Internal structure, density, and mechanical properties of cranberry fruits

Firmness is a critical quality attribute influencing the yield and overall quality of sweetened dried cranberries. This study aimed to seek correlations among the physicochemical characteristics of cranberry fruits, with a focus on internal structure, density, and mechanical properties. Micro-computed tomography imaging, the gravimetric buoyancy test, and the uniaxial compression test of six cranberry groups, which included cultivars with varying firmness levels and different postharvest storage conditions, were performed to investigate the internal structure, density, and mechanical properties of cranberry fruits, respectively. Postharvest freezing significantly affected the volume of interest, porosity, and density. A lower storage temperature resulted in a reduced volume of interest and porosity and an increased density of the Stevens cultivar. The compression force, often used as a measure of the firmness of cranberry fruits, decreased with prolonged storage at a lower temperature within the same cultivar. Additionally, both soft and firmer cultivars showed similar compression forces after freezing, suggesting stronger negative impacts of postharvest freezing on the firmness of the firmer cultivars. The stress and the apparent modulus of elasticity calculated using a conventional method appeared to be overestimated particularly at relatively small strains less than 0.05. Correlation analysis revealed that most examined properties, except density and the apparent modulus of elasticity, showed positive correlations with fruit firmness, expressed as the force at 10% strain. However, these correlations were relatively weak (i.e., correlation coefficients <0.41), suggesting the importance of incorporating the evaluation of various physicochemical properties to provide a basis for understanding the mechanisms that determine cranberry firmness.