|CEN, HAIYAN - Michigan State University|
|BEAUDRY, RANDOLPH - Michigan State University|
Submitted to: Postharvest Biology and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/20/2013
Publication Date: 5/30/2013
Citation: Cen, H., Lu, R., Mendoza, F., Beaudry, R. 2013. Relationship of the optical absorption and scattering properties with mechanical and structural properties of apple tissue. Postharvest Biology and Technology. 83:33-38.
Interpretive Summary: Absorption and scattering are two basic phenomena when light interacts with biological materials like apples. They are dependent on the composition (chemical) and structural (mechanical) characteristics of fruit tissue, which are, in turn, related to the flavor and textural properties of apples. This research was therefore aimed at gaining a quantitative knowledge of the changes of optical absorption and scattering properties in apples during postharvest storage and their relationship with the microstructural and mechanical properties of fruit tissues. Optical absorption and scattering properties for the wavelengths of 500-1000 nm were measured from whole apples of ‘Golden Delicious’ and ‘Granny Smith’ cultivars, which were kept at ~22 degrees Celsius and ~95% relative humidity for up to 30 days, followed with nondestructive acoustic/impact measurements of fruit firmness. Tissue specimens were then excised from the fruit and their mechanical properties (i.e., elasticity, failure strength, etc.) were measured under compression. Moreover, confocal laser scanning and scanning electron microscopic techniques were applied to quantify the morphological characteristics of apple tissues (i.e., size, shape, and area of cells, etc.). Consistent decreases in the absorption coefficient at 675 nm with the storage time were observed for both cultivars due to the decrease of chlorophyll content, while an increase in the absorption coefficient at 525 nm was observed as more anthocyanin was produced. Values of the scattering coefficient generally decreased with storage time. The optical absorption and scattering coefficients had various degrees of correlation with firmness, tissue elasticity, and cell size for both cultivars. This research, for the first time, provided a quantitative understanding of the relationship between the optical and structural/mechanical properties of apple tissues. The findings from the research suggest that optical properties could be used to study or monitor the mechanical properties and microstructural changes of apples during ripening and postharvest storage. However, since the physical and structural measurements are often cultivar dependent, caution should be taken in interpreting their relationship with the optical properties.
Technical 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 changes in, and the relationship between, the optical properties and the mechanical and structural properties of apple tissue. Two apple cultivars, ‘Golden Delicious’ and ‘Granny Smith’, were chosen in this study because of their structural differences. Optical properties measurement, acoustic/impact firmness and compression test, and confocal laser scanning and scanning electron microscopic analyses were carried out to study the changes of optical, mechanical and microstructural properties in the tissue of apples during 30 days of storage at ~ 22 °C and ~ 95% relative humidity. Linear regression analysis was used to correlate acoustic/impact firmness, tissue Young’s modulus, cell morphological parameters (i.e., cell size and shape), and textural features to the absorption and scattering properties of apple fruit. Consistent decreases in the absorption coefficient at 675 nm with the storage time due to the decrease of chlorophyll content were observed for both cultivars, while an increase in the absorption coefficient at 525 nm was observed as more anthocyanin was produced. The reduced scattering coefficient also exhibited a consistent pattern of change during the storage and its values for the fresh apples were considerably higher than those for the stored apples for all test dates. The changes or decreases in the optical absorption and scattering properties were accompanied with decreases in the acoustic/impact firmness, Young’s modulus, and cell size parameters, and with increases in the textural parameters with storage time. The optical property parameters had various degrees of correlation with acoustic/impact firmness, Young’s modulus, cell morphological parameters and textural feature parameters for both cultivars. These findings suggest that optical properties can be used to study or monitor the mechanical properties and microstructural changes of apples during ripening and postharvest storage.