Freezing and Refrigeration Low-temperature storage as a
preservation method probably began when prehistoric humans stored meat
and other foods in ice caves. However, mechanical refrigeration and
large-scale freezing are relatively recent innovations. Mechanical
refrigeration was pioneered by American inventor John Gorrie in 1842,
but a mechanical refrigeration system suitable for widespread commercial
use was not developed until the 1870s. American inventor Clarence
Birdseye developed procedures, equipment, and packaging for
quick-freezing in the 1920s, and in 1953 frozen TV dinners were
introduced by C. A. Swanson and Sons.
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Storage at low temperature slows many of the enzymatic reactions
involved in spoilage and reduces the growth rate of microorganisms
(though it does not kill them). To minimize microbial growth,
refrigerators should be kept at 0° to 4° C (32° to 40° F) and freezers
at or below 0° C (32° F). Refrigeration is advantageous because it does
not cause chemical or physical changes to food. Freezing allows foods to
be stored for longer periods than refrigeration because it inhibits
enzyme activity and microbial growth to a greater degree. The greatest
disadvantage of freezing is that the water in food expands and forms ice
crystals. The ice crystal formation disrupts the structure of plant and
animal cells, giving frozen food a softer texture after thawing. Newer
technologies in which freezing occurs more rapidly help minimize this
problem: faster freezing means that smaller ice crystals form, resulting
in less damage to cells.
Foods that should be refrigerated include meats, fish, eggs, milk, some
fruits, and some vegetables. Many of these foods can also be frozen.
Frozen produce is often high in quality and can rival the flavor of
fresh. In many cases, produce frozen and stored under proper conditions
contains more nutrients than produce picked unripened and allowed to
mature during transportation. Briefly cooking vegetables in boiling
water before freezing, a process known as blanching, inactivates enzymes
altogether and reduces discoloration and nutrient loss.
FControlled Atmosphere Storage Fruits and vegetables are sometimes
stored in sealed warehouses where temperature and humidity are closely
controlled, and perhaps most importantly, the composition of gases in
the atmosphere is altered to minimize spoilage. Usually, the
concentration of oxygen is reduced, the concentration of carbon dioxide
is increased, and ethylene, a gas naturally produced by plants that
accelerates ripening, is removed from the atmosphere. This controlled
environment helps slow the enzymatic reactions that eventually lead to
decomposition and decay, and may increase the time that produce can be
stored by several months. Ripening rooms, in which ethylene gas is added
to the atmosphere, also help produce higher quality fruits and
vegetables. This technology enables produce to be picked before it is
ripe, for easier handling, and then ripened quickly and uniformly under
controlled conditions.
Aseptic Packaging Aseptic packaging is now commonly used for packaging
milk and juice. Like canning, aseptic packaging involves heat
sterilization of food, but unlike canning, the package and food are
sterilized separately. Food can be sterilized more rapidly and at lower
temperatures in aseptic packaging than in canning, allowing the food to
retain more nutrients and better flavor. Containers are sterilized with
hydrogen peroxide rather than with heat, permitting the use of plastic
bags and foil-lined cartons, which would be destroyed by heat
sterilization. These containers cost less than the metal and glass
containers used in canning and also weigh less, reducing transport
costs. Aseptically packaged foods will keep without refrigeration for
long periods of time, perhaps even years. They are growing in popularity
because of their low cost, good taste and nutrition, and convenience.
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Irradiation Irradiation is a process in which food is passed through a
chamber where it is exposed to gamma rays or X rays. These high-energy
rays are strong enough to break chemical bonds, destroy cell walls and
cell membranes, and break down deoxyribonucleic acid (DNA), the
substance that carries genetic information in all cells. Irradiation
kills most bacteria, molds, and insects that may contaminate food.
Irradiation also delays the ripening of fruits and sprouting of
vegetables, permitting produce to be stored for longer periods of time.
Because irradiation involves minimal heating, it has very little effect
on the taste, texture, and nutritive value of food.
The FDA first approved irradiation for use on wheat and wheat flour in
1963, and later approved its use on white potatoes, spices, pork, some
fresh produce (onions, tomatoes, mushrooms, and strawberries), and
poultry. In 1997, in response to several food-borne illness outbreaks
and increasing public concern over the safety of the food supply,
irradiation was approved for use on beef products. Irradiation is also
used to preserve some meals eaten by astronauts during long-term space
missions. Some consumer groups have raised concerns that irradiation
might cause the formation of toxic compounds in food. Because of these
and other concerns, only a limited amount of irradiated food has been
sold in the United States.
Fermentation Fermentation is a chemical reaction carried out by many
types of microorganisms to obtain energy. In fermentation,
microorganisms break down complex organic compounds into simpler
substances. Although chemical changes and microbial growth usually mean
food spoilage, in some cases fermentation is desirable and
microorganisms are actually added to foods. For example, in the
production of beer, wine, and other alcoholic beverages, yeasts convert
sugar into ethyl alcohol and carbon dioxide. In the making of yogurt and
cheese, bacteria convert lactose, a sugar found in milk, to lactic acid.
Alcohol, acids, and other compounds produced in fermentation act as
preservatives, inhibiting further microbial growth. In addition to its
use with alcoholic beverages, cheese, and yogurt, fermentation is used
to produce yeast bread, soy sauce, cucumber pickles, sauerkraut, and
other products.
JPasteurization Pasteurization involves heating foods to a certain
temperature for a specific time to kill harmful microorganisms. Milk,
wine, beer, and fruit juices are all routinely pasteurized. Milk, for
example, is usually heated to 63° C (145° F) for 30 minutes. Ultra-High
Temperature (UHT) pasteurization, a relatively new technique, is used to
sterilize foods for aseptic packaging. In UHT pasteurization, foods are
heated to 138° C (280° F) for 2 to 4 seconds, allowing the food to
retain more nutrients and better flavor.
KGenetic Engineering Genetic engineering is a means of improving the
food supply even before harvest or slaughter by improving yields,
increasing disease resistance, and enhancing the nutritional qualities
of various foods. Broadly speaking, genetic engineering refers to any
deliberate alteration of an organism's DNA. Genetic engineering has been
practiced for thousands of years, ever since humans began selectively
breeding plants and animals to create more nutritious, better tasting
foods. In the past two decades, genetic engineering has become
increasingly powerful as scientific advances have enabled the direct
alteration of genetic material. Genes have been cut and pasted from one
species to another, yielding, for example, disease-resistant squash and
rice, frost-resistant potatoes and strawberries, and tomatoes that
ripen-and therefore spoil-more slowly.
FOOD PACKAGING Regardless of the processing or preservation method used,
proper packaging of food is essential to make sure the food remains
wholesome during its journey from processor to consumer. Packaging
contains food and makes it easier to handle, and protects it from
environmental conditions, such as temperature extremes, during
transport. It locks out microorganisms and chemicals that could
contaminate the food, and helps prevent physical and chemical changes
and maintain the nutritional qualities of food. For example, milk is
often stored in opaque containers to prevent vitamins from being
destroyed by light.
Both the type of food and the processing method used affect the choice
of packaging. For example, since oxygen makes fats go rancid, oils are
packaged in containers that are impermeable to oxygen. On the other
hand, oxygen-permeable plastic wraps allow fruits and vegetables to
"breathe" and ensure that meats will maintain a vibrant red color. Metal
and glass containers have traditionally been used in canning because
these materials can withstand the high temperatures and changes in
pressure that are involved in this processing method.
The development of metal cans in the early 1800s represented the birth
of the modern packaging industry. The first British patent for a
tin-plated steel container was issued in 1810 to British inventor Peter
Durand. Canned foods were produced for the British armed forces in 1812
and offered commercially to the public two years later. Today, food cans
are made of steel with various coatings to resist corrosion. Beverage
cans are made of aluminum because it is lightweight and easy to
manufacture.
In addition to metal, glass is often used for packaging heat-sterilized
foods. Glass is impermeable to oxygen and water and does not change the
flavor of food. Another advantage of glass is that it is transparent,
enabling the consumer to see the product inside. However, glass is not
impact-resistant and is relatively heavy.
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