The basic idea behind all forms of
food preservation is either:
- · To slow down the activity of disease-causing bacteria
- · To kill the bacteria altogether
In certain cases, a
preservation technique may also destroy enzymes naturally found in a food that
cause it to spoil or discolor quickly. An enzyme is a special protein that acts
as a catalyst for a chemical reaction, and enzymes are fairly fragile. By increasing
the temperature of food to about 150 degrees Fahrenheit (66 degrees Celsius),
enzymes are destroyed. A food that is sterile contains no
bacteria. Unless sterilized and sealed, all food contains bacteria.
Different techniques
used for preservation.
Refrigeration and Freezing
In the case of refrigeration,
the idea is to slow bacterial action to a crawl so that it
takes food much longer to spoil. In the case of freezing, the idea is
to stop bacterial action altogether. Frozen bacteria are completely
inactive.
Refrigeration and freezing are used
on almost all foods: meats, fruits, vegetables, beverages, etc. In general,
refrigeration has no effect on a food's taste or texture. Freezing has no
effect on the taste or texture of most meats, has minimal effects on
vegetables, but often completely changes fruits (which become mushy).
Refrigeration's minimal effects account for its wide popularity.
Canning Food
In canning, you boil the
food in the can to kill all the bacteria and seal the can (either before or
while the food is boiling) to prevent any new bacteria from getting in. We generally think of "cans" as being metal,
but any sealable container can
serve as a can. One problem with canning, and the reason why refrigeration
or freezing is preferred to canning, is that the act of boiling food in the can
generally changes its taste and texture (as well as its nutritional content).
Dehydrating Food
Water content in the food is removed.
Since most bacteria die or become completely inactive when dried, dried foods
kept in air-tight containers can last quite a long time. e.g.
·
Powdered
milk
·
Dried fruits
and vegetables
·
Powdered
soups and sauces
·
Pasta
·
Instant rice
Normally, drying completely alters the
taste and texture of the food.
Freeze-Drying
Freeze-drying is a special form of
drying that removes all moisture and tends to have less of an effect on a
food's taste than normal dehydration does.
In freeze-drying, food is frozen and
placed in a strong vacuum. The water in the food then sublimates --
that is, it turns straight from ice into vapor. Freeze-drying is most commonly
used to make instant coffee, but also works extremely well on
fruits such as apples.
Salting and Pickling
Salting, especially of meat, is an ancient
preservation technique. The salt draws out moisture and creates an environment
inhospitable to bacteria. If salted in cold weather (so that the meat does not
spoil while the salt has time to take effect), salted meat can last for years.
Pickling was widely used to preserve
meats, fruits and vegetables in the past, but today is used almost exclusively
to produce "pickles,". Pickling uses the preservative qualities
of salt (see above) combined with the preservative qualities
of acid, such as acetic acid (vinegar). Acid environments inhibit
bacteria.
Pasteurizing, Fermenting, Carbonating
Pasteurizing
It involves heating the food to a
high enough temperature to kill certain (but not all) bacteria and to disable
certain enzymes, and in return you are minimizing the effects on taste as much
as you can. Commonly pasteurized foods include milk, ice cream, fruit juices.
Fermenting
Fermentation uses bacteria like yeast to
produce alcohol. Alcohol is a good preservative because
it kills bacteria. When you ferment grape juice you create wine, which
will last quite a long time (decades if necessary) without refrigeration.
Carbonating
Carbonated water is water in
which carbon dioxide gas has been dissolved under pressure. By
eliminating oxygen, carbonated water inhibits bacterial growth. Carbonated
beverages (soft drinks) therefore contain a natural preservative.
Chemical Food Preservation
There are three classes of chemical
preservatives commonly used in foods:
·
Benzoates (such as sodium benzoate)
·
Nitrites (such as sodium nitrite)
·
Sulphites (such as sulphur dioxide)
Another common preservative is sorbic
acid. All of these chemicals either inhibit the activity of bacteria or kill
the bacteria.
Food Irradiation
Nuclear radiation is able to kill
bacteria without significantly changing the food containing the bacteria. So if
you seal food in plastic and then radiate it, the food will become sterile and
can be stored on a shelf without refrigeration. Unlike canning, however, you do
not significantly change the taste or texture of the food when you irradiate
it.
