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

Abstract

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 ( T m) in the range 96–104°C. Type IIa polymorphs formed at ≥ 90°C had T m = 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 T m = 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 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.