Radio Frequency (RF) Drying

RF drying has had applications mostly in textile and wood drying. With regards to food, RF dry- ing has mainly been used for finish drying applications for cookies, crackers, and pasta (UIE 1992; Mermelstein 1998). Cookies and crackers, fresh out of the oven, have a non-uniform moisture dis- tribution, which may yield to cracking during handling. RF heating can help even out the moisture distribution after baking, by targeting the remaining moisture. Drying application developments are investigating hybrid drying systems, involving RF with heat pumps or fluidized beds, to cater for the special needs of heat-sensitive food stuffs (Zhao et al. 2000; Chou and Chua 2001; Vega-Mercado, Gongora-Nieto, and Barbosa-Canovas 2001).

2.4 ConCLUsIons

The successful development of the use of RF heating in food applications lies in its product-specific design and in the optimal impedance matching between the power generator and the applicator holding the carefully formulated food product. The quality of the applicator’s design is very impor- tant for its efficiency while the dielectric behavior of the processed material requires optimization in careful food formulation. The development of new applications of RF heating requires well- tuned targeted equipment design and automated control systems taking advantage of monitoring the changes in dielectric properties to predict product quality. The industrial potential of RF processing is interesting, with its greater penetration depth than microwave, which has well-designed applica- tors in heating/drying process applications. The potential of RF is even better with hybrid systems, which take the volumetric heating advantages of dielectric heating and couple them with conven- tional processing for efficient, rapid, and high-quality results.

noMenCLAtURe

symbol Units term

c J/kg⋅K Specific heat

dp m Field penetration depth

dT/dt °C/s Time rate of temperature rise

E V/m Electric field strength

f = 1/T cycles/s, Hz Frequency

P W/m3 _Power

T s Period

εo= 8.854188 × 10–12 F/m Absolute permittivity of vacuum

ε F/m Relative complex permittivity

ε′ F/m Relative real permittivity (dielectric constant) ε″ F/m Relative dielectric loss factor (loss factor) tan δ = ε″/ε′ Dielectric loss tangent

ρ kg/m3 _Density

λ m Wavelength

λo=11.1111 m at 27 MHz Wavelength in free space

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3

Ohmic Heating Effects on

Rheological and Functional