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| THE PROCESS |
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Cryogenic freezing is an
upcoming food processing
technology that is gaining
popularity because of the lower
setup costs and improved food
quality when compared to
mechanical freezing.
Cryogenic food freezing differs
widely from mechanical ammonia
of Freon freezing systems. Thus,
it requires a different
procedure in determining the
final exit temperature of a
certain food product. Cryogenic
freezing involves freezing the
outer layers of the food beyond
its actual freezing point, while
the inner part of the product
remains warm. The product's
final temperature is not its
temperature as it exits the
cryogenic tunnel. . Only after
full equilibration or
equalization is its final
temperature known. This usually
occurs after up to 15 minutes
later, Innovative cryogenic
freezing technologies can be
used to produce high quality
frozen foods.
To obtain the maximum
refrigeration benefit, a typical
CO2 system will inject CO2
throughout the length of the
freezer. In N2 systems, the N2
is sprayed into the freezer and
separates as liquid and vapor.
As droplets touch the product
surface, the liquid changes to
vapor, and in the process,
extracts latent heat from the
food surface. The vapor
distribution through the freezer
creates convective currents that
increase the freezing rate. In
this case, about 50% of the
refrigeration effect is supplied
by the N2 phase change from
liquid to vapor. The remaining
heat is removed by the N2 vapor
flowing through the freezer. It
should be noted that heat
removal would not induce phase
change by itself: additional
factors such as the rate of
formation of ice crystals and
the propagation of these in the
food structure are involved.
The freezing of food materials
is more complex than the
freezing of pure water. All food
materials contain solutes such
as carbohydrates, salts,
colorants and other compounds
which affect their freezing
behavior. Most food products
contain animal and/or vegetable
cells forming biological
tissues. The water content of
these tissues is either inside
the cells (intracellular fluid)
or surrounding these (extra
cellular fluid). Since the
lowest concentration of solutes
is found in the extra cellular
fluids, the first ice crystals
are formed there. During a slow
freezing, there will be time for
the cell to lose water by
diffusion and the water will
freeze on the surface of the
crystals already formed. As the
cells keep losing water, the
cell shrinks more and more until
it collapses. The large ice
crystals will exert pressure on
the cellular walls, causing drip
loss during thawing. A rapid
freezing promotes a large number
of small ice crystals
distributed uniformly throughout
the tissue, both inside and
outside the cells .Hence,
products frozen with cryogenic
technologies show a matrix of
small ice crystals and a better
texture than products frozen
using slower heat transfer
processes. |
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