Food spoils because of
the growth and destructive action of microscopic living organisms. They
are cornmonly termed "germs." The various methods of food preservation
are practically all based upon processes that destroy these organisms or
prevent their growth and activity. Because they are microscopic and
because they are living organisms, we shall for convenience call them "microorganisms."
1. Molds. The
molding of food is a common phenomenon. In some cases the food is
completely spoiled; in others, the decomposition is not sufficient to
make the product inedible; and in a few products, the growth of certain
molds is desirable.
The most prevalent mold
and the one causing the most damage is the "blue mold," otherwise known
as "Penicillium expansum." It first appears usually as a white cottony
growth on fruits, cheese, cured meats, vegetables, jellies, wine tanks,
leather left in dark closets, and on other articles favorable to its
growth. This cottony growth of mold threads is known as a "mycelium."
Later, the mold becomes "powdery" in appearance and green or blue in
color. This change in color is due to the formation of billions of
microscopic cells or "spores." The spores are very light and easily
detached. They are carried by the air or other agencies from place to
place. They are floating in the air at all times and places and are
present on the surfaces of all fresh foods. They are capable of
sprouting when conditions become favorable. A large growth may start
from a single cell or spore.
Fruits whose skins become
broken in transit suffer badly from this mold and acquire a moldy taste
and odor. In some such cases the growth will not be apparent because the
mold threads are growing in the pulp or juice of the fruit.
The surface of jellies
may become overgrown by this organism and the upper portion of the jelly
completely spoiled. Leaky jars of fruit may mold from the growth of
penicillium spores gaining entrance through the leaks. Bacon and cheese
may develop green spots of this mold on the surface and still not be
spoiled if the mold is removed in time. The inside of wine or vinegar
barrels may be completely spoiled where this mold is allowed to develop
through improper care of the barrels.
The blue mold can be
controlled, but great care must be taken if it is to be completely
eliminated. Its spores are killed by heating to 180° F. and. growth is
prevented by many chemicals.
"Black Mold," otherwise
known as "Aspergillus niger," often occurs on fruits that have become
moist on the surface or broken; or it may occur on other products
occasionally. It does not produce a moldy taste or odor; it is much less
prevalent, and is easier to control than is the blue mold.
"Pin Mold," or "Gray Mold,"
or "Bread Mold," usually causes the molding of bread stored in a moist
place. It also occurs frequently on fruits and may appear as "whiskers"
on peaches, grapes, and other fruits, shipped long distances in boxes.
It is not especially important in food preservation. It is known
botanically as Mucor.
There are hundreds of
other forms of molds but the above forms are by far the most common on
Molds are not always
deleterious in their action. Camembert, Rocquefort, and some other fancy
cheeses owe their distinctive quality to the growth of special forms of
Penicilliurn molds. A form of Aspergillus mold, known as Aspergillus
oryzae, is used extensively in Japan in making "Saki," Japanese beer. A
Mucor mold is used frequently in distilleries in the production of
alcohol from cereals.
In general, molds are of
interest in food preservation because of their capacity for spoiling
food, their universal occurrence on food products, and the difficulty in
killing their spores by heat or controlling their growth in other ways.
2. Yeasts. When a
fruit juice is allowed to stand a few days it undergoes fermentation.
The sugar is destroyed and alcohol and carbonic acid gas are formed.
This change is brought about by another group of microscopic organisms,
known as yeasts. Yeasts are used in bread making, vinegar manufacture,
and in the production of various fermented beverages.
Unlike molds, they do not
form a mycelium, i. e., a thread-like growth, but only develop as
microscopic cells of various forms. They appear in fermented liquids as
a white sediment or a cloudy growth throughout the liquid.
They are universally
present in the air, on the surfaces of fruits, vegetables, and of
tables, knives, etc., and are capable of growing in and spoiling sugary
liquids, crushed fruits, jellies that do not have sufficient sugar, and
in other products containing from one to 65% sugar. More sugar than 65%
prevents their growth.
Jars and cans of fruit
that become leaky after sterilization become infected with yeast cells
carried in by air passing into the containers. Growth and fermentation
take place and the pressure of. the carbonic acid formed by the yeast
causes the container to swell or burst. Much canned fruit is lost in
this way. The housewife usually attributes the loss to the entrance of
air. It is in reality caused by yeast gaining entrance with the air; air
alone would be incapable of causing fermentation.
Yeasts are easily killed
by heat, a temperature of 60° C. or 140° F. being sufficient, and in
general, yeasts are more easily controlled than molds. Conditions that
will eliminate molds will also remove yeasts.
Yeasts cause the
"souring," "working," or fermenting of spoiled jars or cans of fruit,
bottles of fruit juices, or glasses of jelly. They are therefore of much
importance in the preservation of fruit products.
They are necessary in the
manufacture of all fermented beverages, denatured alcohol, vinegar, and
yeast-risen bread. Yeasts are the most useful of all the microorganisms
met with in food preservation.
3. Bacteria. Milk
sours on standing; meat and many cooked vegetables putrefy unless
spoiling is prevented; dill pickles and sauerkraut undergo certain
characteristic changes. These changes are wholly, or in most part,
brought about by bacteria. They comprise the third main group of "germs"
or microorganisms. Like the other two groups they are universally
distributed. Bacteria are, as a rule, smaller than yeasts and differ
from them in their method of reproduction. Yeasts reproduce by budding
and bacteria by splitting in two, i. e., by "fission." Bacteria prefer
nitrogenous substances of low acid content, such as milk, meat, peas,
and beans, and do not grow readily on fruits or acid vegetables. Molds
and yeasts prefer sugary, acid materials.
Yeast and mold spores are
easily killed by temperatures below 212° F. Many bacterial spores
survive temperatures above 212° F., the boiling point of water. For this
reason, many foods containing such spores are exceedingly difficult to
sterilize by heat, This does not apply to foods high in acid because
these bacteria can not grow readily in the presence of much acid and are
more easily killed in acid foods.
Yeasts and molds produce
relatively harmless compounds in food products. Bacteria on the other
hand may produce in canned vegetables, in meats, and in cheese,
extremely poisonous compounds. These are the ptomaines and botulinus
poison. (See paragraph 25, Part II, on "Spoiling of Canned Foods.") It
is therefore necessary to be sure that such products as canned peas,
beans, corn, and meats, are thoroughly sterilized, in order that
poisoning will not occur.
Several forms of bacteria
are extremely useful in food preservation and food manufacture. The two
most important are vinegar bacteria, necessary in making vinegar, and
lactic acid bacteria, essential in the manufacture and preservation of
sauerkraut, pickled green olives, silage, and cheese. " Vinegar Mother "
is a growth of vinegar bacteria; the sour taste of sauerkraut and sour
milk is brought about by lactic acid formed by lactic acid bacteria.
4. Spoiling of Foods
by Chemical and Physical Changes. Some food products decompose
without the action of organisms. Edible fats and oils become rancid
through the action of the oxygen of the air. Meats are sometimes
practically spoiled by the use of too much salt in salt curing. Dried
fruits may be greatly injured by leaving them too long in the sun on
trays. Canned goods sometimes act upon the tin of the cans to such an
extent that they become poisonous or inedible.
Practically all food
products undergo slow changes through drying or oxidation when left
exposed to the air. Even cereals deteriorate with age in bins,
Changes of this sort are
as a rule slower and more easily controlled than bacterial changes. It
is usually only necessary to exclude moisture or air or control the