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Home and Farm Food Preservation
Chapter 1 - Why Food Spoils


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 food products.

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, elevators, etc.

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 temperature.


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