CHEMISTRY OF COWS MILK 5 



globules. Skim-milk contains fewer and smaller glo- 

 bules than whole milk, while the reverse is true of 

 cream. The large globules do not differ in composi- 

 tion from the small ones. The size and number of 

 fat-globules in milk are influenced by such conditions 

 as advance of lactation, breed of cow, food, age, 

 health, different milkings, different parts of the same 

 milking, etc. 



It was formerly believed generally, and is still by 

 some, that the fat-globules of milk are surrounded by 

 a membranous covering, or else by a semi-liquid, al- 

 buminous layer. We may, however, accept it as es- 

 tablished beyond reasonable doubt that the fat-globules 

 of milk have no special covering of any kind, but are 

 simply minute particles of fat floating free in milk in 

 the form of an emulsion. Fat-globules quite generally 

 retain their individuality even in butter and cheese. 



Amount of fat in milk. Normal milk varies greatly 

 in its fat content, containing from below 2 to over 10 

 per cent., if we consider single milkings of individual 

 cows. The milk from herds of cows varies in fat 

 more commonly between the limits of 3 and 5 per 

 cent. The average amount of milk-fat in milk pro- 

 duced in this country, taking the true average for the 

 entire year, lies somewhere near 4 per cent., perhaps 

 a little under. Many of the conditions that affect the 

 percentage of fat in milk are fairly well known, while 

 others are little understood. We will briefly consider 

 some of the well-recognized conditions that influence 

 the fat content of milk. 



(i) Influence of individuality of cozv on fat con- 

 tent of milk. It is uncommon to find in a herd of 



