Water sorption isotherms of starch powders: Part 1: Mathematical description of experimental data

Adsorption and desorption isotherms for potato, highly amylopectin and highly amylose starch powders were determined at 30, 45 and 60 °C using a gravimetric technique. Samples were equilibrated in desiccators containing sulphuric acid solutions of known water activity (0.05-0.95), and placed in temperature-controlled cabinets for approximately three weeks. The starch powders exhibited Type II behaviour. The sorption capacity decreased with increasing temperature. The data obtained were fitted to several models including two parameter relationships (Halsey, Oswin, Henderson, Modified-BET and Smith), three parameter equations (GAB, Ferro-Fontan) and four parameter equation (Peleg). A non-linear least square regression program was used to evaluate the models constants. The empirical Peleg model followed by the kinetic GAB and the semi empirical Ferro-Fontan models were found to best represent the experimental data in the water activity range 0.05-0.95. In the range of water activity 0.35-0.95 the Smith model was shown to give the closest fit to the experimental data.