Food
preservation
Food preservation
is the process of treating and handling food to stop or slow down Food spoilage,
loss of quality, edibility or nutritional value and thus allow for longer food
storage.
Food preservation has been defined "as the
science which deals with the process of prevention of decay or spoilage of food
thus allowing it to be stored in a fit condition for future use". It has
also been described as the state in which any food may be retained over a
period of time without (1) being contaminated by pathogenic organisms or
chemicals (2) losing optimum qualities of colour, texture, flavor and nutritive
value.
Food preservation usually involves preventing the growth
of bacteria,
fungi
(such as yeasts),
and other micro-organisms (although some methods work by
introducing benign bacteria, or fungi to the food), as well as retarding the oxidation of fats which cause rancidity.
Food preservation can also include processes which inhibit visual
deterioration, such as the enzymatic browning reaction in apples after
they are cut, which can occur during food preparation.
Many processes designed to preserve food will involve a
number of food preservation methods. Preserving fruit by turning it into jam,
for example, involves boiling (to reduce the fruit’s moisture content and to
kill bacteria, yeasts, etc.), sugaring (to prevent their re-growth) and sealing
within an airtight jar (to prevent recontamination). There are many traditional
methods of preserving food that limit the
energy inputs and reduce carbon footprint.
Maintaining or creating nutritional value, texture
and flavour
is an important aspect of food preservation, although, historically, some
methods drastically altered the character of the food being preserved. In many
cases these changes have now come to be seen as desirable qualities – cheese,
yoghurt and pickled onions being common examples.
What is the importance of food
Preservation?
Some foods such as fruits and vegetables are
available in some season and not in others. Other foods are more abundantly
available in some season than in others. Food production and supply does not
always tally with the demand or meets of the people. In some places there is
surplus production of a food product, whereas in some other place there is
inadequate supply. Even foods are perishable and semi-perishable like juicy
fruits, vegetables, mangoes, tomato, papaya and many more, which very quickly
gets spoilt. It is therefore important to improve and expand facilities for
storage and preservation of food. Food preservation helps in:
1. Increasing the self-life of
foods thus increasing the supply. So many perishable foods can be preserved for
a long time.
2. Making the seasonal food
available throughout the year.
3. Adding variety to the diet.
4. Saving time by reducing
preparation time and energy, as the food has already been partially processed.
5. Stabilising prices of food, as
there is less scope of shortage of supply to demand.
6. Decreasing wastage of food by
preventing decay or spoilage of food.
7. Improving the nutrition of the
population. Preserved foods help people to bring a variety in the diet, thereby
decreasing nutritional inadequacies.
What are the essential principles of
food preservation?
Preservation of food by any method is based on the
following principles:
(A) Prevention or delay of microbial
decomposition
(i). By keeping out micro-organisms (asepsis)
Nature provides protective coverings around the
food in the form of shells of nuts, die skins of fruits and vegetables, the
shells of eggs, and the skin or fat on meat or fish. These protective coverings
act as a preservative factor, thereby preventing or delaying microbial
decomposition.
Even in the food industry several aseptic
methods are adopted to prevent the contamination of foods during its
processing. In the canning industry, the load of micro-organisms determines the
heat process necessary for the preservation of food. This is better known as
aseptic canning. In the dairy industry, the quality of milk is judged by its
bacterial content.
Packaging of foods is also an application of
asepsis. The coverings may range from simple wrappers to hermetically sealed
containers of canned foods. Polythene bags and moisture proof wrappings including
heavy foil, heavily mixed papers and cellophane are used.
(ii). By removal of micro-organisms
Filtration is a method used for the complete
removal of microorganisms and is successfully applied only to clear liquids
such as water, fruit juices, beer, soft drinks and wine. The filter used in
this method is made of asbestos pads, unglazed porcelain and similar materials.
This filter is sterilised and made “bacteria proof” before being used as a
filtration device. The liquid is filtered by forcing it under pressure through
the filter.
(iii). By hindering the growth and activity
of micro-organisms
This may be done by low temperature or drying or by
providing anaerobic conditions.
When anaerobic (absence of oxygen) conditions are
created, some aerobic organisms die while the spores of others may survive but
are unable to multiply in the absence of oxygen. This principle is used as a
preservative factor in canned and packaged foods.
Low temperature at which foods are
preserved in cold storage slows down and sometimes prevents bacterial activity.
Drying of foods is a very
effective method of avoiding spoilage of food, since micro-organisms cannot
flourish in the absence of moisture.
Certain chemicals like sodium
benzoate and potassium metabisulphite may be used for preservation, but they
should be used with great care as an excess of any of them may result in
poisoning.
(iv). By killing the micro-organisms using
heat or radiation
In this process gamma rays or high speed electrons
are used to destroy the micro-organisms. Both types of radiations are termed as
ionized radiations. This topic will be dealt with in detail
under “Methods of Food Preservation”.
(B) Prevention or delay of self
decomposition of food
This is done by destruction or inactivation
of food enzymes by blanching. The inactivation affects many plant
enzymes which otherwise might cause toughness and change in colour. All plant
and animal tissues contain enzymes which are highly active at room temperature
and above. For each 10° C (19°F) raises in temperature the rate of the chemical
change doubles. Rancidity of fats is an excellent example of undesirable
oxidation and leads to the deterioration in flavour of foods that may contain
only small quantities of fat. Oxidation also leads to a loss of ascorbic acid.
Plant and animal tissue fiber is softened, the surfaces of cut non-acid fruits
are oxidised and become darkened as a result of enzyme action, thereby changing
the colour, texture and nutritive value.
Before freezing (to prevent the growth of bacteria)
fruits and vegetables are blanched to inactivate the oxidative enzymes, Blanching
is done with hot water or steam and the extent of treatment applied varies with
the kind of food being treated. The brief heat treatment is supposed to
accomplish reduction of the number of micro-organisms on the food, enhancement
of the green colour of vegetables such as peas and spinach, and prevention of
damage because of mechanical causes, insects and animals.
Items of food can be damaged either by insects and
animals or by mishandling. Therefore, meticulous care should be exercised to
minimize any damage to the foods. The entire operation of preserving foods is
divided into three stages of careful handling:
i. Proper packaging
ii. Quick and effective transportation
iii. Providing good storage facilities, like silos
for grains and cold storages for fruits and vegetables.
WHAT ARE THE METHODS OF FOOD PRESERVATION?
Drying
Drying
is one of the most ancient food preservation techniques, which reduces water
activity sufficiently to prevent bacterial growth
Pasteurization
Pasteurization is a process for preservation of
liquid food. It was originally applied to combat the souring of young local
wines. Nowadays it is prominently applied to milk. In this method, milk is
heated at about 70° C for 15 to 30 seconds to kill the bacteria present in it
and cooling it quickly to 10° C to prevent the remaining bacteria from growing.
The milk is then stored in sterilized bottles or pouches in cold places. This
method was invented by Louis Pasteur in 1862.
Refrigeration
Refrigeration preserve foods by slowing down
the growth and reproduction of micro-organisms and the action of enzymes which
cause food to rot. The introduction of commercial and domestic refrigerators
drastically improved the diets of many in the Western world
by allowing foods such as fresh fruit, salads and dairy products to be stored
safely for longer periods, particularly during warm weather.
Freezing
Freezing is also one of the most commonly used processes
commercially and domestically for preserving a very wide range of food
including prepared food stuffs which would not have required freezing in their
unprepared state. For example, potato waffles are stored in the freezer, but
potatoes themselves require only a cool dark place to ensure many months'
storage. Cold stores provide large volume, long-term storage for strategic food
stocks held in case of national emergency in many countries.
Vacuum packing
Vacuum-packing stores food in a vacuum environment,
usually in an air-tight bag or bottle. The vacuum
environment strips bacteria of oxygen needed for survival, slowing spoiling.
Vacuum-packing is commonly used for storing nuts
to reduce loss of flavour from oxidation.
Preservation by oil and vinegar
Oil and Vinegar prevent the growth of food-spoiling
microorganisms. Fruits and Vegetables are often preserved by this method.
Salt
Salting or curing draws moisture from the meat
through a process of osmosis. Meat is cured with salt or sugar, or a combination
of the two. Nitrates and nitrites are also often used to cure meat and
contribute the characteristic pink color.
Sugar
Sugar is used to preserve fruits, either in syrup
with fruit such as apples,
pears, peaches, apricots,
plums or in crystallized
form where the preserved material is cooked in sugar to the point of
crystallisation and the resultant product is then stored dry. This method is
used for the skins of citrus fruit (candied peel), angelica
and ginger.
Smoking
Smoking is used to lengthen the shelf life
of perishable
food items. This effect is achieved by exposing the food to smoke from burning plant
materials such as wood.
Most commonly subjected to this method of food preservation are meats and fish
that have undergone curing. Fruits and vegetables
like paprika,
cheeses,
spices,
and ingredients for making drinks such as malt and tea
leaves are also smoked, but mainly for cooking
or flavoring
them. It is one of the oldest food preservation methods, which probably arose
after the development of cooking with fire.
Artificial food additives
Preservative food additives can be antimicrobial;
which inhibit the growth of bacteria or fungi, including mold, or antioxidant; such as oxygen
absorbers, which inhibit the oxidation
of food constituents.
Pickling
Pickling is a method of preserving food in an
edible anti-microbial liquid. Pickling can be broadly
categorized into two categories: chemical pickling and fermentation pickling.
In chemical pickling, the food is placed in an
edible liquid that inhibits or kills bacteria and other micro-organisms.
Typical pickling agents include brine (high in salt),
vinegar,
alcohol,
and vegetable oil,
especially olive oil
but also many other oils. Many chemical pickling processes also involve heating
or boiling so that the food being preserved becomes saturated with the pickling
agent. Common chemically pickled foods include cucumbers,
peppers,
corned beef,
herring,
and eggs,
as well as mixed vegetables such as piccalilli.
In fermentation pickling, the food itself produces
the preservation agent, typically by a process that produces lactic acid.
Fermented pickles include sauerkraut, nukazuke, kimchi, surströmming,
and cursextido. Some pickled
cucumbers are also fermented.
Lye
Sodium
hydroxide (lye)
makes food too alkaline
for bacterial growth. Lye will saponify
fats in the food, which will change its flavor and texture. Lutefisk
uses lye in its preparation, as do some olive recipes. Modern recipes for century eggs
also call for lye.
Canning and bottling
Canning involves cooking food, sealing it in sterile cans or
jars, and boiling
the containers to kill or weaken any remaining bacteria as a form of sterilization. Foods have varying degrees
of natural protection against spoilage and may require that the final step
occur in a pressure cooker. High-acid fruits like strawberries
require no preservatives to can and only a short boiling cycle, whereas
marginal fruits such as tomatoes require longer boiling and addition of other acidic
elements. Low acid foods, such as vegetables and meats require pressure
canning. Food preserved by canning or bottling is at immediate risk of spoilage
once the can or bottle has been opened.
Jellying
Food may be preserved by cooking in a material that
solidifies to form a gel. Such materials include gelatine,
agar, maize flour and arrowroot
flour. Some foods naturally form a protein
gel when cooked such as eels and
elvers, and sipunculid worms which are a delicacy in Xiamen in
Fujian province of the People's Republic of China. Jellied eels
are a delicacy in the East End of London where they are eaten with mashed potatoes. Potted meats
in aspic,
(a gel made from gelatine and clarified meat broth) were a common way of
serving meat off-cuts in the UK until the 1950s. Many jugged meats are also
jellied.
A traditional British way of preserving meat
(particularly shrimp) is by setting it in a pot and sealing
it with a layer of fat.
Jugging
Meat can be preserved by jugging, the process of stewing
the meat (commonly game or fish) in a covered earthenware
jug or casserole.
The animal to be jugged is usually cut into pieces, placed into a
tightly-sealed jug with brine or gravy, and stewed. Red wine
and/or the animal's own blood is sometimes added to the cooking liquid. Jugging
was a popular method of preserving meat up until the middle of the 20th
century.
Irradiation
Irradiation of food is the exposure of
food to ionizing radiation; either high-energy electrons
or X-rays
from accelerators, or by gamma rays (emitted from radioactive sources as
Cobalt-60
or Caesium-137).
The treatment has a range of effects, including killing bacteria, molds and
insect pests, reducing the ripening and spoiling of fruits, and at higher doses
inducing sterility. The technology may be compared to pasteurization; it is
sometimes called 'cold pasteurization', as the product is not heated. However,
it is fundamentally different from pasteurization as it reduces the microbial
load by incremental (logarithmic) steps whereas in heat treatment the
observation of a minimum temperature and minimum duration of exposure insures
the elimination of the microorganisms under consideration.
Pulsed electric field electroporation
Pulsed electric field (PEF) electroporation is a
method for processing cells by means of brief pulses of a strong electric
field. PEF holds potential as a type of low temperature alternative
pasteurization process for sterilizing food products. In PEF processing, a
substance is placed between two electrodes, then the pulsed electric field is
applied. The electric field enlarges the pores of the cell membranes which
kills the cells and releases their contents. PEF for food processing is a
developing technology still being researched. There have been limited
industrial applications of PEF processing for the pasteurization of fruit
juices.
Modified atmosphere
Modifying atmosphere is a way to preserve food by
operating on the atmosphere around it. Salad crops which are notoriously
difficult to preserve are now being packaged in sealed bags with an atmosphere
modified to reduce the oxygen (O2) concentration and increase the carbon
dioxide (CO2) concentration.
Air-tight storage of grains (sometimes called
hermetic storage) relies on the respiration of grain, insects and fungi which
can modify the enclosed atmosphere sufficiently to control insect pests. A
patented process uses fuel cells to exhaust and automatically
maintain the exhaustion of oxygen in a shipping container, containing, for example, fresh
fish.
Nonthermal plasma
This process subjects the surface of food to a
'flame' of ionised gas molecules such as helium or nitrogen. This causes
micro-organisms to die off on the surface.
High pressure food preservation
High pressure food preservation or pascalization
refers to the use of a food preservation technique which makes use of high pressure.
"Pressed inside a vessel exerting 70,000 pounds per square inch
(480 MPa) or more, food can be processed so that it retains its fresh
appearance, flavour, texture and nutrients while disabling harmful
microorganisms and slowing spoilage. By 2005 the process was being used for
products ranging from orange juice to guacamole
to deli meats
and widely sold.
Burial in the ground
Burial of food can preserve it due to a variety of
factors: lack of light, lack of oxygen, cool temperatures, pH level, or desiccants
in the soil. Burial may be combined with other methods such as salting or
fermentation. Most foods can be preserved in soil that is very dry and salty
(thus a desiccant), or soil that is frozen.
Many root vegetables are very resistant to spoilage
and require no other preservation than storage in cool dark conditions, for
example by burial in the ground, such as in a storage clamp.
Century eggs
are created by placing eggs in alkaline mud (or other alkaline substance)
resulting in their "inorganic" fermentation through raised pH instead
of spoiling. The fermentation preserves them and breaks down some of the
complex, less flavorful proteins and fats into simpler more flavorful ones. Cabbage
was traditionally buried in the fall in northern farms in the USA for
preservation. Some methods keep it crispy while other methods produce sauerkraut.
A similar process is used in the traditional production of kimchi.
Sometimes meat is buried under conditions which cause preservation. If buried
on hot coals or ashes, the heat can kill pathogens, the dry ash can desiccate,
and the earth can block oxygen and further contamination. If buried where the
earth is very cold, the earth acts like a refrigerator.
Controlled use of micro-organism
Some foods, such as many cheeses, wines, and beers will keep for a long
time because their production uses specific micro-organisms that combat
spoilage from other less benign organisms. These micro-organisms keep pathogens
in check by creating an environment toxic for themselves and other
micro-organisms by producing acid or alcohol. Starter micro-organisms, salt,
hops, controlled (usually cool) temperatures, controlled (usually low) levels
of oxygen and/or other methods are used to create the specific controlled
conditions that will support the desirable organisms that produce food fit for
human consumption.
Biopreservation
Biopreservation
is the use of natural or controlled microbiota
or antimicrobials
as a way of preserving food and extending its shelf life.
Beneficial bacteria or the fermentation products produced by these
bacteria are used in biopreservation to control spoilage
and render pathogens
inactive in food.
Hurdle technology
Hurdle
technology is a method of ensuring that pathogens
in food products
can be eliminated or controlled by combining more than one approach. These
approaches can be thought of as "hurdles" the pathogen has to
overcome if it is to remain active in the food. The right combination of
hurdles can ensure all pathogens are eliminated or rendered harmless in the
final product.
Examples
of hurdles in a food system are high temperature during processing, low
temperature during storage, increasing the acidity,
lowering the water activity or redox
potential, or the presence of preservatives
or biopreservatives. According to the type of
pathogens and how risky they are, the intensity of the hurdles can be adjusted
individually to meet consumer preferences in an economical way, without
sacrificing the safety of the product.
No comments:
Post a Comment