Some factors determining the thermal properties of amylose inclusion complexes with fatty acids

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 ∼7. Type I complexes formed at ≤ 60°C had dissociation temperatures (Tm) in the range 96–104°C. Type IIa polymorphs formed at ≥ 90°C had Tm = 114–121°C. Various ratios of types I and IIa were formed at 80°C depending on the duration of heating, but no intermediate form was detected. Annealing of the type IIa complex at 105°C and at 115°C gave rise to increasing proportions of type IIb polymorphs with Tm = 121–125°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 ∼ 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 Tm 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 Tm values intermediate between those of the monoacid complexes. The origin of the endothermic transitions on heating the three types of complexes is discussed.