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FST 151
FOOD FREEZING
FOOD SCIENCE AND TECHNOLOGY 151
Food Freezing – Storage of Frozen Foods
Lecture Notes
Prof. Vinod K. Jindal
(Formerly Professor, Asian Institute of Technology)
Visiting Professor
Chemical Engineering Department
Mahidol University
Salaya, Nakornpathom
Thailand
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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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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 re-crystallization
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.
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Factors Affecting The Quality of Frozen
Foods During Storage
Freezer burn
Freezer burn is caused by the sublimation of ice on the
surface region of the product when the water pressure of ice
is higher than the vapor pressure in the environment.
Freezer burn produces changes in the appearance and
texture on the product’s surface and may be the reason for
off-odors and -flavors.
Moisture migration causes weight losses during freezing and
frozen storage, unless the product is packed using a
material with low water vapor permeability.
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Freezing Burn and Dehydration
With the drop in the temperature air looses its capacity to
hold moisture. The separated water vapors get converted
into frost. The air when and where it has access to product,
starts withdrawing water from the product and the spot or
area from where water is withdrawn gets dehydrated and
appears as white spot called freezer burn. Unpacked foods
are more susceptible to freezer burns. However some
moisture is lost from the product surface, which attributes to
some loss in the weight of product called freezing loss.
Longer freezing time will result in larger freezing losses –
IQF type of freezing results in 2 to 4 %, however, slower
methods may contribute to even 10% or more weight loss.
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Change in the Color of Food
Freezing may also affect natural color of a product due to
difference in light diffraction of ice than water, however, this
change in color is reversible when product is thawed.
Improper freezing results in active enzymes and oxidative
processes. It also denatures pigments and associated
proteins. Proteins are part of pigment complexes and they
get denatured by cooking, or by acids (change of pH either
due to incorporation of acidic elements or by bio-chemical
processes or by bacterial metabolism) or by chemical
reactions. These changes are permanent and irreversible.
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Change in the Color of Food (2)
Mayoglobin, the red pigment in meats gets oxidized either
during freezing or prolonged frozen storage if not
packaged properly, or if packaging is of poor quality and
pervious to atmospheric oxygen or if packaging is not
sealed properly or it gets damaged during handling.
Vegetables have very high level of enzymes and some of
them like katalases and peroxydases are hard in nature
and are active in slow speed in the frozen conditions too.
It is therefore essential that vegetables are flash cooked or
blanched before freezing in order to inactivate enzymes
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Change in the Color of Food (3)
Discoloration in some fruits and vegetables is more
when they are chopped into portions or slices, as they
have high level of enzymes called polyphenolase, which
get converted into melanin when it comes in contact of
oxygen i.e. blackening of potatoes, brinjals and apples.
Proper packing and freezing will arrest this phenomenon.
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Change in the Color of Food (4)
Sea foods specially crustaceans also have high levels of
this enzyme (polyphenol oxidase or PPO) and if they are
not frozen and stored at right temperatures or subjected to
temperature abuse, they undergo this process of
MELANOSIS and render the product with black spots.
Although melanin formation does not affect the edible
qualities of food, it certainly impairs its visual quality.
Some of the fish specially fatty fish develop a red/yellow
discoloration or rust during prolonged storage and
temperature abuse.
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Water absorption and redistribution
Temperature fluctuations lead to a net migration of moisture from
interior towards the surface of the foodstuff or to the package.
As temperature outside the packaging decreases, moisture on
the warmer surface of food sublimes, diffuses and concentrates
as ice crystals on colder surface of the packaging film.
When ambient temperature increases, the ice on the warmer
wrap tends to diffuse back to the colder surface of the food.
However, re-absorption of water from the surface back to its
original location in the food is impossible. This leads to a growing
amount of ice crystals on the surfaces.
The formation of ice crystals inside the package weakens the
appearance of the product and indicates to the customers that
the product has been stored improperly or for a long time.
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Re-crystallization
Physical changes to ice crystals known as re-crystallization
are an important cause of quality loss in some foods.
Small ice crystals, which have been formed in a fast freezing
process, may thaw if the temperature around the product is
increased. When temperature is decreased again, water
molecules join the remaining ice crystals, which get larger
instead of forming new ice crystals. In the end, the amount
of small ice crystals is decreased and the amount of larger
ice crystals is increased and the texture of the product
becomes rough, crystalline and icy.
Especially ice cream is sensitive to fluctuations in
temperature.
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Drip loss
During freezing, frozen water is removed from the original
location in the foodstuff to form ice crystals. During thawing,
water may not be reabsorbed in the original region, leading
to the formation of drip.
Drip loss leads to the loss of nutrients, affects texture and
juiciness and modifies the appearance of the product.
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Re-crystallization of Ice Inside the Product
Temperature fluctuations in the cold store and in the
display freezer cabinets trigger one more process inside
the product itself – re-crystallization of ice crystals. The
small ice crystals that are formed during quick freezing
change their shape and size once temperature of the
product goes up. The core of the product tries to stabilize
with out side air in the store and high temperature in the
store subjects product to similar conditions as witnessed
in the slow freezing - forming bigger ice crystals or recrystallization of smaller ice particles into bigger ones, it
damages tissue cells due to expansion, which ultimately
results in loss of food value.
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Biological and Chemical Changes
The biological changes include reduction of micro flora on
the surface and interior of food. Freezing has inhibiting
effect on the metabolism and reproduction of microbes. As
water gets converted into ice and it is not freely available
to microbes for their metabolic and physiological activities,
they starve and some of them either perish or go into
dormant stage. However as and when they get right
environment like temperature abuse anywhere during the
cold chain, they get active and multiply. Due to this effect
reduction in the bacterial load during freezing is witnessed
however freezing is not a sterilization process.
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Protein denaturation
Freeze-induced protein denaturation and related
functionality losses are commonly observed in frozen fish,
meat, poultry, egg products and dough.
Effects of the protein denaturation may be seen in waterholding capacity, viscosity, gelation, emulsification and
whipping properties of the product.
For example in fish, protein denaturation may be seen as
changes in the texture (toughness). Protein denaturation
may be reduced by keeping the storage temperature as
low as possible.
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Lipid oxidation
Lipid oxidation is identified especially in products with
polyunsaturated fatty acids, particularly phospholipids, like in
fatty fish. Lipid oxidation causes off-odors and -flavors
(rancidity), changes in the appearance (loss of pigment colors)
and loss of nutritional value in the products.
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Lipid oxidation (2)
Lipid oxidation is best prevented by eliminating oxygen from the
product by antioxidants or with vacuum or modified atmosphere
packaging and by using packages with adequate oxygen barrier.
The best way is to use packaging material with good oxygen
barrier tight around the product so that there are no air pockets
inside the package.
Light, especially shortwave UV-light, increases oxidation,
especially in fatty products. It also destroys vitamins and causes
changes in the color. Therefore, packaging material should also
function as a light barrier.
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Conclusion
• Understanding factors related to freezing
should minimize loss in food quality
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5
Frozen Food Components and Chemical
Reactions
Miang H.Lim, Janet E.McFetri dge, and Jens Liesebach
University of Otago, Dunedin, New Zealand
The effect of freezing on the food components is
diverse, and some components are affected more than
others. For example, protein can be irreversibly
denatured by freezing, whereas carbohydrates are
generally more stable. This chapter focuses on
chemical and biochemical reactions that affect the
quality of frozen food systems.
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II. CHEMICAL AND BIOCHEMICAL REACTIONS
IN FROZEN FOOD
A. Protein
Protein may undergo changes during freezing and frozen
storage, primarily because of denaturation. Denaturation can
be defined as a loss of functionality caused by changes in the
protein structure due to the disruption of chemical bonds and
by secondary interactions with other constituents
B. Lipids
Lipids can degrade in frozen systems by means of two wellknown chemical processes: hydrolysis and oxidation. These
processes lead to undesirable changes in the nutritional and
sensory quality of foods, such as the production of rancid
flavors and discoloration.
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C. Carbohydrates
Carbohydrates are susceptible to hydrolysis during frozen
storage, as observed in frozen papaya (44). Sugar
hydrolysis increases the number of moles of solutes in the
food matrix, thus depressing the freezing temperature.
D. Color Pigments
The stability of color pigments during frozen storage is
affected by treatment prior to processing and by processing
and storage conditions (light, oxygen, heavy metals,
temperature, water activity, pH, oxidizing, and reducing
agents).
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E. Flavor Compounds
Food flavors are composed of volatile aroma compounds
and taste components, such as organic acids and sugars.
The effect of freezing and frozen storage on flavor
compounds in food is variable; flavor changes are
affected mainly by enzymatic activities and lipid oxidation.
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F. Vitamins
Freezing is considered as one of the best processing
methods for preserving nutrients in food. In a comparison
of different storage methods, the nutrient content of frozen
beans, sweet corn, and peas was similar to that of fresh
vegetables that had been cooked by boiling. Frozen
vegetables were higher in vitamin C, riboflavin, and thiamin
than canned vegetables.
G. Minerals
Minerals in food matrices may be present in many different
forms such as chemical compounds, molecular complexes,
and even free ions. Minerals present in any form can
dramatically affect the color, texture, flavor, and stability of
foods.
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4 Microbiology of Frozen Foods
C.O. Gill
Agriculture and Agri-Food Canada, Lacombe Research
Centre, Lacombe,
Alberta, Canada
The effects of freezing on microorganisms in foods vary
greatly with the type of microorganism, the physiological state
or stage in the life cycle of the microorganism, the
composition of the food, and the rates of freezing and thawing.
In general, viruses, bacterial spores, and sexual spores of
fungi are likely to be preserved by freezing, irrespective of the
composition of the food and the rates of freezing and thawing.
Other microorganisms are likely to be damaged by freezing,
but the extent to which freezing and subsequent frozen
storage reduces the numbers of any organism may be trivial.
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8
Texture in Frozen Foods
William L.Kerr
University of Georgia, Athens, Georgia, U.S.A.
One critical quality factor influenced by freezing is
food texture. Texture can be defined as those
properties of food determined by the rheological and
structural nature of the food and determined by the
tactile senses.
This chapter discusses the issues regarding the
effects of freezing on the texture of major food groups
e.g. vegetables, fruits, meat, fish and seafood, poultry,
eggs, dairy products and baked goods.
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Due to the chemical and structural differences in
different food groups, each has unique issues
associated with changes in textural quality. With the
exception of bread dough, most food groups suffer
fewest changes in textural quality when frozen at a
rapid rate.
In addition, storage at low temperatures is
preferential, and particular care should be taken to
limit temperature fluctuations during frozen storage.
Methods of thawing can also affect the texture of
foods.
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chapter fourteen
Freezing preservation of fresh
foods: quality aspects
David Reid
General discussion on technological basis of freezing.
Storage stability of frozen vegetables
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From Tables 2–6 for storage stabilities, it
can be seen that different tissue types
exhibit both different absolute stabilities
toward frozen storage and different
temperature sensitivities.
It is also clear that for many products, –
18°C is too high a temperature for
effective long-term frozen storage. There
is a trend within the frozen food industry
to employ lower temperatures for the
storage of many products. Most
refrigerated warehouses operate at
temperatures below –18°C, and indeed
–30°C is not uncommon.
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The effect of storage temperature is most marked
for fish; though well-controlled low-temperature
storage could contribute to a greatly improved
product.
There is growing evidence that the effects of
temperature mishandling of products may be more
solid than previously believed.
A simple trial with frozen green beans that even a
one-time fluctuation in temperature from –18°C to
–15°C for a short time resulted in a detectable color
change in the thawed product after only 3 more
months of storage.
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