Thermal Food Processing

486 Thermal Food Processing: New Technologies and Quality Issues
Cathcart et al. 46 found that while heating packaged bread using RF power,
arcing occurred between the upper electrode and the edges of the cylindrical loaf,
resulting in some burning at those points. This difficulty was overcome by rounding
the edges of the loaf so that there were no corners adjacent to the electrodes.
Moyer and Stotz 5 indicated that when blanching vegetables at 7, 10, and 30 MHz,
arcing and burning occurred between the upper electrode and the vegetables, and
between the individual particles, due to the high voltages at the electrodes. By
decreasing voltage (lowering the electric field across a given sample thickness)
while increasing frequency to 150 MHz to maintain the same output power density
in the medium, the arcing problem was solved. Sanders 47 further demonstrated
the significance of using an air gap between the top electrode and the upper
material surface to lower the voltage, and therefore the electric field across the
sample thickness. By introducing an air gap, some of the voltage between the
electrodes appears in this gap, the exact amount depending on the relative heights
of the gap and of the material, and on the dielectric properties of the material.
The effective voltage is reduced by a factor f p: 47
f p = [(e r tand h a/h s) 2 + (e r h a/h s + 1) 2 ] –1/2 (15.7)
where e r is the relative permittivity (or dielectric constant, e r = e′/e o), and h a and
h s are the heights of the air gap and material (m), respectively. Sanders 47 also
noted that arcing could be controlled by immersing the product in deionized water
or surrounding it with ice, thus minimizing field distortions due to any irregularities,
and again lowering the voltage or effective electric field across the sample
thickness. Dielectric breakdown (arcing) and thermal runaway heating were also
observed during preliminary tests to heat-packaged surimi seafood in a commercial
RF oven. 58
15.6.2 PACKAGING FAILURE
When using RF power to heat packaged food products, it was found that certain
packaging materials failed in an RF field. When heating packaged bread,
Cathcart et al. 46 found that some wrappings, including wax and glassing paper,
were unsatisfactory. Wax paper softened when bread showed an internal temperature
above 38°C, and glassing papers became tacky between 52 and 60°C.
However, they found that cellophane is a satisfactory wrapping material because
it did not break or become unsealed in any of the heat treatments employed. As
stated before, Houben et al. 50,51 found that PTFE, polycarbonate, and borosilicate
glass performed well as tube materials when using RF power to pasteurize
sausage emulsion; however, PVC and PVDC both heated selectively (due to
their polar molecular structures), and therefore could not be used in RF heating.
Burning of packaging material was also observed under some conditions during
preliminary tests with RF heating of vacuum-packaged surimi seafood. 58
Thus, a major challenge in the use of RF power to pasteurize and sterilize
packaged food products is avoiding packaging failure or modification by selecting