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 refrigeratedproblems 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 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 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 foods: • Insulated containers should always be used to reduce heat gain. • Product should always be precooled and held at the required tempera￾ture 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 vehi￾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. Transportation 193