INFLUENCE OF GLOBAL SHAPE AND INTERNAL STRUCTURE OF TOMATOES ON THE RESONANT-FREQUENCY

The firmness of a tomato can be estimated nondestructively from the me asurement of its resonant frequency. The purpose of this research was to investigate the reliability of this technique in relation to the gl obal shape and the internal structure of a tomato. The dynamic behavio ur of spherical and non-spherical tomatoes was studied first by experi mental modal analysis. Mode shapes, excitation techniques and measurem ent points for obtaining high impulse responses are compared. The effe ct of the internal compartmental structure of a tomato on the response s at different excitation points was studied. From the measured freque ncy response functions, the second resonant frequency, with an ''oblat e-prolate'' synchronized motion of the fruit, was selected for further use. The corresponding mode shape incorporates the largest deformatio ns for the small impact excitation used. The best repetitive results w ith greatest discrimination from other modes are obtained by impacting and measuring the response at the equator, at points 180 degrees from each other, for both spherical and non-spherical tomatoes. Impacting and or measuring the tomato on fleshy or juicy parts of the tomato res ulted in a small change (less than 0.5%) in the resonant frequency. Du ring ripening, on the other hand, the resonant frequencies of all mode shapes decreased gradually. A testing procedure based on measurements of the resonant frequencies, namely the acoustic impulse response tec hnique, is proposed. (C) 1997 Silsoe Research Institute.