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Home and Farm Food Preservation
Chapter II - Ways of Preventing Spoiling

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.


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.

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