40. To sterilize milk, place the sealed bottles on a wire rack or a perforated pie tin in a deep vessel, as for the pasteurizing of milk, and pour cold water into the vessel until it nearly covers the bottles. Then raise the temperature of the water quickly to the boiling point, and after it has begun to bubble, allow it to boil for three quarters of an hour. At the end of this time, cool the milk rapidly and then keep it cool until it is used.
41. Although milk thus treated becomes safe, sterilization changes its flavor and digestibility. If milk of this kind must be used, some raw food should be given with it. A diet composed entirely of cooked food is not so ideal as one in which some raw food is included, because raw foods contain substances that are essential to health. The change that takes place in the composition of milk that has been sterilized can be easily observed. Such milk on becoming sour does not coagulate as does pasteurized or raw milk, owing to the fact that the lime salts in the milk are so changed by the high temperature as to prevent the thickening process from taking place. Then, too, sterilized milk is not likely to become sour even after considerable time. Still, such milk is not safe to use except when it is fresh, for instead of fermenting in the usual way it putrefies and is liable to cause such a dangerous sickness as ptomaine poisoning.
42. MODIFIED MILK.--For infants who cannot be fed their normal diet, cow's milk must be used as a substitute, but in order to make it a more nearly ideal food for them it must usually be modified, or changed, by adding other materials. When it is so treated, it is known as modified milk. The materials used to modify milk are sterile water, lime water, barley water, cream, skim milk, milk sugar, or some other easily digested carbohydrate, one of these or a combination of them always being employed. The proportion of these ingredients to use varies with the age of the child that is to be fed and must be constantly changed to meet the child's requirements. In the production of modified milk, a physician's prescription and directions should always be followed closely. Only the best quality of milk should be used, and, in addition, the greatest care should be taken to have all the bottles, utensils, and materials used as clean and sterile as it is possible to make them. If such conditions cannot be met, it is advisable to pasteurize the modified-milk mixture after the materials have been put together.