Submitted to: Journal of Food Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/7/2012
Publication Date: 6/17/2012
Citation: Gao, M., Tang, J., Johnson, J.A., Wang, S. 2012. Dielectric properties of almond shells in the development of radio frequency and microwave pasteurization. Journal of Food Engineering. 112(4):282-287. Interpretive Summary: Almonds are the leading horticultural export for the United States, with about 70% of the total crop shipped for export to more than 80 countries. A major problem for raw almonds is Salmonella contamination of the shell occurring during harvest. Salmonella outbreaks caused by consumption of raw almonds have forced the almond industry to seek effective pasteurization processes, but almond shells are porous and conduct heat poorly when conventional forced hot air methods are used. Because microwave and radio frequency treatments heat rapidly throughout the entire product volume, they are being considered as possible pasteurization treatments. This study measured the dielectric properties of almond shells at different moisture levels and frequencies. The results suggest that if almond shells are wetted prior to treatment, pasteurization temperatures would be rapidly reached and then stabilize as the shells are dried by the treatment. Correlations between dielectric properties and moisture content at microwave frequencies may be useful in moisture measurement of the shells. Dielectric properties were also used to calculate penetration depths at different frequencies, and the commonly available radio frequency of 27 MHz provided sufficient product penetration to allow for the development of continuous commercial pasteurization treatments of almonds.
Technical Abstract: To develop pasteurization treatments based on radio frequency (RF) or microwave energy, dielectric properties of almond shells were determined using an open-ended coaxial-probe with an impedance analyzer over a frequency range of 10 to 1800 MHz. Both the dielectric constant and loss factor of almond shells decreased with increasing frequency, temperature and moisture content. The absolute value of the slopes of log-log plots between frequency and loss factor increased with increasing temperature at low frequencies, especially at high temperatures and moisture contents. The effective electrical conductivity of shell samples was close to zero at the lowest moisture content (6% wet basis) and 3-9 times larger at 90°C than 20°C for the highest moisture content (36% wet basis). A good linear relationship was observed between permittivity and density at 1800 MHz. The power penetration depths at RF range (27 and 40 MHz) were about 6-24 times as deep as those for microwave frequencies (915 and 1800 MHz) at each corresponding temperature and moisture content. It is likely that RF energy may provide uniform heating and high throughput treatments for controlling Salmonella in in-shell almonds after washing.