Nondestructive dynamic testing of apples for firmness evaluation

Two nondestructive dynamic test methods, low-mass impact and acoustic response, were tested and compared with destructive compression and penetration tests to evaluate apple firmness. The purpose of the study was to analyze the performance of the impact test methods for nondestructive firmness evaluation, and to assess whether the acoustic tests could add sorting capacity to low-mass impact testing in apples. A laboratory-instrumented impact hammer and a low-mass impact firmness sensor produced by Sinclair International (SIQ-FT) were used to perform the impact tests, and a piezoelectric-film transducer was applied for the acoustic tests. A new Sinclair internal quality index IQ and two conventional impact parameters C-1 and C-2 were extracted from the impact signals, and a firmness index FI was calculated from the acoustic signal of the samples. The nondestructive tests were followed by parallel-plate compression tests and Magness-Taylor penetration tests. Three apple varieties, 'Golden Delicious', 'Starking Delicious' and 'Granny Smith', were tested by the impact and acoustic methods. The performance of the experimental systems and the test methods were evaluated first by rubber ball calibration tests. The elastic modulus of all apples was well predicted by the acoustic firmness parameter FI (R > 0.91). The impact parameter IQ was equivalent to FI in predicting the elastic modulus of 'Golden Delicious', and less accurate in 'Starking Delicious' and 'Granny Smith'. The conventional indices C-1 and C-2 performed similarly to the 171 and IQ in the rubber balls and the 'Golden Delicious' tests, but failed to predict the elastic modulus of 'Starking Delicious' or 'Granny Smith' apples, probably because of their less uniform shape, and the sensitivity of these empirical parameters to test conditions. The changes of the penetration force during the test period were very low, and their correlation with the elastic modulus and the firmness parameters of all apples was poor. These previously reported findings might indicate that the Magness-Taylor penetration test is not an appropriate measure of apple firmness during shelf life. The test results indicated that the new SIQ-FT device and the IQ parameter offer a new technology that may overcome some basic difficulties associated with the low-mass impact test methods, which have been reported in the literature. The results also indicated that the acoustic method might add sorting capacity to the impact method in some apple varieties. Further R&D efforts are needed, however, to verify these results and adapt the acoustic method for on-line operation. (C) 2003 Elsevier B.V. All rights reserved.