FST 151

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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
  re-crystallization 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 products 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water is withdrawn gets dehydrated and appears as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
ProductTemperature 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
re-crystallization 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 ChangesThe 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 water-holding
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 well-known 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 26 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, 18C
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 18C, and indeed 30C 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 18C to
15C for a short time resulted in a detectable
color change in the thawed product after only 3
more months of storage.