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

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 cep a L. cv. Delta) in solutions of Ca2+ or Sr2+ ions reduced the rate of therm al 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 (A TR) was prepared from these tissues, heated in water or solutions of Ca2+ a nd then extracted in cyclohexane-trans-1,2-diaminetetra-acetate (CDTA) to l eave a residue. The samples were analysed for their carbohydrate compositio n and the molecular size of selected soluble polysaccharides. Divalent cati ons reduced the heat-induced solubilization of wall polymers by reducing th e propensity for beta-eliminative degradation and depolymerization, and the reby increasing the thermal stability of the cross-linked pectic polysaccha rides, including those involved in cell-cell adhesion. Increasing the level s of calcium availability in onion cell-wall material resulted in an increa se in the molecular weight of some species of heat-treated CDTA-soluble, po lysaccharides and a reduction in the thermal solubility of CDTA-insoluble p olysaccharides. (C) 1999 Elsevier Science Ltd. All rights reserved.