Convective drying kinetics of raspberry

Abstract

The drying kinetics of raspberry in a laboratory dryer was studied. Fresh, untreated raspberries were dried at selected temperatures of 50 °C with a constant air velocity of 1.2 m/s. The drying rate curves showed that drying process took place only in the falling rate period. Thin-layer drying models of Newton-Lewis, Henderson and Pabis, logarithmic, Page, Wang and Singh evaluated based on coefficient of determination (R2), reduced chi-square (X2) and root means error (RMSE). The logarithmic model was found to be the best model for describing the characteristics of raspberry. The changes of color on raspberry's surface were determined by original approach, combining digital camera, computer and graphic software. Pearson correlation coefficients and triplicate sample paired t-test on R (red) colour index frequency were calculated during the experiment. A mathematical model to predict the shrinking of geometrical bodies was proposed, assuming unidirectional drying and two-dimensional shrinkage. The model was numerically solved by finite differences, taking into account a convective term in the mass balance equation, which appears as a consequence of non-unidirectional shrinkage. Thermal analysis, by means of differential scanning calorimetry (DSC) and thermogravimetry (TGA) of fresh and dried raspberries have been studied. From results obtained differences in thermal stability of fresh and dried raspberries were shown.