Sorption of water vapor by starch. Thermodynamics and structural changes for dextrin, amylose, and amylopectin

Abstract

Water vapor sorption isotherms of a commercial dextrin prepared by dry acid treatment, amylose recrystallized from n‐butanol, and amylopectin precipitated from methanol were determined at various temperatures in the range of 10–40°C. The differential thermodynamic functions for water vapor sorption on the various materials were calculated, and B.E.T. constants were determined. A striking characteristic of the thermodynamic functions is the strong similarity for amylopectin and amylose. A model for the sorption is proposed. X‐ray patterns for the starch samples equilibrated with water vapor at various relative pressures of water vapor were determined. The major changes are explicable on the basis of intramicellar swelling. As the relative pressures increase, the crystalline patterns gradually fade and disappear. However, some new spacings appear with increased sorption in amylose and amylopectin. These spacings are explained on the basis of the formation of transitory lattices during the swelling of the starch crystallites.