COMPOSITION AND PROPERTIES OF BUTTER 535 



selves with succulent pasture grass, the per cent olein is also 

 high, often amounting to about 50 per cent of the total fat, the 

 melting point of the mixed fat is relatively low and the butter 

 made from this butterfat is relatively soft. 



Physical Structure of Butterfat. In freshly drawn milk and 

 cream the butterfat consists of miscroscopic, liquid fat globules. 

 These fat globules are present in the form of a fairly permanent 

 emulsion in the skim milk which consists of water in which 

 are dissolved the milk sugar, albumen and part of the milk ash, 

 and which contains in suspension the casein. The casein is of 

 colloid nature and the skim milk may logically be considered 

 an emulsion of hydrated colloid. Milk and cream, then, are a 

 fat-in-hydrated colloid emulsion, or a fat-in-skimmilk emulsion. 



The fact that the butterfat globules remain as independent 

 units, and that they form this emulsion, is due to the fact that 

 nature produces them in this fine state of division in the first place. 

 Fisher and Hooker very interestingly show that the secretion 

 of butterfat is the result of fatty degeneration of the cells in the 

 alveoli of the mammary gland or udder. In this fatty degenera- 

 tion the cells break down, liberating the minute fat globules in 

 a fat-in-hydrated colloid emulsion, in which they retain their 

 individuality because of the forces of surface tension, adsorption 

 and viscosity, as explained under "Philosophy of Churning," 

 Chapter X. 



Size of Fat Globules. As previously stated the butterfat, 

 or milk fat, is present in milk and cream in the form of very 

 minute fat globules. These fat globules vary in size from 

 about one micromillimeter to about 17.4 micromillimeters ; they 

 average about from three to five micromillimeters in diameter. 

 One micromillimeter, or one micron represents about one twenty- 

 five tousandth of one inch. 



The size of the fat globules is controlled by breed and 

 period of lactation of the cows, and it is influenced by temporary 

 indisposition of the cows and abrupt changes in feed. 



The Channel Island breeds, the Jerseys and Guernseys, 

 produce milk in which the fat globules average nearly three 

 times as large in diameter as those in the milk from the Hoi- 

 steins and Ayrshires. 



At the beginning of the period of lactation the fat globules 

 are largest. As the period of lactation advances the average 



