New Star Software Collection/NSS_Collection.iso/3-251 oo2 english program/1.ima/RR.ZIP/PMETHODS.SEQ

Text File | 1991-06-30 | 7.8 KB | 144 lines

Two of the major causes of food spoilage are the growth of
microorganisms and the action of enzymes within the food cells.
Microorganisms are dependent on certain environmental and
nutritional factors. They need moisture, warmth, oxygen and a
source of energy. Without these things, they are unable to live
or grow, and the food is thereby preserved from decay.

Food preservation is certainly not a new idea. Keeping food
cold, for example, is an ancient technique, for caves and other
cool places have always been used for food storage. Likewise,
drying, preserving in sugar, pickling and salting have been
practised since the earliest times. However, ancient man could
not have clearly understood why particular methods were
effective. During the past century, food science has gradually
increased our knowledge about the causes of and the remedies for
food decay. As a result, several new preservative methods have
been developed, such as canning, artificial dehydration,
freezing, freeze-drying, irradiation and the addition of
chemicals. These new techniques have had a huge impact on the
world's food supply systems.

Keeping food cool slows the growth of microorganisms. Therefore,
in the past, people kept food in cool caves or in cool rooms
built under their houses. Today, refrigeration uses the same
technique. A refrigerator cools food to about 4°C., but does not
freeze it. Food can be stored in the main part of a refrigerator
for several days, because the microorganisms are relatively
inactive at that temperature. However, refrigeration is not
suitable for long-term preservation of food.

Drying can preserve food for much longer than refrigeration, and
is perhaps the oldest preservative method. It is still the most
widely used. Where climatic conditions allow, some foods,
especially grain and fruit, can simply be exposed to sunlight
and dry winds, which together will carry away 90% of the
moisture. Today, in regions of the world with unpredictable
weather, an artificial method of drying, known as dehydration,
is more suitable. It is more expensive but also more efficient.
A current of warm air is passed over the food while it is inside
large machines called dehydrators. These machines do their work
in a short space of time and can therefore process a relatively
large volume of food. Another way of drying food is by smoking
it. Smoke has some bactericidal properties, but its main role in
the preservative process is to dry out the food in the same way
that warm, dry winds do.

Several of the older methods of food preservation, namely,
preserving in sugar, pickling and salting, make use of a natural
process known as osmosis. The cell walls of living organisms are
semi-permeable membranes: they allow certain substances to pass
through while blocking others. By the process of osmosis, water
molecules pass through the cell wall until there is an equal
concentration of water on either side. If some foods are
immersed in vinegar, a salt solution or a sugar solution, some
of that liquid will enter the food cells while some of the water
inside the cells flows outward. These actions are simultaneous
and continue until the liquids are said to be balanced. Sugar in
concentrations exceeding 50% stops the growth of most spoilage
micro- organisms, and is used for preserving fruit to make jams
and jellies.

Pickling involves the use of vinegar, which is composed mainly
of acetic acid and can kill most microorganisms. Pickling is
often used for preserving green tomatoes, cucumbers and other
vegetables which taste good when sour.

Salting is often done with salt crystals rather than a salt
solution, especially for foods such as beef and fish. Large
quantities of crystals can be rubbed over the surface of the
food. Some of the salt mixes with the moisture on the surface
and enters the cells. However, because most of the crystals are
so large, they cannot reach very far into the food tissue: they
stay on the surface absorbing the water from the food. Bacteria
cannot live easily in the dry, salty cells on the surface of the
meat or fish. The enzymes Inside the cells are inactivated
because they are deprived of water and also are subjected to
higher concentrations of salt than occur naturally inside the
cells.

One of the commonest and most familiar methods of food
preservation is canning or bottling. Since the vast majority of
micro organisms cannot live in extremely hot conditions, food is
placed in metal or glass containers and sealed so that no air
can enter. The containers are then heated to temperatures in
excess of 100 degrees centigrade. The heat sterilizes the food
while the seals ensure that no other micro organisms can enter.
The length of time that the food is subjected to heat is
determined by the nature of the food and the type of bacteria to
be killed.

Freezing (i.e., the cooling of food to temperatures below 0°C.)
does not kill micro organisms, but it does inactivate them. Food
can, therefore, be preserved effectively for quite long periods
by freezing. If frozen food is to retain its flavour, texture
and nutrient value, it must be frozen rapidly. Slow freezing
produces large ice crystals which break the cell walls of the
food, destroying its structure, reducing its nutrient value and
giving it an unpleasant taste and appearance. Rapid freezing, on
the other hand, produces crystals that are smaller and better
distributed. Once food has been frozen, it has to be kept at sub
zero temperatures or else the micro organisms are reactivated
and are able to continue the spoilage process. The invention and
development of artificial refrigeration and freezing have made a
major contribution to food storage technology.

An extremely efficient, but costly, method of food preservation
is freeze-drying. This process combines the best of freezing and
drying techniques. The food is first cooked, then quick-frozen
to a temperature of about 30 degrees centigrade. After that, it
is placed in a special container where the pressure is reduced
by strong vacuum pumps and heat is applied to dry it. When the
process has been completed, the food can be stored at normal
room temperatures. Furthermore, the food retains its taste and
nutrient value (after being reconstituted with water) and is
light enough to be easily transported in large quantities.

Certain inorganic chemicals, such as sulphur dioxide, sulfites,
nitrites and nitrates, can be added to food to delay the growth
of microorganisms and the action of enzymes. Many food
scientists believe that the inorganic preservatives may have
undesirable long-term effects on the health of consumers.
Research is still being done, but meanwhile, most governments
insist that concentrations of these additives are kept low.

Another very modern technique may involve similar health
problems. At the present time scientists and food technologists
are experimenting with the sterilizing effects of radiation. In
the United States, the government's Food and Drug Administration
has approved irradiation for use on some vegetables and meats.
In this technique, fresh foods are subjected to radiation from
gamma rays. The radiation kills the bacteria which cause
spoilage. However, little is known about the chemical changes
caused in the food by this process, and many people fear that
even a short exposure to radiation may destroy important
nutrients in the food, or even create potentially dangerous
chemicals in the food.

Methods of food preservation will continue to develop. We have
added canning and freezing to the older methods of preserving
food. It appears that, for several more years, we shall continue
to use mainly the familiar, effective and safe methods that have
long been available to us, and we will accept the newest methods
only after careful testing.