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  • Author or Editor: W.P. Wergin x
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Calcium is an important constituent of the cell wall and plays roles in maintaining firmness of fruit and reducing postharvest decay. The modification of the cell wall is believed to be influenced by calcium that interacts with acidic pectic polymers to form cross-bridges. Infiltrating apples with CaCl2 has been suggested as an effective postharvest treatment for increasing the calcium content. Three different methodologies were used to analyze the effects of calcium on the cell walls: 1) nickel staining of polygalacturonate on free-hand sections, 2) cationic gold labeling of anionic binding sites in the cell walls, and 3) analytical detection of calcium ions (40Ca, 44Ca) using a secondary ion mass spectrometry. The combination of these methods allowed us to directly visualize the cellular features associated with the infiltration of calcium. Treatment resulted in significant enrichment in the cell wall of the pericarp, transformed the acidic pectins in calcium pectates, and resulted in new calcium cross-bridges. Evidence now suggests that exogenously applied calcium affects the cell wall by enhancing its strength and reinforcing adhesion between neighbor cells; therefore, calcium infiltration delays fruit degradation.

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Prestorage heat treatment of apples has been shown to have a positive effect on fruit quality in storage. Postharvest treatment of apples with CaCl2 also beneficially affected fruit during storage. However, calcium uptake seems limited in heat-treated apples which indicates that the surface of the fruit may have been affected by the heat treatment. This study examined the effect of heat treatment on the subsequent uptake of CaCl2 solutions and related this process to the ultrastructure of the epicuticular wax surface of the fruit. Apples were pressure infiltrated with a 4% CaCl2 solution either without heat treatment or following 4 days at 38°C. Examination of the apple surfaces with low temperature scanning electron microscopy revealed that heat treatment changed the pattern of epicuticular wax. The epicuticular wax of non heated fruit exhibited numerous deep surface cracks. The epicotictdar wax of heated fruit did not exhibit similar cracks. This apparent obstruction or elimination of deep cracks may limit the CaCl2, solutions from entering the fruit. The heated fruit contained significantly less calcium compared to the fruit that were pressure infiltrated with the CaCl2 solution but not heated. These results indicate that cracks on the fruit surface may be a” important pathway for the penetration of CaCl2 solutions.

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