Reducing produce damage during transit
Aug 1, 2008 12:00 PM
The Floor of a highway truck is in constant vibration as it travels. When this vibration is excessive, products such as fresh fruits and vegetables or frozen items can be damaged by individual pieces moving with respect to each other or their package.
A session during the Scientists Speak portion of this year's IARW-WFLO-IRTA Annual Convention discussed how to diagnose transport vibration problems and offered some thoughts to minimizing damage. The presenter was James Thompson of the University of California, Davis' Biological & Agricultural Engineering Department.
Vibration levels must exceed the acceleration of gravity to cause individual items to bounce, rotate, or otherwise move in their package, he explained. With the exception of an occasional impact caused by a truck traveling over a pothole or separation in the roadway, a truck's floor rarely exceeds one G.
Vibration damage occurs because nearly all commonly used boxes amplify low frequency vibration as they transmit it from the wood pallet through boxes stacked upon it, said Thompson. The top boxes on a pallet can experience four to ten times the vibration of the first layer of boxes.
The area of a trailer most likely to experience transport vibration damage is near the rear doors, especially the top rear cases. The lowest vibration levels are near the trailer's center, between the wheels of the trailer and the tractor.
The front of a trailer usually doesn't experience high vibration levels because most long-haul tractors are equipped with air-ride suspension systems that remove much of the low frequency accelerations that cause product damage.
Bruising, surface marking, and other physical damage to produce can be caused by many steps in the handling process, Thompson said. Pickers can squeeze product too tightly, packinghouse equipment can allow pieces to fall too far, or product can be exposed to excessive vibration in transport.
The key difference between transport vibration damage and other causes is that the product damage is localized, he said. Because of the variation in vibration levels in a trailer, and the vibration amplification of the package, damage is usually limited to the contents of the top one or two layers of boxes in the one or two rows of pallets nearest the rear door.
If bruising damage is fairly consistent throughout the load, then the damage is not caused by transport vibration.
Prevention tips
Naturally, produce items vary in their susceptibility to vibration damage. For example, he noted that oranges and iceberg lettuce are typically not subject to this, whereas delicate items like strawberries and raspberries are very subject to vibration damage in a truck shipment.
One way to protect susceptible products from exposure to damaging levels of vibration is to use tractors and trailers equipped with air-ride suspensions, advised Thompson. Doing so reduces the levels of vibration that reach the trailer's floor.
Further, he suggested that sensitive items not be loaded in the two pallet rows close to the trailer doors, as that is where vibration levels are the greatest.
Specially designed packaging can also prevent fruit from rubbing against neighboring fruit or packaging, Thompson said. The key here is to immobilize it.
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