Freezing Food Guide
According to a
study, an American consumes on average 71 frozen foods a year, most of which
are pre-cooked frozen meals. Freezing food preserves it from the time it is
prepared to the time it is eaten. Since early times, farmers, fishermen, and
trappers have preserved their game and produce in unheated buildings during the
winter season. Freezing food slows down decomposition by turning residual
moisture into ice, inhibiting the growth of most bacterial species. In the food
commodity industry, there are two processes: mechanical and cryogenic (or flash
freezing). The freezing kinetics is important to preserve the food quality and
texture. Quicker freezing generates smaller ice crystals and maintains cellular
structure. Cryogenic freezing is the quickest freezing technology available due
to the ultra low liquid nitrogen temperature (-196 °C).
The ice
crystallization that occurs during freezing processes, and along frozen
storage, causes the major important physic and chemical modifications which
decrease food quality. Freezing, or Solidification, is a phase
transition in which a liquid turns into a solid when its temperature is lowered
below its freezing point.
PHYSICAL CHANGES ON FROZEN FOOD
The main
physical changes of foods verified during freezing processes are related to the
risk of freeze cracking, moisture migration, recrystallization of ice crystals
and drip loss during thawing.
1) Freeze Cracking
The small ice
crystals formed with high freezing rates obtained with cryogenic freezers,
allow preservation of food structure. However, products may crack under those
conditions. This may happen when the internal stress of unfrozen food is higher
than the frozen material strength at food surface. To avoid cracking, a previous cooling step should be applied prior to
freezing. The reduction of the temperature gradients between the product
and the freezing medium or a pre-cooling step decrease significantly the risk
of freeze cracking.
2) Moisture Migration
During freezing
processes, when cell contents are super cooled, moisture movements may occur by
an osmotic mechanism. The occurrence
of temperature fluctuations results in vapor pressure differences, which are
responsible for moisture migration. If frozen products are stored without an
adequate moisture barrier, the ice on the food surface sublimes, since ice
water pressure is higher than the environment vapor pressure. An opaque
dehydrated surface is formed (microscopic cavities previously occupied by ice
crystals) with an unsightly white color. This leads to freezer burn. Learn how to How to Freeze Your Food Without Getting Freezer Burn?
If temperature
increases, water moves from the product; ice sublimes and water diffuses
through the packaging film. If temperature decreases, the ice on the wrap tends
to diffuse back to the food surface, however, the water reabsorption to the
original location is very improbable. To reduce moisture migration, temperature
fluctuations and internal temperature gradients should be minimized and
internal barriers within the product and within the packaging should be
included.
3) Recrystallization
Modifications in
the size, shape or orientation of the ice crystals are known as
"recrystallization" and usually lead to quality losses in some
products. Recrystallization reduces the advantages of fast freezing leading to
physico-chemical changes of food products. This process may happen in three
different ways:
(i)
changes in surface shape or internal structure (isomass recrystallization);
(ii)
linkage of two adjacent ice crystals to form a large crystal (accretive recrystallization),
and
(iii)
increase of the average size of the crystal (migratory recrystallization).
Migratory recrystallization is the most important and it is mainly related to
temperature fluctuations during storage. If temperature increases, the
product's surface warms slightly, the ice crystals melt, moisture moves to
regions of lower vapor pressure and some areas will be dehydrated. When
temperature decreases, water vapor does not form new nuclei points and links to
the existing ice crystals. This originates a reduction of the number of small
crystals and an increase of large crystals, disrupting the cellular structure.
The recrystallization during storage and transportation may lead to freeze-dried packaged product
or to toughening of animal tissue.
4) Drip Loss
During ice
formation, water is removed from the original location. However, during
thawing, water may not be reabsorbed in the same regions, and usually drip loss
is observed. Size and location of ice crystals, rate of thawing, the extent of
water reabsorption, the status of the tissue before freezing, and the water-holding capacity of the tissue have a
great influence on drip losses. The time required for thawing should be
longer than the one used for freezing (for comparable temperature driving
forces). In frozen meats, a slow thawing process at low temperatures will
permit a better water diffusion in the thawed tissue and its relocation in the
fibers. In vegetable tissues, the water is not reabsorbed.
CHEMICAL CHANGES ON FROZEN FOOD
During freezing,
changes in temperature and concentration (due to ice formation) play an
important role in enzymatic and nonenzymatic reactions rates. Ice crystals may
release the enclosed contents of food tissues, such as enzymes and chemical substances,
affecting the product quality during freezing and frozen storage. The main
chemical changes verified during freezing and frozen storage are related with
lipid oxidation, protein denaturation, enzymatic browning, and degradation of
pigments and vitamins.
1) Impacts on Food Texture,
Color and Flavor
Lipid oxidation
and protein denaturation are the major important causes of quality loss in
frozen meat and fish. Flavor, appearance, nutritional content and protein
functionality are usually degraded by lipid
oxidation. The solutes concentration during freezing processes catalyzes
the initiation of oxidative reactions, which disrupt and dehydrate cell
membranes, exposing membrane phospholipids to the oxidation process.
Food products
stored in contact with air, mainly fish and poultry that have significant
amounts of polyunsaturated fatty acids, also are susceptible to oxidation. The
decomposition of hydroperoxides of fatty acids in aldehydes and ketones results
in the formation of volatile compounds that gives rise to the aroma and taste
characterized as "rancid". Lipid oxidation also has an impact in
terms of pigment degradation and color quality deterioration of the products.
Freezing and thawing accelerate pigment oxidation. For example, the metmyoglobin
formation in red meats (brown color) and the carotenoid bleaching in fish and poultry
favor parallel fat oxidation. Hydrolytic rancidity, textural softening, and
color loss are also direct consequences of hydrolytic enzyme activities, which
can be inactivated by heat.
In relation to
fruits and vegetables, the ice crystals formation leads to undesirable losses
in texture, such as loss of turgor during thawing. The semi-rigid nature of the
cells and the less orderly packaging of the cells are mainly responsible for
the textural damage observed in frozen/thawed fruits and vegetables. Low
storage temperatures and slow thawing should be guaranteed to minimize losses of
membrane semi-permeability and cellular disruption. Also to avoid tissues
softening, pretreatments can be applied. The most important chemical changes
verified in frozen products are associated with the reactions that produce
off-odors and off-flavors, pigment degradation, enzymatic browning and
autoxidation of ascorbic acid.
Water that does
not freeze, even at very low temperatures, is responsible for deteriorative and
enzymatic reactions, particularly during
frozen storage. In non-blanched products, enzymatic oxidation of phenolic
compounds by polyphenoloxidase, leads to discoloration (browning) of food
products. However, ascorbic acid can be introduced as an inhibitor of enzymatic
reactions. The salts precipitation in concentrated solutions conduces to
changes of anthocyanins color. During frozen storage of green blanched products,
chlorophylls and carotenes are also degraded, the rate of pigment degradation
being dependent on the extent of tissues damaged prior to freezing. The action
of lipases and lipoxygenases leads to flavor alterations due to the
accumulation of volatile compounds (carbonyl compounds and ethanol) in vegetable
tissues.
2) Impact on Nutritional
Quality of Frozen Food
Commonly,
freezing is considered less destructive than any other preservation process and
frozen products have a nutritional quality comparable to fresh products.
Several unsaturated fatty acids (nutritionally essential or beneficial) are one
of the major substrates for lipid oxidation, but the losses are not limiting in
most of the frozen foods. Protein denaturation is mainly due to ice crystals formation
and recrystallization, dehydration, solutes concentration and oxidation. Thus,
several losses in protein functionality are reported in frozen fish, meat,
poultry and egg products, and some texture deterioration in frozen muscle
tissues may be attributed to protein damage. However, protein denaturation in
frozen products is considered minimal when compared to the total available
protein.
In terms of
nutritional value, vitamins (essentially B and C) are the compounds that suffer
a major negative impact with freezing and frozen storage conditions. Ascorbic
acid losses are attributed to oxidative mechanisms during frozen storage.
Blanching also affects negatively this quality indicator and the rates of
deterioration are extremely slow when compared to ambient or chilled storage.
Vitamin
Content of Frozen Foods
Vitamin C: Usually lost in a higher
concentration than any other vitamin. A study was performed on peas to
determine the cause of vitamin C loss. A vitamin loss of ten percent occurred
during the blanching phase with the rest of the loss occurring during the cooling
and washing stages. The vitamin loss was not actually accredited to the
freezing process. Another experiment was performed involving peas and lima
beans. Frozen and canned vegetables were both used in the experiment. The
frozen vegetables were stored at −10 °F (−23 °C) and the canned
vegetables were stored at room temperature (75 °F). After 0, 3, 6, and 12
months of storage, the vegetables were analyzed with and without cooking.
O'Hara, the scientist performing the experiment said, "From the view point of the vitamin content
of the two vegetables when they were ready for the plate of the consumer, there
did not appear to be any marked advantages attributable to method of
preservation, frozen storage, processed in a tin, or processed in glass."
Vitamin B1
(Thiamin): A
vitamin loss of 25 percent is normal. Thiamin is easily soluble in water and is
destroyed by heat.
Vitamin B2
(Riboflavin): Not
much research has been done to see how much freezing affects Riboflavin levels.
Studies that have been performed are inconclusive; one study found an 18
percent vitamin loss in green vegetables, while another determined a 4 percent
loss. It is commonly accepted that the loss of Riboflavin has to do with the
preparation for freezing rather than the actual freezing process itself.
Vitamin A
(Carotene): There
is little loss of carotene during preparation for freezing and freezing of most
vegetables. Much of the vitamin loss is incurred during the extended storage
period.
3)
Microbiological Aspects
During the
pre-freezing stage, microorganisms can grow but very slowly (have a long
generation time) when the temperature is approaching the minimum growth
temperature. If the temperature is kept below the minimum temperature for
growth, some microorganisms may die. However, above 0°C the loss of
microorganisms' viability is limited and in practice, is negligible. When
bacteria, in the exponential growth phase, are cooled quickly it is expected
that microorganisms inactivation is more pronounced, but an abrupt temperature drop
may lead bacteria to form cold shock proteins that protect them against other
stresses such as heating, low pH or low water activity.
The freezing
stage causes the apparent death of 10%-60%
of the viable microorganisms and these values increase during frozen storage.
Factors such as low temperature, extracellular ice formation, intracellular ice
formation, concentration of solutes and internal pressure may be involved in
the microbial inactivation. The sensitivity of microorganisms to the freezing
process differs considerably
Thus, the main
concern is related to the microorganisms that survive during the freezing step,
and with the ones that can grow when the product is thawed. Usually, the less
resistant microorganisms are the Gram-negative bacteria followed by the
Gram-positive bacteria. Nonsporulating rods and spherical bacteria are the most
resistant ones, and spores (such as Clostridium and Bacillus) remain unaffected
by freezing. Bacteria in the stationary phase are more resistant than those in the
exponential phase.
The freezing
process causes damage mainly in the microorganisms membrane, which loses some barrier
properties at temperatures below 15°C, leading to leakage of internal cell
material. The dissociation of lipid-proteins may injure the cells during the
freezing process. Cell membranes may also suffer mechanical damage due to ice
crystals formation. After storage at different temperatures, it is common to
observe higher microbial inactivation for warmer storage (e.g., -8°C)
temperatures than for colder storage temperatures (e.g., -18°C or lower). Freezing
and storage at very low temperatures (-150°C or even colder) seems to result in
increased survival. The long-time exposure to concentrated solutions (both
internal and external) may lead to microbial death in both conditions. However,
the recrystallization of ice observed if temperature fluctuations occur,
increases the solutes concentration and consequently provokes damage to
microorganisms. Temperature fluctuations at lower storage temperatures generate
smaller ice crystals than at higher storage temperatures.
Technology
Used in Freezing
The freezing
technique itself, just like the frozen food market, is developing to become
faster, more efficient and more cost-effective. Mechanical freezers were the
first to be used in the food industry and are used in the vast majority of
freezing / refrigerating lines. They function by circulating a refrigerant,
normally ammonia, around the system, which withdraws heat from the food
product. This heat is then transferred to a condenser and dissipated into air
or water. The refrigerant itself, now a high pressure, hot liquid, is directed
into an evaporator. As it passes through an expansion valve, it is cooled and
then vaporizes into a gaseous state. Now a low pressure, low temperature gas
again, it can be reintroduced into the system.
Cryogenic or
(flash freezing) of food is a more recent development, but is used by many
leading food manufacturers all over the world. Cryogenic equipment uses very low
temperature gases – usually liquid nitrogen or solid carbon dioxide – which are
applied directly to the food product.
Effectiveness of Freezing as Food Preservation
Freezing is an
effective form of food preservation because the pathogens that cause food
spoilage are killed or do not grow very rapidly at reduced temperatures. The
process is less effective in food preservation than are thermal techniques,
such as boiling, because pathogens are more likely to be able to survive cold
temperatures rather than hot temperatures. One of the problems surrounding the
use of freezing as a method of food preservation is the danger that pathogens
deactivated (but not killed) by the process will once again become active when
the frozen food thaws. Read... Food Storage Shelf Life Chart
Foods may be
preserved for several months by freezing. Long-term frozen storage requires a
constant temperature of -18 °C (0 °F) or less.
Freezing is one
of the oldest and most common processes used in food preservation and one of
the best methods available in the food industry.
There are several methods and types of equipment that can be used and adapted according
to the different types of foods. Freezing usually retains the initial quality
of the products. However, during freezing and frozen storage, some physical,
chemical and nutritional changes may occur. To avoid this impact on fresh
products, mainly in fruits and vegetables, some pretreatments may be required
to inactivate enzymes and microorganisms.
Vacuum packing combined with freezing will increase the storage life of
food. Learn the correct way to vacuum pack food at... How To
Vacuum Pack Meat, Poultry and Sea Food Properly?
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