112 Chapter 5
comes the problem of uneven air distribution,
since each item is subjected to the same
velocity/time profi le. Some meat products
are frozen on racks of trays (2 m high), while
pulled or pushed through a freezing tunnel by
mechanical means. For larger operations, it
is more satisfactory to feed meat on a con-
tinuous belt through linear tunnels or spiral
freezers.
In the past decade, the use of impingement
technology to increase the surface heat
transfer in freezing systems has received
attention (Newman 2001 ; Sundsten et al.
2001 ; Everington 2001 ). Impingement is the
process of directing a jet or jets of fl uid at a
solid surface to effect a change. The very
high velocity (20 – 30 m s^ −^1 ) impingement gas
jets break up the static surface boundary
layer of gas that surrounds a meat product.
The resulting medium around the product is
more turbulent, and the heat exchange
through this zone becomes much more effec-
tive. Impingement freezing is best suited for
products with high surface - area - to - weight
ratios (i.e., hamburger patties or products
with one small dimension). Testing has
shown that products with a thickness less
than 20 mm freeze most effectively in an
impingement heat transfer environment.
When freezing products thicker than 20 mm,
the benefi ts of impingement freezing can still
be achieved; however, the surface heat trans-
fer coeffi cients later in the freezing process
should be reduced to balance the overall
process effi ciency. The process is also very
attractive for products that require very rapid
surface freezing and chilling.Contact Freezing Methods
Contact freezing methods are based on heat
transfer by contact between products and
metal surfaces (which in turn are cooled by
either primary or secondary refrigerants) or
direct immersion in a refrigerated liquid.
Modern plate cooling systems differ little
in principle from the fi rst contact freezercon vection, and evaporation/condensation.
Conduction requires a good physical contact
between the meat to be cooled and the cooling
medium, and this is generally diffi cult to
achieve with carcasses and other irregular
meat cuts. Radiation does not require any
physical contact, but a large temperature dif-
ference is required between the surface of the
meat being cooled and that of surrounding
surfaces to achieve signifi cant heat fl ow. In
primary freezing, radiation is only important
in the initial stages of the process in a system
where the meat is not surrounded by other
product. Again, in the initial stages of the
freezing of cooked meat products (e.g., pies,
pasties, joints), radiant heat loss can be sub-
stantial if the products are surrounded by
cold surfaces. Evaporation from a meat
surface reduces yield and is not desirable in
most meat refrigeration operations but can be
useful again in the initial cooling of cooked
meat products. Convection is by far the most
important heat transfer mechanism employed
in the majority of meat refrigeration systems.
In most cases, refrigerated air is the transfer
medium; however, in some cases water,
brine, or a cryogenic gas can be used.
Air Freezing Methods
Air is by far the most widely used method of
freezing meat, as it is economical, hygienic,
and relatively noncorrosive to equipment.
The big advantage of air systems is their ver-
satility, especially when there is a require-
ment to freeze a variety of irregularly shaped
products or individual products. However,
relatively low rates of heat transfer are
attained from product surfaces in air systems.
Systems range from the most basic, in which
a fan draws air through a refrigerated coil and
blows the cooled air around an insulated
room, to purpose - built conveyerized blast
freezing tunnels or spirals. In a continuous
system, meat is conveyed through a freezing
tunnel or refrigerated room, usually by an
overhead conveyor or on a belt. This over-