Influence of processing conditions of atmospheric freeze-drying/low-temperature drying on the drying kinetics of sliced fruits and their vitamin C retention

Sliced fruits (apple and kiwi) were dried under atmospheric freeze-drying (AFD) and/or low-temperature drying (LTD) conditions, and the drying kinetics, resultant degree of shrinkage, and vitamin C retention were evaluated. The air temperature programs were set in the range of −20 to 10 °C, and the impact of the sub-zero temperature setting was investigated. As expected, shortening the sub-zero temperature time and increasing the air temperature led to a shorter total drying time. Notably, the application of freezing had an effect to reduce the drying time. The application of the LTD condition without freezing significantly increased the total drying time compared with that of the drying condition with freezing. When the temperature was increased stepwise from −20 to 10 °C, the total drying time was effectively reduced when the sub-zero temperature was maintained for a relatively long time. This was the most practical drying program for maintaining product quality. It was found that drying at sub-zero temperatures has advantages in reducing product shrinkage; however, the impact was not remarkable. The application of sub-zero temperature conditions was advantageous for vitamin C retention for both apple and kiwi drying. However, if the negative effect of freezing on quality is significant, simple LTD conditions may be a good compromise for balancing total drying time and product quality.