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 stiffness indexes based on acoustic methods and involving Young's modulus and sound propagation velocity are proposed. The theoretical background leading to their formulation from physical considerations is presented. The effectiveness of the considered indexes is tested by means of an experimental setup built with two piezoelectric transducers contacting several samples of kiwifruits during their ripening process. The dependency on the geometrical position of the contact and the robustness against simplifying assumptions, such as uniform density or roundness, is investigated. The obtained experimental results show that stiffness indexes based on propagation delays are more rapid and reliable than those based on fruit resonance in assessing the ripeness degree.