Moisture desorption isotherms and glass transition temperatures of osmo-dehydrated apple and pear

The aim of this work was to evaluate osmo-dehydrated pear and apple stability on the basis of combined data of moisture desorption isotherms and glass transition temperatures (Tg) investigated in the temperature range of 30–60 °C. Parallelipedic pieces of pears and apples were osmo-dehydrated at 30 °C in sucrose syrups at 70% during 65 min. To study their hygroscopic properties behavior, the osmo-dehydrated fruits were stored in static desiccators at 30, 45 and 60 °C, and relative humidities ranging from 6 to 80%. The GAB model satisfactory described the relationship between the water activity (aw) and water content (X) of osmo-dehydrated fruits (r² ≥ 0.994). Differential scanning calorimetry was used to measure Tg of osmo-dehydrated fruits conditioned at various water activities. A strong plasticizing effect of water on Tg was found with a large reduction of Tg when the water content increased. The dependence of Tg on water activity data was satisfactory correlated by the Roos model (0.993 ≤ r² ≤ 0.999). Whereas the Tg ～ X_w was adequately described by Gordon and Taylor model (0.898 ≤ r ≤ 0.990). The changes of Tg ～ aw and Tg ～ X_w were established and the critical values of water contents were determined for the investigated range of temperature.