Acoustic impulse-response technique for evaluation and modelling of firmness of tomato fruit

The firmness of tomato fruit was monitored using an acoustic impulse-response technique. This technique provides a stiffness factor S, based on the first resonance frequency and the mass of the intact fruit. A logarithmic relation was found between objective stiffness measurements on tomatoes by the acoustic impulse-response technique and subjective firmness measurements, suggesting that subjective measurements distinguish firmness differences more easily in soft than in firm tomatoes. Accuracy and repeatability of the acoustic impulse-response technique and of subjective scores by market experts were equally high. However, the advantage of using S is that it is objective and no scaling problems between different experts occur. In soft fruit only a stiffness difference as large as 2.5 x 10(6) Hz(2) g(2/3) could be distinguished with satisfactory accuracy by all experts. Tomatoes with a stiffness of 2.0 x 10(6) Hz(2) g(2/3) or less had more than 50% chance of being rejected. The acoustic impulse-response technique was also used to study firmness changes of tomatoes during storage, and the influence of variety, producer, season, production method, maturity at the time of picking, and storage conditions. Change in stiffness as a function of time could be expressed as a decreasing exponential function. Tomatoes stored at 20 degrees C had the same deterioration constant, regardless of maturity at harvest. However, when fruit were stored at 12 degrees C, the deterioration constant of the three tested varieties was greater for red than for orange fruit. The deterioration constant was much higher for fruit harvested in spring than in the other seasons. The influence of temperature could be expressed in the form of an Arrhenius equation. The model was validated for tomatoes subjected to fluctuating temperatures. (C) 1999 Elsevier Science B.V. All rights reserved.