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.
10. Cooking temperatures of different starches.
Experiment: Cooking starch.
Pare and grate one or more potatoes. Wash the gratings by placing them in a cheesecloth bag and immersing them in cold water. Squeeze and press the contents of the bag until no more starch seems to pass through the cloth. Let it settle, pour off the water; add clear water and let the starch settle again. Pour off the second water. Take one tablespoonful of the starch, mix it with one cupful of cold water. Heat it slowly over a moderate fire, stirring it constantly, and recording the temperature at which the mixture becomes noticeably clearer and thickens.
Repeat this experiment with corn-starch; wheat starch, washed from wheat flour, as is done with the grated potato; with starch washed from rye flour; and, if desired, with rice, bean, pea, oat and tapioca starches, also.