38. To pasteurize milk in the home, proceed in the manner illustrated in Fig. 3. Place the rack or invert the perforated pie tin in the bottom of the vessel, and on it place the bottles of milk from which the caps have not been removed. Make a hole through the cap of one bottle, and insert the thermometer into the milk through this hole. Then fill the vessel with cold water to within an inch or so of the top of the bottles, taking care not to put in so much water as to make the bottles float. Place the vessel over the fire, heat it until the thermometer in the bottle registers a few degrees over 150 degrees Fahrenheit, and keep the milk at this temperature for 15 to 20 minutes. At the end of this time, the milk will be sufficiently pasteurized and may be removed from the fire. As soon as it is taken from the water, cool it as rapidly as possible by running cold water into the vessel slowly or by placing the bottles in several changes of water, taking care not to place the hot bottles in very cold water at first, as this may cause them to crack.
When the milk has been cooled by some rapid method, keep it cool until it is used. This precaution is necessary because of the nature of pasteurized milk. The temperature at which milk is pasteurized is sufficient to kill all fully developed bacteria, but those which exist in an undeveloped state, or in the form of spores, develop very rapidly after pasteurization unless the milk is kept cold and clean. If these bacteria were allowed to develop, the purpose of pasteurization would be lost, and the milk would become as dangerous as it was originally. The advantage of cooling milk rapidly will be fully appreciated upon referring to Fig. 4, which illustrates the development of a single germ in milk that is cooled rapidly and in milk that is cooled slowly.
39. STERILIZED MILK.--By sterilized milk is meant milk in which all germs are destroyed by sterilization. Such milk is not sold by dealers, but the process of sterilization is resorted to in the home when pasteurization is not sufficient to render milk safe. This process, which is the only positive means of destroying all germs, consists in bringing the milk to the boiling point, or 212 degrees Fahrenheit, allowing it to boil for three quarters of an hour, and then cooling it rapidly. One who undertakes to treat milk in this way should remember that it is difficult to boil milk, because the solids in the milk adhere to the bottom and sides of the vessel and soon burn. However, this difficulty can be overcome by sterilizing the milk in the bottles in which it is bought.
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.
PRESERVED MILK
43. Besides milk that is commonly sold by dairymen and milk dealers, it is possible to buy in the market many grades of so-called PRESERVED MILK. Such milk is produced by driving off all or part of the water contained in milk, and it is sold as condensed, evaporated, and powdered milk. Usually, it is put up in tin cans, and while it is not used so extensively as regular milk, many firms are engaged in its preparation.