Study on Moisture Content Changes during Atmospheric Freeze Drying of Fresh-cut Japanese Radish and its Quality after Drying and Rehydration

Abstract

We conducted atmospheric freeze drying (AFD) of fresh-cut Japanese radish (daikon) at different temperatures (0, −5, −10, −18 ℃) with an air speed of 0.43 m･s −1 , and evaluated moisture content changes. We assessed samples dried by AFD for surface color, soluble solids content ( °Brix), rehydration ratio, and texture (breaking stress and initial elasticity) after rehydration. In the rehydration and texture tests, we compared AFD using −5 ℃ samples and samples dried by hot air at 60 ℃. An exponential model was used to describe the sample moisture content during AFD, which decreased exponentially with drying constant with an Arrhenius-type temperature dependency. Compared to the sample dried at 60 ℃, the rehydration ratio was 1.29 times higher for AFD at −5 ℃, and the sample showed a large number of voids. There were no significant differences between the −5 ℃ and 60 ℃ samples in terms of breaking stress or initial elasticity. These results indicate that AFD is an effective drying method for preserving the quality of products.