Shrinkage of papaya (Carica papaya L.) during convective drying: Influence of glass transition phenomenon

The aim of the present work was to study the shrinkage phenomenon during convective drying of papaya cubes and relate its extension with glass transition. A convective tray dryer was used for the experiments, which were carried out at air temperatures of 40 and 70°C and air velocity of 1.0m/s. The shrinkage behaviour was accomplished by total area and apparent volume alterations along drying. Samples were photographed during process, and their lateral areas were measured using the ImageJ software. Sample temperature (Tp) was monitored using thermocouples. The values of glass transition temperatures (Tg) of papaya as a function of moisture content were based on another work. The moisture ratios were fitted to the Fick’s model, obtaining the moisture diffusivities, which varied from 2.6×10 to 3.0×10 m/s. The experimental data of shrinkage were fitted by Suzuki et al. model, which satisfactorily described the results. 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 Tp was above the Tg along the process. At lower temperature (40°C), shrinkage stopped at a critical value of moisture content (0.21g water/g total), which coincided with the point in which the Tg was close to Tp.