Freezing/Thawing 119Thermal Thawing/Tempering Methods
Air
Air is used in the vast majority of thawing/
tempering applications. Use of still air is
limited to thin products; otherwise, thawing
times are excessively long. Although little or
no equipment is needed, considerable space
is required to lay out individual items of
product. Moving air is more commonly used,
providing more rapid heat transfer as well as
improved control of temperature and humid-
ity. Two - stage air thawing with high initial
air temperature followed by a second stage at
an air temperature below 10 ° C has also been
used. The duration of the high temperature
stage is limited to 1 or 2 hours to avoid exces-
sive bacterial growth, but the increase in heat
input during this time considerably reduces
the overall process time.
Immersion
Immersion in liquid media allows much more
rapid heat transfer, especially if pumped or
agitated to avoid temperature stratifi cation in
the liquid and grouping together of products.
Thawing times are therefore greatly reduced.
Practical limitations are that boxes and other
packaging (unless vacuum pack or shrink
wrap) must be removed before immersion,
bulk blocks are liable to break up, leaching
of product surfaces can lead to poor appear-
ance, and frequent changing of water for
hygiene reasons requires disposal or treat-
ment of large quantities of effl uent.
Plate
Plate thawing takes place between metal
plates through which warm liquid is piped.
The plates and product may also be immersed
in water to improve thermal contact between
them. Shape is important for reasonable
contact with the fl at plates, although immer-
sion helps by fi lling gaps. If immersion is
used, frequent water changes are required to
prevent bacterial accumulation.Vacuum
Vacuum thawing relies on the transfer of
latent heat of condensation of steam onto
product surfaces at low pressure and tem-
perature. For example, if a pressure of
1704 Nm^ −^2 is maintained, steam can be gen-
erated at 15 ° C and will condense at this tem-
perature onto the frozen product surfaces.
This ensures that although large amounts of
latent heat are added, the product will not rise
above 15 ° C. The process is rapid, but evacu-
ation to sub - atmospheric pressure restricts it
to batch operation. It is more effective for
thin products where the heat released at the
surface is quickly conducted through the
product.High Pressure
High pressure decreases the phase change
temperature of pure water (down to − 21 ° C
at 210 MPa). The lowering of the melting
point allows the temperature gap between the
heat source and the phase change front to
increase, and thus enhances the rate of heat
fl ux (LeBail et al. 2002 ; IIR 2006 ). The pres-
sure is released when the food temperature is
a little above 0 ° C. High pressure thawing has
been applied experimentally to pork and beef
(Suzuki et al. 2006 ). There is some evidence
that the high pressure has the additional
benefi t of reducing the number of microor-
ganisms (LeBail et al. 2002 ; IIR 2006 ).Electrical Thawing/Tempering
Methods
Resistance
Resistance to the passage of a current (50 –
60 Hz) creates heating effect (ohmic heating).
Electrical contacts are required and product
structure must be uniform and homogeneous;