Exploring the variability and heterogeneity of apple firmness using visible and near-infrared hyperspectral imaging

In this work, the variability and heterogeneity of apple firmness and their effects on the prediction performances of developed models based on hyperspectral imaging (HSI) at 380-1010 nm were explored. Both the intervariability of cultivars and the intra-variability of single apples presented intensive changes of firmness. A large heterogeneity of firmness inside single apples was located at the equator of 'Red Fuji', the calyx of 'Cream Fuji' and the stem of 'Huaniu', with the variation coefficient of 15.6%, 15.4% and 17.5%, respectively. Besides, the least square support vector machines (LS-SVM) models based on the HSI spectra from different cultivars and spatial regions had the best prediction performances of firmness, with the Rc 2 , Rp 2 of 0.85 and 0.81, respectively. This study indicated the modelling calibration of apple firmness should particularly consider both apple variability and heterogeneity at the stem, equator and calyx regions to obtain accurate HSI determinations of firmness. Furthermore, the development of a multi-cultivar model for apple fruit is significant, for improving the performance of the firmness model.