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