Influence of global shape and internal structure of tomatoes on the resonant frequency

The firmness of a tomato can be estimated nondestructively from the measurement of its resonant frequency. The purpose of this research was to investigate the reliability of this technique in relation to the global shape and the internal structure of a tomato. The dynamic behaviour of spherical and non-spherical tomatoes was studied first by experimental modal analysis. Mode shapes, excitation techniques and measurement points for obtaining high impulse responses are compared. The effect of the internal compartmental structure of a tomato on the responses at different excitation points was studied. From the measured frequency response functions, the second resonant frequency, with an ''oblate-prolate'' synchronized motion of the fruit, was selected for further use. The corresponding mode shape incorporates the largest deformations for the small impact excitation used. The best repetitive results with 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 resulted in a small change (less than 0.5%) in the resonant frequency. During 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 technique, is proposed. (C) 1997 Silsoe Research Institute.