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

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How many factors influence the freezing time? What are they? ... liquid may become sufficiently high to cause crystallization of the solute. ... – PowerPoint PPT presentation

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


1
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

2
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?

3
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 ,

4
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

5
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

6
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.

11
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.

12
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

13
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

14
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.

37
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.

38
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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40
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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44
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.

45
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 .

46
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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50
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)

51
Some of Specific Quality Attributes as Evaluating
Factors
  • 1. Degradations of pigments
  • 2. Loss of vitamins
  • 5. Enzyme activity
  • 4. Oxidation of lipids

52
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.

53
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

54
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.

55
Shelf Life of Frozen Foods
  • Practical storage life (PSL)
  • High quality life (HQL)
  • Just noticeable difference (JND)

56
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

57
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?
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