Factors influencing dynamic mechanical properties of red 'Delicious' apple tissue.

This research measured tissue failure stress, failure strain, and shock wave speed in dynamic axial compression rests to assess, with respect to these properties: (I) the homogeneity of Red 'Delicious' apple tissue samples (10 mm dia., 15.2 mm long) taken around the apple equator; (2) the isotropy between tissue samples taken radially and parallel to the stem-calyx axis; and (3) the effects of strain rate and fruit size on dynamic (impact) properties. Results were compared with Magness-Taylor penetrometer measurements. At strain rates from 20 to 150 s(-1) (equivalent to drop heights of 5 to 265 mm, respectively), the measured tissue properties were homogeneous around the periphery of the apple, but not isotropic for radial versus parallel sample orientation, Radial samples had lower failure strain but slightly higher failure stress than parallel samples. Smaller apple tissue had lower failure strain, higher failure stress, higher elastic modulus, and higher shock wave speed than that of larger apples. Failure stress and strain both increased as strain rate increased from 20 to 150 s(-1). However the failure strain was largest at the quasi-static strain rate (0.027 s(-1), likely due to a bio-yield occurring at the quasi-static strain rate, but nor at dynamic strain rates. As expected very poor linear correlations occurred between the Magness-Taylor penetrometer force and dynamic failure properties. Generally, correlations were better at the lower than at the higher strain rates, which is logical considering the viscoelastic nature of apples and the quasi-static loading rates used in Magness-Taylor measurements, important conclusions are that measurement of dynamic failure properties must take into consideration fruit size and strain rare, and that Magness-Taylor tests cannot predict such properties, at least in the apples we measured.