The preservation of fruit and other foods has been a household industry for generations, and it is now an important commercial industry. The old-time farm had its smoke-house where hams and beef were "cured," the barrel of brine stood in the cellar for pork and corned beef, apples and corn were dried for winter use, and rows of preserve jars stood upon the shelves. Food was preserved by simple processes long before the reason for the decay and spoiling of food was fully understood, but with larger knowledge and better appliances, we now preserve food more effectively and in quantities larger than were possible in former days.
Fruit is the food material now most commonly preserved in the home kitchen. Vegetables need to be subjected to heat for a much longer time than fruit, and many people prefer to buy canned vegetables rather than to go to the trouble and expense of canning them at home. Where there is an oversupply of vegetables in the home garden, it is sometimes economy to can them, and this may be done if care is exercised. The cost of fuel and labor must be counted in, when studying the question of home preserving versus buying the canned product.
Whatever the food material, and the process, the principles of preservation are the same for all.
Why does food spoil? - The decay and moldiness of fresh fruit are matters of common observation; and the housekeeper knows that mold is liable to cover the top of a jelly glass, and that a can of fruit will ferment at times, even to the point of bursting the can.
We recognize another kind of deterioration in meat and fish that have become tainted, even when no mold is visible, and there is no opportunity for ordinary fermentation. The microscope has given us eyes to see, and as a result of the patient work of the scientist with this instrument we now know that the difficulties in keeping food are caused by the presence of minute vegetable organisms known as molds, yeasts, and bacteria. It is impossible in some cases to draw a sharp line between these different forms of lower life, yet we are able to distinguish them sufficiently for practical purposes.
Fig. 28. - Three species of mold.
Masses of mold that can be seen with the naked eye are distinguished by a feathery appearance and bright color. Figure 28 shows three species of the green mold that affects jam and jellies. Other species are found in Roquefort and Camembert cheese, and give the flavors characteristic in these cheeses.
The presence of yeast can be detected by its action, but it cannot itself be seen without the microscope. When canned fruit or homemade fruit juice "works," yeasts cells are present in great number. Figure 29 shows one form of yeast, highly magnified, and Fig. 30 shows a single yeast cell. The yeast cake is a mixture of thousands of such cells with some flour or flour and meal, and the cells lie dormant in the cake, until we are ready to use them in bread. (See Chapter XII (Yeast Bread).) The actual yeast, however, is what Fig.
Fig. 29. - One form of yeast.
30 shows it to be, a tiny, one-celled plant, increasing in number by the division of the single cell, or by the budding out of one cell from another. When conditions are favorable the yeast cells increase in number with great rapidity, and some of the sugar that is present is broken down into carbon dioxide gas and alcohol. It is this gas that causes the familiar bubbling when fermentation is taking place. We put yeast cells into bread and cultivate it for this gas. But how does it occur in canned fruits, when its presence is not desired? Wild yeast floats in the air, and lies upon the surface of fruit. All cultivated yeast has been derived from wild yeast. In old-fashioned ways of bread-making no yeast was introduced, a, cell wall. b, vacuole. c, granules. d, nucleus. e and e, buds.
Fig. 30. - A yeast cell.
a soft dough being left in a warm place to ferment naturally, the yeast cells probably being present in the flour. The yeast that spoils the canned fruit is present in the fruit, in the utensils, or can, and has not been killed as it should be in the canning process.
The bacteria are also one-celled microorganisms, smaller than the yeast. Figure 31 shows the four types of bacterial cells. Their size is measured by the unit used in the microscope, called the micron, which is about 1/25000 of one inch. Bacteria may measure from one to three or four of these microns in length. Some bacteria are reproduced by means of spores which form within the cell. Bacteria, as they develop in some material, produce substances from the material that may or may not be injurious to us. One important truth about the bacteria is this : that many of them are harmless, and may even be made useful, as in the manufacture of fruit vinegar. The pleasant acid of buttermilk and of sour milk is due also to bacteria which are not harmful to us. However, there may be disease producing bacteria present in milk that is not clean, and their presence must not be tolerated. Other bacteria, developing in meat and fish, produce substances known as ptomaines, which are dangerous poisons; or, more often, the kinds of bacteria which thrive in meats and fish may themselves be directly injurious to man.
Fig. 31. - The four types of bacterial cells. A, cocci. B, bacilli. C, spirilla. D, branched filamentous organism.
It is evident, therefore, that the problem before us is the control of these lower organisms, that we may increase or destroy them as we will.