Time-resolved reflectance spectroscopy reveals different texture characteristics in ‘Braeburn’, ‘Gala’ and ‘Kanzi®’ apples

Time-resolved reflectance spectroscopy (TRS) has been used for non-invasive assessment of maturity and texture characteristics in intact apples. The absorption coefficient measured at 670 nm can be considered a maturity index for apples while the reduced scattering coefficient (µ’s) changes according to pulp texture characteristics, increasing with apple softening and when fruit texture changes from very firm-crispy to mealy. This work aimed at studying the relationships between the pulp optical properties measured by TRS in three apple cultivars and the fruit texture measured through mechanical-acoustic and sensory analysis. ‘Braeburn’, ‘Gala’ and ‘Kanzi®’ apples were picked at three times, stored in air for 1 month (1°C, RH~97%) and analyzed after 7 days of shelf life at 20°C (RH~78%). Each fruit was measured by TRS in the 580-1064 nm wavelength range, obtaining the chlorophyll (CHL) and water (H2O) contents and the scattering coefficient (µ’s). On the same samples, intercellular spaces (RISV), mechanical-acoustic parameters as well as sensory attributes were determined. ‘Kanzi®’ apples showed the lowest µ’sand the highest H2O, ‘Braeburn’ the highest CHL with intermediate values of H2O and µ’s, while ‘Gala’ had the highest values of µ’sand intermediate H2O and CHL. PCA on all data extracted three functions. PC1 opposed flesh firmness (Fmax), energy, average sound value (MEAN), acoustic linear distance (LD), sound at Fmax, sensory firmness, crispness and juiciness to mealiness, RISV and µ’s. PC1 scores were highest in ‘Kanzi®’ and lowest in ‘Braeburn’ apples and decreased with picking time. PC2 grouped modulus of deformability, stiffness and CHL opposed to MEAN and number of acoustic peaks, showing the highest scores in ‘Braeburn’ apples and the lowest in ‘Gala’. PC3 opposed sensory juiciness and firmness to LD and highest acoustic peak, and had the lowest scores in ‘Gala’ apples. Our results showed that TRS optical properties, especially µ’s, were able to distinguish the three apple cultivars, indicating that they could have different textures, as actually shown by acoustic-mechanical and sensory analyses.