Introduction
Food preservation is defined as the process of addressing and dealing with
food in a way that maintains its taste and preserves its nutritive value as
long as possible via stopping or slowing spoilage with the aim of preventing
food borne illnesses. The process of food preservation include inhibiting the
growth of bacteria, fungi and other microorganisms besides slowing down the
process of oxidation of fats, which causes food rancidity. The most common food
preservation methods include drying, freezing, vacuum packing, canning,
preservation in syrup, sugar crystallization, salting, irradiation, addition of
preservatives and inert gases.
Food Spoilage
There are plenty of factors that cause food deterioration and spoilage. Chief
among these are
Microorganisms, which consist of bacteria, yeasts and fungi (molds.)
Enzymes, which occur naturally in plants, will cause foods to deteriorate
in time resulting in the loss of nutritional value, flavor and palatability.
Exposure to light will cause the
destruction of some vitamins, and the rate of all chemical and biological
reactions, including the actions of enzymes and the growth of microbes, will
increase as the temperature increases.
So all preserved foods will keep better and longer when protected from
light and stored at cooler temperatures.
For any type of food preservation method, the retention of nutritional
components is a concern, but Freezing is probably the least destructive when
properly done.
Freezing
Freeing is method of food preservation whereby:
- The heat is removed
- Temperature of the food is
reduced below its freezing point
- And a portion of water in
food undergoes a change in state to form ice crystals (aw lowered)
In freezing preservation is achieved by:
- Low temperature
- Reduced water activity due
to ice formation and high concentration of solutes in unfrozen water
- Blanching of some foods
Chemical Effects of Freezing
Concentration of chemicals in liquid
phase
Increased acidity
Low pH- Protein denaturation
Effect more pronounced during storage
and slow freezing
Types of changes:
- Flavour and odor
deterioration
- Pigment degradation
- Enzymatic brownng
- Auto oxidation of ascorbic
acid
- Protein insolubilization
- Lipid oxidation
The above changes are dependant on initial substrate concentration,
pH,Aw,O, handling and processing, and time and temperature.
Prevetion of Chemical Changes can be done by:
Inactivation of enzymes
Low temperature storage
Alternation of pH
Exclusion of oxygen
Freezing slows down, but does not destroy, enzymes in fruits and
vegetables. That is why it is important to stop enzyme activity before
freezing. The two methods you can use are blanching and adding chemical
compounds such as ascorbic acid.
(Enzymes are proteins present in plants and animals. While
the plant or animal lives, enzymes help speed up the ripening and maturing
processes. Even after we harvest plants or slaughter animals, enzyme reactions
can continue and result in undesirable color, flavor and texture changes in the
food).
Enzyme activity does not harm frozen meats or fish and is neutralized by
the acids in frozen fruits
(Plant products contain enzymes which give off ethylene gas,
so exposure to anaerobic conditions (e.g. freezer storage) can lead to off
colours and flavours. That's why pre-frozen fruits and vegetables have
generally been blanched to kill the enzymes)
Fats in meat, fish and poultry become rancid during freezer storage. This
is caused by contact with air left in the package or air that enters the
package because proper storage materials were not used. Control rancidity by
trimming excess fat from meat before freezing, using a wrapping material that
prevents air from reaching the product, and by storing foods for the
recommended length of time.
Slow Freezing & Rapid Freezing
Slow freezing causes a larger loss of drip on thawing, a larger loss of
nitrogenous constituents and nucleic acid derivatives to the drip, and a larger
loss of water-holding capacity of meat, than fast freezing.
In addition, slow freezing, as compared to fast freezing, increases protein
breakdown. Rapid freezing preserves the integrity of muscle proteins to a
greater extent than slow 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. This causes
meat to "drip" and lose juiciness. Ideally, food 2-inches thick
should freeze completely in about 2 hours
Slow Freezing
- Rates of cooling is less
than 1 degree C/ minute
- Ice crystal form in
extracellular locations
- Large ice crystal
formation
- Maximum dislocation of
water
- Shrinkage
- Less than maximum
attainable food quality
- Produces both
extracellular and intracellular (mostly) locations of ice crystals
- Small ice crystals
- Numerous ice crystals
- Minimum dislocation of ice
crystals
- Frozen appearance similar
to the unfrozen state
- Food qulity usually
superior to that attained by slow freezing.
The location of ice crystal in tissue is dependant on:
Slow
Rapid
- Specimen temperature
- Nature of the cell
Biological Effects of Freezing:
- Growth of mos is
temperature dependant
- No pathogens can grow
around 5 degree C
- No mos growth <-5degree
C
Freezing cannot kill pathogens if food is already contaminated. However,
some mos are killed, some are injured and some are stable.
Controlling initial microbial load inaddition to rapid freezing, and
storage at -18 degree C (constant) will reduce the detrimental effects of
microorganisms.Frozen food should be thawed rapidly (low temperature) and to be
used immedialtely or store at 5 degree C or cook for safety.
Quality Concerns in frozen foods includes:
- Freezer Burn
- Recrystallization
- Drip loss
- Loss of functionality
- Chemical reactions
- Color changes.
The bright red color of meat as purchased usually turns dark or pale brown
depending on its variety. This may be due to lack of oxygen, freezer burn or
abnormally long storage.
Freezing doesn't usually cause color changes in poultry. However, the bones and
the meat near them can become dark. Bone darkening results when pigment seeps
through the porous bones of young poultry into the surrounding tissues when the
poultry meat is frozen and thawed.
The dulling of color in frozen vegetables and cooked foods is usually the
result of excessive drying due to improper packaging or over-lengthy storage.
Best Practices
All foods have a limited shelf-life, no matter how they are stored or
preserved. It is imperative that you have a plan for rotating your food stash
to keep it fresh.
Proper packaging helps maintain quality and prevent freezer burn. Freeze unopened vacuum packages as is. If a package
has accidentally been torn or has opened while food is in the freezer, the food
is still safe to use; merely overwrap or rewrap it. Freezer burn does
not make food unsafe, merely dry in spots. It appears as grayish-brown leathery
spots and is caused by air coming in contact with the surface of the food. Cut
freezer-burned portions away either before or after cooking the food. Heavily
freezer-burned foods may have to be discarded for quality reasons.
Never stack packages to be frozen. Instead, spread them out in one layer
on various shelves, stacking them only after frozen solid.
For best results, freeze foods at -18 degrees Celsius or lower as soon as they
are packaged and sealed. Set the temperature control at -10 degrees Fahrenheit
or lower about 24 hours in advance to help food freeze rapidly. Do not overload
your freezer with unfrozen food. Add only the amount that will freeze within 24
hours.
Place packages in contact with refrigerated surfaces in the coldest part of
the freezer. Leave a little space between packages so that air can circulate
freely. When the food is frozen, packages can be restacked close together.
