are cooled in distant pack houses and transported across the oceans in highly developed refrigerated transport systems. They then pass through refrigerated port stores and via refrigerated road transport to distribution depots from which, either directly or indirectly, they are despatched to the catering facility. This is shown diagrammatically in Fig. 4.1 Here, refrigerated stores maintain quality prior to use. Some raw materials ay be frozen rather than chilled, and will require thawing equipment Following the cooling operation, the food is chilled using blast chillers or,in some cases, immersion chillers, and will then be stored under refrigeration before distribution in insulated or refrigerated vehicles. It may then be held in refrigerated storage or display cabinets before re-heating. In addition, the waste produced during the food preparation may be stored under refrigeration. The
4.1 Introduction Chilled foods are foods which are cooled to a temperature above their freezing point and which need to be maintained at that temperature to preserve quality. Generally such foods will lose value if frozen, and in many cases freezing will destroy them. From the refrigeration viewpoint, the range of foods regarded as chilled is very wide. In this chapter they are taken to include fresh fruits and vegetables, both temperate and tropical in origin, the whole range of meat, fish and dairy products, and prepared complete meals. Frequently a narrower definition covering only prepared foods is used (Anon. 1997). It is immediately obvious that refrigeration is essential for the production, storage and distribution of chilled foods. However, the range and variety of refrigeration equipment required is less readily apparent. Consider, for example, the operation of a cook-chill catering facility. Raw materials from around the world are cooled in distant pack houses and transported across the oceans in highly developed refrigerated transport systems. They then pass through refrigerated port stores and via refrigerated road transport to distribution depots from which, either directly or indirectly, they are despatched to the catering facility. This is shown diagrammatically in Fig. 4.1. Here, refrigerated stores maintain quality prior to use. Some raw materials may be frozen rather than chilled, and will require thawing equipment. Following the cooling operation, the food is chilled using blast chillers or, in some cases, immersion chillers, and will then be stored under refrigeration before distribution in insulated or refrigerated vehicles. It may then be held in refrigerated storage or display cabinets before re-heating. In addition, the waste produced during the food preparation may be stored under refrigeration. The 4 The refrigeration of chilled foods R. D. Heap, Cambridge Refrigeration Technology
80 Chilled foods 心89 Field/orchard Meat works/packhouse bution depot ReAs A Fig, 4.1 The chill chain general lack of problems in refrigeration machinery, which is so essential, is a tribute to the reliability of the technology. Refrigeration is almost a forgotten part of the chilled food preparation process in the mind of the consumer; it taken for granted Food refrigeration is not new. Natural ice and evaporative cooling have been used for millennia, and the relatively recent use of mechanical refrigeration to
general lack of problems in refrigeration machinery, which is so essential, is a tribute to the reliability of the technology. Refrigeration is almost a forgotten part of the chilled food preparation process in the mind of the consumer; it is taken for granted. Food refrigeration is not new. Natural ice and evaporative cooling have been used for millennia, and the relatively recent use of mechanical refrigeration to Fig. 4.1 The chill chain. 80 Chilled foods
The refrigeration of chilled foods 81 store foodstuffs at chill temperatures in fact dates back to the US apple stores of the 1870s(Thevenot 1979). Refrigerated transport of chilled(as distinct from frozen) meat started between the Us and the uk around 1875, and longer- distance chilled transport from Australasia to Europe dates from 1895(Critchell and Raymond, 1969). By 1901, the Uk was importing over 160,000 tonnes of hilled beef annual 4.2 Principles of refrigeration The basic principles of vapour compression refrigeration were established in the 19th century, and this form of refrigeration is almost universally adopted nowadays. At its simplest, such a refrigeration system has four interlinked components(Fig. 4.2). A refrigerant fluid in the vapour state is compressed to a higher pressure, and consequently a higher temperature. The high temperature restrictor to a lower pressure area, cooling further in the process. The cold liquid can then be used to extract heat from a storage space or cooling area, this heat vaporising the cold, low pressure liquid in an evaporator. The cold vapour is chen fed back to the compressor to complete the cycle The compressor, condenser, expansion restrictor, and evaporator form the basic components. Whilst heat is extracted from a process at the evaporator, the extracted heat plus the heat equivalent of the compression energy must be rejected at the condenser. This means that any refrigeration device must reject a quantity of heat, which is greater than the heat energy removed from the product or space being cooled. The energy used by a vapour compression refrigeration machine depends on its design, but generally the larger the temperature difference between evaporator and condenser, the greater the energy used in the ompressor for a given amount of cooling duty. Also, the greater this temperature difference is, the smaller will be the refrigerating capacity of the system Theoretical analysis of refrigeration cycles and full details of components may be found in numerous refrigeration textbooks( Gosney 1982, AshraE and-books, Alders 1987) and would be out of place in the present publication Nevertheless, a basic appreciation of the principles outlined above is useful for all users of refrigeration equipment 4.3 Safety and quality issues Food safety is concerned with freedom from pathogens and toxins- food should not make people ill, nor should it poison them. Food quality is the nutritional value and the perception of taste, texture and appearance of a foodstuff that is safe. Ideally, food safety is subject to legislative controls, whereas food quality is an issue best left to market forces
store foodstuffs at chill temperatures in fact dates back to the US apple stores of the 1870s (The´venot 1979). Refrigerated transport of chilled (as distinct from frozen) meat started between the US and the UK around 1875, and longerdistance chilled transport from Australasia to Europe dates from 1895 (Critchell and Raymond, 1969). By 1901, the UK was importing over 160,000 tonnes of chilled beef annually. 4.2 Principles of refrigeration The basic principles of vapour compression refrigeration were established in the 19th century, and this form of refrigeration is almost universally adopted nowadays. At its simplest, such a refrigeration system has four interlinked components (Fig. 4.2). A refrigerant fluid in the vapour state is compressed to a higher pressure, and consequently a higher temperature. The high temperature gas is cooled and liquefied in a condenser. The cool liquid then passes through a restrictor to a lower pressure area, cooling further in the process. The cold liquid can then be used to extract heat from a storage space or cooling area, this heat vaporising the cold, low pressure liquid in an evaporator. The cold vapour is then fed back to the compressor to complete the cycle. The compressor, condenser, expansion restrictor, and evaporator form the basic components. Whilst heat is extracted from a process at the evaporator, the extracted heat plus the heat equivalent of the compression energy must be rejected at the condenser. This means that any refrigeration device must reject a quantity of heat, which is greater than the heat energy removed from the product or space being cooled. The energy used by a vapour compression refrigeration machine depends on its design, but generally the larger the temperature difference between evaporator and condenser, the greater the energy used in the compressor for a given amount of cooling duty. Also, the greater this temperature difference is, the smaller will be the refrigerating capacity of the system. Theoretical analysis of refrigeration cycles and full details of components may be found in numerous refrigeration textbooks (Gosney 1982, ASHRAE hand-books, Alders 1987) and would be out of place in the present publication. Nevertheless, a basic appreciation of the principles outlined above is useful for all users of refrigeration equipment. 4.3 Safety and quality issues Food safety is concerned with freedom from pathogens and toxins – food should not make people ill, nor should it poison them. Food quality is the nutritional value and the perception of taste, texture and appearance of a foodstuff that is safe. Ideally, food safety is subject to legislative controls, whereas food quality is an issue best left to market forces. The refrigeration of chilled foods 81
82 Chilled foods Expansion device Evaporator pressor ig. 4. 2 The basic vapour compression refrigeration circuit For chilled foods, safety and quality may or may not be interlinked. For fresh fruits and vegetables, spoilage will make the food unpalatable but there is unlikely to be a health risk. For many prepared foods including cooked meats growth of food poisoning pathogens will take place to an extent dependent on temperature and time, leading to injurious food which may look and taste satisfactory. For some dairy products, pathogen growth and off-flavours may develop together. In every case, the maintenance of safety and quality depends on maintaining as low a temperature as is possible without damaging the food This has to be achieved throughout all stages of the chill chain shown in Fig 4. 4 Refrigerant fluids and the environment Until the early 1990s, the choice of refrigerant fluids for use within the closed vapour compression refrigeration cycle was a matter of little concern equipment users. Unfortunately, it is now realised that those fluids developed over the years for efficiency and for safety have unexpected environmental side effects when they are released into the atmosphere Ozone depletion and global warming are two quite separate environmental problems. The ozone layer, which protects the surface of the earth from excessive ultraviolet radiation, is damaged by the emission of stable chemicals containing chlorine or bromine. These chemicals include CFC (chlorofluoro- carbon) and HCFC (hydrochlorofluorocarbon) refrigerants, which contribute to ozone depletion in the stratosphere, and also to atmospheric global warming Global warming is a natural phenomenon, in which heat from the sun is trapped
For chilled foods, safety and quality may or may not be interlinked. For fresh fruits and vegetables, spoilage will make the food unpalatable but there is unlikely to be a health risk. For many prepared foods including cooked meats, growth of food poisoning pathogens will take place to an extent dependent on temperature and time, leading to injurious food which may look and taste satisfactory. For some dairy products, pathogen growth and off-flavours may develop together. In every case, the maintenance of safety and quality depends on maintaining as low a temperature as is possible without damaging the food. This has to be achieved throughout all stages of the chill chain shown in Fig. 4.1. 4.4 Refrigerant fluids and the environment Until the early 1990s, the choice of refrigerant fluids for use within the closed vapour compression refrigeration cycle was a matter of little concern to equipment users. Unfortunately, it is now realised that those fluids developed over the years for efficiency and for safety have unexpected environmental sideeffects when they are released into the atmosphere. Ozone depletion and global warming are two quite separate environmental problems. The ozone layer, which protects the surface of the earth from excessive ultraviolet radiation, is damaged by the emission of stable chemicals containing chlorine or bromine. These chemicals include CFC (chlorofluorocarbon) and HCFC (hydrochlorofluorocarbon) refrigerants, which contribute to ozone depletion in the stratosphere, and also to atmospheric global warming. Global warming is a natural phenomenon, in which heat from the sun is trapped Fig. 4.2 The basic vapour compression refrigeration circuit. 82 Chilled foods