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._
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.
“Food and the Principles of Dietetics,” by Hutchison, gives, on page 378, a list of different starches and the temperatures at which they gelatinize.
In a bulletin entitled “Digestibility of Starch of Different Sorts as Affected by Cooking,” by Edna D. Day, Ph.D. (U. S. Dept, of Agriculture, Office of Experiment Stations, Bulletin No. 202, page 40), we read that starch takes up water at 60 degrees to 80 degrees Centigrade (140 degrees to 176 degrees Fahrenheit) and forms a sticky, colloidal substance known as starch paste, in which form it is very easily digested. Long boiling, at least to the extent of three hours, does not make it more quickly digestible.
There is something to be considered besides the mere starch in cooking starchy foods, and the fact that potato starch will form paste at 149 degrees while rice starch requires 176 degrees does not mean that less cooking will be needed for potatoes than for rice. The woody fibre or other constituents of foods, as well as their density and difference in size, must be taken into account.