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 forcesstore 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 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 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