Freezing of raw and processed foods

1. FREEZING OF RAW AND
PROCESSED FOODS
OBT554
Unit 2
Dr K.Geetha
Associate Professor, Dept of
Biotechnology, Kamaraj College of
Engg & Tech, Madurai

2. INTRODUCTION
 Freezing provides: extended shelf life and long-term
preservation.
 Freezing: most widely used methods of food
preservation
 Changes the physical state of a substance by
changing water into ice when energy is removed in
the form of cooling below freezing temperature.
 The terminologies of the freezing process:
precooling, supercooling, freezing, tempering,
eutectic, ice nucleation, and glass transition

3. PRINCIPLE
 The freezing of foods slows down, but does not stop, the
physicochemical and biochemical reactions that govern
the deterioration of foods.
 Deteriorative properties: Organoleptic quality,
physicochemical properties such as ionic strength, pH,
viscosity, water activity, surface and interfacial tension, and
oxidation–reduction potential.
 Microbial growth - completely stopped below –18°C.
 The freezing process reduces the random motion and
rearrangement of molecules in the matrix.
 Oxygen is almost totally expelled from ice crystals as they
are formed.

4. FREEZING RATE AND QUALITY
 Fast freezing produces better quality frozen food than
slow freezing.
 Rate of freezing determines the size of the ice crystals,
cell dehydration, and damage to the cell walls.
 In foods containing microbial cultures, it is important to
maintain their activity.
 Rapid freezing causes detrimental effects on yeast
activity of frozen dough due to the formation of
intracellular ice crystals.
 Yeast activity decreased significantly when the rate of
cooling was increased from 0.98 to 1.57°C/min

5. MICROBIAL ASPECTS
 Effects of freezing on microorganisms: desirable or undesirable
- depending on the types of food products.
 Food devoid of microbes: no contamination should happen
 Food containing microbes (like curd/dough): care must be taken to
reduce the damage in cells during freezing of foods.
 The maximum recommended storage temperature at which
microbiological spoilage ceases: -9°C and -12°C.
 Freezing causes the death of 10%–60% of the microbe population.
 Genera commonly encountered in frozen food include
Pseudomonas, Achromobacter, Flavobacter, Micrococcus,
Lactobacillus.

6. PHYSICAL CHANGES
 Different types of water are present in frozen foods.
 Broadly categorized as free and unfreezable water.
 Major cause of product degradation: amount of unfrozen
water present in the frozen matrix.
 Weight Loss: Foods lose moisture during the freezing
 Recrystallization: the rate of recrystallization is highest
between the melting and glass transition* temperatures. Large
fluctuation in temparature during storage may lead to
recrystallization of ice.
 Others include: Retrogradation, Protein Denaturation,
Freezer Burn, Glass Formation, textural, mechanical,
consistency, appearance and sensory attributes and
water-holding capacity.

7.  *Glass–liquid transition, or glass transition:
the gradual and reversible transition in
amorphous materials (or in amorphous regions
within semicrystalline materials), from a hard
and relatively brittle "glassy" state into a viscous
or rubbery state as the temperature is increased.

8. CHEMICAL CHANGES
 Rancidity –
 Rancidity generally is the complete or incomplete oxidation or hydrolysis of
fats and oils when exposed to air, light, or moisture or by bacterial action,
resulting in unpleasant taste and odor.
 Example: The degradation of lipids in frozen peas during storage at 18°C led
to flavor damage due to the formation of hydroperoxides, thiobarbituric acid,
and fatty acids, particularly in unblanched samples
 Color, Flavor, and Vitamin Loss –
 Example: freezing, and thawing lead to cell disintegration, pigment
degradation, and isomerization of carotenoids in Pineapple.
 Freezing of strawberries is usually associated with a reduction in aroma and
the development of off-flavor.
 The destruction of vitamin C (ascorbic acid) occurs during freezing and frozen
storage.
 Release of Enzymes –
 Example: Beef and pig skeletal muscle contain glutamic-oxalacetic
transaminase.
 Freezing and thawing cause a remarkable increase of glutamic-oxalacetic
transaminase activity in the muscle press juice.
 Hydrolysis-
 In melons, total cell wall polysaccharides decreases more during the first 5
months than during the second 5 months of frozen storage due to hydrolysis
 Acetaldehyde Formation - The formation of acetaldehyde in frozen
vegetables increases during storage and is thus an indication of shelf
life

9. PROCESSING AND PACKAGING FACTORS
 Pretreatments for Freezing:
 Blanching,
 Heat Treatments,
 Dipping Pretreatments,
 Antifreeze Proteins,
 Osmotic Concentration (Partial removal of water by
osmotic treatment),
 Cryoprotection,
 Irradiation

10. Blanching

11. Heat Treatments

12. Dipping Pretreatments

13.  Storage and Display:
A package for frozen product should
(a) be attractive and appeal to the consumer,
(b) protect the product from external contamination during
transport and handling, and from permeable gases and
moisture vapor transfer,
(c) allow rapid, efficient freezing and ease of handling, and
(4) be cost effective.

16.  Thawing:
Microbiologically safe thawing process includes:
 (a) inside a refrigerator at temperatures below 5°C,
 (b) microwave oven, or
 (c) as part of the cooking treatment

17. FREEZING METHODS
1. Freezing by Contact with a Cooled Solid: Plate Freezing
2. Freezing by Contact with a Cooled Liquid: Immersion
Freezing
3. Freezing by Contact with a Cooled Gas:
a) Cabinet Freezing - cold air is circulated in a cabinet
where product is placed in a tray
b) Air-Blast Freezing – the temperature of food is reduced with cold
air flowing at a relatively high speed.
Fluidized Bed Freezing, Belt Freezing, Spiral Freezing,
Tunnel Freezing
4. Cryogenic Freezing- liquefied gases are placed in direct
contact with the foods. Food is exposed to an atmosphere below
-60°C through direct contact with liquid nitrogen or liquid
carbon dioxide or their vapor

18. Plate Freezing

20. PLATE FREEZING
 Plate freezers are used to freeze flat products, such as pastries, fish fillets,
and beef patties, as well as irregular-shaped vegetables that are packaged in
brick-shaped containers, such as asparagus, cauliflower, spinach, and broccoli.
 Plate freezers consist of a series of parallel flat plates through which a coolant
is circulated.
 The plates can be mounted either horizontally or vertically.
 A hydraulic system is used to both open the space between plates for loading
and unloading,
 Vertical plate freezers are best suited to freezing unpackaged deformable
products such as fish and meat.
 Blocks are formed by direct gravity feeding of the product between the plates.
 Plate heating and block ejection systems are required to remove the block at
the end of the freezing process and cleaning may be required before reloading.
 Horizontal plate freezers are commonly used for either product packed into
rectangular cartons or product formed into rectangular shapes by metal molds
or trays.
 Major advantages: Rate of freezing is high even for packaged products; the
product has very consistent size and shape and can be easily bulk stacked
with high packing density and stability for subsequent transportation; they
are very compact; infrequent defrosting of the plates is required.
 Major disadvantages: high capital cost, especially if they are automated and
the limitation on product types that can be handled, refrigerants.

21. Immersion Freezing

22. IMMERSION FREEZING
 In immersion freezers the product is immersed
directly in, or sprayed with, a cold liquid such as a
brine or glycol.
 The product is usually packaged to prevent cross-
contamination between the liquid and the product.
 Products with irregular shapes are easily handled.
 Although high rates of freezing can be achieved, these
types of freezers are now seldom used except for some
fish, meat, and poultry products.
 The liquid is refrigerated either by circulation through
a heat exchanger or by cooling coils and/or a jacket
built into the liquid tank.

23. Cabinet Freezing

24. CABINET FREEZING
 Cabinets are intended for freezing and cooling of food
products.
 Cabinet freezers are used for freezing (cooling) of the
following products – ready-to-eat meals, confectionary,
seafood and meat products as well as for freezing of large-
sized products and products with extended storage time.
 Cabinet freezers have up to 10 various freezing modes
depending on the product in use.
 The door frame is equipped with a heating system so the
door can be easily opened both in the process of freezing
and at the end of this process, so the operation losses will be
kept to a minimum.
 Chamber floor is equipped with a special detachable ramp
for the comfortable loading/unloading of the cabinet.

25. Fluidized Bed Freezing

27. FLUIDIZED BED 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.

28. Belt Freezing
Continuous flexible mesh is
used in a linear line.
Packaged food is brought
up by a chilled room on the
belt.

29. Spiral Freezing
Continuous flexible mesh is
used in a spiral-shaped
rows. Packaged food is
brought up by a chilled
room on the belt.

31. Tunnel Freezing
The air is directed from a pressure
chamber through small nozzle
outlets on the top and bottom
of the product.
As a result, the product
is completely encompassed by
surrounding air.
This very efficient heat transfer
likewise supports rapid freezing.

32. Cryogenic Freezing

33. CRYOGENIC FREEZING
 Cryogenic freezing is used to freeze food at an extremely
fast rate.
 The food is moved through a spray of liquid nitrogen or
directly immersed in liquid nitrogen.
 The liquid nitrogen boils around the food at a temperature
of −196 °C (−321 °F) and extracts a large amount of heat.
 Other important properties of such cryogens are that they
are colorless, odorless, chemically inert, and nontoxic in
normal concentrations.
 Therefore they are safe for direct contact with food.
 The product is either sprayed with, or immersed in, the
cryogen at atmospheric pressure.
 Special care must be exercised with CO2 because it forms a
low density snow.
 Cryogenic freezers can operate continuously with the
product being conveyed through a tunnel

34.  Main advantages: high rates of freezing achieved by
the very cold temperatures and low refrigerant-to-
product surface heat transfer resistance (resulting in
lower weight loss and higher quality); ease of
operation; compact size; low cost of the equipment;
rapid installation and start-up; mechanical simplicity;
and low maintenance cost.
 Main disadvantage: High cost of the cryogens.
 Cryogenic freezing (or alternatively liquid immersion
freezing) can be used for rapid crust freezing with
completion of freezing in an air-blast freezer

35. EMERGING FREEZING TECHNIQUES
 Several kinds of high-pressure ices with different
chemical structures and physical properties are
reported.
 A pressure-shift or high-pressure freezing process
can generate small and uniform ice crystals.
 Improved structures by pressure-shift freezing are
reported for tofu (soybean curd) and carrot.

36. DEVELOPMENTS IN FREEZING TECHNOLOGY
 Pretreatment:
 Dehydrofreezing - partial dehydration and formulation
stages prior to freezing.
 Advantage: Energy saving; Better quality & stability
 Freezer technology:
 Impingement heat transfer technology – forcing air
at high velocity to impinge on the food perpendicular to
the food surface
 Advantage: to increase heat transfer during freezing
 Immersion chilling and freezing (ICF) - similar to
osmotic dehydration.
 Advantage: rapid heat transfer and lower operating and
investment costs.

37. REFERENCE
 Handbook of Food Preservation - Second Edition - M.
Shafiur Rahman
 Food preservation techniques - Peter Zeuthen and Leif
Bùgh-Sùrensen
THANK YOU