Radio-frequency treatment for stabilization of wheat germ: Dielectric properties and heating uniformity

Wheat germ (WG) is a valuable by-product of wheat milling but is highly susceptible to lipid rancidity induced by lipase (LA) activity. Therefore a stabilization step is required to reduce LA activity. Radio-frequency (RF) heating is an alternative thermal technology suitable for processing food and has great potential for WG stabilization. However, in order to develop effective RF stabilization treatments, a knowledge of the dielectric properties (DPs) (i.e. dielectric constant (epsilon '') and dielectric loss factor (epsilon '')) of WG at frequencies used for heating is required. Furthermore, an understanding of the influence of RF processing conditions on heating uniformity and LA inactivation is needed. Findings in the present study show that epsilon' and epsilon '' of WG increased with increasing temperature and moisture content (MC). Furthermore, the relationship between DPs and temperature/MC can be described by quadratic order models. RF heating uniformity in WG gradually improved with decreasing heating rate from 19.6 to 6.3 degrees C/min and MC from 15.96 to 7.05% (wet basis, w.b.). Placing the WG on a moving conveyor at 4 m/h slightly improved RF heating uniformity in the interior layers of the WG. Placing a pair of rectangular polyetherimide plates (12 x 8 cm) above and below the container during RF heating at 6.3 degrees C/min further improved temperature distribution leading to a more uniform LA inactivation in WG. Overall, the results obtained from this study are useful in computer simulation and optimizing process parameters for WG stabilization by RF heating.