Dielectric spectroscopy of low-losses sugar lyophiles: III: The influence of moisture on the dielectric response of freeze-dried lactose

This paper presents the results of a dielectric spectroscopy study of freeze-dried lactose with a range of moisture contents. Dielectric properties were measured over a wide range of frequency (10⁻¹ to 10⁶ Hz) and temperature (−120 °C to 120 °C). Four relaxation processes were analysed with respect to moisture content and corresponding relaxation mechanisms were suggested. Two processes (γ and β) were observed in the sub-T_g range of temperature and another two processes were observed near to and above the glass transition temperature, T_g. The relatively high-frequency γ-process was ascribed to the mobility of pendant hydroxymethyl groups and exhibited only a weak dependence on moisture content. The most moisture sensitive process was the second sub-T_g (Johari-Goldstein) β-process, whereby the relaxation time changed by 2 orders of magnitude as the moisture was increased by 7%. Also the third process (α-relaxation, near T_g) was sensitive to moisture content and was in good agreement with DSC data measured for freeze-dried lactose. The fourth process was a proton percolation process at the micro-crystals formed at the surface of amorphous particles during heating at the temperatures higher than T_g and shows the moisture dependence.