a typical microwave heating cavity and its attachments of the mats system
a single mode cavity
two micro wave transparent windows at the top and bottom of the cavity
one of the two horns at bottom
a tee waveguide junction
h-bend waveguide elbow
one of the six ebend waveguide elbow
connected with generator
dielectric and thermal properties of whey protein gel
thermal conductivity
relative loss factor
relative dielectric constant
specific heat
simulation
case
microwave generator
oparating
transmitted
cavity
typical
occupied
The Federal Communications Commission (FCC) of the United
States designated 915 ± 13 MHz and 2450 ± 50 MHz for industrial,
scientific, and medical uses other than telecommunications. However,
the operating peak frequency of a magnetron may vary
within or beyond the allocated bandwidth. The variations are
caused by differences in design and manufacture of magnetrons
and the generators. A magnetron may also experience frequency
shifts as it ages (Cooper, 2009). An important reason for the frequency
shift would be the reduction of strength of the permanent
magnet in the magnetron (Decareau, 1985). Finally, the operating
frequency of a microwave generator also changes with the power
setting during operation.
The heating pattern of food in a microwave heating system is
determined by the microwave propagations and resonant modes
within the microwave heating cavities. Each mode has a matched
frequency. In a multimode microwave heating cavity with fixed
dimension, the mode type is determined by the microwave
frequency. A small shift in frequency may result in a different
mode type (Dibben, 2001), which can lead to unpredictable heating
patterns. For industrial microwave assisted thermal processes that
require regulatory acceptance for food safety purposes, it is highly
desirable that the systems provide predictable and repeatable
heating patterns in the processed foods to allow accurate monitoring
of temperature history at the cold spots. A 915 MHz single
mode microwave assisted thermal sterilization (MATS) system
was developed at Washington State University (WSU) with the
ultimate goal for industrial implementation (Tang et al., 2006).
The MATS system was powered by four high-power magnetron
generators. Since its inception, several MATS processes for different
foods in either rigid trays or flexible pouches were developed
by the WSU research team and accepted by the United States Food
and Drug Administration (FDA) or the United States Department of
Agriculture Food Safety and Inspection Service (USDA, FSIS). After
monitoring the operating frequencies of the four generators of
the MATS system over one year (2009–2010), we noticed changes
in their peak frequencies. Although no change in the heating
patterns were observed during microwave processing, it is necessary
to systematically investigate the effect of the operation peak
frequency on microwave heating of foods and determine the