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Temperature management is critical during long distance transport, so loads must be stacked to enable proper air circulation to carry away heat from the produce itself as well as incoming heat from the atmosphere and off the road. Transport vehicles should be well insulated to maintain cool environments for pre-cooled commodities and well ventilated to allow air movement through the produce. During transport, produce must be stacked in ways that minimize damage, then be braced and secured. An open air vehicle can be loaded in such a way that air can pass through the load, and provide some cooling of the produce as the vehicle moves. Traveling during the night and early morning can reduce the heat load on a vehicle that is transporting produce. Drivers of vehicles used for shipping produce must be trained in how to load and handle their cargoes. There tends to be a large turnover in drivers (in the US the average time on the job is only 3.5 years) so training is a constant concern (Hagen, et al., 1999).
Recent documents report that carrying mixed loads in North America is still a very common practice, especially with vegetable shipments (Hagen et al, 1999). Mixed loads can be a serious concern when temperature optima are not compatible (for example, when transporting chilling sensitive fruits with commodities that require very low temperatures) or when ethylene producing commodities and ethylene sensitive commodities are transported together. High ethylene producers (such as ripe bananas, apples, cantaloupe) can induce physiological disorders and/or undesirable changes in color, flavor and texture in ethylene sensitive commodities (such as lettuce, cucumbers, carrots, potatoes, sweet potatoes).
A wide range of pallet covers are available for covering cooled product during handling and transportation. Polyethylene covers are inexpensive and lightweight, and protect pallet loads from dust, moisture and some loss of cold. Lightweight insulated covers can protect the load from heat gain for several hours (for example, during a delay in loading). Heavyweight covers are sometimes used to protect tropical products from the cold when shipped during winter. Visit the internet for more information (keywords: produce pallet covers) or order a catalog from the International Ripening Company ( http://www.QAsupplies.com or 800-472-7205).
Bulk loads of produce should be carefully loaded so as not to cause mechanical damage. Vehicles can be padded or lined with a thick layer of straw. Woven mats or sacks can be used in the beds of small vehicles. Other loads should not be placed on top of the bulk commodity.
Source: Wilson , J. No date. Careful Storage of Yams: Some Principles to Reduce Losses. London : Commonwealth Secretariat/ International Institute of Tropical Agriculture.
Cooling open loads is desirable whenever possible. A truck ventilating device can be constructed for an un-refrigerated open vehicle by covering the load loosely with canvas and fashioning a wind catcher from sheet metal. The scoop should be mounted at the front of the bed and should reach somewhat higher than the height of the cab. High transportation speeds and/or long distance transport run the risk of causing excess drying of the crop.
Source: Pantastico, Er. B 1980. FAO/UNEP Expert Consultation on Reduction of Food Losses in Perishable Products of Plant Origin, Working Document 2: Fruits and Vegetables. (6-9 May, Rome : FAO)
This ventilating system was designed for hauling bulk loads of fresh fava beans in Iran . The wind catcher and ducts were constructed using wooden crates. After removing their end panels the crates were wired together into the pattern shown below. Air flows upward through the load during transport, helping to keep the produce from overheating. This system has also been used in pick-up trucks, and for hauling bulk greens and green beans. Best results are obtained when transporting during the early morning hours, before sunrise.
Source: Kasmire, R.F. 1994. Personal Communication.
The USDA post harvest cooling program stresses to small growers the need to:
This small cooler uses a 12,000 BTU/hr (1 ton) 110 volt room window air conditioner to cool air inside the insulated box. The cool air inside the front of the box is forced through the produce by a pressure fan in a second wall inside. The return air passes under a false floor to the front of the box.
The Portacooler was built by a cooperating team at USDA, Beltsville , Maryland , to cool highly perishable berries and other air cooled produce. Approximate cost is $US 1200, but the cost can be considerably lower if it is built using a used air conditioning unit. Complete plans are available on the internet on the ATTRA website http://www.attra.org
Source: Ross, D.S. 2002 E-mail:[email protected]
For optimum transport temperature management, refrigerated trailers need insulation, a high capacity refrigeration unit and fan, and an air delivery duct. The checklist below includes these and other desired features in a top-air delivery trailer.
Refrigerated Vehicles-- Pre-loading Checklist
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Refrigeration unit operating properly? |
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Thermostat calibrated? |
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Refrigeration air chutes and ducts properly installed and in good repair? |
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Door seals in good condition? |
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Doors seal tightly when closed? |
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Walls free of cracks and holes? |
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Front bulkhead installed? |
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Floor drains open? |
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Inside of vehicle clean and odor-free? |
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Floor grooves free of debris? |
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Inside height, width, length adequate for load? |
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Load braces and other devices available to secure load? |
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Is the vehicle trailer pre-cooled (or pre-warmed)? |
Source: Ashby, 1995
The condition of the inside of a refrigerated trailer affects its ability to maintain desired temperatures during transport. Handlers should inspect the trailer before loading, and check these features:
Source: Kasmire, R.F. and Hinsch, R.T. 1987. Maintaining Optimum Transit Temperatures in Refrigerated Truck Shipments of Perishables. University of California Perishables Handling Transportation Supplement No. 2.
Bushels (very sturdy, uniformly made, stackable baskets) of produce can be loaded into a refrigerated trailer using a pattern of alternately inverted layers that leave plenty of space between rows for air circulation.
Source: Ashby, B. H. et al. 1987. Protecting Perishable Foods During Transport by Truck. Washington , D.C. : USDA, Office of Transportation, Agricultural Handbook No. 669.
Produce transported in cartons should also be stacked so as to allow adequate air circulation throughout the load. The diagram below illustrates cross-wise offset loading of partial telescopic containers. On the floor of the truck, pallets or other supports should be used to keep the cartons out of direct contact with the floor.
Source: Ashby, B. H. et al. 1987. Protecting Perishable Foods During Transport by Truck. Washington , D.C. : USDA, Office of Transportation, Agricultural Handbook No. 669.
When cartons of various sizes must be loaded together, the larger, heavier containers should be placed on the bottom of the load. Parallel channels should be left for air to move through the length of the load.
Source: Nicholas, C.J. 1985. Export Handbook for U.S. Agricultural Products. USDA, Office of Transportation, Agricultural Handbook No. 593
Often the large containers used for cut flower packaging must be hand-stacked when loaded into a transport vehicle. The best loading pattern for cut flowers is known as the pigeon hole pattern, where boxes are stacked in alternating solid and open layers, and channels are left down both side walls. This pattern provides channels for air circulation lengthwise through the load, and allows every box to be in direct contact with refrigerated air.
Source: Rij, R et al. 1979. Handling, Precooling and Temperature Management of Cut Flower Crops for Truck Transportation. USDA Science and Education Administration, AAT-W-5, UC Leaflet 21058.
Containers should be loaded so that they are away from the side walls and the floor of the transport vehicle in order to minimize the conduction of heat from the outside environment. In the diagrams below, the numbers of cartons refer to how many cartons would be in contact with the walls and floor of the truck when fully loaded.
Only the load on the bottom right is fully protected from heat transfer. The use of pallets keeps the cartons off the floor, while center-loading leaves an insulating air space between the pallet loads and the outside walls.
Source: Ashby, B. H. et al. 1987. Protecting Perishable Foods During Transport by Truck. Washington , D.C. : USDA, Office of Transportation, Agricultural Handbook No. 669.
When stacking containers, be sure to align them properly. Whenever possible, stack them so that corner matches corner on both the cartons and the pallet. Most of the strength of corrugated fiberboard containers is in their corners, so an over-hang of only 1 inch will decrease stacking strength by 15 to 34%.
Well-aligned stack of cartons has the strongest stacking strength possible
There should always be a void between the last stack of produce and the back of the transport vehicle. The load should be braced to prevent shifting against the rear door during transit. If the load shifts, it can block air circulation, and fallen cartons can present great danger to workers who open the door at a destination market. A simple wooden brace can be constructed and installed to prevent damage during transport.
Source: Nicholas, C.J. 1985. Export Handbook for U.S. Agricultural Products. USDA, Office of Transportation, Agricultural Handbook No. 593
Bracing the load can be accomplished using wooden braces, load rails, air pillows or Styrofoam blocks. The key is to immobilize stacked produce to reduce damage during transport.
Source: Thompson, J.F. 2002. Transportation. In. Kader, A.A. Postharvest Technology of Horticultural Crops (3rd Edition). UC Publication 3311. University of California , Division of Agriculture and Natural Resources. pp.259-269.
To prevent shifting of the load in a cargo container for air transport, a piece of solid foam or folded fiber-board should be placed along the curved or triangular portion of the floor of the container. Cartons stacked on top will be much better supported and be held upright.
Source: McGregor, B. 1987. Tropical Products Handbook. USDA Office of Transportation Agricultural Handbook Number 668.
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