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

Starch-paraffin wax compositions were prepared by passing aqueous two-phase mixtures of cornstarch and paraffin wax through a steam jet cooker under e xcess steam conditions. Jet cooking converts the paraffin wax to micron-siz ed droplets that remain suspended in the aqueous dispersion and do not coal esce, due to an adsorbed layer of interfacial starch that surrounds each dr oplet. Solidified droplets of starch-coated paraffin wax were isolated by d ilution of jet cooked dispersions with excess water followed by centrifugat ion. Wax droplets, having specific gravity lower than that of water, were c ollected 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 extra ction with cyclohexane. Starch percentages varied from about 3-8%, dependin g upon whether waxy, normal, or high amylose starch was used, and whether j et cooked dispersions were diluted with hot or cold water prior to centrifu gation. The effect of small amounts of lipid material (normally present in cereal starches) on weight percent interfacial starch was determined by exa mining products prepared from starch that was solvent-extracted to remove t he lipid component (i.e. defatted). Although defatted normal cornstarch pro duced a product having a lower percentage of interfacial starch than a comp arable product prepared from starch that still contained native lipid, defa tting had little effect when waxy starch was used. The morphology of the st arch layer, as observed by SEM, was affected by the presence or absence of lipid in the starting starch. Amylose was preferentially adsorbed at the pa raffin-water interface when native lipid was present in the starting starch , suggesting that lipid enhances the adsorption of amylose through the form ation of helical inclusion complexes. X-ray diffraction patterns of interfa cial starch were consistent with this interpretation and showed the Vh-patt ern commonly attributed to amylose-lipid complexes. Nitrogen analyses sugge sted that proteins, present in cornstarch in small amounts, may also adsorb along with starch at the paraffin-water interface. (C) 2001 Elsevier Scien ce Ltd. All rights reserved.