Freezing/Thawing 113liquid ice. Such systems may achieve higher
rates of heat transfer than the single - state
liquids (Maria et al. 2005 ).
Contact freezing offers several advantages
over air cooling — for example, much better
heat transfer and signifi cant energy savings.
However, disadvantages are the need for
regularly shaped products with large fl at sur-
faces with plate systems, and the need to
wrap and wash off the immersion liquid in
immersion systems.
Cryogenic freezing is essentially a subset
of immersion freezing, in that it directly uses
cryogenic refrigerants, such as liquid nitro-
gen or solid carbon dioxide. The method
of cooling is essentially similar to water -
based evaporative cooling, where cooling
is brought about by boiling off the refriger-
ant, the essential difference being the tem-
perature required for boiling. As well as
using the latent heat absorbed by the boiling
liquid, sensible heat is absorbed by the result-
ing cold gas. Due to very low operating tem-
peratures and high surface heat transfer
coeffi cients between product and medium,
cooling rates of cryogenic systems are often
substantially higher than other refrigeration
systems.patented in 1929 by Clarence Birdseye.
Essentially, product is pressed between
hollow metal plates containing a circulating
refrigerant. A hydraulic cylinder is used
to bring the freezing plates into pressure
contact with the product. These plates can
be either horizontal or vertical (Fig. 5.1 ).
Good heat transfer is dependent on product
thickness, good contact, and the conductivity
of the product. Plate freezers are often limited
to a maximum thickness of 50 to 70 mm.
Good contact is a prime requirement. Air
spaces in packaging and fouling of the plates
can have a signifi cant effect on cooling
time; for example, a water droplet frozen
on the plate can lengthen the freezing time
in the concerned tray by as much as 30%
to 60%.
An immersion freezer is made up of a tank
with a cooled freezing liquid that can be any
nontoxic salt, sugar, or alcohol solution in
water, and a means of conveying the wrapped
meat through the tank. The freezing process
is often completed in an air blast system. Ice
slurries are being considered as an alternative
to conventional immersion liquids. Such
binary systems are described in the scientifi c
literature as fl ow ice, fl uid ice, slush ice, or
Figure 5.1. Vertical plate freezer for freezing of blocks of boned meat.