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Visualizing the quality variation inside loquat fruit at pixel level is important to deeply understand its ripening process and further optimize its preharvest planting pattern and postharvest storage strategy. Total soluble solids (TSS) is a major quality attribute of loquat fruit and detecting changes in its content can be used to monitor the ripening and postharvest senescence of loquat fruit. The refractometer method cannot provide detailed TSS distribution within loquat flesh, because the analysis focuses on measuring the mean TSS of only a flesh cube, which is used to make juice for the refractometer measurement. In this study, hyperspectral imaging was used to measure and visualize internal TSS distribution within loquat fruit. Loquat fruits with different TSS contents were selected and cut at different cross sections for imaging. Different calibration and wavelength selection algorithms were applied and compared. The uninformative variable elimination by partial least square regression model with preprocessing from spectral set I (468-1026 nm) was identified as the best model for the TSS determination of loquat flesh, which had a high prediction ability with a correlation coefficient of 0.960 and residual predictive deviation of 3.513. On the basis of the best model, the quantitative TSS distribution at different cross sections inside loquat fruit was visualized at pixel level. The results showed that hyperspectral imaging is a feasible way of visualizing the spatial changes of TSS distribution inside loquat fruit at pixel level, which would be helpful to understand the detailed TSS change inside postharvest loquat fruit.

Softening of fruit during the postharvest storage, which is mainly associated with both compositional and spatial changes of polysaccharides within cell wall, affects the texture and quality of fruit. Current research on the fruit softening mechanism lacks an understanding of the overall softening at the cell level. The objective of this work was to investigate the change in the spatial distribution of cell wall polysaccharides in peach flesh cells at both single- and multiple-cell levels in a label-free way during the postharvest storage. Nonmelting peaches (Prunus persica L. Batsch cv.”Zhonghuashoutao”) at commercial maturity were stored at 0 °C and 20 °C. Firmness measurement and chemical analysis were performed at each storage time. In addition, three molecular imaging techniques, namely confocal Raman microspectroscopy (CRM), Fourier transform infrared microspectroscopy (FTIRM), and stimulated Raman scattering microscopy (SRS) were used to visualize changes in the spatial distribution of cell wall polysaccharides of peach fruit in a label-free way during the postharvest storage. The combination of CRM and FTIRM provided complementary spectral information to visualize the spatial changes of cellulose, hemicellulose, and pectin in the cell wall of peach flesh during softening at the single-cell level, and found that the cell wall polysaccharides tended to be concentrated in the cell corner of parenchymal cells at the late stage. Furthermore, SRS, which is an ultrafast Raman imaging technique (approximately three or four orders of magnitude faster than CRM), was used for high-throughput cell wall phenotypes measurement. Different degradation degrees of parenchymal cells during fruit softening were found based on the gray-scale statistical analysis of SRS data. In general, cell wall polysaccharides decreased during softening and tended to be concentrated in the cell corner for most parenchymal cells at the late stage, but there were also some cells not in line with the whole softening trends. The results show that there were differences in the content and spatial changes of cell wall polysaccharides among parenchymal cells of peach fruit during the softening process, and the hybrid use of CRM, FTIRM, and SRS is a promising method for simultaneous visualization of changes in cell wall polysaccharides of peach.

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