Sonic vibrational characteristics of intact apples are related to flesh elasticity which is, in turn, related to firmness. Firmness changes in Golden Delicious and Delicious apples were followed during accelerated ripening and under storage conditions. Firmness was measured by Magness-Taylor puncture force and by compression of tissue cylinders (modulus of elasticity and rupture strength) for comparison with sonic vibrational characteristics of intact apples. Influences of apple temperature, size, shape, and skin on sonic spectra were investigated. Sonic resonant frequencies were significantly correlated with destructive firmness measurements and decreased as storage time increased. Sonic amplitudes were not closely related to firmness. Regression equations incorporating sonic data and size were developed to predict Magness-Taylor force. Use of sonic vibrational characteristics is proposed as a rapid nondestructive method for firmness sorting of apples.
`Delicious' apples (Malus domestica Borkh.) from five major U.S. production areas were tested after ≈3 months of commercial storage. Soluble solids concentration (SSC), titratable acidity (TA), Magness-Taylor (MT) firmness, and sonic transmission spectra were compared with ripeness (maturity in trade terminology) scores assigned by six U.S. Dept. of Agriculture (USDA)-licensed apple inspectors according to USDA Grades and Standards inspection procedures. USDA ripeness categories are defined by textural and flavor terms. Inspectors in this test used visual, manual, oral, and auditory sensations to make their judgments, but firmness was the paramount characteristic judged. SSC and TA did not correlate with inspectors' scores, MT, or sonic measurements and thus are not satisfactory indices of ripeness for stored apples. Sonic resonance functions correlated significantly with mean inspectors' scores and with MT firmness. Inspectors' scores correlated slightly better with MT firmness than with sonic terms. MT is destructive and site-specific; in contrast, sonic measurements are nondestructive and representative of the entire fruit.