Combined Heat and Mass Transfer Associated with Kinetics Models for Analyzing Convective Stepwise Drying of Carrot Cubes

Stepwise drying is an effective technique that promotes energy saving without additional capital cost. The stepwise drying mode was investigated for energy consumption and dried product qualities using a coupled heat and mass transfer model associated with kinetics equations of volume shrinkage and degradation of beta-carotene in carrot cubes. Simulations were performed using a finite element method with extension of a chemical species transport. Validation experiments were carried out under constant drying modes at 60 degrees C, 70 degrees C and 80 degrees C using a lab-scale convective hot air dryer. The verified models were subsequently employed to investigate the effects of two step-up drying modes (60 to 70 degrees C and 60 to -80 degrees C). The optimal drying condition was determined using the synthetic evaluation index (SI) with criteria of high specific moisture evaporation rate (SMER), low shrinkage ratio and beta-carotene degradation. Simulated results showed comparable agreement with experimental data of moisture content, shrinkage ratio and beta-carotene ratio. Step-up drying of 60 to 70 degrees C gave the highest SMER of 0.50 x 10(-3) kg of water evaporated per kWh, while the operation at constant temperature of 80 degrees C gave the lowest value of 0.19 x 10(-3) kg of water evaporated per kWh. Model-predicted results showed less shrinkage of carrot cubes, but higher degradation of beta-carotene under step-up drying compared to single-stage drying under temperature of 60 degrees C. Based on the highest SI value (0.36), carrot cubes were optimally dried under step-up mode of 60 to 70 degrees C.