Sorption and thermodynamic properties of dry crystallized Palada payasam mix prepared using manual and mechanical...

An attempt was made to develop mathematical models to describe the sorption behaviour and determine the thermodynamic properties of sorption of dry-crystallized Palada payasam mix at different water activities (between 0.113 and 0.973) and temperature ranging from 25 to 35 °C. Samples were prepared using both manual and mechanical mixing during the crystallization process and the equilibrium moisture content (EMC) was recorded using the isopiestic technique to compare the sorption behaviour of both samples. The sorption isotherms were found to follow a J-shape-type III plot, with the sorption capacity decreasing with temperature. The isotherms also exhibited a typical temperature inversion of EMC at water activity > 0.70. The sorption data obtained was fitted to 12 mathematical models for sorption and the Guggenheim, Anderson and de Boer (GAB), Peleg and Iglesias and Chirife models were found to describe the data over the entire range of water activity with a good fit. The isosteric heat of sorption and spreading pressure were determined as a function of moisture content. Isosteric heat of the samples was computed and found to decrease from 51.75 to 47.16 kJ/mol for control (manually stirred) and 49.38 to 47.58 kJ/mol for experimental sample (mechanically stirred) for a range of moisture content up to 29% (d.b). The spreading pressures increased with increasing water activity but decreased with increasing temperature. No significant difference was observed between the sorption properties of the dry crystallized samples prepared using manually and mechanically stirring.