Modelling the kinetics of drying of d'Agen plums (Prunus domestica)
A two-stage model has been used successfully to predict the drying curves for d'Agen prunes. The model assumes the drying process occurs in two distinct stages. First, a period where the evaporation of water from the surface of the drying fruit is the limiting factor for the moisture loss is assumed. This results in a constant rate of water loss. A second period of drying occurs when a water concentration gradient has been established within the plum. During this time, the rate of moisture loss is limited by the mass transfer of water through the fruit, leading to a falling rate of evaporation of water at the fruit surface. This model predicts reasonable values for the moisture loss as a function of drying time. Significant discrepancies between the experimental results and the model occur only for a longer drying time at the higher temperature range (e.g. 90°C and above). This is due mainly to uncertainty in the value for the equilibrium moisture loss caused by other weight loss processes in the latter stages of drying such as thermal degradation of carbohydrates.