FoSB 6201_Refrigeration and Freezing foo

Refrigeration of food and food materials

1. POSTHARVEST HANDLING AND
PROCESSING I
FoSB 6201

2. Refrigeration and Freezing
Heat transfer unit operation

3. 1.Chilling
2.Freezing
3. Freeze-drying and
4.Freeze concentration
Low temperature processing

4. •Refrigeration slows down the chemical
and biological processes in foods and the
accompanying deterioration and the loss of
quality.
•The storage life of fresh perishable foods
such as meats, fish, fruits, and vegetables
can be extended by several days by cooling,
and by several weeks or months by
freezing.
•There are many considerations in the
design and selection of proper refrigeration
and heat transfer mechanisms.
•Micro-organisms and enzymes are inhibited
at low temperatures, but unlike heat
processing they are not destroyed.
•The need to maintain chill- or frozen
temperatures throughout the distribution
chain is of importance.

5. Industrially, food refrigeration temperatures may be divided into the following
principal ranges:
i.Moderate cooling involves bringing the food to a moderately low
temperature (typically 15 to 20 °C), which can be done cheaply with
cold water or evaporative cooling.
ii.Chilling involves cooling the food to a temperature not far above its
freezing point, which for most fresh foods is in the range −1 to 8 °C.
iii.Conventional freezing usually attempts to bring the food down to the
range −12 to −40 °C, which is readily done by mechanical refrigeration.
•Frozen meat is usually kept at −18 °C, fish in the range −18 to
−30 °C, while ice cream is brought down to −35 to −40 °C for
hardening and then stored below −25 °C to prevent crystal growth
and texture loss.
iv.Cryogenic freezing covers the lowest temperature ranges.
•The most common cryogenic refrigerants are dry ice (CO
2),
with a sublimating temperature of −78.5 °C, and liquid
nitrogen, with a boiling point of −196°C.

6. Chilling is a physical operation in which temperature of a food is
reduced
i.Chilling involves cooling the food to a temperature not far
above its freezing point, which for most fresh foods is in the
range −1 to 8°C.
ii.It is used to reduce the rate of biochemical and
microbiological changes hence extension of shelf life of
fresh and processed foods.
iii.It causes minimal changes to sensory and nutritional
properties of foods.
iv.Chilling is often used in combination with other physical
processes such as pasteurization to extend shelf life of
mildly processed foods.
1. Chilling

7. Chilled food are grouped into 3 categories according to their
storage temperature;
i.−1 °C to 1°C (fresh fish, meats, sausages and ground meats,
smoked meats and breaded fish).
ii.0 °C to 5 °C (pasteurized canned meat, milk, cream,
yoghurt, prepared salads, sandwiches, baked goods etc.).
iii. 0 °C to 8 °C (full cooked meats, cook or uncooked cured
meats butter, margarine, hard cheese, fruit juices and soft
drinks).
•Chilled food supply is heavily dependent on sophisticated
and relatedly expensive distribution systems involving chill
stores, refrigerated transport and retail chill display cabinets,
together with widespread of ownership of domestic
refrigerators.
•Precise temperature control is essential at all stages.

8. •Chilling reduces the rate of enzymatic and microbiological
changes and retards respiration of fresh foods.
•The factors that control the shelf-life of fresh crops in chill
storage include;
i.The type of food variety or cultivar.
ii.The part of crop selected (e., spinach vs carrots vs
onions).
iii.The condition of the food at harvest.
iv.The temperature at harvest, storage, distribution and
retail display.
v.The relative humidity of the storage atmosphere, which
influences dehydration losses.

9. ❑Refrigeration is used for
•Cooling/chilling of foods
❑Low temperatures are applied mainly for preservation of
processed and fresh foods by;
•Reducing activity of microorganism,
•Prevent enzyme activities,
•Retard chemical and biological reactions.
❑Typical application include preservation of ;
•Fresh products e.g., vegetable thru pre-cooling & chilling
•Cold storage of vegetables, meat, and fish.
•Freezing of meat and fish.
❑Life of many foods may be increased by storage at temperatures
below 4° C.
Refrigeration

10. Refrigeration…
✓Refrigeration retards spoilage but cannot improve the initial
quality of the product.
✓Unlike thermal sterilization, refrigeration is not a method of
permanent preservation.
✓Refrigerated and even frozen foods have a definite ‘shelf life, the
length of which depends on the storage temperature.
✓The preserving action of cold exists only as long as low
temperature is maintained, i.e., ‘cold chain’.
✓Refrigeration must often be combined with other preservation
processes.
✓Best preservation method…, but expensive!!

11. Equipments
✓Chilling equipment is classified by the method of removing heat.
a.Mechanical refrigerators
b.Cryogenic systems (solid CO
2 Liquid CO
2 or N
2)
c.Vacuum cooling
d.Hydrocooling (direct immersion in chilled water)
✓Batch and continuous operation is possible

12. 1.Moisture movement: In air cooling and freezing, moisture loss
from food is often a major consideration.
•Dehydration, or moisture loss, causes a product to shrivel
or wrinkle and lose quality.
•The loss of moisture from fresh fruits and vegetables is
also called transpiration.
•Moisture loss can be minimized by;
(i) keeping the storage temperature of
food as low as possible,
(ii) keeping the relative humidity of the
storage room as high as possible, and
(iii) avoiding high air velocities.
Changes in foods during refrigeration

13. 2.Ice formation and crystal growth: Usually the water in the food
does not turn into ice until it is cooled several degrees below
the thermodynamic freezing point.
3.Physiological changes: Chilling may cause damage to the cell
membrane of fruit and vegetables, leading to other reactions
that damage or kill the cells and cause loss of texture, colour
and other quality factors.
•Chilling injury may happen at temperatures well above
freezing, especially with tropical fruit and vegetables.
4.Biochemical reactions are influential in affecting sensory
qualities: texture, colour, taste and flavour.
•Biochemical reactions such as ripening, starch–sugar
reactions in vegetables, autolysis (enzymic tissue
destruction), oxidation and vitamin degradation slow
down at lower temperatures and most are stopped by
freezing.

14. v.Microbial growth: Refrigeration aims at stopping the growth of both
pathogenic bacteria, which are dangerous to health, and spoilage
bacteria, which cause an unpleasant taste, smell or appearance.

15. Numerous changes in physical properties caused by exposure to
low temperatures may have significant effects on food quality.
Some of these changes are:
•Increase in viscosity.
•Decrease in solubility, resulting in crystallization,
precipitation, cloudiness (e.g. in beer).
•Hardening, transition to rubbery and glassy state in
carbohydrate systems.
•Hardening of fats.
•Decomposition of colloidal systems such as emulsions
and gels.
•Freezing.
The effect of low temperature on physical properties

16. Methods for chilling or freezing
▪Basic methods for reducing temperature/cooling of liquid and solid
foods:
i.Mechanical compression of refrigerants, most important
method – consist of a compressor, a condenser, an
expansion (throttling) valve, and an evaporator.
ii.Evaporation of cryogenic fluids/liquids e.g., liquid nitrogen
and liquid/solid carbon dioxide.
iii.Vacuum cooling – removal of large quantities of heat by
evaporation of water at low temperature. It is applied for
cooling foods with a larger external surface containing free
water e.g., leafy vegetables
iv.Use of ice (ice blocks, ice tubes, or ice flakes) e.g., fresh
foods such as fish and vegetables; processed foods such as
poultry and cut meet also in smaller applications. The
cooling effect is produced by the melting ice.

17. During cooling or freezing, heat is removed from the
food usually by the combined mechanisms of
convection, radiation, and evaporation, and the rate
of heat transfer between the food and the surrounding
medium at any time can be expressed as;
Where;
h average heat transfer coefficient for combined
convection, radiation, and evaporation, W/m2 · °C
As exposed surface area of the food, m2
Tsurface surface temperature of the food, °C
Tambient temperature of the refrigerated fluid (air,
brine, etc.) away from the food surface, °C.
The heat transfer coefficient h is not a property of the
food or refrigerated fluid. Its value depends on the
shape of the food, the surface roughness, the type of
cooling fluid, the velocity of the fluid, and the flow
regime.

18. Cold storage (industrial setting)

19. Compression (Compressor):
❑Refrigerant is in gaseous state (Ammonia).
❑Its temperature and pressure are elevated, resulting in a superheated gas
with increased enthalpy
❑Discharge line: A discharge line is used for delivering the high
pressure, high temperature vapour from the discharge of the
compressor to the condenser.
Condensation (Condenser):
•The superheated gas enters a heat exchanger, commonly referred as a
temperature exchanger or condenser.
•Using either a air-cooled or water-cooled atmospheres, the refrigerant
gives up heat to the surroundings and condenses to form a saturated
liquid.
•By the time all the refrigerants has liquefied, its temperature may fall
below the point and a subcooled liquid may be obtained.
Receiver tank: Receiver tank is used to provide storage for a condensed
liquid so that a constant supply of liquid is available to the evaporator as
required.
Liquid line: A liquid line transmits the liquid refrigerant from the receiver
tank to the refrigerant flow control.
Mechanical compression of refrigerants

20. Expansion valve:
•The proper amount of refrigerant liquid to
the evaporator is controlled by the
expansion valve.
•A pressure and temperature drop and
a mixture of liquid and gas leaves …
Evaporation (Evaporator):
•An evaporator provides a cooling effect
to the refrigerated space.
•The liquid refrigerant absorbs heat
from the refrigerated surface and gets
converted into refrigerant vapour.
•The saturated vapor may gain more
energy and become superheated before
entering the compressor.

21. https://www.youtube.com/watch?v=XscHn6GPWO0

22. Working principle
A simple vapour compression system follows the following chain,
Compression – condensation – expansion – vaporization
•The vapour at low temperature and low pressure enters the compressor
where it is compressed which makes the temperature and pressure to
increase considerably.
•From the compressor the vapour enters the condenser where it is
condensed to a very high pressure liquid and it is collected in a receiver
tank.
•From the receiver tank, the refrigerant slowly passes through the
expansion valve, where it is throttled down to a lower pressure.
•Finally it reaches the evaporator where it extracts heat from the
surrounding where it is being refrigerated and vaporizes to low pressure
vapour. Thus the surrounding is cooled. From the evaporator the low
pressure refrigerant vapour enters the compressor. This process is
repeated as a cycle.
Refrigeration – How it works https://www.youtube.com/watch?v=wzqTWv8zGlM

23. 2. Freezing
Freezing is a physical process in which the temperature of a
food is reduced below its freezing point.
Freezing is one of the most widespread industrial methods of
food preservation today.
▪Based on the rapid crystallization of free water into
ice.
▪Storage of food for several months at temp near -18
°C.
▪Frozen food usually packaged to prevent moisture loss
and quality changes.
▪Preservation is achieved by a combination of low
temperatures, reduced water activity and in some
foods, pre treatments (by blanching).
▪Only small changes in nutritional or sensory qualities
of foods when correct freezing and storage procedures
are followed.

24. Food preservation at low temperature
•Freezing; uses temperatures below the freezing point,
conventionally below -18 °C.
The stronger preserving action of freezing is due not only to the lower
temperature but also and mainly to the depression of water activity as
a result of conversion of part of the water to ice.
The use of refrigeration in the food industry is not limited to
preservation.
•other purposes such as hardening (butter, fats)
[https://www.youtube.com/watch?v=DWd3062qR_Q],
•freeze concentration, freeze drying,
•air conditioning including air dehumidification and cryomilling
(see size reduction).
•Commercially frozen foods include: fruits, vegetables, fish, meats,
baked goods, prepared foods.

25. •The main changes to frozen foods during storage are as
follows;
i.Degradation of pigments (Chloroplast and chlomoplasts
broken down and to chlorophyll degraded to brown
pheophytin)
ii.Loss of vitamins – water soluble vitamins e.g., vit C, B1, B2,
B6, niacin
iii.Residual enzyme activity
•polyhenoloxidase activity cause browning,
•lipoxygenases activity produces off-flavors and off-odours from
lipids
•Proteolytic and lipolytic activity in meats alter the texture and off-
flavor over longer storage.
iv.Oxidation of lipids – cause off-odours and off-flavours

26. ❑The method of freezing is an important consideration in the
freezing of foods.
❑The methods for chilling or freezing may be classified according
to the mechanism of heat transfer.
a.The use of a steam of cold air (Blast chillers or freezers)
b.Contact with cold surfaces (Contact freezers)
c.Immersion in cold liquid/low temperature brine
(Immersion coolers/chillers or freezers)
d.Evaporative cooling/Cryogenic (Cryogenic cooling or
freezers)
Methods of Freezing

27. Industrial cooling..
Industrial cooling system is divided in 4 main categories;
a.Plate contact systems
b.Gas contact systems
c.Immersed and liquid contact systems
d.Cryogenic freezing systems

28. Food is placed in contact with a cold surface and the temperature
difference transfers heat from the food. Foods are cooled by
conduction.
i.Plate freezers; Food is positioned between two cold metallic
plates. Commonly used in production lines for solids and
packaged food e.g., fish fillets, meat patties and chopped
vegetables.
ii.Conveyor belts : Food (packaged) is placed on a temperature-
controlled metallic conveyors to the cold end of the conveyor
run
iii.Rotating drums: Use to cool or freeze high viscosity liquids
by introducing them on the top of the rotating refrigerated drum
and scraping off as the drum rotates through about 270 °
1.Plate contact systems (Contact freezers):

29. A cold gas (air) flows through the food, absorbing heat from it.
Depending on the gas velocity and temperature the system can
be used either for freezing or for cold storage.
i.Tunnel refrigerator: food is transported through a cold
chamber on a conveyor,. Cold air, forced inside the tunnel
establishing contact with the food and causes the heat
transfer.
2.Gas contact refrigerators:

30. •The product is brought into contact with a fast-flowing
chilled liquid.
•Chilled water, brines (salt & sugar solutions), and other types
of liquids such as alcohol or ethylene glycol solution are
commonly used.
•Food sometimes unpackaged are brought into contact with
the flowing liquid in two ways ;
i.Immersion (not commonly used as it is difficult to control
mass transfer from the liquid to sample also effects on the
products characteristics.
ii.Spraying – the liquid is sprayed over the food, which
makes heat transfer more efficient.
Liquid contact is more preferred as less liquid is used, cutting costs
and lowering environmental impact
3.Immersed and liquid contact refrigeration

31. •Quick freezing preserves the texture of fruit and
vegetables tissues more successfully.
•It allows temperatures as low as -20 and -30 C
Kumar et al., 2018, Int. J. Pure App. Biosci. 6 (2): 1343-1346.DOI: http://dx.doi.org/10.18782/2320-7051.6458

32. Kumar et al., 2018, Int. J. Pure App. Biosci. 6 (2): 1343-1346.DOI: http://dx.doi.org/10.18782/2320-7051.6458

33. Are used in the fast (cooling) or freezing of food at a very low temp.
•Liquid nitrogen and liquid carbon dioxide are used as
refrigerants.., which freeze the product by direct contact,
either as vapors or liquids.
•The boiling of liquid nitrogen and liquid carbon dioxide are -
196 °C and -79 °C respectively.
•Cryogens are colorless, odorless, and chemically inert
•Give very large temperature difference and high heat.
•Cryogenic cooling is used in limited applications because of
its high cost.
•Sometimes cryogenic cooling is used in combination with air-
blast freezing for improved quality and reduced cost. The food
product is first crust-frozen in a bath of nitrogen to seal moisture and
flavor in, and then transferred into the air-blast freezing section,
where the freezing process is completed at a lower cost.
4.Cryogenic freezing systems

35. ▪Cryogenic freezing is mainly used for small products such
as burgers or ready meals.
▪The most common method is by direct spraying of liquid
nitrogen onto a food product while it is conveyed through
an insulated tunnel

37. https://www.youtube.com/watch?v=AHV6SuRhtY8

38. •The main advantages of cryogenic liquids are;
oHigh cooling or freezing speed (high capacity, better quality).
oLow product weight loss during cooling or freezing (0.1 –
1.0%)
• The main advantages of cryogenic freezing are:
oCompact, inexpensive equipment.
oLow initial capital investment (1/3 of that of mechanical
systems)
oExtremely rapid freezing, very low moisture loss
o Versatile, quick to install anywhere (Simple construction).

39. The disadvantages are:
oHigh cost of the cryogens
oDependence on a reliable supply of cryogens
oNot suitable for large items. The very large temperature
gradients may create large mechanical stress, cracks and
breakage.
oMore expensive than mechanical refrigeration.
Nevertheless, cryogenic freezing is valuable in the following
situations:
•The freezing line is used occasionally
•Economy of floor-space is of essence (need to accommodate
the line in existing, crowded plant layout, location with
expensive land cost)
•Experimentation, product development at pilot scale.

40. FREEZING EQUIPMMENTS
•Depends on type, size and shape of raw materials, finished
product quality, space availability, investment; and cooling
medium
•Freezing equipment can be grouped as follows
1.Direct contact with cold surface
-Packed/unpacked food will be in direct contact with a
metal surface .
-Plate freezers and scrapped surface freezers
(liquid/semisolid foods)

41. 2.Air as cooling medium
-Air at very low
temperature is used for
freezing the food
products.
•Still air freezer
•Air blast tunnel
•Spray freezer,
fluidized bed freezer,
and
•impingement
freezers: efficiently
freeze thin, flat food
products.

42. 3.Liquids as coolants
-Very low temperature liquids are used for freezing the
product
-The liquid may be sprayed on the product or the product
immersed in the liquid.
-E.g., Immersion type and cryogenic freezers

44. 3. FREEZE DRYING AND FREEZE
CONCENTRATION
Freeze drying and freeze concentration
https://www.sciencedirect.com/topics/agricultural-and-biological-
sciences/freeze-dryers

45. •In freeze drying and freeze concentration, preservation effect is
achieved by reduction in water activity without heating the
food
•Nutritional and sensory qualities are better retained
•Freeze drying and freeze concentrations processes are slower
than the conventional dehydration, evaporation or membrane
concentration
•Refrigeration energy costs are higher, relatively high capital
investment, results in high production costs for freeze-dried
and freeze-concentrated foods
Freeze drying and freeze concentration

46. FREEZE DRYING
•Freeze drying is the more important operation commercially and
is used to dry expensive foods which have delicate aromas or
textures (coffee, mushrooms, herbs, spices, fruit juices, meat,
seafoods, vegetables and complete meals for military rations or
expeditions).
•Freeze dried foods are of superior high quality hence price.
•Microbial culture used in food processing are freeze dried for
long term storage prior to inoculum generation.
•Freeze drying reduces >90% moisture from food

47. The important parts of freeze-drying apparatuses are –
i.Refrigeration System
ii.Vacuum System
iii.Control System
iv.Product Chamber or Manifold
v.Condenser
EQUIPMENT OF FREEZE DRYING

48. • Removal of water in the frozen state from frozen material
by sublimation.
• Foods in frozen state are sublimed under very high
vacuum to low moisture levels/make food stable at room
temperature

49. 1.Intermittent Freezer dryers
2.Continuous free dryers
https://www.sciencedirect.com/topics/agric
ultural-and-biological-sciences/freeze-
dryers
FREEZE DRYING EQUIPMENTS

50. Modern freeze concentration processes consist of freezing followed
by mechanical separation (centrifugation), i.e..,
•A crystallization section where part of the water is converted
into solid ice crystals using a refrigeration system.
•The ice crystals are then separated by filters, centrifuges.
•The degree of concentration achieve is higher than in
membrane process.
•Freeze concentration is not widely used in food processing
but has some applications for liquid foods such as fruit juices,
coffee extracts, beer, wine, milk, vinegar etc.
FREEZE CONCENTRATION

51. •Four major methods for production of concentrates are;
i.Freeze-concentration
ii.Vacuum concentration
iii.Dehydro-Freezing DF- food dehydration & freezing
iv.Freeze drying
•.

52. Typical application are;
a.Dairy processing (milk and milk products) involves
cooling and preservation
b.Breweries, wineries, distillers (alcohols)
c.Ice making (ice blocks, flake, cube) used in cooling
d.Fisheries, poultry and meat processing (chilling, freezing
and storage).
e.Fruits and vegetables processing units (cooling, freezing)
f.Freeze-drying system for certain types of foods
g.Precooling plants for fresh and fruits vegetables
h.Cold stores for various types of foods (medium/low
temperature ranges.
i.Refrigerated transport
j.Appliances for storage and distribution of foods

53. 1.Accelerated freeze driers
•Food is held btn two layers of expanded metal mesh and subjected to
slightly pressure on both sides.
•Heating is by conduction.

2.Radiation freeze driers
•Infrared radiation from radiant heaters is used to heat shallow
layers of food on flat trays.
•Heating is more uniform than in conduction types, since surface
irregularities on the food have a smaller effect on the rate of heat
transfer.
3.Microwave and dielectric freeze driers.
•Radio frequency heaters are used
EQUIPMENTS - FD
https://www.youtube.com/watch?v=bbvxYGB7XMg

54. The basic components of a freeze concentration unit;
•A direct freezing system (e.g., solid CO2) or indirect equipment
e.g., a scraped surface heat exchanger to freeze the food.
•A mixing vessel to allow ice crystal to grow.
•A separator to remove the crystals fro the concentrated solution .
•Separation can be achieved by centrifugation, filtration, filter press
EQUIPMENTS - FC

55. 1.James G. Brennan, Alistair S. Grandison () Food Processing
Handbook
2. Nirmal K. Sinha, Y. H. Hui, E. l Evranuz, E. Özgül Evranuz,
Muhammad Siddiq, Jasim Ahmed () Handbook of Vegetables
and Vegetable Processing
3. Zacharias B. Maroulis, George D. Saravacos () Food Process
Design
4. R. S. Agrarwal () Emerging Technologies in Air conditioning
and Refrigeration.
5. https://www.slideshare.net/roycechua/food-processing-
technology
6. Fundamentals of Food Freezing -
https://www.youtube.com/watch?v=O3kb7sb0aRg
REFERENCES

56. Seminar
Topic:
Food Packaging: Functions of food packaging, types of
packaging, packaging systems, Modified atmosphere
packaging (MAP), and safety aspects of packaging.

57. Freezing
▪Based on the rapid crystallization of free water into ice
▪Storage of food for several months at temp near -18 C
▪Frozen food usually packaged to prevent moisture loss and quality
changes
Freezing Equipment (methods)
1.Contact freezers : Either direct contact with freezing medium
(immersed) or indirectly by contact with a belt or plate which is
in the freezing medium

58. a)Direct immersion method
•Immersion of product in the low temperature
•Offers direct contact and faster freezing
•Contamination problem
b)Indirect contact freezers;
•Plate freezing, common contact freezer (horizontal &
vertical)
The product is placed btn two metal plates which are
refrigerated by a refrigerant passing through a hollow passage
created in the plates

59. 2.Air blast freezing;
•Widely used to freeze large variety of products
•Freezing time 4-10 hrs.
•Blast freezers are designed as batch freezers or continuous
freezers in the forms of tunnels
•Take advantage of the heat transfer, efficiency of rapidly
circulation of air over the product
•Is for packed and unpacked
E.g.,
i.Batch cabinet chiller/freezer
ii.Continuous tunnel coolers/freezers (trolley/conveyors belts

61. •Types of air blast freezers
i.Spiral freezer; materials moves on the perforated
belts in spiral path subjected to cold air
ii.Fluidized bed freezer
iii.Fluidized belt freezer
iv.Straight bed freezers
3.Cryogenic freezing

62. References
1.Vapour compression refrigeration system/Refrigeration System.
Accessed from http://www.mechanicalengineeringblog.com/4818-
vapour-compression-refrigeration-system-refrigeration-system/
2.Fundamentals of Food Freezing -
https://www.youtube.com/watch?v=O3kb7sb0aRg