Non-destructive analyses of apple quality parameters by means of laser-induced light backscattering imaging

Abstract

Monitoring of apple fruit development is necessary to determine appropriate production measures. Furthermore, consumer acceptance of apples depends on internal quality especially regarding the soluble solids content (SSC) and fruit flesh firmness. In the present work, laser-induced light backscattering imaging was applied to analyze fruit SSC and firmness during apple development in different growing locations by means of non-destructive readings, showing potential for rapid site-specific fruit evaluation during production. Spectral images of the backscattering of light on the fruit surface were obtained from ‘Elstar’ and ‘Pinova’ apples using laser diodes emitting at five wavelengths (10 nm bandpass) between 600 and 1100 nm, addressing the fruit absorption and scattering properties. Different multivariate calibration methods were tested on the frequency of different intensities of backscattering each for the five wavelength bands to analyze SSC and firmness. The method was applied on two cultivars picked at weekly intervals during fruit development grown in sites with different plant water availability. During fruit development the mean values of SSC data of drought stressed and sufficiently irrigated regions of orchard ranged from 11.1 to 15.4 and 10.5 to 14.5 °Brix, respectively. The mean values of fruit firmness at the two different field regions developed from 130.9 to 71.6 and 116.1 to 68.3 N/cm 2, respectively. Using partial least squares regression, calibration uncertainty in cross-validation ranged between 6 and 2% for SSC and firmness, while a validation on a test-set gave a percentage error of prediction in the range of 10% for SSC and 9% for firmness with respect to refractometrical SSC readings and the Magness–Taylor firmness test, respectively. Variation in fruit parameters due to slight drought stress was found in the range of 12–13%. A calibration on the specific fruit material is necessary for assessing the spatial distribution of fruit quality parameters in the orchard, however, laser-induced backscattering imaging is an inexpensive method for rapidly receiving relevant information for site-specific measures.