120 Chapter 5
be produced during the process, such as
when energy is absorbed at the surface rather
than penetrating all of the product. In the
extreme, such warming can lead to some
parts of the food being cooked while others
remain frozen. These problems, as well as the
capital cost of equipment, have greatly
limited commercial use. Attempts to avoid
runaway heating have involved low - power
(and hence longer duration) microwaving,
cycling of power on and off to allow equal-
ization periods, and cooling of surfaces with
air or liquid nitrogen. Penetration depth
depends upon temperature and frequency,
being generally much greater at frozen tem-
peratures and greater at lower frequencies.Ultrasonic
In some work, ultrasound has been merely
used to assist heat transfer during immersion
thawing. However, research has shown that
ultrasound is more highly attenuated in
frozen meat than in unfrozen tissue, and that
the attenuation increases markedly with tem-
perature, reaching a maximum near the initial
freezing point of the food (Miles et al. 1999 ).
The ultrasound attenuation - temperature
profi le therefore appears to be better suited
to producing stable rapid thawing than micro-
wave. Miles et al. (1999) has demonstrated
that using 300 kHz ultrasound at an intensity
of 1 Wcm^ −^2 , a 15 cm thick block of meat can
be thawed in less than 1 hour.Tempering and Crust - Freezing
Systems for Meat
Tempering can be a process in which the
temperature of the product is either raised or
lowered to a value that is optimal for the next
processing stage. Tempering systems where
the temperature of frozen product is raised
have been covered in the thawing and tem-
pering section above. Tempering operations
are used to produce the optimum texture in aotherwise the path of least resistance will be
taken by the current, resulting in uneven tem-
peratures and runaway heating. Frozen foods
do not readily conduct electricity at low tem-
peratures, but this improves at higher tem-
peratures, so uniformity of initial temperature
distribution is also important to avoid
runaway heating.
Dielectric
This is split into two frequency bands: radio
frequency and microwave. The fi rst uses
more typical electrical techniques, with con-
ductors, electrodes, etc. The second relies
more on electromagnetic wave technology,
with waveguides to “ beam ” the waves into a
cavity.
Radio Frequency
This uses the application of alternating
electric e.m.f. (3 – 300 MHz), using elec-
trodes. Product requirements are similar to
resistance methods: uniform structure, homo-
geneity, and uniformity of temperature distri-
bution. The fi eld is created between two or
more electrodes, but the product need not be
in direct contact with them. Conveyorized
systems have been applied to thawing of
meat and offal, in some cases using water
surrounding the material to aid temperature
uniformity.
Microwave
Electromagnetic (900 – 3000 MHz) waves are
directed at the product through waveguides
without the use of conductors or electrodes.
Potentially very rapid, the application is
limited by thermal instability and penetration
depth. Instability results from preferential
absorption of energy by warmer sections and
by different ingredients, such as fat. Warmer
sections may be present at the start of the
process; for example, the surface temperature
may be warmer than the middle, or they may