Characteristics. Milk from healthy, well-nourished cows should be of full white color, opaque, and with a slightly yellowish tinge sometimes described as "cream white." It should vary but slightly in composition from the above analyses. The fat should not be less than 2.5%. The amount of fat may be easily determined with a Feser's lactoscope (Eimer and Amend, New York), directions for the use of which come with the instruments. It will generally vary from 3% to 4% in good milk. Should it fall below 2.5% the milk should be rejected as too poor for use. Such milk has probably been skimmed, or comes from unhealthy or poorly fed cows.
The specific gravity of milk should be from 1.027 to 1.033. This may be found with a Quevenne's lactometer. If it falls below 1.027, one has a right to claim that the milk has been watered or that the cows are in poor condition.[16]
The reaction of good milk varies from slightly alkaline to slightly acid or neutral. That from the same cow will be different on different days, even under the same apparent conditions of care, varying from one to the other, probably because of some difference in the nature of the food she has eaten. However, if the reaction is decidedly alkaline, and red litmus-paper becomes a distinct blue, the milk is not good, and possibly the animal is diseased. Should the reaction be decidedly acid, it shows that the milk has been contaminated, either from the air by long exposure, or from the vessels which held it, with those micro-organisms which by their growth produce an acid, a certain amount of which causes what is known as "souring."
Milk from perfectly healthy and perfectly kept cows is neutral, leaving both red and blue litmus-paper unchanged; but as a general thing milk is slightly acid, even when transported directly from the producer to the consumer and handled by fairly clean workmen in fairly clean vessels. Such milk two or three hours old when examined microscopically is found to contain millions of organisms. Milk is one of the best of foods for bacteria, many of the ordinary forms growing in it with exceeding rapidity under favorable conditions of temperature. Now it has been found that such milk, although it may not contain the seeds of any certain disease, sometimes causes in young children, and the sick, very serious digestive disturbances, and may thus become indirectly the cause of fatal maladies.[17]
All milk, unless it is positively known to be given by healthy, well-nourished animals, and kept in thoroughly cleaned vessels free from contamination, should be sterilized before using. Often the organisms found in milk are of disease-giving nature. In Europe and America many cases of typhoid fever, scarlatina, and diphtheria have been traced to the milk-supply. In fact milk and water are two of the most fruitful food sources of disease. It therefore immediately becomes apparent that, unless these two liquids are above suspicion, they should be sterilized before using. Boiling water for half an hour will render it sterile, but milk would be injured by evaporation and other changes produced in its constituents by such long exposure to so high a degree of heat. A better method, and one which should be adopted by all who understand something of the nature of bacteria, is to expose the milk for a longer time to a lower temperature than that of boiling.
To Sterilize Milk for Immediate Use. (1) Pour the milk into a granite-ware saucepan or a double boiler, raise the temperature to 190° Fahr., and keep it at that point for one hour. (2) As soon as done put it immediately into a pitcher, or other vessel, which has been thoroughly washed, and boiled in a bath of water, and cool quickly by placing in a pan of cold or iced water. A chemist's thermometer, for testing the temperature, may be bought at any pharmacy for a small sum, but if there is not one at hand, heat the milk until a scum forms over the top, and then keep it as nearly as possible at that temperature for one hour. Do not let it boil.
To Sterilize Milk which is not for Immediate Use. Put the milk into flasks or bottles with narrow mouths; plug them with a long stopper of cotton-wool, place the flasks in a wire frame to support them, in a kettle of cold water, heat gradually to 190° Fahr., and keep it at that temperature for one hour. Repeat this the second day, for although all organisms were probably destroyed during the first process, spores which may have escaped will have developed into bacteria. These will be killed by the second heating. Repeat again on the third day to destroy any life that may have escaped the first two.
Spores or resting-cells are the germinal cells from which new bacteria develop, and are capable of surviving a much higher temperature than the bacteria themselves, as well as desiccation and severe cold.[18] Some writers give a lower temperature than 190° Fahr. as safe for sterilization with one hour's exposure, but 190 may be relied upon. Milk treated by the last or "fractional" method of sterilization, as it is called, should keep indefinitely, provided of course the cotton is not disturbed. Cotton-wool or cotton batting in thick masses acts as a strainer for bacteria, and although air will enter, organisms will not.
All persons who buy milk, or in any way control milk-supplies, should consider themselves in duty bound to (1) ascertain by personal investigation the condition in which the cows are kept. If there is any suspicion that they are diseased, a veterinary surgeon should be consulted to decide the case. If they are healthy and well fed, they cannot fail to give good milk, and nothing more is to be done except to see that it is transported in perfectly cleansed and scalded vessels. (2) If it is impossible to obtain milk directly from the producer, and one is obliged to buy that from unknown sources, it should be sterilized the moment it enters the house. There is no other means of being sure that it will not be a bearer of disease. Not all such milk contains disease-producing organisms, but it all may contain them, and there is no safety in its use until all bacteria have been deprived of life.