Chpter%206%20FREEZING%20AND%20FROZEN-FOOD%20STORAGE

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Chpter 6 FREEZING ANDFROZEN-FOOD STORAGE

• Principles of freezing
• DESCRIPTION OF FOOD FREEZING SYSTEMS
• INDRECT-CONTACT FREEZING SYSTEMS
• DIRECT-CONTACT FREEZING SYSTEMS
• INDIVIDUAL QUICK FREEZING (IQF)
• ESTIMATION OF FREEZING TIME
• FOOD FREEZING AND PRODUCT QUALITY
• SUMMARY

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Problem

• Explain the cause of freezing temperature
  depression.
• Why should we use rapid freezing process?
• How many factors influence the freezing time?
  What are they?
• What changes occur to the product during
  freezing?
• Why the storage temp. should be kept in constant?

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vocabulary

• freeze, ice crystals , irreversible, negative
  changes, latent heat of fusion , depression of
  the freezing temperature , supercooling region ,
  freezing plateau , solutes , depression of the
  freezing temperature, freezing--point depression
  , freezing--temperature depression, eutectic
  temperature ,

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vocabulary

• barrier, air blat system, spiral conveyor,
  fluidized bed, immersion, cryogenic, tunnel,
  spray of liquid refrigerant, intimate contact ,
  fundamental, estimation, critical, individual,
  freezing medium, Plank's equation, temperature
  gradient, thermal conductivity, cylinder,
  cylindrical, sphere, spherical, infinite plate,
  convective heat transfer coefficients

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Freezing Process

• The freezing process is the removal of thermal
  energy from the food product to the extent
  required to reduce the temperature below the
  freezing temperature of water. The thermal energy
  removed as a part of freezing is primarily latent
  heat of fusion required to convert water to ice
  within the product

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DEFINITION OF FREEZING AND FROZEN-FOOD STORAGE

• Food freezing is the preservation process that
  depends on the reduction of product temperature
  to levels well below the temperature at which ice
  crystals begin to form within the food. By
  reducing the temperature of the product to -10 to
  -20?, the normal reactions that cause
  deterioration of foods are reduced to negligible
  or minimal rates.

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• The two curves compare the characteristic
  temperature/time relationship during freezing of
  water with that of a food product. The first
  characteristic of the food--freezing curve is the
  depression of the freezing temperature. All food
  products will exhibit an initial ice crystal
  formation temperature below that of pure water.
  The second feature of the food-freezing curve is
  the gradual decline in temperature during removal
  of the latent heat of fusion.

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Freezing-temperature Depression

• The formation of ice crystals within the water
  phase results in a concentration of solutes
  within the water phase. The ice crystals are pure
  water in the solid state. The result of the
  increased concentration of the solute in the
  liquid phase causes depression of the freezing
  temp. , a lower temp. at which ice crystals will
  form. Since these changes occur on a continuous
  basis, the gradual temp. decline with time is a
  direct result of freezing-temperature depression.

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Liquid Concentrated in Foods

• In very pure solutions, the solute concentration
  in unfrozen liquid may become sufficiently high
  to cause crystallization of the solute. This
  change occurs at the eutectic temp. In frozen
  foods, the temp. used for food freezing are
  typically well above the eutectic temp. for
  solutes found in food products. Most often,
  eutectic temp. are in -55 to -75? range. It
  should be recognized that all frozen foods, even
  at temp. as low as -40?, will contain small
  fractions of unfrozen water.

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Definition of Freezing Time

• time required to reduce the product temperature
  from the initial freezing temperature to a
  temperature of 5 degrees below the initial
  freezing temperature

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Effect of Freezing Rate on Product Quality

• The formation of large ice crystals causes
  greater damage to the product structure and
  negative impact on the quality attributes of that
  product. As might be anticipated, more rapid
  freezing and formation of small ice crystals
  usually result in a more desirable product after
  freezing and frozen--food storage

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INDIRECT CONTACT FREEZING SYSTEMS

• Plate freezing system
• Indirect-contact freezing system
• Cabinet freezing system
• Scraped surface, continuous system

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cabinet freezing system

• The product is placed in a package prior to
  freezing, and the packages are p1aced 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.

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Plate freezing system

• 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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Indirect-contact freezing system

• 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

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Scraped surface, continuous system

• 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.

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DIRECT-CONTACT FREEZING SYSTEMS

• Air blat system
• Continuous spiral conveyor system
• Continuous fluidized bed system
• Continuous immersion freezing system
• Continuous cryogenic freezing systems

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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.

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Air Blat System

• 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.

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Continuous Spiral Conveyor System

• 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

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Continuous Fluidized Bed System

• 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.

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Continuous Immersion Freezing System

• 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.

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Continuous Cryogenic Freezing Systems

• 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.

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Individual Quick Freezing (IQF)

• 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.

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Estimation of Freezing Time

• The primary dimension of the product is the
  thickness (L) in the horizontal dimension. The
  freezing medium temp. is (Tm) and temp. of
  unfrozen product in the middle of the diagram is
  the initial freezing temp. of the product
  (TF).The vertical sections of 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.

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Factors Influencing Freezing Time

• 1.Thermal conductivity of frozen food (k)
• 2.Convective heat transfer coefficient (h)
• 3.Area for heat transfer (geometry)
• 4.Temperature of the freezing medium (TF)
• 5.Size of the product defined as the thickness (L)

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Food Freezing Product Quality

• 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.

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Influence of Freezing Rate

• In slow rate freezing, ice crystals will become
  much larger.
• The movement of water from one cell to another
  leads to dehydration of the cell and irreversible
  changes .
• Large ice crystals within the product will lead
  to a frozen product with a rough texture.
• But extremely high freezing rates lead to stress
  cracking .

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STORAGE OF FROZEN FOODS

• 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.

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Changes in quality

• Microbial activity (negligible at 18C)
• Biochemical reaction
• Enzymatic reaction (blanching for vegetables,
  sulfur dioxide/ reduction of oxygen/ addition of
  acid for fruits)

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Some of Specific Quality Attributes as Evaluating
Factors

• 1. Degradations of pigments
• 2. Loss of vitamins
• 5. Enzyme activity
• 4. Oxidation of lipids

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Fluctuation of Storage temp. on Product Quality

• An increase in the product temperature results in
  conversion of ice to liquid state, with the
  possibility of recrystallization 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
  recrystallization results in an increase in ice
  crystal size and the impacts on quality.

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Types of recrystallization

• 1. Isomass, a change in shape of the ice crystal
  resulting in a reduction of the surface-to-volume
  ratio
• 2. Accretive, the joining of two ice crystals to
  a form a much larger crystal
• 3. 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

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Freezer Burn

• 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.

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Shelf Life of Frozen Foods

• Practical storage life (PSL)
• High quality life (HQL)
• Just noticeable difference (JND)

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SUMMARY

• Food freezing systems can be divided into major
  categories indirect-contact and direct-contact
  systems.
• The times required for freezing of a product can
  be estimated by Plank's equation.
• Frozen-food quality is influenced by freezing
  process, and frozen food storage.
• The most critical factor is maintaining a uniform
  temperature during storage to avoid the negative
  impacts on product-quality

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Problems

• How to improve the quality of frozen food?
• Why should we increase the freezing rate at the
  freezing operation?
• How to estimate the freezing time for a product,
  what factors should be considered when estimate
  the freezing time?
• What changes in quality will occurs during
  storage of frozen foods?