Experimental Investigation and Mechanism Analysis on Microwave Freeze Drying of Stem Lettuce Cubes in a Circular Conduit

摘要

Stem lettuces are an important vegetable due to its nutritional characteristics. Stem lettuce was used as the model in this investigation. The ice-melting and freezing point temperature as well as distribution and migration of unfrozen water in stem lettuce cubes during the microwave freeze drying (MFD) step were studied using differential scanning calorimetry (DSC) and low-field pulsed nuclear magnetic resonance (LF-NMR). MFD of stem lettuce cubes was carried out in three circular conduits with different diameters (40, 55, and 70 mm) at a microwave frequency of 2,450 MHz. The total drying time for MFD was almost the same for all circular conduits; that is, approximately 4.5 h. This was less than the 7.0 h required for conventional radiation freeze drying (FD) in a 40-mm-diameter circular conduit and far less than the 20 h required for FD in a flat slab model. The microwave energy supplied and specimen temperature profiles during MFD were strongly influenced by the tissues, unfreezable water content, moisture distribution, and ionic characteristics of the samples. It was also found that the MFD process involving the conduits can be divided into three steps based on the amount and distribution of unfrozen water in the frozen samples and how heat is supplied in order to prevent corona discharge.