An Objective Evaluation of Changes in Firmness of Ripening Bananas Using a Sonic Technique

SUMMARY— Firmness in fruits is sometimes evaluated as being the force necessary to attain a given deformation within the product. Since modulus of elasticity is defined as the ratio of stress to strain, it should measure resistance to force and, hence, the firmness of a material. Using a sonic technique, the resonant frequencies of cylindrical specimens of flesh from Valery bananas were measured and Young's modulus of elasticity was calculated. Softening of the banana during ripening was associated with a decrease in Young's modulus of elasticity from 272 × 10⁵ dynes/cm² at ripeness corresponding to a light green peel color to 85 × 10⁵ dynes/cm² at the yellow stage. Modulus of elasticity was significantly and directly correlated with starch content, but inversely correlated with luminous reflectance and the logarithm of percent reducing sugars.