Moisture sorption isotherms and net isosteric heat of sorption for spray-dried pure orange juice powder

- Moisture sorption isotherms for spray-dried pure orange juice powders were determined.
- GAB equation was determined to be the best fit for the moisture sorption isotherms.
- The net isosteric heat of sorption was calculated from the Clausius-Clapeyron equation.
- The heat of sorption was used as a limiting condition for direct regression method.
- The parameters of the GAB equation were determined using direct regression method.

The aim of this study was to measure the moisture sorption isotherms of spray-dried pure orange-juice powders. These isotherms have been fitted using eleven different sorption equations. The GAB model was found to be the best fit. The GAB parameters were estimated both by direct and indirect regression methods. The net isosteric heat of sorption was also determined using the Clausius-Clapeyron equation and was successfully used as a limiting condition for estimating the six unknown parameters of the GAB equation using the direct regression method. The net isosteric heat of sorption decreased from 9.05 kJ/mol at 0.05 kg/kg dry basis to approximately 0.0 kJ/mol at 0.7 kg/kg dry basis. These values, as expected, were comparable to the enthalpy difference between the monolayer and multilayer moisture (8.16 kJ/mol), and the difference between the latent heat of condensation for pure water and the heat of sorption of multilayer moisture (= −0.88 ± 0.69 kJ/mol).