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 degrees C. Parallelipedic pieces of pears and apples were osmo-dehydrated at 30 degrees 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 degrees 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 (r(2) >= 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 <= r(2) <= 0.999). Whereas the Tg similar to X-w was adequately described by Gordon and Taylor model (0.898 <= r <= 0.990). The changes of Tg similar to aw and Tg similar to X-w were established and the critical values of water contents were determined for the investigated range of temperature. (C) 2012 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.