Glass transition phenomenon on shrinkage of papaya during convective drying

Abstract

A differential scanning calorimeter was used to determine the Tg of papaya pieces equilibrated with several water activities. Thermograms revealed the existence of two Tg. The first, which presented lower value, is due to the matrix formed by sugar and water. The second one, less visible and less plasticized by water, probably corresponds to macromolecules. The data of Tg were satisfactory correlated by the Gordon–Taylor model. A convective tray dryer was used for the drying experiments, which were carried out at air temperatures of 40°C and 70°C and air velocity of 1.0m/s. The shrinkage behavior was accomplished by total area and apparent volume alterations along drying. Samples were photographed during process, and their length and lateral areas were measured using the ImageJ® software. As result, the effect of air temperature on shrinkage was significant and the highest temperature (70°C) induced higher extent of shrinkage values. At this condition, papaya sample was in a rubbery state, characterized by great matrix mobility, and remained so until the end of process, since product temperature (Tp) was above the Tg along the process. At lower temperature (40°C), shrinkage stopped at a critical value of moisture content (0.21g/g, wet basis), which coincided with the point in which the Tg was close to Tp.