In this sense, research has been carried out for many years to determine the resistance of fruit and vegetables to compression force. Kaufmann (1970) measured the effect of water potential and temperature on the extensibility of citrus peel. Sarig and Nahir (1973) reported the initial and permanent deformation of a creep test on citrus fruit to indicate firmness, and Gyasi et al. (1981) determined Poisson's ratio for citrus fruit peel and pulp. Singh and Reddy (2006) provided information on changes in postharvest physico-mechanical properties of orange peel and fruit under ambient and refrigerated storage conditions in order to help to determine the appropriate handling, packaging, storage and transportation systems. Although penetrometer measurements are widely used, since they are moderately well correlated with human perception of firmness and with storage life, there are some problems related to the use of probes of varied geometries (rounded, hemispherical or flat tips) under the generic term Magness–Taylor (Abbott, 1999). On the other hand, devices used for deformation test measurements are expensive, and these techniques are slow, destructive and not very adaptable for on-line sorting of horticultural products.