Effects of main drying temperature on drying characteristics and quality of freeze-dried scallion (Allium fistulosum)

The main drying link is the longest time-consuming stage in vacuum freeze-drying (VFD) technology. This study evaluated the effects of main drying temperature (constant temperature, variable temperature (VT)) on the drying characteristics and quality of scallion in VFD. The results showed that there was no significant difference between the drying time of VT and 4 °C group. However, the drying time of VT group was 48.38% shorter than that of −20 °C group. After VT-VFD, the scallion sample was brighter, the volume shrinkage and degree of tissue structure damage was lower, and the bioactives content was higher. Also, after rehydration, the VT-VFD scallion better retained its flavor than the constant temperature-VFD scallion. Therefore, VT drying can significantly improve the drying characteristics and product quality of VFD scallion.