The document discusses freezing as an operation to preserve food by lowering its temperature below the freezing point. It describes the freezing process and freezing curve, including nucleation, crystal growth, and how solute concentration changes. It discusses factors that influence freezing time like thermal conductivity and size/shape of food pieces. Methods to calculate freezing time are presented, including Planck's equation and Pham's method. Different freezing systems are outlined like air blast freezers, plate freezers, and fluidized bed freezers.

1. Freezing Curve, Freezing System
& Freezing time calculation
Muneeb
Central university Of Punjab

2. Freezing
• Freezing is an unit operation
• In which the temperature of the food is reduced below the freezing point.
• Proportion of water undergoes change in system to form icecrystalls.
• Immobilisation of the water to ice and resulting concentration of dissolved solutes in
unfrozen water will lower the water activity.
• During freezing preservation is achieved by low temperature and reduced water
activity.
• Small changes to nutritional qualities or sensory quality.
• During freezing sensible heat is first removed to lower the temperature to the freezing
point.
• In fresh foods the heat produced by the respiration also removed.

3. The material to be
frozen first cools
down to the
temperature atwhich
nucleation starts.
Before ice can form, a
nucleus, is required
upon which the
crystal can grow; the
process of producing
this seed is definedas
nucleation.
Once the firstcrystal
appears in the
solution, a phase
change occurs from
liquid to solid with
further crystal
growth.
FREEZING PROCESS

4. Freezing curve

5. AS The food is cooled to below the freezing point ᶱf . At the point S the water remains
liquid, although the temperature is below the freezing point. The phenomenon is
known as supercooling and may be as much as 100c below freezing point
SB The temperature rises rapidly to the freezing point as ice crystals begin to form and
latent heat of crystallisation is released.
BC heat is removed as same rate as before, but it is latent heat is being removed as ice
forms and the temperature therefore remains almost constant. The freezing point
is gradually depressed by the increase in solute concentration in the unfrozen
liquor and the temperature falls slightly.
CD One of the solutes become supersaturated and crystallises out, the latent heat of
crystallisation is released and the temperature rises to the eutectic temperature
for that solute.

6. DE Crystallisation of water and solute continues. The total time tf taken is
determined by the rate at which is heat is removed
EF The temperature of the ice–water mixture falls to the temperature of the
freezer. A proportion of the water remains unfrozen at the temperatures used
in commercial freezing; the amount depends on the type and composition of
the food and the temperature of storage.

7.  Crystallization occurs at point ‘B’ of the freezing curve and consists of
nucleation and crystal growth
 Nucleation: occurs by combining molecules into an ordered particle of a size
sufficient to survive and serve as a rate for crystal growth
 Homogeneous nucleation: in puresystems
 Heterogeneous nucleation: nucleus formation around suspended particle or
at a cell wall, in food systems, takes place during supercooling
 Crystal growth: enlargement of nucleus by the orderly addition of molecules

8. IMPORTANT POINTS TO KNOW
1. Freezing point is defined as the temperature at which the first
ice crystal appears and the liquid at that temperature is in
equilibrium with the solid.
2. Freezing time is defined as time required to lower product
temperature from its initial temperature to a given temperature at
its thermal center.
3. Freezing rate is defined as the ratio of difference between initial
and final temperature of product to freezing time.

9. Calculation of freezing time
• During freezing, heat is conducted from the interior of a food to the
surface and is removed by the freezing medium.
• The factors which influence the rate of heat transfer are:
• the thermal conductivity of the food
• the area of food available for heat transfer
• the distance that the heat must travel through the food (size of
the pieces)
• the temperature difference between the food and the freezing
medium
• the insulating effect of the boundary film of air surrounding the
food
• packaging, if present, is an additional barrier to heat flow.

10. It is difficult to define the freezing time precisely but two approaches are taken.
The effective freezing time measures the time that food spends in a freezer and is used to
calculate the throughput of a manufacturing process.
nominal freezing time can be used as an indicator of product damage as it takes no
account of the initial conditions or the different rates of cooling at different points on the
surface of the food
The calculation of freezing time is complicated for the following reasons
• differences in the initial temperature, size and shape of individual pieces of food
• differences in the freezing point and the rate of ice crystal formation within different
regions of a piece of food
• changes in density, thermal conductivity, specific heat and thermal diffusivity with a
reduction in temperature of the food.

11. • HOW TO DETERMINE FREEZING TIME ??
• ONE OF THE WAYTO ESIIMATE FREEZING TIME ISTHROUGH
• PLANCK’S EQUATION.
• THISAPPROCCH WAS DEVELPOED BY PLANCKS IN 1913.
• Plank’s equation was derived based on energy balance principle. He
developed the equation to find out the freezing point of water.
• Although this equation has many limitations but it enables to find out
freezing point with those limitations with spreadsheet or Calculator.

12. This involves the following assumptions:
 freezing starts with all water in the food unfrozen but at its freezing point, and
loss ofsensible heat is ignored.
heat transfer takes place sufficiently slowly for steady-state conditions to operate
the freezing front maintains a similar shape to that of the food (for example in a
rectangular block the freezing front remains rectangular)
there is a single freezing point
the density of the food does not change
the thermal conductivity and specific heat of the food are constant when
unfrozen and then change to a different constant value when the food is frozen.
𝑡𝑓 =
λ 𝜌
𝜃𝑓−𝜃𝑎
[
𝐿
6
1
ℎ
+
𝑥
𝑘1
+
𝐿2
24𝐾2
]
ℎ =
L
6
[
𝑡𝑓 𝜃𝑓−𝜃𝑎
λ 𝜌
−
𝐿𝑥
6𝑘1
−
𝐿2
24𝐾2
]

13. • where tf (s) freezing time
• L (m) length of the cube,
• h (Wm2K1) surface heat transfer coefficient,
• 𝜃f (ºC) freezing point of the food,
• 𝜃a (ºC) temperature of the freezing medium,
• λ(J kg1) latent heat of crystallisation,
• 𝜌(kg m3) density of the food,
• x (m) thickness of the packaging,
• k1 (Wm1K1) thermal conductivity of the packaging,
• k2 (W m1K1) thermal conductivity of the frozen zone,
• 6 and 24 are factors which represent the shortest distance between the centre and
the surface of the food. Other shapes require different factors; these are 2 and 8
for a slab, 4 and 16 for a cylinder and 6 and 24 for a sphere

14. Pham’s Method to Predict Freezing Time
Pham proposed a method for predicting food freezing and thawing times.
His method can be used for finite-size objects of irregular shapes by
approximating them to be similar to an ellipsoid.
Another advantage of this method is that it is easy to use, yet it provides answers
with reasonable accuracy.
The following assumptions are used in developing this method:
The environmental conditions are constant.
The initial temperature, T i, is constant.
The value for the fi nal temperature, T c, is fi xed.
The convective heat transfer at the surface of an object is described by
Newton’s law of cooling.

15. Tfm= 1.8+0.263Tc +0.105Ta
• where Tc is final center temperature (°C), and Ta is freezing medium temperature
• This is an empirically derived equation that is valid for most water-rich biological materials
• The time for freezing of any simple-shaped object is calculated from the following equation:
𝑡 =
𝑑 𝑐
𝐸 𝑓ℎ
∆𝐻1
∆𝑇1
+
∆𝐻2
∆𝑇2
1 +
𝑁 𝐵𝑖
2
where dc is a characteristic dimension, either shortest distance to the center, or radius (m),
h is the convective heat transfer coefficient (W/[m 2 K]),
Ef is the shape factor, an equivalent heat transfer dimension. Ef 1 for an infinite slab, Ef 2 for
an infinite cylinder, and Ef 3 for a sphere.
• ΔH1 is the change in volumetric enthalpy (J/m 3) for the precooling period,
• ΔH2 is the change in volumetric enthalpy (J/m 3) for the phase change and post cooling
period

16. • TYPES OF FREEZING
FAST FREEZING
 Quick or fast freezing occurs
at –25ºC or less. Ice crystals
are small and do not damage
food cells.
SLOW FREEZING
 Slow freezing occurs at -24
ºC or above. Ice crystals are
big and damage the food cells
causing loss of texture,
nutrients, colour & flavour on
thawing.

17. TYPES OF FREEZERS

18. Freezing system
• Indirect contact systems
1. Plate freezers
2. Air blast freezers
3. Freezers for liquid foods
• Direct - contact systems
1. Air blast
2. immersion

19. 1) Cooled air freezer:
Chest freezer
Air blast freezer
Fluidized bed freezer
belt freezer
Tunnel freezer
2) Cooled Liquid
Immersion freezers.
3) Cooled surface freezer
Plate freezer
Scraped surface freezer
Contact freezer
4) Cryogenic Freezers
Liquid nitrogen freezers
Liquid carbon dioxide freezers

20. Cooled air freezers
Chest freezer
• The food is frozen in naturally circulated stationery air at temperature between -20oc to -30oc.
• Chest freezer takes longer time (3-72 h) for freezing
• Loss of product quality
• Air is circulated by fans for uniform distribution of temperature
• The heat transfer coefficients are low.

21. Blast freezers,
• Air is recirculated over food at between -30C and -40C at a velocity
of 1.5–6.0 m/s.
The high air velocity reduces the thickness of boundary films surrounding the food and
thus increases the surface heat transfer coefficient
• In batch equipment, food is stacked on trays in rooms or cabinets. Continuous
equipment consists of trolleys stacked with trays of food or on conveyor belts which
carry the food through an insulated tunnel
• relatively economical and highly flexible in that
foods of different shapes and sizes can be frozen
• The equipment is compact and has a
relatively low capital cost and a high throughput

22. • Fluidised bed freezer
• Fluidised-bed freezers are modified blast freezers in which air at between 25ºC
and 35ºC is passed at a high velocity (2–6 m s1) through a 2–13 cm bed of food,
• contained on a perforated tray or conveyor belt.
• Food comes into greater contact with the air than in blast freezers, and all
surfaces are frozen simultaneously and uniformly
• The equipment therefore needs less frequent defrosting.
• However, the method is restricted to particulate foods (for example peas,
sweetcorn kernels, shrimps, strawberries or French fried potatoes).

23. Belt freezers (spiral freezers)
• have a continuous flexible mesh belt which is formed into spiral tiers and carries
food up through a refrigerated chamber
• Cold air or sprays of liquid nitrogen are directed down through the belt stack in a
countercurrent flow,
• which reduces weight losses due to evaporation of moisture.
• Spiral freezers require relatively small floor-space and have high capacity
• Other advantages include automatic loading and unloading,
• low maintenance costs and flexibility to freeze a wide range of foods including
pizzas, cakes, pies, ice cream, whole fish and chicken portions.

24. • Cooled-liquid freezers
• In immersion freezers, packaged food is passed through a bath of refrigerated
propylene glycol, brine, glycerol or calcium chloride solution on a submerged
mesh conveyor.
• In contrast with cryogenic freezing, the liquid remains fluid throughout the
freezing operation and a change of state does not occur.
• The method has high rates of heat transfer and capital costs are relatively low.
• It is used commercially for concentrated orange juice in laminated card–
polyethylene cans, and to pre-freeze filmwrapped poultry before blast freezing.

25. Cooled-surface freezers
• Plate freezers
• consist of a vertical or horizontal stack of hollow plates, through which refrigerant
is pumped at 40ºC
• They may be batch, semi-continuous or continuous in operation. Flat, relatively
thin foods (for example filleted fish, fish fingers or beef burgers) are placed in
single layers between the plates and a slight pressure is applied by moving the
plates together.

26. Advantages
• good economy and space utilisation,
• relatively low operating costs compared with other methods,
• little dehydration of the product and therefore minimum defrosting of
condensers,
• high rates of heat transfer
Disadvantages
• relatively high capital costs,
• and restrictions on the shape of foods to those that are flat and relatively thin.

27. Scraped surface freezing
• These are used for liquid as well as for semi-solid foods like ice cream.
In ice-cream manufacture, the rotor scrapes frozen food from the wall of the freezer barrel and
incorporates air.
• In scraped surface freezers, the freezing is very fast and up to 50% of the water is frozen within a
few seconds. this results in very small crystals which are not detectable in the mouth and thus given
a smooth creamy consistency to the product.
• The temperature is reduced between -4oc and -7oc. the frozen aerated mixture is then pumped into
containers and finally the freezing is completed in hardening room.

28. Cryogenic freezing
• Freezers of this type are characterised by a change of state in the refrigerant (or cryogen)
as heat is absorbed from the freezing food.
• The cryogen is in intimate contact with the food and rapidly removes heat from all
surfaces of the food to produce high heat transfer coefficients and rapid freezing.
• The two most common refrigerants are liquid nitrogen and solid or liquid carbon dioxide
THE PRODUCT CAN BE EXPOSED TO A CRYOGENIC MEDIUM IN THREE
WAYS:
1. Directly sprayed on the product in a tunnel freezer.
2. Vaporized and blown over the food in a spiral freezer or batch freezer.
3. The product is immersed in the cryogenic liquid in an immersion freezer.

29. • LIQUID NITROGEN FREEZER
• The refrigerant consumption is in the range of 1.2-kg refrigerant per kg of the product.
• Liquid nitrogen refrigerants are colorless and odorless.
• Product is cooled by gaseous nitrogen and frozen by liquid nitrogen spray.
• Typical food products used in this system are fish fillets, seafood and fruits like berries.
• The use of gaseous nitrogen reduces the thermal shock to the food and recirculation fans
increases the rate of heat transfer.

31. References
• Introduction to food engineering r.paul singh , dennis r.heldman
• Food science Norman N. Potter , Joseph H. Hotchkiss
• Food processing technology, Principle and practices P J Fellows.
• https://en.wikipedia.org/wiki/frozen_food
• www. slideshare.net

32. Thank you.