MECHANICAL PROPERTIES AND TEXTURE OF STORED APPLES

Abstract

ABSTRACT Granny Smith, Delicious and Jonathan apples were placed in cool store immediately after harvest and were tested for ultimate tensile strength, fracture resistance and hysteresis of tensile specimens, and bruise resistance of whole apples under slow and impact compression over a period of 18 weeks. Sensory panel assessments of apple flesh texture were also made. Ultimate tensile strength and fracture resistance showed strong negative correlations with time for all varieties. Ultimate tensile strength decreased from 200–300 kPa at the beginning of the test period to 10 kPa at the end. Fracture resistance fell from 200–300 Jm−2 to 10 Jm−2 over the same time. The hysteresis ratio was unaffected by storage time but showed some effect of conditioning at 20° C for one week. Hysteresis ratios were between 0.1 and 0.2. Bruise resistance also remained fairly constant over the storage period for all varieties for both slow and impact compression. The explanation of this behaviour is the increase in the number of ruptured cells per unit volume of bruised tissue as storage time increases. Sensory panel assessment showed an inability to sense long term textural changes. There was, however, good correlation between panel assessment, ultimate tensile strength and fracture resistance for varietal effects and storage treatments. The tensile properties of apple tissue, namely, ultimate tensile strength and fracture resistance, provide a good measure of deterioration of apples during storage.