The first milk given after parturition is called colostrum. It differs in composition from normal milk, and is fit only for food for the newly born calf.
' The only precise method bf determining the quality of milk is by a complete gravimetric analysis, but for practical purposes the knowl edge of the specific gravity of milk and its fat content is usually sufficient. The specific grav ity can be ascertained quickly by the use of a lactometer, and the fat content can be quite accnrately determined by the use of-the Babcock test. In making this test, a definite amount of milk is mixed with sulphuric acid in a special bottle having a graduated neck. This is whirled rapidly in a machine and the fat is separated from the rest of the mixture by centrifugal force. By the addition of water heated to about 130° F. the fat is brought up into the neck of the bottle, where its exact percentage tan be read. Simple tests have been devised for de termining the acidity of milk, and whether' or not it has been heated.
Milk is a very delicate product and is sub j i ect to taints and changes in great number. It readily absorbs strong odors with which it comes in contact, and objectionable flavors are easily imparted to it when the cows are allowed to eat certain foods within a few hours before milking. The flavors caused by garlic and rag weed are well known; rape and green rye are mid to produce bad flavors in the milk when they are fed in certain conditions. Flavors due to the causes named are most noticeable when the milk is first drawn from the udder, and may be largely reduced by aeration or the exposure of milk to the air.
When held, milk undergoes many kinds of changes. The most common is souring, but it may become slimy or *ropy,* curdle without souring, become asoapy,s turn to a red, blue or other color, develop peculiar and disagreeable odors, form poisonous products, or become altered in other ways. Most of these changes are directly due to bacteria, but some are caused by enzymes or unorganized ferments.
Bacteria find in milk an exceptionally favor able medium for growth. They enter milk in so many ways that it is a practical impossibility to produce milk which is germ-free. But the
character and number of the bacteria that get into milk and their development are largely controllable and have much to do with the suc cess or failure of all operations with milk and its products. Many species of bacteria found in milk are perfectly harmless. Some are essential to certain dairy operations, for example, when it is desired to ripen cream to make butter, lactic acid bacteria are needed; they produce lactic acid from milk sugar and cause the milk or cream to sour. Other species have their peculiar effects in the ripening processes of the various forms of cheese. Numerous varieties of bacteria are objectionable and some are dan gerous. The latter include pathogenic bacteria and other forms that may be the direct or in direct cause of disease to the consumer. Well authenticated cases are on record of typhoid fever, diphtheria and other disease germs being carried by milk, and there seems to be proof that tuberculosis is sometimes spread in this manner.
Bacteria are introduced into milk from many sources. As secreted, milk is probably sterile, but it becomes contaminated in the teats of the cow, so that milk as it leaves the udder always has a few bacteria. In some cows this number may run into the hundreds of thousands per cubic centimeter, although more often the count is in the hundreds. After leaving the udder it is subject to external contaminations, the most serious of which from the standpoint of num bers is unsterilized utensils. The next im portant source of contamination is manure. Stable air ordinarily plays little or no part in introducing large numbers of bacteria into milk. It is possible for the average dairyman on the average farm to produce milk free from visible dirt, which when fresh has a low bacterial count, by the use of three simple factors; namely, sterilized utensils, clean cows with clean udders and teats and the small-top pail. To facilitate keeping the cow clean the stable should be kept clean and the cows bedded. It is desirable that the udders and teats be washed before each milking.