In situ Raman monitoring of dielectric-heating-enhanced freeze-drying under different electromagnetic wave frequencies.

We studied the effect of dielectric heating on the enhancement of freeze-drying by electromagnetic waves (EMWs) under different frequencies: 2.45 GHz microwaves (MWs), and 27 and 200 MHz radio frequencies (RFs). The irradiation with RFs, particularly at 27 MHz, reduced the duration of freeze-drying by 67%. We further analysed the water structure by in situ Raman spectroscopy during freeze-drying under EMWs. The phase transition from ice to water occurred soon after starting irradiation by MWs at 2.45 GHz, while the ice phase was almost maintained at an RF of 27 MHz.