Starch–paraffin wax compositions prepared by steam jet cooking. Examination of starch adsorbed at the paraffin–water interface

Abstract

Starch–paraffin wax compositions were prepared by passing aqueous two-phase mixtures of cornstarch and paraffin wax through a steam jet cooker under excess steam conditions. Jet cooking converts the paraffin wax to micron-sized droplets that remain suspended in the aqueous dispersion and do not coalesce, due to an adsorbed layer of interfacial starch that surrounds each droplet. Solidified droplets of starch-coated paraffin wax were isolated by dilution of jet cooked dispersions with excess water followed by centrifugation. Wax droplets, having specific gravity lower than that of water, were collected from the dispersion surface, washed with water, and dried. Weight percent interfacial starch in isolated wax droplets was calculated from the weight of residual starch remaining after removal of paraffin wax by extraction with cyclohexane. Starch percentages varied from about 3–8%, depending upon whether waxy, normal, or high amylose starch was used, and whether jet cooked dispersions were diluted with hot or cold water prior to centrifugation. The effect of small amounts of lipid material (normally present in cereal starches) on weight percent interfacial starch was determined by examining products prepared from starch that was solvent-extracted to remove the lipid component (i.e. defatted). Although defatted normal cornstarch produced a product having a lower percentage of interfacial starch than a comparable product prepared from starch that still contained native lipid, defatting had little effect when waxy starch was used. The morphology of the starch layer, as observed by SEM, was affected by the presence or absence of lipid in the starting starch. Amylose was preferentially adsorbed at the paraffin–water interface when native lipid was present in the starting starch, suggesting that lipid enhances the adsorption of amylose through the formation of helical inclusion complexes. X-ray diffraction patterns of interfacial starch were consistent with this interpretation and showed the V h-pattern commonly attributed to amylose–lipid complexes. Nitrogen analyses suggested that proteins, present in cornstarch in small amounts, may also adsorb along with starch at the paraffin–water interface.