Piezoelectric transducers for real-time evaluation of fruit firmness. Part I:Theory and development of acoustic techniques

This paper presents a non-destructive method for evaluating the firmness of fruits. Three novel stiffnessindexes based on acoustic methods and involving Young's modulus and sound propagation velocity areproposed. The theoretical background leading to their formulation from physical considerations is pre-sented. The effectiveness of the considered indexes is tested by means of an experimental setup builtwith two piezoelectric transducers contacting several samples of kiwifruits during their ripening pro-cess. The dependency on the geometrical position of the contact and the robustness against simplifyingassumptions, such as uniform density or roundness, is investigated. The obtained experimental resultsshow that stiffness indexes based on propagation delays are more rapid and reliable than those based onfruit resonance in assessing the ripeness degree.