8. The Efficiency Of Home-Made Refrigerating Boxes Compared With Other Means Of Keeping Foods Cold

Materials

One box fitted as for fireless cooking, with two or three covered crocks of at least one-half gallon capacity, packed as directed on page 37, with either sawdust, hay, straw, excelsior or paper. Sawdust is specially recommended.

Thermometer

Ice

Notebook and pencil

Fill the central crock with a weighed quantity of ice. Fill one or both of the other crocks with water at room temperature. Cover the crocks and close the box. Record the temperature of the water at the end of six, twelve, twenty-four, and forty-eight hours.

Make repeated observations of the temperatures found in ordinary household refrigerators, cellars, cold storage rooms, and any other places used for keeping foods cold. Compare these with the temperatures obtained with a homemade refrigerating box. Is there any economy in using these boxes?

Bacteriology Of Insulating Boxes. 9. Temperatures Which Kill Disease And Putrefactive Germs, Or Check Their Growth

It is taken for granted that the student of this subject will be more or less familiar with the nature of bacteria and the elements of bacteriology. It will be recalled that bacteria are a vegetable form of life; that, like all plants, they have, under certain conditions, the power of growth which is shown, largely, by their reproduction; and that under other conditions they are killed. When their growth is merely checked, they are in a dormant state, or perhaps form spores, in either of which cases they are ready to develop as soon as their environment permits. Temperature has much to do with the state of bacteria. If the temperature and other conditions are such that they are in an active or growing state, they will multiply with enormous rapidity. When in food stuffs they effect certain changes by reason of the products which they form as a result of their life processes, or of the alteration in the food materials, owing to their abstraction of some chemical elements or compounds used for their nutrition. When bacteria form unpleasant smelling or tasting substances we speak of them as "putrefactive bacteria." Those which, if introduced into the bodies of humans or animals, will cause diseases, are called "disease bacteria." Foods are liable to contain both kinds; and, therefore, it is, obviously, wise to do all that is possible to kill them or prevent their growth. Most forms occurring in foods grow best at from 80 degrees to 98 degrees Fahrenheit. Few bacteria grow at above 100 degrees, and, if kept at 125 degrees, the weaker ones soon die. After subjection to a temperature of 150 degrees to 160 degrees Fahrenheit, for ten minutes, if water is present, almost all kinds are killed unless they are in the spore state. Prolonged boiling will often be resisted by spores. Dry heat is not as effective in killing bacteria as moist, and a higher temperature must, therefore, be reached to effect this end. Below 70 degrees Fahrenheit the growth of bacteria is more and more retarded, but not entirely checked until freezing point is reached. The popular idea that freezing may be relied upon to destroy bacteria is not true.

The bearing of these facts upon the subject of bacteria in foods cooked in insulating boxes is evident. Whether foods are cooked or kept cold, care must be taken that such a temperature is reached that bacteria may not grow.

In application of these principles we see that foods must be heated sufficiently to kill bacteria before it will be safe to subject them to the comparatively low temperature of the cooker for the long period necessary. This is one reason why foods in large pieces, such as roasts of meat, whole vegetables, and moulds containing a mass of food, must be boiled for a considerable time before being put into the cooker. Heat will not penetrate at once to the centre of such foods, and they would be likely to ferment or putrefy unless boiled long enough to heat the centre beyond the point where bacteria thrive. The fact that meats, cereals, and other foods have been known to sour or ferment, even after such boiling, if left in the cooker for a very long time, may be explained by the fact that, though all growing bacteria were killed, spores, which resisted the boiling, might have been present in the food, and when it cooled to a point conducive to the germination of these spores, and remained at this temperature for long, they might have developed, become active, and produced the objectionable changes characteristic of their kind.

In the case of foods to be kept in refrigerating boxes, a temperature considerably below 70 degrees Fahrenheit must be maintained. 50 degrees Fahrenheit, or lower, will be found an excellent preventive of germ growth.

Mr. L. A. Rogers has written a clear and concise description of the nature, growth, and conditions necessary to combat bacteria such as are found in food, in his paper entitled "Bacteria in Milk," published in the Yearbook of the Department of Agriculture, 1907, pages 180 to 196.

Other books which give information on this subject are "Bacteria Yeasts and Molds in the Home," by Conn, and "Household Bacteriology," by S. Maria Elliott.

Yeasts and moulds also may take part in the changes which spoil foods; but the temperature conditions which control bacteria would be practically the same for them.