Indirect contact freezing systems
Module- 12
Lec- 12
Dr. Shishir Sinha
Dept. of Chemical Engineering
IIT Roorkee
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Plate freezing system
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Indirect-contact freezing system
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Cabinet freezing system
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Scraped surface, continuous system
Figure 1 Schematic diagram of indirect-contact freezing system
Cabinet freezing system
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The product is placed in a package prior to freezing, and the packages are placed on
trays before they are moved into the freezing system. These types of freezing systems
operate as batch systems, with ti1e freezing time established by the length of time that
the product remains in the cabinet. The environment in the room is maintained at a
low temperature, and air movement is established by fans within the cabinet.
Figure 2 Cabinet freezing system
Plate freezing system
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In these types of freezing systems, the product is held firmly between two plates
throughout the period of time required for product temperature reduction. The plates
are the primary barrier between the cold refrigerant and the product. These types of
freezing systems have a definite advantage when the product configuration allows for
direct and close contact between the plate surface and the product surface.
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Figure 3 Plate freezing system
Indirect-contact freezing system
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Indirect-contact freezing system using high-velocity air. In these types of systems the
product package is the barrier between the cold air as a freezing medium and the
product within the package. As illustrated, the systems are designed to be continuous,
with continuous product movement in a direction concurrent to air flow.
Figure 4 Indirect-contact freezing system
Scraped surface, continuous system
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These types of freezing systems utilize a scraped surface heat exchanger as a primary
component of the continuous system used to convert liquid product into a frozen
slurry. In these systems, the outer wall of the heat exchanger barrel represents the
barrier between the product and the low-temperature refrigerant used for product
freezing.
Figure 5 Scraped surface, continuous system
Direct-contact freezing systems
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Air blat system
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Continuous spiral conveyor system
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Continuous fluidized bed system
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Continuous immersion freezing system
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Continuous cryogenic freez
ing
systems
Figure 6 Direct-contact freezing systems
Direct Contact Freezing System
Direct contact between the product and medium used for reduction of product temperature
is appropriate and effective. In these types of systems, there is no barrier between
product surface and the medium utilized for product freezing.
Figure 7 Direct Contact Freezing System
Air Blat System
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One type of air blat system is used for direct contact freezing. The product is carried
through the entire freezing system on a conveyor, and low temperature air is directed
over the product throughout the period of time the product is held in the freezing
tunnel.
Figure 8 Air Blat System
Continuous Spiral Conveyor System
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In this type of system the product is carried on a spiral conveyor from the time it
enters the low temperature environment until it leaves the system. For these types of
systems, the freezing time is established by the rate of movement of the conveyor
systems through the low-temperature environment
Figure 9 Continuous Spiral Conveyor System
Continuous Fluidized Bed System
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In these types of freezing systems, the product moves on a conveyor into the cold
environment in a manner similar to air blast systems. In a fluidized bed system, the
cold air used as a freezing medium is directed upward through the mesh conveyor at
velocities sufficient to cause vibration and movement of product on the conveying
system. The vibration or movement of product while being conveyed, increases the
contact between cold air and the product and reduces the time required for freezing.
Figure 10 Continuous Fluidized Bed System
Continuous Immersion Freezing System
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For products where rapid freezing is appropriate, direct contact between a liquid
refrigerant such as nitrogen or carbon dioxide may be used. The product is carried on
a conveyor through a bath of liquid refrigerant to establish direct and intimate contact
with the liquid refrigerant.
Figure 11 Continuous Immersion Freezing System
Continuous Cryogenic Freezing Systems
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The product on a conveyor moves through a tunnel where it is exposed to a spray of
liquid refrigerant as it changes phase to vapor state. The length of time for freezing is
established by the rate of conveyor movement through the tunnel where the product is
exposed to the cryogenic refrigerant.
Individual Quick Freezing (IQF)
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The concept of IQF involves exposure of individual pieces of the product to a low
temp. medium for a relatively short period of time. In some situations, the rate of
freezing is increased by promoting more intimate contact between the product pieces
and the cold refrigerant. In other situations, IQF is achieved by using very low
temperature cryogenic refrigerants as a freezing medium.
Estimation of Freezing Time
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The primary dimension of the product is the thickness (L) in the horizontal dimension.
The freezing medium temp. is (T ) and temp. of unfrozen product in the middle of
m
the diagram is the initial freezing temp. of the product (T ).The vertical sections of
F
product in contact with the freezing medium are referred to as frozen zones. As the
freezing process continues, the thickness of the frozen zones increases. When the two
zones meet the freezing process is complete.
Figure 12 Schematic diagram for development of Plank’s equation
Factors Influencing Freezing Time
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Thermal conductivity of frozen food (k)
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Convective heat transfer coefficient (h)
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Area for heat transfer (geometry)
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Temperature of the freezing medium (T )
F
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Size of the product defined as the thickness (L)
Food Freezing & Product Quality
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An unfrozen product could have 70% water and 30% total solids. Within a
temperature range of 5 degrees below the initial freezing point, a product might have
30% unfrozen water, 40% ice or frozen water, and the same 30% total solids. The
changes occur gradually, As the temperature continues to decrease, the percentage of
water in the frozen state increases. At a temperature low enough, a small fraction of
the water will remain in the liquid state, The changes will have impact on the product
quality attributes.
Influence of Freezing Rate
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In slow rate freezing, ice crystals will become much larger.
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The movement of water from one cell to another leads to dehydration of the cell and
irreversible changes .
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Large ice crystals within the product will lead to a frozen product with a rough
texture.
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But extremely high freezing rates lead to stress cracking .
Storage of frozen foods
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The quality of a frozen-food is influenced by storage conditions. The changes in
quality decrease as temperature is decreased, maintaining low storage temperatures
increases the cost of frozen-food storage. Higher temperatures in frozen-food storage
must be avoided due to the sensitivity of the frozen-food to temperature. Experience
has established that a frozen food storage temperature of -18℃ is accepted as a safe
storage temperature for extended shelf life of a frozen food.
Changes in quality
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Microbial activity (negligible at-18C)
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Biochemical reaction
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Enzymatic
reaction (blanching
for vegetables,
sulfur dioxide/reduction of
oxygen/addition of acid for fruits)
Some of Specific Quality Attributes as Evaluating Factors
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1. Degradations of pigments
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2. Loss of vitamins
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5. Enzyme activity
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4. Oxidation of lipids
Fluctuation of Storage temperature on Product Quality
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An increase in the product temperature results in conversion of ice to liquid state, with
the possibility of re-crystallization when the temperature decreases. Small ice crystals
will tend to melt as the temperature rises and change back to ice when the temperature
is lowered. The re-crystallization results in an increase in ice crystal size and the
impacts on quality.
Types of re-crystallization
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Isomass, a change in shape of the ice crystal resulting in a reduction of the surface-tovolume ratio
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Accretive, the joining of two ice crystals to form a much larger crystal
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Migration, an increase in size of crystals and an overall reduction in the number of ice
crystals as a result of liquid water migrating from one crystal to another
Freezer Burn
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Reduced air temperature in the frozen-food storage environment is at a very low
relative humidity. This creates a significant vapor pressure gradient between frozen
product surface and the surrounding low-temperature air. A loss of moisture from the
product surface and a negative quality impact occurs.
Shelf Life of Frozen Foods
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Practical storage life (PSL)
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High quality life (HQL)
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Just noticeable difference (JND)
Summary
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Food freezing systems can be divided into major categories indirect-contact and
direct-contact systems.
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The times required for freezing of a product can be estimated by Plank's equation.
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Frozen-food quality is influenced by freezing process, and frozen food storage.
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The most critical factor is maintaining a uniform temperature during storage to avoid
the negative impacts on product-quality
Problems
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How to improve the quality of frozen food?
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Why should we increase the freezing rate at the freezing operation?
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How to estimate the freezing time for a product, what factors should be considered
when estimate the freezing time?
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What changes in quality will occurs during storage of frozen foods?
Reference
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http://books.google.co.in/books?id=vg10zQP0FkwC&pg=PA455&lpg=PA455&dq=I
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http://www.fao.org/docrep/008/y5979e/y5979e03.htm
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
http://www.fao.org/docrep/008/y5979e/y5979e03.htm