GLASS TRANSITIONS AND STATE DIAGRAMS FOR TYPICAL NATURAL FRUITS AND VEGETABLES

Differential scanning calorimetry (DSC) was used to measure phase transitions and unfreezable water in fresh samples of onion, grape and strawberry after equilibration at different relative humidities. From the DSC trace for each product, the glass transition and melting temperatures, specific heat capacities, latent heats of melting and unfrozen water contents were obtained. It was found that annealing of the sample was necessary to allow devitrification of water and a minimal amount of non-frozen water in the amorphous matrix. The effect of freeze-concentration was observed in an increased glass transition temperature, a gradual disappearance of the ice formation exotherm, and a consequent increase in the ice-melting endotherm. State diagrams for these food samples were defined. The Gordon-Taylor equation was able to predict the glass transition temperature for the water-food systems studied, from the corresponding pure component values.