Time-resolved reflectance spectroscopy nondestructively reveals structural changes in ‘Pink Lady®’ apples during storage

With the aim of studying the optical properties of ‘Pink Lady®’ apples measured by TRS during storage in normal atmosphere and of relating them to fruit maturity and structural characteristics, 60 apples, measured at harvest by TRS at 670 nm and in the spectral range 740−1100 nm, were ranked on the basis of decreasing μa670 (increasing maturity) and randomized into 6 batches corresponding to 6 times of analysis: at harvest and after 7, 15, 29, 66 and 91 days of storage at 1 °C. At each storage time, apples were measured by TRS in the same spectral range as at harvest, and firmness, intercellular space volume (RISV), soluble solid content (SSC) and starch hydrolysis were measured; Streif Index and weight loss were computed. Overall, the μa spectra showed a maximum at 670 nm (chlorophyll-a) and at 980 nm (water), while the μ’s spectra had a decreasing trend with the wavelength increase. The μa670 and μa980 decreased with storage time as a consequence of a decrease in chlorophyll and water contents, respectively. The size and the density of the scattering centers were affected by the interplay of various phenomena: starch hydrolysis, flesh softening, water loss and RISV increase. High correlation coefficients were found between μa in the spectral region 920−1100 nm and firmness, Streif Index and RISV, while low correlation coefficients were observed between μa and SSC, and between μ’s and all the quality parameters. Using PLS analysis, good prediction for firmness, RISV and Streif Index were obtained using μa spectra; models’ performance improved when μa and μ’s are combined.