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
The refrigeration of chilled foods 83 in the atmosphere by, particularly, carbon dioxide and water vapour. Fears of excessive global climate change are associated with high emissions of carbon dioxide(mainly from burning fossil fuels in power stations and elsewhere)and of other more powerful but much less abundant 'greenhouse gases including HFCs(hydrofluorocarbons) Under the auspices of the Montreal Protocol(Anon. 1987), the developed world ceased the production of ozone-depleting CFCs during the 1990s. This was made possible by the substitution of less environmentally harmful HCFCs The HCFCs themselves are expected to be phased out by 2010-20, if not earlier and in some applications there are no known effective substitutes at present available In Europe, the use of CFCs in existing equipment will be banned, and the supply of new equipment using HCFCs will be prohibited. At the time of writing. dates for these limitations are still uncertain. These matters have two direct impacts on users of chilling equipment. Firstly, every change of technology costs money, and may in some cases result in an increase in running costs as well as re-equipment costs. Secondly, it may be necessary in the future to move from locally safe but globally harmful CFCs and HCFCs to globally safe but potentially locally hazardous substances such as ammonia and propane. These latter substances can be used safely, but there are added costs and added needs for proper training of equipment users. HFCs(hydrofluoro- carbons) have been developed as alternatives. These do not deplete ozone and are widely available, but are being targeted by some environmentalists, as they are greenhouse gases within the Kyoto Protocol The purchaser of equipment needs to be aware of these matters, as he may otherwise obtain machinery that will have to be modified or even replaced long before its expected economic life is over. There could also be financial implications at the time of machinery disposal. The reduction of CFC and HCFC use in insulating foams in storage cabinets and stores has been well publicised but at the time of writing the implications for refrigeration machiner fficiently widely appreciated A further related issue is the relation between global warming, energy nd energy efficiency. New refrigerant fluids may be less efficient, but future environmental concerns may penalise excessive energy use. The equipment specifier is likely to face some difficult choices over the next few years. The user will face new responsibilities for minimising refrigerant leakage, ensuring fficient operation, and using only properly qualified maintenance staff. For a fuller discussion see(Heap 1998) 4.5 Chiled foods and refrigeration The benefit of chilled storage is the extension of life of the foodstuff in good condition, by slowing down the rate of deterioration. Chilling, it must be emphasised, cannot improve the quality of a poor product, neither can it stop the processes of spoilage- it can only slow them down(see Chapters 7, 9, 10)
in the atmosphere by, particularly, carbon dioxide and water vapour. Fears of excessive global climate change are associated with high emissions of carbon dioxide (mainly from burning fossil fuels in power stations and elsewhere) and of other more powerful but much less abundant ‘greenhouse’ gases including HFCs (hydrofluorocarbons). Under the auspices of the Montreal Protocol (Anon. 1987), the developed world ceased the production of ozone-depleting CFCs during the 1990s. This was made possible by the substitution of less environmentally harmful HCFCs. The HCFCs themselves are expected to be phased out by 2010–20, if not earlier, and in some applications there are no known effective substitutes at present available. In Europe, the use of CFCs in existing equipment will be banned, and the supply of new equipment using HCFCs will be prohibited. At the time of writing, dates for these limitations are still uncertain. These matters have two direct impacts on users of chilling equipment. Firstly, every change of technology costs money, and may in some cases result in an increase in running costs as well as re-equipment costs. Secondly, it may be necessary in the future to move from locally safe but globally harmful CFCs and HCFCs to globally safe but potentially locally hazardous substances such as ammonia and propane. These latter substances can be used safely, but there are added costs and added needs for proper training of equipment users. HFCs (hydrofluorocarbons) have been developed as alternatives. These do not deplete ozone and are widely available, but are being targeted by some environmentalists, as they are greenhouse gases within the Kyoto Protocol. The purchaser of equipment needs to be aware of these matters, as he may otherwise obtain machinery that will have to be modified or even replaced long before its expected economic life is over. There could also be financial implications at the time of machinery disposal. The reduction of CFC and HCFC use in insulating foams in storage cabinets and stores has been well publicised, but at the time of writing the implications for refrigeration machinery are insufficiently widely appreciated. A further related issue is the relation between global warming, energy use, and energy efficiency. New refrigerant fluids may be less efficient, but future environmental concerns may penalise excessive energy use. The equipment specifier is likely to face some difficult choices over the next few years. The user will face new responsibilities for minimising refrigerant leakage, ensuring efficient operation, and using only properly qualified maintenance staff. For a fuller discussion see (Heap 1998). 4.5 Chilled foods and refrigeration The benefit of chilled storage is the extension of life of the foodstuff in good condition, by slowing down the rate of deterioration. Chilling, it must be emphasised, cannot improve the quality of a poor product; neither can it stop the processes of spoilage – it can only slow them down (see Chapters 7, 9, 10). The refrigeration of chilled foods 83
84 Chilled foods For the international land transport of chilled (and frozen) foods, the UNECE Treaty Agreement on the International Carriage of Perishable Foodstuffs and on the Special Equipment to be used for such Carriage(ATP) lays down various provisions. Foods are classified and maximum temperatures are stated in the ATP agreement as follows(UNECE 1998) Red offal Butte Game +4°C Milk for immediate consumption +4°C Industrial milk Yoghurt kefir. cream. fresh cheese +4C Fish, molluscs, crustaceans in melting ice(0C) Unstabilized meat products +6°C Meat(not offal) +7°C Poultry, rabbits This list excludes prepared vegetable foods with or without dressings and fresh fruit and vegetables There are two quite distinct applications of refrigeration to chilled foods These are the chilling operation itself, in which the foodstuff is cooled from either an ambient temperature of maybe 30C or a cooking temperature of over 0oC, and the chilled storage, at a closely controlled temperature of between 1.5C and +150oC depending on the foodstuff Chilling equipment and chilled storage equipment are quite different in their requirements and their design, and although some chilling equipment may be used for chilled storage, storage equipment is not designed to cool products, only to maintain temperature Transport refrigeration for chilled food distribution is a special case of storage and transport equipment should not be expected to provide rapid cooling 4.6 Chilling The rate at which heat can be extracted during chilling is dependent on many factors. The size and shape of the pack or container will affect the rate of heat transfer to the cooling air (or, in some cases, water). The temperature and speed of the air will also affect this. Within the pack, the weight, density,water content, specific heat capacity, thermal conductivity, latent heat content, and initial food temperature will each play a part In the case of unpackaged foods, the factors leading to rapid cooling also lead to rapid loss of moisture, so it may seem that slow cooling is better. Generally his is not the case, as the extended cooling time is also an extended drying-out time. More rapid chilling is possible with thinner packs, with higher airspeeds, and with lower air temperatures. All these lead to higher operating costs, so equipment design has to be a compromise to give the best overall operating ystem. This means that available for different
For the international land transport of chilled (and frozen) foods, the UNECE Treaty Agreement on the International Carriage of Perishable Foodstuffs and on the Special Equipment to be used for such Carriage (ATP) lays down various provisions. Foods are classified and maximum temperatures are stated in the ATP agreement as follows (UNECE 1998): Red offal +3ºC Butter +6ºC Game +4ºC Milk for immediate consumption +4ºC Industrial milk +6ºC Yoghurt, kefir, cream, fresh cheese +4ºC Fish, molluscs, crustaceans in melting ice (0ºC) Unstabilized meat products +6ºC Meat (not offal) +7ºC Poultry, rabbits +4ºC This list excludes prepared vegetable foods with or without dressings and fresh fruit and vegetables. There are two quite distinct applications of refrigeration to chilled foods. These are the chilling operation itself, in which the foodstuff is cooled from either an ambient temperature of maybe 30ºC or a cooking temperature of over 70ºC, and the chilled storage, at a closely controlled temperature of between 1.5ºC and +15.0ºC depending on the foodstuff. Chilling equipment and chilled storage equipment are quite different in their requirements and their design, and although some chilling equipment may be used for chilled storage, storage equipment is not designed to cool products, only to maintain temperature. Transport refrigeration for chilled food distribution is a special case of storage, and transport equipment should not be expected to provide rapid cooling. 4.6 Chilling The rate at which heat can be extracted during chilling is dependent on many factors. The size and shape of the pack or container will affect the rate of heat transfer to the cooling air (or, in some cases, water). The temperature and speed of the air will also affect this. Within the pack, the weight, density, water content, specific heat capacity, thermal conductivity, latent heat content, and initial food temperature will each play a part. In the case of unpackaged foods, the factors leading to rapid cooling also lead to rapid loss of moisture, so it may seem that slow cooling is better. Generally, this is not the case, as the extended cooling time is also an extended drying-out time. More rapid chilling is possible with thinner packs, with higher airspeeds, and with lower air temperatures. All these lead to higher operating costs, so equipment design has to be a compromise to give the best overall operating system. This means that a range of equipment is available for different 84 Chilled foods
The refrigeration of chilled foods 85 applications, and an appropriate choice must be made, dependent on the planned operation 4.7 Chilling equipment 4.7.1 Cooling systems For most prepared foods, air blast cooling chambers or tunnels are used. Water immersion(hydrocooling)is used for some vegetables, and for fresh, leafy produce, vacuum coolers may be appropriate. For some fresh produce which has relatively long storage life, cooling may be achieved using storage chambers, but frequently cooling rates will be enhanced by the use of special air circulation arrangements. Each of these systems will be considered in turn below 4.7.2 Blast chillers Blast chillers operate by passing cold air over foodstuffs at high speed. For cook-chill catering and similar operations, there are various guidelines, such as those issued by the dhss in the UK. These recommend that equipment should be capable of chilling foods of up to 50 mm thickness from 70oC down to a core temperature of 3C or below within 90 minutes. This requires an air speed of at least 4 metres per second and an air temperature of around -4oC Small,reach-in chillers taking batches of up to 30 kg are available, for buffer'supplies in catering and for teaching and research. Larger models with capacities of up to a quarter of a tonne of foodstuffs are designed to accommodate wheel-in trolleys of trays. a typical single trolley unit might have a nominal capacity of 45 kg, typically accommodated on a trolley taking 20 trays of food. The evaporator and fans are located to the side of the interior chamber and the compressor and condenser may be located either in the top of the unit or remotely, depending on whether the heat and noise emitted can be accommodated locally or not. Controls will permit the unit to be used as a chilled store at 0-3C, or will operate the chill ing cycle using any combination of air temperature, product probe temperature, or simple timer. At the end of the illing cycle, a defrosting cycle to remove ice and frost from the evaporator is operated. Total power draw for a 45 kg unit is about 7kw With a two-hour load/chill/defrost cycle, it is convenient to operate a four- batch shift, with the final batch being left in the cabinet as overnight storage Optionally, temperature recorders may be fitted to monitor operation. For larger units, there may be doors at each side, so that chilled trolleys may be rolled through into a chilled food holding store at 0-3.C. It is also possible to obtain combination units with a frozen food storage cabinet alongside a chiller/chill storage unit. This allows a caterer to remove frozen food, cook and portion it, then chill and finally store the completed portions Other forms of blast chillers have been developed for the chilling of fresh poultry, which use a carbon dioxide tunnel in which CO, snow is used to provide
applications, and an appropriate choice must be made, dependent on the planned operation. 4.7 Chilling equipment 4.7.1 Cooling systems For most prepared foods, air blast cooling chambers or tunnels are used. Water immersion (hydrocooling) is used for some vegetables; and for fresh, leafy produce, vacuum coolers may be appropriate. For some fresh produce which has a relatively long storage life, cooling may be achieved using storage chambers, but frequently cooling rates will be enhanced by the use of special air circulation arrangements. Each of these systems will be considered in turn below. 4.7.2 Blast chillers Blast chillers operate by passing cold air over foodstuffs at high speed. For cook-chill catering and similar operations, there are various guidelines, such as those issued by the DHSS in the UK. These recommend that equipment should be capable of chilling foods of up to 50 mm thickness from 70ºC down to a core temperature of 3ºC or below within 90 minutes. This requires an air speed of at least 4 metres per second and an air temperature of around 4ºC. Small, ‘reach-in’ chillers taking batches of up to 30 kg are available, for ‘buffer’ supplies in catering and for teaching and research. Larger models with capacities of up to a quarter of a tonne of foodstuffs are designed to accommodate wheel-in trolleys of trays. A typical single trolley unit might have a nominal capacity of 45 kg, typically accommodated on a trolley taking 20 trays of food. The evaporator and fans are located to the side of the interior chamber, and the compressor and condenser may be located either in the top of the unit or remotely, depending on whether the heat and noise emitted can be accommodated locally or not. Controls will permit the unit to be used as a chilled store at 0–3ºC, or will operate the chilling cycle using any combination of air temperature, product probe temperature, or simple timer. At the end of the chilling cycle, a defrosting cycle to remove ice and frost from the evaporator is operated. Total power draw for a 45 kg unit is about 7 kW. With a two-hour load/chill/defrost cycle, it is convenient to operate a fourbatch shift, with the final batch being left in the cabinet as overnight storage. Optionally, temperature recorders may be fitted to monitor operation. For larger units, there may be doors at each side, so that chilled trolleys may be rolled through into a chilled food holding store at 0–3ºC. It is also possible to obtain combination units with a frozen food storage cabinet alongside a chiller/chill storage unit. This allows a caterer to remove frozen food, cook and portion it, then chill and finally store the completed portions. Other forms of blast chillers have been developed for the chilling of fresh poultry, which use a carbon dioxide tunnel in which CO2 snow is used to provide The refrigeration of chilled foods 85
86 Chilled foods cooling. Although this can achieve good results, there is considerable risk of surface freezing which would be unacceptable for many products. Liquid nitrogen is another total-loss' refrigerant that may be used to cool cabinets. As the temperature of liquid nitrogen at atmospheric pressure is -196C, careful control is necessary. An alternative may be synthetic liquid air (SLA)(Waldron and Pearce 1998), which overcomes the danger of asphyxiation that exists with other cryogens Alltotal-loss systems depend on the availability of compressed, liquefied ases, and it should be noted that the total energy use of such systems(including that needed for liquefaction) is much greater than that of equivalent mechanical refrigeration systems, so running costs may be high. In some applications, either reduced capital costs or increased chilling speed may make such systems 4.7.3 Hydrocoolers The use of chilled water, either sprayed down through a chamber or in an immersion tank, provides very rapid cooling with no risk of freezing. It is normally only applicable to fresh fruits and vegetables that can withstand water immersion, and so is a little outside the scope of the general range of chilled foods, though it may be applied to vacuum packs of prepared foodstuffs. Water is normally recirculated in such systems, so great care is necessary to ensure continued cleanliness by regular flushing out, addition of fungicides,or whatever may be necessary for the particular product. It is of course possible to combine a degree of hydrocooling with normal cleaning operations for items such as root vegetab 4.7.4 Vacuum coolers Vacuum coolers are highly specialised and expensive pieces of equipment, well suited to the rapid cooling of pre-packaged leafy vegetables. They operate at low pressure with wet produce in a sealed chamber, under which conditions the ooling is mostly achieved by low temperature evaporation of moisture. The process is in batches, with cooling times of about 15-30 minutes, and typical equipment can accommodate several tonnes of produce, normally on pallets or trolleys 4.7.5 Store cooling For large volumes of live produce, particularly fresh fruits and vegetables, cooling may be achieved by placing cartoned or binned produce in a cool store and allowing the circulation of air in the store to provide all the cooling that is necessary. This is a slow process, taking several days and dependent on the store air circulation and the stacking of the produce. In many fruit stores,a combination of store extract fans, curtains, and planned stacking as shown
cooling. Although this can achieve good results, there is considerable risk of surface freezing which would be unacceptable for many products. Liquid nitrogen is another ‘total-loss’ refrigerant that may be used to cool cabinets. As the temperature of liquid nitrogen at atmospheric pressure is 196ºC, careful control is necessary. An alternative may be synthetic liquid air (SLA) (Waldron and Pearce 1998), which overcomes the danger of asphyxiation that exists with other cryogens. All ‘total-loss’ systems depend on the availability of compressed, liquefied gases, and it should be noted that the total energy use of such systems (including that needed for liquefaction) is much greater than that of equivalent mechanical refrigeration systems, so running costs may be high. In some applications, either reduced capital costs or increased chilling speed may make such systems attractive. 4.7.3 Hydrocoolers The use of chilled water, either sprayed down through a chamber or in an immersion tank, provides very rapid cooling with no risk of freezing. It is normally only applicable to fresh fruits and vegetables that can withstand water immersion, and so is a little outside the scope of the general range of chilled foods, though it may be applied to vacuum packs of prepared foodstuffs. Water is normally recirculated in such systems, so great care is necessary to ensure continued cleanliness by regular flushing out, addition of fungicides, or whatever may be necessary for the particular product. It is of course possible to combine a degree of hydrocooling with normal cleaning operations for items such as root vegetables. 4.7.4 Vacuum coolers Vacuum coolers are highly specialised and expensive pieces of equipment, well suited to the rapid cooling of pre-packaged leafy vegetables. They operate at low pressure with wet produce in a sealed chamber, under which conditions the cooling is mostly achieved by low temperature evaporation of moisture. The process is in batches, with cooling times of about 15–30 minutes, and typical equipment can accommodate several tonnes of produce, normally on pallets or trolleys. 4.7.5 Store cooling For large volumes of live produce, particularly fresh fruits and vegetables, cooling may be achieved by placing cartoned or binned produce in a cool store and allowing the circulation of air in the store to provide all the cooling that is necessary. This is a slow process, taking several days and dependent on the store air circulation and the stacking of the produce. In many fruit stores, a combination of store extract fans, curtains, and planned stacking as shown in 86 Chilled foods
The refrigeration of chilled foods 87 区X⑧ 区 Fig. 4.3 Cooling tunnel arrangement within a store Fig 4.3 is used to provide a simple form of something approaching a blast chiller. Air is extracted through a uniform thickness of cartons, with air entry other than through the cartons blocked by the curtain. If necessary, pallet bases osed to air movement by the insertion of plastic foam or other convenient 4.7.6 Summary of equipment These types of equipment may be summarised as follows Blast chillers: preferred equipment for most applications, design must be matched to production requirements Hydrocoolers: an excellent alternative in those cases where they may be used, especially fresh fruits and vege Vacuum coolers: specialist equipment for limited application Store cooling: with suitable stacking and curtains, a good method with 4. 8 Chilled storage Chilled storage equipment may be seen around the world in a wide rang sizes, each suited to the particular operation for which it is designed smallest, it may be an absorption cycle refrigerator in a caravan or boat
Fig. 4.3 is used to provide a simple form of something approaching a blast chiller. Air is extracted through a uniform thickness of cartons, with air entry other than through the cartons blocked by the curtain. If necessary, pallet bases are closed to air movement by the insertion of plastic foam or other convenient material. 4.7.6 Summary of equipment These types of equipment may be summarised as follows: Blast chillers: preferred equipment for most applications, design must be matched to production requirements. Hydrocoolers: an excellent alternative in those cases where they may be used, especially fresh fruits and vegetables. Vacuum coolers: specialist equipment for limited application. Store cooling: with suitable stacking and curtains, a good method with wide application. 4.8 Chilled storage Chilled storage equipment may be seen around the world in a wide range of sizes, each suited to the particular operation for which it is designed. At its smallest, it may be an absorption cycle refrigerator in a caravan or boat. There Fig. 4.3 Cooling tunnel arrangement within a store. The refrigeration of chilled foods 87