Numerical Simulation and Quality Analysis of Carrot Crisps Drying Based on Freezing Pretreatment

Freezing pretreatment is one of the novel pretreatments for fruit and vegetable drying, and its treatment intensity has a direct impact on the effectiveness of color protection and drying promotion. However, there are major limitations on transformations of dynamic changes of and water status of material, which is crucial for judging processing intensity. Here a mathematical model was presented to characterize heat transfer in freezing pretreatment of carrot crisps. The model accounts for freezing phase change heat transfer. And a further analysis on the effect of freezing pretreatment on product quality was made according to experimental results. Results showed that the heat transfer process of freezing pretreatment is divided into four stages based on the temperature changes, namely fast cooling segment, phase transition segment, secondary cooling segment, constant temperature holding segment. In the phase transition segment, the presence of latent heat is the core element that the temperature of the material can be maintained constant. In the constant temperature holding segment, the effect of processing time on the material was significant. The optimal pre-freezing conditions for carrots was a temperature of −15°C for the duration of 45 min plus an additional 10 h, resulting in dried products with minimal color difference, maximum rehydration ratio, and optimal puffing degree.