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A. Temporary Prevention of Spoiling
It is often desirable to
preserve foods for a relatively short time only. In such cases methods
are usually employed, which will alter the original qualities of the
product as little as possible. The methods will vary with the character
of the food and other conditions.
5. Asepsis. Milk,
fresh fruit juices and the surfaces of fruits, vegetables, meats, and
other food products contain great numbers of microorganisms capable of
causing spoiling. By handling these products in a careful and cleanly
way, by using containers that are clean and preferably sterilized by
heat, and by washing or otherwise cleansing certain products of adhering
dust, etc., the molds, yeasts, and bacteria will be kept down to small
numbers, and their multiplication will be hindered. Often such treatment
will greatly prolong the keeping of food products, especially of fresh
fruits and vegetables. Cleanliness and care in handling in order that
excessive contamination by "germs" will be prevented is termed
"asepsis." The extreme care taken in modern certified dairies in the
production of certified milk is one of the best examples of the
application of asepsis in the handling of a food product.
6. Cold Storage.
The organisms that cause spoiling, grow most rapidly and are most active
at warm temperatures. At temperatures near the freezing point their
growth is almost completely stopped. The storage of eggs, meats, and
fruits at low temperatures has become an enormous industry. It has made
these foods available over a longer period of the year than would
otherwise be possible. Fresh meat is exported in great quantities from
South America, Australia, and the United States in ships equipped with
cold storage facilities.
Cold storage is principally an industrial
undertaking but is used extensively in a small way by housewives and
farmers in the use of the various forms of household ice chests and in
the winter storage of vegetables on farms. Meat is often allowed to
freeze in the winter air and is kept in a cold place till needed.
In all cases the principle involved is the
same; namely, reducing the activity of the microorganisms of spoiling by
a reduction of temperature. Lowering of temperature also slows up
deleterious chemical changes such as the rancidifying of fats, and oils,
and the deterioration of cereals.
7. Exclusion of Moisture. Moisture is
necessary to the growth of microscopic organisms. Fruits often spoil
during shipment because of the collection on their surfaces of moisture,
in which molds develop. Perfectly dry surfaces will not support mold or
other " germ " growth. For this reason, dried fruits, meats, dried
vegetables, and cereals should be stored in a dry atmosphere. Cars for
the shipment of fresh fruits are well ventilated in order that moisture
in excessive amounts will not collect on the fruit, and permit growth of
mold. The same principle applies to the storing of bread, meats,
cereals, flour and many other foods. As in the shipment of fruit or the
keeping of bread, exclusion of surface moisture involves ventilation;
that is, a supply of circulating air to carry away any moisture given
off by the food product.
8. Use of Mild Antiseptics. Food
products may often be preserved a short time by the use of small amounts
of antiseptics. These inhibit, that is, decrease or prevent the activity
of the organisms of spoiling but do not destroy them. For example, meat
may be kept by the addition of salt. If small amounts are used, the
preservation will be temporary; if large quantities are added, the meat
will be permanently preserved. The same applies to butter preserved with
salt. Sugar in amounts less than 65% exerts a temporary preservative
effect. Small amounts of sodium benzoate are used in ketchups, etc., as
a means of temporary preservation. Milk is sometimes illegally preserved
temporarily by the addition of formaldehyde or borax in small amounts.
These are examples of the use of various antiseptics as a means of
temporary preservation.
9. Pasteurization as a Means of Temporary
Preservation. By pasteurization is meant heating a food product to a
temperature which kills most of the organisms present, but does not
destroy all. It also greatly weakens those not killed and retards their
normal development. The most familiar example of this method is in the
pasteurization of milk. The temperature used kills typhoid and
tuberculosis bacteria, but does not destroy certain spore-bearing
bacteria. The milk so treated will usually be free from bacteria capable
of producing serious diseases but will not keep permanently, because the
spores of the resistant bacteria will finally develop and cause
spoiling. Milk pasteurization for market dairy milk is compulsory in
many cities and states. Many other food products are heated to keep them
for a few days; for example, meats, cooked vegetables, jams, etc., are
often so treated by simply heating them in an open pot. This preserves
them for several days. Pasteurization may, then, be taken to mean the
heating of a food product to a sufficiently high temperature to kill
many of the microorganisms, but not all that are present, and results in
temporary preservation only. It is usually carried out at temperatures
below 212° F., the boiling point of water.
The term " pasteurization
" is frequently applied to the sterilization of fruit juices or other
products at temperatures below 212° F. In most of these cases, however,
the products are actually sterilized; that is, all living organisms are
killed. Sterilization thus differs from pasteurization in that
sterilization is complete destruction of all life present and
pasteurization is only destruction of part of the organisms present.
10. Exclusion of Air. The keeping
qualities of some food products are enhanced if air is effectively
excluded. This is true of pickles, such as dill pickles, and green olive
pickles; also of butter, cheese, olive oil, wine, and vinegar. In some
cases the effect is due to the exclusion of organisms, e. g., eggs
sealed with water glass or paraffin; in others, to the exclusion of
oxygen necessary to the growth of molds or bacteria that would destroy
such foods as pickles and wine, and in other cases to the exclusion of
oxygen essential to the deleterious chemical changes that take place in
such food products as olive oil and other vegetable and animal oils and
fats. The popular
idea that air itself causes the spoiling of canned fruits, vegetables,
and various other canned products by its entrance through leaks is
erroneous. This can be proven by sterilizing food products in bottles
plugged with cotton. Air can go in but " germs " are retained on the
cotton. The sterilized product will keep indefinitely under these
conditions. It is the growth of molds, yeasts, and bacteria gaining
entrance with the air that causes spoiling.
B. PERMANENT PREVENTION OF SPOILING
There are several important principles
applied in the permanent preservation of food products. Like methods of
temporary preservation, the process must be adapted to the product and
the existing conditions. In the following paragraphs the most important
principles are discussed.
11. Preservation by Sterilization by
Heat. Sterilization by heat means complete destruction of all forms
of life in the product sterilized. If the sterilized material is to be
kept for any appreciable length of time, sterilization must be
accompanied by sealing the product in air-tight containers. The
exclusion of air is necessary in order that microorganisms shall not
gain entrance to the food.
The temperature necessary for sterilization
will depend almost entirely upon the composition of the food. Foods high
in acid are very easily sterilized; those low in acid are difficult to
sterilize. This rule holds, apparently without exception. Meats, milk,
and vegetables of low acidity, such as peas, corn, pumpkin, and, beans
are exceedingly difficult to sterilize by heat. Acid' products, such as
most fruits and tomatoes, are easily sterilized. On all of these
products are found numbers of spore bearing bacteria of great resistance
to heat; but apparently in the presence of acid they are easily killed
or are not able to develop. Regardless of which theory is true, it
remains a fact that acidity very positively affects sterilizing
temperatures. If products low in acid are acidified with some harmless
acid substance such as lemon juice or vinegar, they will become
relatively easy to sterilize. This principle is made use of in the
"lemon juice method " described later.
Fruit juices and most fruits are readily
sterilized by a few minutes' heating to 165° F.; pickled olives must be
heated a short time at 212° F., the boiling point of water; string beans
require two hours or longer at 212° F.; corn, three hours or longer at
212° F.; and meats, four hours or longer at 212° F. There is thus a
gradation in the length of heating and the temperature necessary for
complete-sterilization.
Sterilization in boiling water at 212° F. is
made more effective if the time of sterilization is broken up into two
or three periods separated by intervals of 24 hours. For example, corn
or meat may be readily sterilized by heating in cans or jars in boiling
water for one hour on each of three successive days. Between the first
and second heating, most of the spores that have survived the first
heating will germinate because of the softening effect of the heat.
These will be very tender and will be easily killed when the second
heating occurs. The third heating will kill all the spores left from the
second heating. This method is known as "intermittent sterilization," or
the "three-day method." It is well adapted to the household
sterilization of meats and certain vegetables. Its application is
described later.
Steam confined in a closed space and heated will reach temperatures
above 212°. The spores of bacteria, in or on products difficult to
sterilize, will be quickly killed if the materials are confined in a
steam retort and heated under steam pressure of several pounds per
square inch. This method is used to the exclusion of all others in
commercial canneries for the sterilization of such materials as peas,
beans, corn, meats, and milk. It requires factory-made equipment, but
simple inexpensive steam pressure sterilizers for home use are
available. The home application of steam pressure sterilization will be
discussed later.
12. Preservation by Use of Antiseptics. Salt, sugar, vinegar
acid, and lactic acid are used commonly as preservatives for foods and
prevent spoiling by their poisonous action upon microorganisms. These
are all harmless antiseptics. Various chemicals are also used as food
preservatives but most of them are considered harmful to health.
Examples are benzoic acid, sodium benzoate, salicylic acid,
formaldehyde, sulphurous acid, and sodium fluoride.
Sugar will prevent spoiling if it is present
to the extent of 65%. Sirups, honey, jellies, candies, and marmalades do
not spoil because they contain enough sugar to prevent molding or
fermentation. Salt
must usually be present to the extent of at least 15% to act as a
permanent preservative. An apparent exception would appear to be butter,
where 5% is sufficient. But in this case, the real preservative effect
of the salt takes, place in the buttermilk and brine in the butter
because the butter fat itself is inert and does not dissolve the salt.
The liquid portion of the butter constitutes only 15% or less of the
butter. Therefore; 5% of salt calculated on the weight of the butter
would give a 30% solution in the liquid part of the butter. The same
principle applies to other salted products. It is the salt in solution
that exercises an antiseptic effect in the preservation of salted meats
and salted vegetables.
Vinegar acid is a more effective antiseptic
than salt or sugar. For most food products 3% acetic acid is sufficient
to preserve them. Vinegar is used in the preservation of many forms of
pickles. Benzoate
of soda is the most common and least harmful chemical preservative used.
It is allowed by pure food laws in quantities up to one-tenth of 1%. It
is used for the preservation of sirups and fruit preserves used in soda
fountains and for the preservation of ketchup.
Sulphurous acid from the fumes of burning
sulphur is allowed in small quantities in food products. Other chemical
preservatives are prohibited by law and are therefore of little interest
to the housewife and farmer. Sulphurous acid from burning sulphur is
used in fruit drying to prevent darkening. Its use for this purpose is
universal and is permitted by pure food laws. It is doubtful, however,
whether the amount used is sufficient to act as a permanent
preservative.
13. Preservation by Drying. Microorganisms that cause spoiling
require a certain minimum amount of moisture for growth. If the moisture
falls below this minimum in a food product the food will not spoil by
molding, fermentation, or putrefaction. This principle is made use of in
the drying of fruits, vegetables, and meats, and making dessicated
liquids, such as dessicated milk, dried coffee extract, etc.
The amount of evaporation necessary will
depend upon the composition of the food. Foods impregnated with salt
need not be dried so much as those not containing salt, because the salt
exerts an antiseptic effect in addition to the preservative effect of
the drying. The same applies to smoked meats. Fruits and vegetables must
be dried to the point where the juice or sap in the dried product
contains more than 65% sugar, or its equivalent in other soluble
compounds. It is actually the high concentration of sugar in these cases
that exerts the preservative action. This point will be reached for
prunes and figs when 23/ pounds have been dried to 1 pound; for
apricots, peaches, and pears, when about 5 or 6 pounds have been dried
to 1; for grapes, about 4 pounds to 1; for beets, about 7 to 1; for
turnips, carrots, and tomatoes, about 14 to 1; and for onions, about 16
to 1. The ratio will depend upon the original water content of the
product dried. This varies with the locality, method of growing, degree
of ripeness, and other conditions.
In practice the dried products are not dried
by determining the loss in weight, but they are dried until the texture
is attained at which experience has shown that the product will keep.
14. Preservation by Smoking. Meats
are smoked to impart an agreeable flavor and to preserve them. The
preservative action is brought about principally by the antiseptic
effect of compounds of a creosotic nature existing in the smoke, but is
also due in part to the drying effect of the heat accompanying the
smoking process.
15. Preservation by Fermentation. Microorganisms usually cause
spoiling, but under certain conditions and with certain food products,
their activity can be utilized as a means of food preservation.
Preservation of foods in this manner may be accomplished by the action
of a number of different microorganisms, which carry on various sorts of
changes in the food product. These changes are designated as
fermentations, the term in- eluding alcoholic fermentation, vinegar
fermentation, and lactic avid fermentation.
Fruit juices may be changed to wines and
hard ciders by yeast fermentation. If air is excluded the fermented
products will not spoil, because of the preservative effect of the
alcohol, and also because the yeast has destroyed the sugar and other
food compounds upon which other organisms might develop.
Vinegar is formed by acetic acid
fermentation of alcoholic liquids. This fermentation is carried on by
vinegar bacteria. The acetic acid formed will preserve the liquid
itself, or fruits, vegetables, and meats stored in the liquid, provided
air is excluded after vinegar fermentation is over. The acetic acid of
the vinegar is the preservative agent.
Lactic acid fermentation occurs in the
manufacture of sauerkraut, fermented string beans, and similar fermented
vegetables. It is carried out by lactic acid bacteria, which form lactic
acid from the sugar of the vegetables. Vegetables so fermented will keep
indefinitely after lactic fermentation is over, provided air is
excluded. Silage owes its keeping qualities largely to lactic acid
formed by lactic acid fermentation in the silo.
16. Exclusion of Air. Certain food
products are spoiled by the action of the oxygen of the air. Oils and
fats are of this type. Such products will not spoil if air is excluded.
Other food products are spoiled by the
combined action of various microorganisms and the air. Wine, eggs, and
vinegar belong to this class. If eggs are sealed with water glass they
will keep for a year or longer. If wine and vinegar are sealed in
completely filled bottles they will keep for scores of years. Therefore,
the simple exclusion of air may be termed a means of permanently
preserving some food products. |
