Transportation Developments in frozen transport in the 19th century established the inter- national food market. In 1877, a cargo of frozen meat was sent from Buenos Aires to France. The following year 5000 frozen mutton carcasses were transported from Paraguay to France. In 1880, the SS Strathleven arrived in London with a cargo of 40 tons of frozen Australian beef, and by 1910 Great Britain was importing 600000 tons of frozen meat. Further develop- ments in temperature controlled transportation systems for chilled prod ucts have led to the rapid expansion of the 'fresh' food market. The sea transportation of chilled meat from Australia to European and other distant markets, and road transportation of chilled products throughout Europe and the Middle East is now common practice. Air freighting is increasingly being used for high value perishable products such as strawberries, aspara gus and live lobsters(Sharp, 1988). However, foods do not necessarily have to fall into this category to make air transportation viable since it has been shown that 'the intrinsic value of an item has little to do with whether or not it can benefit from air shipment, the deciding factor is not price but mark-up and profit 9.1 Sea transport Historically it was the need to preserve meat during sea transport that lead to the development of mechanical refrigeration and the modern interna- tional trade in foodstuffs. Developments in temperature control, packaging and controlled atmospheres have substantially increased the range of foods that can be transported around the world in a chilled condition. With
9 Transportation Developments in frozen transport in the 19th century established the international food market. In 1877, a cargo of frozen meat was sent from Buenos Aires to France. The following year 5000 frozen mutton carcasses were transported from Paraguay to France. In 1880, the S. S. Strathleven arrived in London with a cargo of 40 tons of frozen Australian beef, and by 1910 Great Britain was importing 600000 tons of frozen meat. Further developments in temperature controlled transportation systems for chilled products have led to the rapid expansion of the ‘fresh’ food market. The sea transportation of chilled meat from Australia to European and other distant markets, and road transportation of chilled products throughout Europe and the Middle East is now common practice. Air freighting is increasingly being used for high value perishable products such as strawberries, asparagus and live lobsters (Sharp, 1988). However, foods do not necessarily have to fall into this category to make air transportation viable since it has been shown that ‘the intrinsic value of an item has little to do with whether or not it can benefit from air shipment, the deciding factor is not price but mark-up and profit’. 9.1 Sea transport Historically it was the need to preserve meat during sea transport that lead to the development of mechanical refrigeration and the modern international trade in foodstuffs. Developments in temperature control, packaging and controlled atmospheres have substantially increased the range of foods that can be transported around the world in a chilled condition. With
192 Meat refrigeration Table 9.1 Guidelines for meat shipment Vacuum pack,0°C Vacuum pack,-15°C CO2,-1.5°C 6 weeks 10 week >12 weeks Beef 10 weeks Source: Heap, 1997. conventional vacuum packing it is difficult to achieve a shelf-life in excess of 12 weeks with beef and 8 weeks for lamb(Gill, 1984). Controlled/mod ified atmospheric packaging can extend this by many weeks. Work in Nev Zealand has shown that a she wee achieved in cuts of lamb(Gill and Penney, 1986). The cuts were individu- ally packed in evacuated bags of linear polyethylene, and then placed in a foil laminate bag that was gas flushed and filled with a volume of carbon dioxide(Co2) approximately equal to that of the meat. Similar storage lives are currently being achieved with beef primals transported from Australia and South Africa to the EU Heap(1997) stated that assuming good stan- dards of preparation and prompt cooling, the times given in Table 9.1 could be used as approximate guidelines for long distance meat shipment These times rely on the meat being at or below the storage temperatur e before loading. The two to four week advantage of transporting meat at -15'C rather than 0C is lost if the meat is loaded at a temperature above 0C. Cooling in the centre of a load of meat is very slow and the meat will be well into its journey before the desired temperature is achieved Most International Standard Organisation(ISO) containers for food transport are either 6 or 12m long, hold up to 26 tonnes of product and can be 'insulated' or 'refrigerated(Heap, 1986). The refrigerated containers incorporate insulation and have refrigeration units built into their structure The units operate electrically, either from an external power supply on board the ship, or in dock, or from a generator on a road vehicle Insulated ontainers utilise either plug-type refrigeration units or may be connected directly to an air-handling system in a ship's hold or at the docks Close temperature control is most easily achieved in containers that are placed in insulated holds and connected to the ships refrigeration system. However, suitable refrigeration facilities must be available for any overland sections of the journey. When the containers are fully loaded and the cooled air is forced uniformly through the spaces between cartons, the maximum difference between delivery and return air can be less than 0.8C. The entire product in a container can be maintained to within +1.0'C of the set Refrigerated containers are easier to transport overland th n the Insu- lated types, but often have to be carried on deck when shipped because of
conventional vacuum packing it is difficult to achieve a shelf-life in excess of 12 weeks with beef and 8 weeks for lamb (Gill, 1984). Controlled/modified atmospheric packaging can extend this by many weeks. Work in New Zealand has shown that a shelf-life of up to 23 weeks at -2 °C can be achieved in cuts of lamb (Gill and Penney, 1986). The cuts were individually packed in evacuated bags of linear polyethylene, and then placed in a foil laminate bag that was gas flushed and filled with a volume of carbon dioxide (CO2) approximately equal to that of the meat. Similar storage lives are currently being achieved with beef primals transported from Australia and South Africa to the EU. Heap (1997) stated that assuming good standards of preparation and prompt cooling, the times given in Table 9.1 could be used as approximate guidelines for long distance meat shipment. These times rely on the meat being at or below the storage temperatur e before loading. The two to four week advantage of transporting meat at -1.5 °C rather than 0 °C is lost if the meat is loaded at a temperature above 0 °C. Cooling in the centre of a load of meat is very slow and the meat will be well into its journey before the desired temperature is achieved. Most International Standard Organisation (ISO) containers for food transport are either 6 or 12m long, hold up to 26 tonnes of product and can be ‘insulated’ or ‘refrigerated’ (Heap, 1986). The refrigerated containers incorporate insulation and have refrigeration units built into their structure. The units operate electrically, either from an external power supply on board the ship, or in dock, or from a generator on a road vehicle. Insulated containers utilise either plug-type refrigeration units or may be connected directly to an air-handling system in a ship’s hold or at the docks. Close temperature control is most easily achieved in containers that are placed in insulated holds and connected to the ship’s refrigeration system. However, suitable refrigeration facilities must be available for any overland sections of the journey. When the containers are fully loaded and the cooled air is forced uniformly through the spaces between cartons, the maximum difference between delivery and return air can be less than 0.8°C. The entire product in a container can be maintained to within ±1.0 °C of the set point. Refrigerated containers are easier to transport overland than the insulated types, but often have to be carried on deck when shipped because of 192 Meat refrigeration Table 9.1 Guidelines for meat shipment Vacuum pack, 0 °C Vacuum pack, -1.5 °C CO2, -1.5 °C Pork 6 weeks 8 weeks – Lamb 7 weeks 10 weeks >12 weeks Beef 10 weeks 14 weeks – Source: Heap, 1997
Transportation 193 problems in operating the refrigeration units within closed holds On board ship, they are therefore subject to much higher ambient temperatures and consequently larger heat gains which make it far more difficult to control product temperatures. For bulk transportation of frozen meat, refrigerated cargo ships are com monly used(Heap, 1997). Frozen meat is generally stored and transported loading and unloading can be tolerated with frozen me a c at -18C or below. Unlike chilled meat, small temperature changes during 9.2 Air transport In the 1990s, the volume of perishables transported by air increased by 10-12%per year(Stera, 1999). Although airfreighting of foods offers a apid method of serving distant markets, there are many problems because the product is unprotected by refrigeration for much of its journey. Up to 80%of the total journey time is made up of waiting on the tarmac and transport to and from the airport. During flight the hold is normally between 15 and 20%C Perishable cargo is usually carried in standard con- tainers, sometimes with an insulating lining and/or dry ice but is often unprotected on aircraft pallets(Sharp, 1988) Sharp's studies in Australia have led to the following recommendations for air transport of chilled food Insulated containers should always be used to reduce heat gai Product should always be precooled and held at the required tempera ture until load With products that deteriorate after any surface freezing, dry ice should ot be used Containers should be filled to capacity. A thermograph should accompany each consignment 9.3 Overland transport Overland transportation systems range from 12m refrigerated containers for long distance road or rail movement of bulk chilled or frozen products to small uninsulated vans supplying food to local retail outlets or even directly to the consumer. Some of the first refrigerated road and rail veh cles for chilled product were cooled by air that was circulated by free or forced systems, over large containers of ice( Ciobanu, 1976). Similar systems using solid carbon dioxide as the refrigerant have also been used for cooling of transport vehicles. However, most overland vehicles for long distance transport are now mechanically refrigerated
problems in operating the refrigeration units within closed holds. On board ship, they are therefore subject to much higher ambient temperatures and consequently larger heat gains which make it far more difficult to control product temperatures. For bulk transportation of frozen meat, refrigerated cargo ships are commonly used (Heap, 1997). Frozen meat is generally stored and transported at -18 °C or below. Unlike chilled meat, small temperature changes during loading and unloading can be tolerated with frozen meat. 9.2 Air transport In the 1990s, the volume of perishables transported by air increased by 10–12% per year (Stera, 1999). Although airfreighting of foods offers a rapid method of serving distant markets, there are many problems because the product is unprotected by refrigeration for much of its journey. Up to 80% of the total journey time is made up of waiting on the tarmac and transport to and from the airport. During flight the hold is normally between 15 and 20 °C. Perishable cargo is usually carried in standard containers, sometimes with an insulating lining and/or dry ice but is often unprotected on aircraft pallets (Sharp, 1988). Sharp’s studies in Australia have led to the following recommendations for air transport of chilled foods: • Insulated containers should always be used to reduce heat gain. • Product should always be precooled and held at the required temperature until loading. • With products that deteriorate after any surface freezing, dry ice should not be used. • Containers should be filled to capacity. • A thermograph should accompany each consignment. 9.3 Overland transport Overland transportation systems range from 12 m refrigerated containers for long distance road or rail movement of bulk chilled or frozen products to small uninsulated vans supplying food to local retail outlets or even directly to the consumer. Some of the first refrigerated road and rail vehicles for chilled product were cooled by air that was circulated by free or forced systems, over large containers of ice (Ciobanu, 1976). Similar systems using solid carbon dioxide as the refrigerant have also been used for cooling of transport vehicles. However, most overland vehicles for long distance transport are now mechanically refrigerated. Transportation 193
194 Meat refrigeration Types of refrigeration system ajority of current road transport vehicles for chilled foods are refrig- using either mechanical, eutectic plates or liquid nitrogen cooling 93.1. Mechanical units Many types of independent engine and/or electric motor driven mechani cal refrigeration units are available for lorries or trailers. One of the most common is a self-contained 'plug unit which mounts in an opening pro- vided in the front wall of the vehicle. The condensing section is on the outside and the evaporator on the inside of the unit, separated by an insu- lated section that fits into the gap in the wall. Units have one or two com- pressors, depending upon their capacity, which can be belt driven from vehicle but are usually driven direct from an auxiliary engine. This engine may use petrol from the vehicle's supply, an independent tank, or liquid petroleum gas. Many are equipped with an additional electric motor for standby use or for quiet running, for example when parked or on a ferry Irrespective of the type of refrigeration equipment used, the product will not be maintained at its desired temperature during transportation unless it is surrounded by air or surfaces at or below that temperature. This is gravity, around the load. Inadequate air distribution is probably the princ usually achieved by a system that circulates moving air, either forced or ple cause of product deterioration and loss of shelf-life during transport Conventional forced air units usually discharge air over the stacked or sus- pended products either directly from the evaporator or through ducts towards the rear cargo doors. Because air takes the path of least resistance it circulates through the channels which have the largest cross-sectional area. These tend to be around rather than through the product. If products have been cooled to the correct temperature before loading and do not gen erate heat, then they only have to be isolated from external heat ingress. Surrounding them with a blanket of cooled air achieves this purpose. Care has to be taken during loading to avoid any product contact with the inner surfaces of the vehicle because this would allow heat ingress during trans- port. Many trucks are now being constructed with an inner skin that forms a return air duct along the side walls and floor, with the refrigerated air being supplied via a ceiling duct 9.3.1.2 Eutectic plates Eutectic plate cooling systems are used in refrigerated vehicles serving local distribution chains. The eutectic plate consists of a coil, through which a primary refrigerant can be passed, mounted inside a thin tank filled with a eutectic solution. Standard eutectic solutions freeze at temperatures between-3 and -50C and Table 9.2 lists some that have been applied in food chilling systems. A number of these plates are mounted on the wall and ceilings or used as shelves or compartment dividers in the vehicles. Two
9.3.1 Types of refrigeration system The majority of current road transport vehicles for chilled foods are refrigerated using either mechanical, eutectic plates or liquid nitrogen cooling systems. 9.3.1.1 Mechanical units Many types of independent engine and/or electric motor driven mechanical refrigeration units are available for lorries or trailers. One of the most common is a self-contained ‘plug’ unit which mounts in an opening provided in the front wall of the vehicle. The condensing section is on the outside and the evaporator on the inside of the unit, separated by an insulated section that fits into the gap in the wall. Units have one or two compressors, depending upon their capacity, which can be belt driven from the vehicle but are usually driven direct from an auxiliary engine. This engine may use petrol from the vehicle’s supply, an independent tank, or liquid petroleum gas. Many are equipped with an additional electric motor for standby use or for quiet running, for example when parked or on a ferry. Irrespective of the type of refrigeration equipment used, the product will not be maintained at its desired temperature during transportation unless it is surrounded by air or surfaces at or below that temperature. This is usually achieved by a system that circulates moving air, either forced or by gravity, around the load. Inadequate air distribution is probably the principle cause of product deterioration and loss of shelf-life during transport. Conventional forced air units usually discharge air over the stacked or suspended products either directly from the evaporator or through ducts towards the rear cargo doors. Because air takes the path of least resistance it circulates through the channels which have the largest cross-sectional area. These tend to be around rather than through the product. If products have been cooled to the correct temperature before loading and do not generate heat, then they only have to be isolated from external heat ingress. Surrounding them with a blanket of cooled air achieves this purpose. Care has to be taken during loading to avoid any product contact with the inner surfaces of the vehicle because this would allow heat ingress during transport. Many trucks are now being constructed with an inner skin that forms a return air duct along the side walls and floor, with the refrigerated air being supplied via a ceiling duct. 9.3.1.2 Eutectic plates Eutectic plate cooling systems are used in refrigerated vehicles serving local distribution chains. The eutectic plate consists of a coil, through which a primary refrigerant can be passed, mounted inside a thin tank filled with a eutectic solution. Standard eutectic solutions freeze at temperatures between -3 and -50 °C and Table 9.2 lists some that have been applied in food chilling systems. A number of these plates are mounted on the walls and ceilings or used as shelves or compartment dividers in the vehicles.Two 194 Meat refrigeration
Transportation 195 Table 9.2 Freezing point and latent heat of fusion of ome eutectic solutions Eutectic solution Freezing Latent heat of oint(C) fusion(kWhm-) Carbonate of soda Potassium bicarbonate Potassium chloride Ammonium chloride Sodium chloride ource: Lenotre. 1988. methods are commonly used for charging up the plates: (1) when the vehicle is in the depot the solutions are frozen by coupling the plates to stationary refrigeration plants via flexible pipes and(2)a condensing unit on the vehicle is driven by an auxiliary drive when the vehicle is in use and an electric motor when stationary To provide the required cooling capacity, the plates should be mounted so that air can circulate freely over both sides and over the product. Most systems rely on gravity circulation but some are equipped with fans, ducts and dampers for temperature control Eutectic systems are chosen for the simplicity, low maintenance and quietness of their operation but can suffer from poor temperature control. 9.3.1.3 Liquid nitrogen A typical liquid nitrogen system consists of an insulated liquid nitrogen storage tank connected to a spray bar that runs along the ceiling of the transport vehicle. Liquid nitrogen is released into the spray bar via a ther mostatically controlled valve and vaporises instantly as it enters the body of the vehicle. The air is then cooled directly utilising the change in the latent and sensible heat of the liquid nitrogen. Once the required air tem- perature has been reached the valve shuts off the flow of liquid nitrogen and the temperature is subsequently controlled by intermittent injections of liquid nitrogen Many advantages are claimed for liquid nitrogen transport systems (Table 9.3). It is also claimed thatthat long hauls can be carried out sin vehicles are available that will maintain a chilled cargo at 3C for 50h after a single charge of liquid nitrogen and that overall costs are comparable with mechanical systems. 9.3.2 Observations of transport Gill and Phillips(1993)found that the deep temperature in beef sides and quarters at the time of their loading into transport vehicles in three USA plants ranged from 6 to 18C(Fig 9. 1). Maximum surface temperatures
methods are commonly used for charging up the plates: (1) when the vehicle is in the depot the solutions are frozen by coupling the plates to stationary refrigeration plants via flexible pipes and (2) a condensing unit on the vehicle is driven by an auxiliary drive when the vehicle is in use and an electric motor when stationary. To provide the required cooling capacity, the plates should be mounted so that air can circulate freely over both sides and over the product. Most systems rely on gravity circulation but some are equipped with fans, ducts and dampers for temperature control. Eutectic systems are chosen for the simplicity, low maintenance and quietness of their operation but can suffer from poor temperature control. 9.3.1.3 Liquid nitrogen A typical liquid nitrogen system consists of an insulated liquid nitrogen storage tank connected to a spray bar that runs along the ceiling of the transport vehicle. Liquid nitrogen is released into the spray bar via a thermostatically controlled valve and vaporises instantly as it enters the body of the vehicle. The air is then cooled directly utilising the change in the latent and sensible heat of the liquid nitrogen. Once the required air temperature has been reached the valve shuts off the flow of liquid nitrogen and the temperature is subsequently controlled by intermittent injections of liquid nitrogen. Many advantages are claimed for liquid nitrogen transport systems (Table 9.3). It is also claimed that that long hauls can be carried out since vehicles are available that will maintain a chilled cargo at 3 °C for 50h after a single charge of liquid nitrogen and that overall costs are comparable with mechanical systems. 9.3.2 Observations of transport Gill and Phillips (1993) found that the deep temperature in beef sides and quarters at the time of their loading into transport vehicles in three USA plants ranged from 6 to 18 °C (Fig. 9.1). Maximum surface temperatures Transportation 195 Table 9.2 Freezing point and latent heat of fusion of some eutectic solutions Eutectic solution Freezing Latent heat of point (°C) fusion (kW h m-3 ) Carbonate of soda -3 92 Potassium bicarbonate -6 85 Potassium chloride -10 94 Ammonium chloride -15 89 Sodium chloride -21 74 Source: Lenotre, 1988
196 Meat refrigeration Table 9.3 Claimed advantages of liquid nitrogen refrigerated vehicles Advantages of nitrogen refrigerated vehicles Minimum maintenance Improved vehicle utilization Accurate temperature control Rapid initial temperature reduction Uniform cargo temperature Modified atmosphere transportation Silent Environmentally acceptable Low capital costs Multicompartment triple-temperature operation Low operating costs Flexible refrigeration Source: Smith. 1986 6 6.57.58.59.510.511.512513.514.515.516.51751 Temperature(C) Fig 9.1 Deep leg temperatures when loaded into railway wagons or road trailers (source: Gill and Phillips, 1993) 420 86420 0.5 Fig 9.2 Maximum surface on beef sides and quarters when loaded(source: Gill and Phillips, 1993) were also high and ranged from 0.5 to 6.5C (Fig 9.2 ). In rail wagons the surface temperature decreased during the first 24 h and was subsequentl maintained at a temperature of 0+1C. In the road vehicles the surface emperature fell slowly during the whole journey and had not attained a steady minimum value when unloaded. On average the deep temperature of sides in rail wagons reached 1C after 72h. Temperatures in quarters in road vehicles were still above 2 C after 120h
were also high and ranged from 0.5 to 6.5°C (Fig 9.2). In rail wagons the surface temperature decreased during the first 24 h and was subsequently maintained at a temperature of 0 ± 1 °C. In the road vehicles the surface temperature fell slowly during the whole journey and had not attained a steady minimum value when unloaded. On average the deep temperature of sides in rail wagons reached 1 °C after 72 h. Temperatures in quarters in road vehicles were still above 2 °C after 120 h. 196 Meat refrigeration Table 9.3 Claimed advantages of liquid nitrogen refrigerated vehicles Advantages of nitrogen refrigerated vehicles Simple, dependable operation Improved pay loads Minimum maintenance Improved vehicle utilization Accurate temperature control Rapid initial temperature reduction Uniform cargo temperature Modified atmosphere transportation Silent operation Environmentally acceptable Low capital costs Multicompartment triple-temperature operation Low operating costs Flexible refrigeration Source: Smith, 1986. 0 2 4 6 8 6.5 7.5 8.5 9.5 10.5 11.5 12.5 13.5 14.5 15.5 16.5 17.5 18.5 Temperature (°C) Frequency Rail Road All Fig. 9.1 Deep leg temperatures when loaded into railway wagons or road trailers (source: Gill and Phillips, 1993). 0 2 4 6 8 10 12 14 Frequency 0.5 1.5 2.5 3.5 4.5 5.5 6.5 Temperature (°C) Rail Road All Fig. 9.2 Maximum surface on beef sides and quarters when loaded (source: Gill and Phillips, 1993)
Transportation 197 20.5 145 125 Frequency Fig. 9.3 Predicted proliferation of pseudomonads during rail and road trans- portation of beef(source: Gill and Phillips, 1993) o Journey times varied from 3. 8 to 6.7 days and it was calculated that Iring that time pseudomonads could proliferate by between 8 and 22 generations(Fig. 9.3). Adequate air movement through the load in the rail vehicles produced even temperature distributions. The authors thought that by better policing of the temperature set point, rail transportation could be substantially improved. However, road vehicles could not cool the meat during transport and had to be loaded with cold meat. In general it is not advisable to rely on product cooling during trans- portation, however, in the Netherlands 'in transport cooling is an integral part of a processing system for pork carcasses that allows pigs to be dis- patched on the same day they are killed. Studies have been made on the long distance chilled transportation of retailer-ready'portions of beef and their subsequent retail display life (Bell et aL., 1996). Cuts 10cm thick weighing between 750 and 1000 g were packed in clear plastic high oxygen barrier film, metallised film or conven- tional vacuum bags. Cuts in clear plastic or metallised film were further packed in oxygen-free saturated carbon dioxide atmosphere. Cuts were hen stored and transported for between 39 and 89 days at between 0 and -"C. However, colour stability of the fats limited the retail display life to ca. 48h. The lean meat colour and sensory attributes remained acceptable for 48h after the cuts were rejected because of grey-green fat discolouration 9.3.3 Problems particular to local delivery vehicles In a 1970-71 survey of vehicles used to transfer chilled meat from small abattoirs to shops(Table 9. 4 ), almost 70% were unrefrigerated and 20% had no insulation(Cutting Malton, 1972) Eight of the mechanically refrigerated vehicles had propane-driven R 12 compressors, and one diesel, and they could be mains-operated when static
Journey times varied from 3.8 to 6.7 days and it was calculated that during that time pseudomonads could proliferate by between 8 and 22 generations (Fig. 9.3). Adequate air movement through the load in the rail vehicles produced even temperature distributions.The authors thought that by better policing of the temperature set point, rail transportation could be substantially improved. However, road vehicles could not cool the meat during transport and had to be loaded with cold meat. In general it is not advisable to rely on product cooling during transportation, however, in the Netherlands ‘in transport cooling’ is an integral part of a processing system for pork carcasses that allows pigs to be dispatched on the same day they are killed. Studies have been made on the long distance chilled transportation of ‘retailer-ready’ portions of beef and their subsequent retail display life (Bell et al., 1996). Cuts 10 cm thick weighing between 750 and 1000 g were packed in clear plastic high oxygen barrier film, metallised film or conventional vacuum bags. Cuts in clear plastic or metallised film were further packed in oxygen-free saturated carbon dioxide atmosphere. Cuts were then stored and transported for between 39 and 89 days at between 0 and -1 °C. However, colour stability of the fats limited the retail display life to ca. 48 h. The lean meat colour and sensory attributes remained acceptable for 48 h after the cuts were rejected because of grey-green fat discolouration. 9.3.3 Problems particular to local delivery vehicles In a 1970–71 survey of vehicles used to transfer chilled meat from small abattoirs to shops (Table 9.4), almost 70% were unrefrigerated and 20% had no insulation (Cutting & Malton, 1972). Eight of the mechanically refrigerated vehicles had propane-driven R 12 compressors, and one diesel, and they could be mains-operated when static. Transportation 197 8.5 10.5 12.5 14.5 16.5 18.5 20.5 0 12 3456 Frequency Generations Road Rail Fig. 9.3 Predicted proliferation of pseudomonads during rail and road transportation of beef (source: Gill and Phillips, 1993)
198 Meat refrigeration Table 9.4 Types of road vehicle investigated in Meat Research Institute(MRI) Survey(1970-71) System No of cases Liquid nitrogen Insulated non r 9287 Uninsulated Source: Cutting and Malton, 1972. The uninsulated vehicles were mostly 10cwt(0.5 ton) delivery vans, with no partition between driver and load Since that time the intensifying demand from legislation and retailers for ower delivery temperatures, has put increasing pressure on fleet operators to improve temperature control. However, there are substantial difficulties n maintaining the temperature of chilled foods transported in small refrig erated vehicles that conduct multi-drop deliveries to retail stores and cater ers. The vehicles have to carry a wide range of products and operate under diverse ambient conditions. During any one delivery run, the chilled product can be subjected to as many as fifty door openings, where there is heat ingress directly from outside and from personnel entering to select and remove product. The design of the refrigeration system has to allow for extensive dif- ferences in load distribution, dependent on different delivery rounds, days of the week and the removal of product during a delivery run. The ability of a refrigeration system to respond to sudden demands for increased refrig- eration is often restricted by the power available from the vehicle. All these problems combine to produce a complex interactive system In the UK, current sales of chilled foods are expanding. The overall market worth for chilled foods increased from f5. 8 billion in 1992 to f6.2 billion in 1993. In the meat industry the traditional range of pies, pasties, sausages and cooked meats has been rapidly developed with the addition of fermented meats, restructured products, Kievs, salads, stir fry products, and vegetarian burgers and rissoles. Traditional meat product manufactur- ers are now aiming to extend their range of products to include items such as gourmet-style meals without artificial colours, flavours or preservatives. Retailers are discovering that considerable quality and economic advan tages can be derived from maintaining chilled products at temperatures far closer to their initial freezing point. Increasingly, fleet operators will be forced to deliver chilled foods at temperatures between 0 and 2C. 9.3.4 Design and operation of local distribution vehicles There will be few real advances in the design of chilled distribution vehicles until there is a firm understanding of the interaction between the
The uninsulated vehicles were mostly 10cwt (0.5 ton) delivery vans, with no partition between driver and load. Since that time the intensifying demand from legislation and retailers for lower delivery temperatures, has put increasing pressure on fleet operators to improve temperature control. However, there are substantial difficulties in maintaining the temperature of chilled foods transported in small refrigerated vehicles that conduct multi-drop deliveries to retail stores and caterers. The vehicles have to carry a wide range of products and operate under diverse ambient conditions. During any one delivery run, the chilled product can be subjected to as many as fifty door openings,where there is heat ingress directly from outside and from personnel entering to select and remove product.The design of the refrigeration system has to allow for extensive differences in load distribution, dependent on different delivery rounds, days of the week and the removal of product during a delivery run.The ability of a refrigeration system to respond to sudden demands for increased refrigeration is often restricted by the power available from the vehicle. All these problems combine to produce a complex interactive system. In the UK, current sales of chilled foods are expanding. The overall market worth for chilled foods increased from £5.8 billion in 1992 to £6.2 billion in 1993. In the meat industry the traditional range of pies, pasties, sausages and cooked meats has been rapidly developed with the addition of fermented meats, restructured products, Kievs, salads, stir fry products, and vegetarian burgers and rissoles. Traditional meat product manufacturers are now aiming to extend their range of products to include items such as gourmet-style meals without artificial colours, flavours or preservatives. Retailers are discovering that considerable quality and economic advantages can be derived from maintaining chilled products at temperatures far closer to their initial freezing point. Increasingly, fleet operators will be forced to deliver chilled foods at temperatures between 0 and 2 °C. 9.3.4 Design and operation of local distribution vehicles There will be few real advances in the design of chilled distribution vehicles until there is a firm understanding of the interaction between the 198 Meat refrigeration Table 9.4 Types of road vehicle investigated in Meat Research Institute (MRI) Survey (1970–71) System No. of cases Mechanical 9 Liquid nitrogen 2 Insulated, non refrigerated 18 Uninsulated 7 Source: Cutting and Malton, 1972
Transportation 199 → No food 0 mm to 5 kw 42186420 50100150200 Fig 9.4 Heat extracted by the refrigeration plant during the standard journey for vans with different thickness of insulation, with and without food(source refrigeration system, the vehicles construction, the air movement within he vehicle, the external environment, the operation of the vehicle and the temperature of the foodstuff. a project recently completed by the Food Refrigeration and Process Engineering Research Centre(FRPERC)as part of a MAFF LINK scheme has produced a predictive model that will assist fleet operators in specifying the design of and the equipment for small delivery vehicles(Gigiel, 1998) Refrigerated vehicles are developed and tested in carefully controlled conditions. Owing to the large number of interacting variables, as many as possible are held constant during the tests. The prediction programme allows for systematic alteration of one or more variables whilst simulating the operation of a vehicle in a complex, realistic way. The verified model has provided valuable data on the factors influenc- ing food temperature and van performance. 93. 4. Van insulation The heat extracted by the refrigeration system during the journey is she plotted against the thickness of insulation in Fig. 9.4. Only a small thickness of insulation greatly reduces the amount of heat that has to be extracted, the amount decreasing with the reciprocal of the thickness of insulation. In all cases, van and food temperatures were maintained at less than 5C. This was only achieved in the case with no insulation by fitting the vehicle with a refrigeration system with a nominal capacity of 10kW. The food in the van modified the action of the thermostat and reduced the running time of he refrigeration system and the heat extracted by it. The reduction was 43% when the insulation was 75 mm thick 9.3. 4.2 Infiltration The heat extracted from a poorly sealed van was 86% more than from a well-sealed van(Fig. 9.5). However, infiltration during the time that the
refrigeration system, the vehicle’s construction, the air movement within the vehicle, the external environment, the operation of the vehicle and the temperature of the foodstuff. A project recently completed by the Food Refrigeration and Process Engineering Research Centre (FRPERC) as part of a MAFF LINK scheme has produced a predictive model that will assist fleet operators in specifying the design of and the equipment for small delivery vehicles (Gigiel, 1998). Refrigerated vehicles are developed and tested in carefully controlled conditions. Owing to the large number of interacting variables, as many as possible are held constant during the tests. The prediction programme allows for systematic alteration of one or more variables whilst simulating the operation of a vehicle in a complex, realistic way. The verified model has provided valuable data on the factors influencing food temperature and van performance. 9.3.4.1 Van insulation The heat extracted by the refrigeration system during the journey is shown plotted against the thickness of insulation in Fig. 9.4. Only a small thickness of insulation greatly reduces the amount of heat that has to be extracted, the amount decreasing with the reciprocal of the thickness of insulation. In all cases, van and food temperatures were maintained at less than 5 °C. This was only achieved in the case with no insulation by fitting the vehicle with a refrigeration system with a nominal capacity of 10 kW. The food in the van modified the action of the thermostat and reduced the running time of the refrigeration system and the heat extracted by it. The reduction was 43% when the insulation was 75 mm thick. 9.3.4.2 Infiltration The heat extracted from a poorly sealed van was 86% more than from a well-sealed van (Fig. 9.5). However, infiltration during the time that the Transportation 199 2 1.8 0.8 0.6 0.2 0.4 0 0 1.6 1.4 1.2 1 50 100 150 200 No food 1 tonne food 0 mm to 5 kW Thickness (mm) Heat extracted (J) Fig. 9.4 Heat extracted by the refrigeration plant during the standard journey for vans with different thickness of insulation, with and without food (source: Gigiel, 1998)
200 Meat refrigeration 品品杀工 8642 Amount of infiltration Fig 9.5 Heat extracted by the refrigeration plant during the standard journey for a well, and a poorly sealed van(source: Gigiel, 1998) door is closed is a relatively small proportion of the total refrigeration load In this vehicle, fitted with a nominal 2kW cooling system, the state of the seals did not cause the temperature of the food to increase to more than 5° C during the journey 9.3.4.3 Weight of fittings and thermal mass of lining The weight of the fittings and the thermal mass of the lining form a size able refrigeration load and take a finite amount of time to cool. However, n a sales van this normally takes place late in the evening or in the early hours of the morning when ambient temperatures are low and no other loads are imposed on the van. The load is therefore smaller than the size of the refrigeration plant fitted and the short pull down time from 28 to 5C would not warrant keeping the refrigeration system running all night. However, if the vehicle was in continuous use for the transport of foods at different temperatures(chilled and frozen) then pull-down times between changing loads could be a serious disadvantage. 9.3.4.4 Door openings The heat extracted from a closed van is very small(Fig. 9.6). Door open ings greatly increase the heat load and, when the van engine drives the refrigeration system, this extra heat must all be removed during the period when the van is moving. Several factors interact when the number of door openings increases. The complete journey takes longer and during the extended journey the ambient temperature and the solar radiation on the van is different from the early part of the journey. If the length of time that he door is left open at each stop is also increased from 5 to 10min then the temperature of the air around the food in the van increases more during each stop. The refrigeration plant therefore operates at a higher evaporat
door is closed is a relatively small proportion of the total refrigeration load. In this vehicle, fitted with a nominal 2 kW cooling system, the state of the seals did not cause the temperature of the food to increase to more than 5 °C during the journey. 9.3.4.3 Weight of fittings and thermal mass of lining The weight of the fittings and the thermal mass of the lining form a sizeable refrigeration load and take a finite amount of time to cool. However, in a sales van this normally takes place late in the evening or in the early hours of the morning when ambient temperatures are low and no other loads are imposed on the van. The load is therefore smaller than the size of the refrigeration plant fitted and the short pull down time from 28 to 5 °C would not warrant keeping the refrigeration system running all night. However, if the vehicle was in continuous use for the transport of foods at different temperatures (chilled and frozen) then pull-down times between changing loads could be a serious disadvantage. 9.3.4.4 Door openings The heat extracted from a closed van is very small (Fig. 9.6). Door openings greatly increase the heat load and, when the van engine drives the refrigeration system, this extra heat must all be removed during the period when the van is moving. Several factors interact when the number of door openings increases. The complete journey takes longer and during the extended journey the ambient temperature and the solar radiation on the van is different from the early part of the journey. If the length of time that the door is left open at each stop is also increased from 5 to 10min then the temperature of the air around the food in the van increases more during each stop. The refrigeration plant therefore operates at a higher evaporat- 200 Meat refrigeration 20 18 16 14 12 10 8 6 4 2 0 good bad Amount of infiltration Heat extracted (MJ) Fig. 9.5 Heat extracted by the refrigeration plant during the standard journey for a well, and a poorly sealed van (source: Gigiel, 1998)
