Influence of Two-Stage Drying Methods on the Physical Properties and Drying Characteristics of Sweet Potato Slices

The effects of single-stage freeze-drying, two-stage mid-infrared assisted freeze-drying and vacuum pre-drying and freeze finish-drying on the operational time, energy uptake, color and hardness properties of sweet potatoes were investigated. Prior to the drying procedure, the samples were unpretreated and pretreated in citric acid solution. Drying curves of sweet potato were determined for mid-infrared and vacuum pre-drying at different drying temperatures (40, 50 and 60°C). The combination of mid-infrared and freeze-drying showed the largest reduction in both drying time (52%) and specific energy consumption (52%) compared to single-stage freeze-drying. Four different empirical thin layer drying models were compared based on their coefficient of determination and root mean square error to estimate the drying curves. The results showed that the Polynomial and Henderson-Pabis models were the most appropriate to describe the drying of sweet potato slices. The total color difference and hardness values of sweet potato degraded while increasing the pre-drying temperature. The citric acid pretreatment enhanced the texture and color of dried sweet potato