Desorption Isotherms and Net Isosteric Heat of Chestnut Flour and Starch

The desorption isotherms of chestnut flour and chestnut starch were determined at different temperatures (20°C, 35°C, 50°C, and 65°C) using gravimetric method. Desorption isotherms of potato starch were also determined in order to establish a comparison against desorption isotherms of chestnut starch. Several saturated salt solutions were selected to generate different water activities in the range of 0.09 to 0.91. Obtained desorption isotherms were of type II, according to Brunauer’s classification. Three-parameter Brunauer–Emmett–Teller and Guggenheim–Anderson–de Boer (GAB) models satisfactorily fitted the experimental data for all systems studies, although the last one can be considered better based on obtained statistical parameters and because it is applicable in a broader water activity range. The average monolayer moisture content (kilograms per kilogram d.b.) calculated by GAB model was 0.059 ± 0.007 for chestnut flour, 0.103 ± 0.021 for chestnut starch, and 0.060 ± 0.028 for potato starch. The net isosteric sorption heat, calculated by means of Clausius–Clapeyron equation, decreased when moisture content increased. The maximum values of net isosteric sorption heat (kilojoules per mole) were approximately 27.5 for chestnut flour, 16.0 for chestnut starch, and 33.0 for potato starch in the range of temperature from 20°C to 50°C.