Mechanical damage occurs in postharvest handling primarily in two modes, impact forces and compressive
forces. When apples are handled in bulk bins, the compressive forces are static when stationary or dynamic during the
handling and transport process. Developing an understanding of the characteristics of the actual compressive forces and
interactions between apples, will enable bulk systems to be designed to eliminate potentially damaging forces. Prototype
low profile force sensors were developed to measure static and dynamic compressive forces on individual fruit in bulk
bins. The sensors provide a uniform sensitivity over a 73 mm (2.9 in.) diameter surface. Using 10 sensors, compressive
forces were measured in the floor of five bulk bin designs (one hardwood, one plywood and three plastic) as they were
transported over a 1.3 km (0.8 mile) orchard course using a short tined tractor-mounted fork-lift. The compression data
collected was used to develop force spectral density (FSD) representations for each force sensor. Damage-free apples
were placed on the floor of each bin, and accumulated damage was measured. Bruise damage was higher in all bin
designs for test fruit positioned at the section of the bin floor furthest from the tractor. The range of bruise damaged fruit
was 15.0 to 47.5% and graded 67.5 to 92.5% U.S. Extra Fancy. The magnitude of compressive forces measured with each
sensor was inconsistent among bin designs; however, higher forces were recorded in the middle of the bin and the end of
the bin away from the tractor. Predominant FSD levels were found at frequencies between 1.0 to 2.0 Hz and 3.5 to 4.5 Hz
for all 5 bin designs. Bruise damage best correlates to the amplitude of FSD at a frequency of 4 Hz.