Non-destructive Measurement of Chlorophyll Concentration of Fruit Skin in Apple

In order to develop a non-destructive method to measure the chlorophyll concentration of fruit skin in apple (Malus × domestica Borkh.), reflectance of diffused light through the skin of fruit was measured using a portable spectrometer. Reflectance was acquired in the 650–740 nm wavelength range to establish a model to predict chlorophyll a and b concentrations using partial least squares regression. The chlorophyll concentration was precisely predicted by each model (r2 = 0.918–0.982) in ‘Akane’, ‘Sansa’, ‘Kitaro’, ‘Jonagold’, and ‘Fuji’ in 2018. Chlorophyll concentrations of ‘Akane’, ‘Sansa’, ‘Toki’, ‘Kitaro’, ‘Jonagold’, and ‘Fuji’ in 2019 were also estimated with small errors using the prediction model of ‘kitarou’ in 2018. The chlorophyll concentration showed a relatively high correlation with the ground color index (r2 = 0.783–0.903) and starch index (r2 = 0.694–0.947) in all cultivars. The prediction model was not significantly affected by the temperature of fruit. From these results, the chlorophyll concentration could be non-destructively, precisely, and rapidly estimated using a portable spectrometer, and there is possibility that this can be used to confirm the best time to harvest apple.