Effect of temperature on sorption isotherms and heats of sorption of quince jam

The water sorption isotherms of two different brands of ‘marmelada’ quince jam at different temperatures were determined experimentally and the parameters for the GAB estimated. Results obtained showed that the typical high sugar content of these products leads to an inversion of the usual temperature-dependence of water sorption isotherms for values of water activity above 0.6. This confirmed a behaviour already reported by other authors for high sugar products (e.g. dried fruits). From the data obtained for each brand of Portuguese ‘marmelada’ at different temperatures it was also possible to estimate net isosteric heats of sorption and their dependence on moisture content.