J. agric. Engng Res. (1996) 65, 261 – 267 Models of Ultrasonic Parameters to Assess Avocado Properties and Shelf Life A. Mizrach;* N. Galili;† S. Gan-mor;* U. Flitsanov;* I. Prigozin* * Department of Post-Harvest Technologies, Institute of Agricultural Engineering, ARO, The Volcani Center, P.O. Box 6, Bet-Dagan, Israel 50250 † Agricultural Engineering Department, Technion - Israel Institute of Technology, Haifa 32000, Israel (Recei ed 13 March 1995; accepted in re ised form 29 May 1996) An ultrasonic testing technique was applied for non- destructive quality evaluation of avocado fruit and quantitative relations were established between ultra- sonic parameters and maturity related factors. The fruits of the same batch and picking date softened at dif ferent storage times, a phenomenon which was attributed to dif ferent biological ages of the fruit at the picking date. Time-shifting of the test results of each fruit according to its firmness reduced the scatter of the experimental data. Second-order polynomial expressions represented well the changes in attenua- tion of the ultrasonic signals and the firmness of the fruit with storage time. A cubic equation represents fairly well the wave velocity of the ultrasonic signals versus time. A linear relationship was found between attenuation of the ultrasonic signal and firmness as measured in a cone penetrometer test. It was con- cluded that equations of this type may be used for prediction of ripening and shelf-life of avocado fruit, and could replace the commonly used penetrometer method. Further steps, such as the averaging of repeated measurements, may be needed to reduce scatter when predicting the maturity of individual fruits. ÷ 1996 Silsoe Research Institute 1. Introduction Non-destructive assessment of avocado maturity is important for both growers and customers. Indications of maturity may include firmness, oil content and dry weight. Studies have shown that avocado firmness correlates well with its maturity and expected storage time (Lewis 1 ). The firmness decay rate is moderate at the beginning, increases and then stops in full maturity * Presented at Ag Eng 94, Milan, Italy, 29 August – 1 September 1994. (Zauberman and Fuchs 2 ). Firmness dif ferences in avocado fruits are good predictors of the dif ference in their ripening stages, since a softer fruit will fully ripen sooner than a harder one. Classification of fruits according to firmness would allow proper distribution of avocado fruit to distant markets and a longer shelf-life. Nondestructive firmness control is impor- tant, as even a small quantity of ripe fruits in a shipment may trigger a chain reaction, causing prema- ture ripening of the whole shipment due to a high level of ethylene production (Zauberman and Fuchs 3 ). Distinction between hard (unripe), medium (firm ripe), and soft (over ripe) fruit may be made by hand. However, it is almost impossible for the human sorter to distinguish between high-quality freshly-picked fruit and fruit collected from the ground which have low quality and a short shelf-life. This classification is commonly carried out by a penetrometer, but even if this is a reliable method, it is a destructive technique. A non-destructive method for sorting avocado fruit into two or more firmness classes would therefore be of considerable economic interest. Mizrach et al. 4 evaluated the use of high-power, low-frequency ultrasonic excitation for determination of the fruit tissue properties. An experimental system was designed for determination of the basic ultrasonic properties of some fruit and vegetables, namely, velocity of wave propagation and wave attenuation. Further studies by Mizrach et al. 5,6 correlated ultraso- nic properties and some ripening parameters of the fruit tissue. Their strong inter-dependence indicated the potential use of this ultrasound method for detec- tion of firmness properties in fruit tissue. Galili et al. 7 examined ultrasonic waves developed on the peel and flesh of a whole fruit by ultrasonic excitation and their connection with the internal properties of the fruit. Using an ultrasonic head with two inclined probes, pressed gently on the external face of the fruit, the wave velocity and signal attenua- 261 0021-8634 / 96 / 120261 + 07 $25.00 / 0 ÷ 1996 Silsoe Research Institute