Assessment of two- and three-parameter Lewicki models for description of sorption phenomena of starch materials

Experimental moisture sorption isotherm data for raw potato, starch gels (potato, starch-sugar and starch-salt) and starch powders (potato, highly amylopectin and highly amylose) were determined using a gravimetric static method at 30, 45 and 60°C and over a relative humidity range 0.10-0.84. The raw potato, starch powders and starch gels exhibited type II behaviour, with type III form being observed on incorporation of sugar, or salt, into the starch gel matrix. The two-parameter Lewicki relationship, and the three-parameter Lewicki and GAB equations were considered for description of the experimental data. A nonlinear least square regression technique was used to evaluate the constants for the models. The three-parameter Lewicki model was found to be the most suitable for describing the sorption characteristics of raw potato, potato starch, starch-sugar and starch-salt gels within the investigated temperature and water activity range. However, both Lewicki models gave an inadequate representation of the data for potato, high amylopectin and high amylose starch powders.