The thermal properties of water-insoluble amylose-stearic acid (18:0) complexes prepared under various conditions were studied by differential scanning calorimetry (DSC). Complexes were studied normally at a concentration of 5% in water at pH similar to 7. Type I complexes formed at less than or equal to 60 degrees C had dissociation temperatures (T-m) in the range 96-104 degrees C. Type IIa polymorphs formed at greater than or equal to 90 degrees C had T-m = 114-121 degrees C. Various ratios of types I and IIa were formed at 80 degrees C depending on the duration of heating, but no intermediate form was detected. Annealing of the type IIa complex at 105 degrees C and at 115 degrees C gave rise to increasing proportions of type IIb polymorphs with T-m = 121-125 degrees C and dissociation enthalpies of 32-34 J/g of amylose, depending on the temperature and time of annealing. Conversion into the higher polymorphs was retarded at a higher concentration (10%) of the complex under identical conditions, and was delayed at pH similar to 4.7. The dissociation temperatures of amylose complexes with the cis-unsaturated fatty acids oleic (18:1), linoleic (18:2), and linolenic (18:3) also depended on the temperature of formation, and three distinct types were obtained (I, IIa, and IIb). Significant decreases in the T-m of the three polymorphs were observed for each double bond in the fatty acid guest molecule. When type I and type II complexes were made using various proportions of 18:0 and 18:2, mixed acid complexes were obtained with T-m values intermediate between those of the monoacid complexes. The origin of the endothermic transitions on heating the three types of complexes is discussed.
