Calcium-mediated Postharvest Changes in Texture and Cell Wall Structure and Composition in `Golden Delicious' Apples

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Abstract
Changes in texture, cell wall structure and composition during storage of Ca-treated and untreated `Golden Delicious' apple fruit (Malus domestics Borkh.) were investigated. The cell wall region of Ca-treated fruit showed no swelling during storage and cell-to-cell contact was maintained, whereas regions of the middle lamella in untreated tissue stained lightly, appeared distended, and eventually separated. In control fruit, microfibril orientation was lost in distended regions of the cell wall, especially in the outer wall region adjacent to the middle lamella. Furthermore, the middle lamella was fenestrated and in some cases was completely degraded. These changes during storage of control fruit were accompanied by a decrease in arabinose and galactose moieties of the cell wall and an increase in soluble pectin. Calcium treatment of fruit inhibited solubilization of polyuronide and arabinose moieties and reduced the loss in galactose content during storage. Tensile strength and firmness were positively correlated to Ca content of the fruit cortex. Excessive tensile stress caused tissue failure in control fruit when cells of the cortical tissue separated at the middle lamella. In contrast, cylinders of Ca-treated fruit fractured through cortical cell walls.