Relationship of the optical absorption and scattering properties with mechanical and structural properties of apple tissue

Abstract

Optical absorption and scattering properties of fruit change with the physiological and biochemical activities in the tissue during ripening and postharvest storage. But it has not been well understood on how these changes are related to the structural and mechanical properties of fruit. This research was therefore aimed at quantifying the relationship between the optical and mechanical properties of apple tissue and gaining a preliminary understanding of how the optical properties are related to the microstructural characteristics of apple tissue. Optical properties measurement and acoustic/impact firmness and compression tests were carried out for ‘Golden Delicious’ (GD) and ‘Granny Smith’ (GS) apples over a 30-day period of storage at ∼22°C and ∼95% relative humidity (RH), followed with confocal laser scanning microscopic analysis of tissue specimens. Linear regression analysis was then used to correlate acoustic/impact firmness, Young's modulus and the cell's area and equivalent diameter with the absorption and scattering properties of apple fruit. Consistent decreases in the absorption coefficient at the chlorophyll absorption waveband of 675nm with storage time were observed for both cultivars, while an increase in the absorption coefficient at the anthocyanin absorption waveband of 525nm was observed. Values of the reduced scattering coefficient generally decreased with storage time. The changes or decreases in the optical absorption and scattering parameters were accompanied with decreases in the acoustic/impact firmness and Young's modulus. Using the mean values of each test date, the optical parameters were correlated with acoustic/impact firmness (r=0.870–0.948 for GD and r=0.334–0.993 for GS) and Young's modulus (r=0.585–0.947 for GD and r=0.292–0.694 for GS). The preliminary confocal laser scanning microscopic analysis showed that the optical absorption and scattering parameters were also positively correlated with the cell's area and equivalent diameter. These findings suggest that optical properties could be used to study the mechanical properties of apples and possibly their microstructural changes during ripening and postharvest storage.