FOOD PROCESSING AND PRESERVATION BY LOW TEMPERATURE

1. Food Processing
and
Preservation
Technology

5. Principle of Food Preservation
The basic principle of food preservation includes:
• Preservation or delay of microbial decomposition: That is archived by :
1. Keeping out microorganisms (asepsis)
2. Removal of microorganisms,i.e., by filtration
• Hindering the growth of activity of microorganisms,i.e., by low
temperature, drying, anaerobic conditions, &
• Killing the microorganisms,i.e., by heat or radiations.
• Preservation or delay by self – decomposition of food. This is brought
about by :
1. Destruction or inactivation of food enzymes i.e, by blanching
2. Preservation or delay of chemical reactions i.e, preservation of oxidation by means of an
antioxidant.
3. Preservation of damage caused by insects, animals & mechanical causes.

6. Bread
Meat, fish, poultry,
vegetables

22. Methods for
preservation

23. Preservation of foods by low
temperature
Lowering the storage temperature of the food
will reduce or prevent spoilage by
microorganisms and/or chemical reactions.

24. • Refrigeration slows down the biological, chemical, and
physical reactions that shorten the shelf life of food.
• Temperatures typically between 45 - 32°F (7.2 - 0°C).
Preferably below 38°F.
• All perishable foods should be refrigerated as soon as
possible, preferably during transport, to prevent bacteria
from multiplying.

26. Vapor compression cycle

27. Refrigeration cycle

28. TRADITIONAL REFRIGERATED FOODS
1. Fresh foods (unprocessed), fruits and
vegetables, (fresh meats, poultry, fish)
2. Processed foods: doughs, minimally
processed vegetables
3. Refrigerated foods containing fruits and
vegetables: entrees, dinners, salads.
(Pasteurized dairy products cured meats)

29. Effect of refrigeration on food
• Refrigeration slows bacterial growth but not
inhibits.
• Loss in turgor pressure
• Intermixing of flavor of milk or sulphur
containing foods
• Loss of moisture
• Wilting/yellowing of leaves
• Changes in textural and taste parameters

30. II. FREEZING
• Freezing is the unit operation in which the temperature of a
food is reduced below its freezing point and a proportion
of the water undergoes a change in state to form ice
crystals.
• Principle of Freezing: When a substance changes its state
from liquid to solid, the evolution of heat takes place from
the body to its surroundings.
• The immobilization of water to ice and the resulting
concentration of dissolved solutes in unfrozen water lower
the water activity (aw) of the food
• < 32oF (0°C)

31. Freezing curve
• AS: The food is cooled below its freezing point
of 0º C. At point S, the water remains liquid.
• SB: The temperature rises rapidly to the
freezing point, because ice crystals will form
and at a higher speed release latent heat, which
is extracted by freezing the food.
• BC: The heat is removed, eliminating the latent
heat, formation of ice, temperature remaining
constant. Increasing concentration of solutes in
the unfrozen water fraction causes the decrease
of freezing point, so the temperature decreases.
This is the phase in which most of the ice
forms.
• CD: The crystallization of water and solutes
continues. The temperature falls for the
mixture of water and ice.

32. Types of Freezing
• Rapid freezing prevents undesirable
large ice crystals from forming
throughout the product because the
molecules don't have time to form into
the characteristic six-sided snowflake.
• Slow freezing creates large, disruptive
ice crystals. During thawing, they
damage the cells and dissolve
emulsions.

33. Methods of freezing
- The use of cold air blasts or other low temperature
gases coming in contact with the food, e.g. blasts,
tunnel, fluidized bed, spiral, belt freezers.
- Indirect contact freezing, e.g. plate freezers, where
packaged foods or liquids are brought into contact
with metal surfaces (plate, cylinders) cooled by
circulating refrigerant (multi-plate freezers).
- Direct immersion of the food into a liquid refrigerant,
or spraying liquid refrigerant over the food (e.g. liquid
nitrogen, and freon, sugar or salt solutions).

34. 1. AIR FREEZING - Products frozen by
either "still" or "blast" forced air.
• cheapest (investment)
• "still" slowest, more changes in product
• "blast" faster, more commonly used
2. INDIRECT CONTACT - Food placed in
direct contact with cooled metal surface.
• relatively faster
• more expensive

35. 3. DIRECT CONTACT - Food placed in
direct contact with refrigerant (liquid
nitrogen, "green" freon, carbon dioxide
snow)
• faster
• expensive
• freeze individual food particles

36. Freezing equipment
• Mechanical Freezers
- Evaporate and compress the refrigerant in
a continuous cycle
• Cryogenic Systems
- Use solid and liquid CO2, N2 directly in
contact with the food

38. Batch Freezer
Source: Unit operations for food the food industries by: W.A. Gould
Blast
Type

39. Double Contact Plate Freezer
Hydraulic
Pump
Top
Pressure
plate
Connecting
Linkage
Corner
Headers
Refrigerant
hoses
Trays
Contact
plates
Polyurethane and polystyrene
insulated doors

41. Immersion freezing
• In immersion freezing, food is placed in a
refrigerant prior to freezing.
• Brine is often used for fish, and a glycol,
sugar solution for fruits.
• This provides a layer which protects the food
from the dry atmosphere of the freezer.

43. Cryogenic freezers
• Freezers of this type are characterized by a change of state
in the refrigerant (or cryogen) as heat is absorbed from the
freezing food.
• The heat from the food therefore provides the latent heat of
vaporization or sublimation of the cryogen.
• 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
(-196℃) and solid or liquid carbon dioxide (-72 ℃).

44. Cryogenic Freezing
• Uses liquid nitrogen which is very cold (-196ºC)
Food passes through a tunnel where nitrogen gas is
sprayed downwards. A beefburger will be frozen in
1 minute at these extreme temperatures.
• This produces small crystals, and little moisture loss.
• This method is used when freezing prawns. The
prawns are first dipped in liquid nitrogen to freeze the
outside layer. This prevents the prawns sticking
together and from sticking to the freezer belts.

45. Cryogenic freezer
Source: Fellows (2000)

46. Ultra rapid:
Direct Contact Liquid Nitrogen Tunnel
Freezer
Source: Unit operations for food the food industries by:
W.A. Gould
IQF
Cryogenic
freezer

48. Fluidized bed freezing (IQF: Individual quick freezing)
• Used to freeze particulate foods such as
peas, cut corn, diced carrots, and
strawberries.
• The foods are placed on a mesh
conveyor belt and moved through a
freezing zone in which cold air is
directed upward through the mesh belt
and the food particulates begin to
tumble and float.
• This tumbling exposes all sides of the
food to the cold air and minimizes the
resistance to heat transfer at the surface
of the food.

50. Effect of Freezing on Food
• Low temperatures do not significantly affect the
nutritional value of food, but thiamin and vitamin
C may be destroyed when vegetables are blanched
(briefly immersed in boiling water) before
freezing.
• If fish is frozen too slowly, some of its cells may
rupture and release nutrients into the liquid that
drips from the fish when it thaws.
• Some flavours become weaker and some become
stronger when food is frozen.

51. Advantages of Freezing
• Many foods can be frozen.
• Natural color, flavor, and nutritive value
retained.
• Texture usually better than other methods
of food preservation.
• Foods can be frozen in less time than they
can be dried or canned.

52. How Freezing Affects Food
Chemical changes
– Enzymes in vegetables
– Enzymes in fruit
– Rancidity
Texture Changes
– Expansion of food
– Ice crystals

53. ISSUES with FROZEN FOODS (cont.)
2. Undesirable physical changes
A. Fruits and vegetables lose crispness
B. Drip loss in meats and colloidal type foods
(starch, emulsions)
• Freeze product faster
• Control temperature fluctuations in storage.
• Modify starch, egg systems, etc.

54. 2. Undesirable physical changes (cont.)
C. Freezer burn
• Package properly
• Control temperature fluctuations in storage.
D. Oxidation
• Off-flavors
• Vitamin loss
• Browning
E. Recrystallization

55. Dehydrofreezing
• Dehydrofreezing is a method of food preservation that combines the
techniques of drying and freezing. The process of freezing partially
dehydrated foods is known as dehydrofreezing.
• Fruits or vegetables dried at home have had 85 to 90% of their
moisture removed to prevent mold growth. However, by removing
only 70 percent of the moisture and storing the fruit or vegetable in
the freezer, a tastier product results. The freezer's low temperature
inhibits microbial growth.
• Fruits and vegetables processed this way have good flavor and color
and reconstitute in about half the time it takes for dried foods.

56. Thawing
Thawing is the process of taking a frozen product from
frozen to a temperature (usually above 0°C) where there is
no residual ice, i.e. “defrosting”. Thawing is often considered
as simply the reversal of the freezing process.

57. Methods of thawing

59. • Damaging to the texture of the frozen fruit
• Increase in drip loss and a decrease in firmness
• Loss of cell wall materials in middle lamella.
• Loss of flavor
• Loss of nutrients
• Loss of turgor pressure
Effect of thawing-freezing cycle