As a rule the flavor and odor of milk are not changed by heat until the temperature reaches nearly to the boiling point. A scum then forms on boiling milk, composed of casein 50.86%, fatty matter 45.42%, ash 4.72% (Rosenau). Prolonged boiling changes the color of milk from a creamy white to a yellowish brown which deepens with boiling. This is due to the caramelization of the milk sugar. Cream will not rise (or its rise will be very slow) on milk which has been subjected to a temperature of 150 deg. F. for thirty minutes or more because the fat droplets are broken down so that they cannot hold together at that temperature and become more completely distributed throughout the fluid.[32] ~Pasteurization~ is acknowledged to be preferable to sterilization in milk used for infant feeding because the higher the temperature the greater the change in the chemical composition of the fluid. According to Morse and Talbot[33] the temperature of the pasteurization should be as low as possible. Pasteurization at 140 deg. F. for 20 minutes is sufficient; lower temperatures are not. "At this temperature there is no change in the taste, odor, or color of the milk, no noteworthy changes in the chemical composition are produced, the ferments and bactericidal action are unaffected and bacterial toxins and non-spore-bearing microorganisms are destroyed."[34] Rosenau[35] states that the bacillus of typhoid, diphtheria, and dysentery, as well as the cholera vibrio and other pathogenic non-spore-bearing bacteria which are often found in milk, are destroyed at a temperature of 140 deg. F. for twenty minutes, and at higher temperatures for shorter lengths of time. Sommerfield's[36] investigations prove that butyric acid bacilli are destroyed at a temperature of 212 deg. F. for from 1 to 2 minutes. It must be understood that no matter what method is used to insure purity in milk, nothing does away with the necessity for keeping the milk both clean and cold. The receptacles in which the milk is allowed to stand, the vessels in which it is measured, and the person who handles it must be absolutely clean, and the nurse must keep in mind the fact that pasteurization does not completely destroy the bacterial growth in milk, that it merely diminishes it, and she must see that the milk which has undergone the pasteurizing process is kept cold, otherwise the microoerganisms which are present, even if to a less extent than in raw milk, will undoubtedly multiply.