Modification of cell-wall polymers of onion waste: III. Effect of extrusion-cooking on cell-wall material of outer fleshy tissues

The polymers of onion cell walls are known to be modified by heating, but there is little information on the effects of extrusion-cooking. This work investigates the effects of extrusion-cooking on the physico-chemical characteristics and microstructure of cell walls of onion waste in relation to cell-wall chemistry. Cell-wall material from white fleshy outer scale leaves of waste onions was extruded at a range of moisture contents, barrel temperatures and screw speeds through a co-rotating twin-screw extruder. Extrusion-cooking had little effect on the carbohydrate composition of cell-wall material. However, it resulted in an increase in the solubility of pectic polymers and hemicelluloses, and this was accompanied by an increase in swelling of the cell-wall material. The degree of solubility of the pectic polysaccharides was largely dependent on the barrel temperature, and involved depolymerisation.