Measuring the Surface Area of Apples for Expression of Pesticide Residues

Several methods of measuring the surface area of apples have been reported in the literature, but with little supporting data for their selection or by which to judge the magnitude of the errors associated with these methods. The present study was undertaken to evaluate the accuracy and precision of the use of weight, volume displacement, and diameter using planimeter measurement as a standard for determining the surface area of apples. Because methods of calculating surface area assume apples to be perfect spheres, ping-pong balls were used as working standards. McIntosh and Delicious apples were sampled from two areas in each tree through the season for measurement and supplemented by weight and volume displacement data collected routinely from apple residue studies in previous years. All methods used were found comparable on ping-pong balls. Surface areas of apples computed from volume displacement and the planimeter were not significantly different. Location of the fruit in a tree, development of the fruit during the season, and the variation from year to year were found to affect specific gravity significantly. Surface areas using weight and a constant specific gravity were significantly different from planimeter values on Golden Delicious but not on McIntosh. Both volume displacement and the weight method were superior to the others studied. Sample size, sources of variation, and bias were evaluated and are discussed.