Principal component analysis of chewing sounds to detect differences in apple crispness

An investigation was made to establish the basic relationship between the crispness of 'Cox's Orange Pippin' apples (Malus domestica Borkh.) and recorded chewing sounds. Crispness groups were created by submitting apples to varying storage conditions. After carrying out a fast Fourier transformation on the time signal of the generated sound, principal component analysis (PCA) was carried out on the power spectra of a training set, and a calibration matrix for group prediction was created. The PC values were compared with mechanical parameters, including apple firmness measured with the acoustic impulse response technique, maximum force and slope of the force-deformation curve during a penetrometer measurement, and tensile strength in a ring tensile test. PCA on Fourier-transformed chewing sounds appeared to be a promising technique to separate apple crispness groups. By further development this technique has potential as an objective measure for crispness evaluation. Mealy and crisp apples could be distinguished by PCA. The frequencies between 100 and 500 Hz and between 800 and 1100 Hz contributed most to the PCs calculated from the original power spectra, corresponding to peaks in power spectra of crisp apples. When PCA was carried out on the logarithm of the original power spectra, all frequencies contributed to some degree to the PCs. In further experiments on a variety of stored fruit, the position of the power spectra in the PC1-PC2 space was correlated with apple tensile strength. Prediction of group belonging, using a calibration matrix based on the first 15 PCs, gave much better results for the logarithm of the power spectra than for the original signals. Chewing sounds from apples stored under normal air composition or under ULO conditions could be distinguished relatively well, while two RH groups could not be separated. This corresponded to results from sensory analysis. (C) 2000 Elsevier Science B.V. All rights reserved.