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

The extraction of polymers from onion cell walls and the mechanical properties of onion tissues can be modified by heating. However, there is little information on the effects of thermal treatments on the chemistry of cell-wall polysaccharides of onion tissues. The present work reports pressure-cooking-induced changes in cell-wall polymers of onion waste. The outer fleshy scale leaves of onion (Allium cepa L. cv. Delta and Hysam) were subjected to a range of pressure-cooking times (0, 10, 20, 30 and 50 min, 120°C). Cell-wall material was prepared as cold-alcohol-insoluble residues and subsequently extracted with buffered phenol. The cell-wall material was then extracted sequentially with water, NaCl, imidazole, cyclohexane-trans-1,2diaminetetraacetate (CDTA), Na2CO3 and KOH to leave a residue. The samples were analysed for their carbohydrate composition, their degree of methylesterification and the molecular size of selected soluble polysaccharides. Pressure-cooking-induced softening of onion tissue involved cell-wall swelling and cell separation. This was accompanied by an increase in water-soluble pectic polysaccharides and a decrease in the pectic polymers of the imidazole, CDTA, Na2CO3 and KOH-soluble fractions and insoluble residue. Pressure-cooking also resulted in a slight decrease in the peak molecular weight of water-soluble polysaccharides.