Modification of cell-wall polymers of onion waste—Part II. Effect of divalent cations

Abstract

The purpose of this study was to investigate the effect of divalent cations on the chemistry and mechanical properties of onion cell walls as modified by heating. Pre-soaking the outer fleshy scale leaves of onion ( Allium cepa L. cv. Delta) in solutions of Ca 2+ or Sr 2+ ions reduced the rate of thermal softening. This was accompanied by a reduction in wall swelling, and an enhancement of cell adhesion, particularly at the edges of the cell faces. To investigate this effect on wall polymer chemistry, cell-wall material (AIR) was prepared from these tissues, heated in water or solutions of Ca 2+ and then extracted in cyclohexane-trans-1,2-diaminetetra-acetate (CDTA) to leave a residue. The samples were analysed for their carbohydrate composition and the molecular size of selected soluble polysaccharides. Divalent cations reduced the heat-induced solubilization of wall polymers by reducing the propensity for β-eliminative degradation and depolymerization, and thereby increasing the thermal stability of the cross-linked pectic polysaccharides, including those involved in cell–cell adhesion. Increasing the levels of calcium availability in onion cell-wall material resulted in an increase in the molecular weight of some species of heat-treated, CDTA-soluble, polysaccharides and a reduction in the thermal solubility of CDTA-insoluble polysaccharides.