Nondestructive Measurement of Sugar Content and Firmness in Korla Fragrant Pears by Using Their Dielectric Spectra
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Abstract:
Dielectric spectra of Korla fragrant pears were used for nondestructive measurement of their sugar content and firmness. For this purpose, the dielectric constant and dielectric loss factor at 201 discrete frequencies across the frequency range of 10 to 4500 MHz were measured in 168 Korla fragrant pears from different areas, by means of an open-ended coaxial-line probe and vector network analyzer. Patterns of changes in the dielectric properties were analyzed. Sample-set partitioning based on the method of joint x-y distances was used to group 112 samples as a calibration set and 56 samples as a prediction set. To the full dielectric spectra (FS), uninformative-variable elimination and successive projection algorithm (SPA) were applied in order to extract 144 and 20 characteristic variables (CVs) for prediction of sugar content, and to extract 52 and 9 CVs for firmness prediction, respectively. The FS and extracted CVs were subjected to partial least squares, back propagation network, least square support vector machine, and extreme learning machine (ELM) analyses to establish sugar content and firmness measurement models for Korla fragrant pears. The results revealed that the SPA-ELM model showed the best prediction for sugar content of the Korla fragrant pears: correlation coefficients of the calibration and prediction sets were 0.943 and 0.876, respectively, and the root-mean-squared errors of the calibration and prediction sets were 0.298°Brix and 0.330°Brix, respectively. The correlation coefficients of the calibration and prediction sets of all predicted models for firmness were less than 0.6. The study indicates that dielectric spectra can be used to nondestructively measure sugar content of Korla fragrant pears. However, this approach shows poor predication performance on firmness. Further studies should be conducted to evaluate feasibility of the use of dielectric spectra to nondestructively measure the firmness of Korla fragrant pears.
