THE PHYSICAL PROPERTIES OF MILK 55 



However, in many localities, creaming is done by the "deep- 

 setting system," which seems to overthrow the idea of creaming 

 in shallow pans and at high temperature. In the deep-setting 

 system the milk is stored in cans about 20 inches deep and kept 

 at a temperature of 40° to 55° F. While in shallow pans of 2 to 

 4 inches deep the skimmed milk rarely contains less than 0.5 per 

 cent, fat, the skimmed milk in the deep cans retains but 0.2 per 

 cent. It is difficult to explain why milk at low temperature, with 

 increased viscosity and with a greater volume for the fat globules 

 to traverse, should yield a richer cream than milk kept in shallow 

 pans at higher temperature. Possibly an explanation is found 

 in the fact that the rising globules come in contact with a larger 

 number of globules in deep layers than in shallow layers, thus 

 facihtating the formation of clusters. 



5. The Viscosity of Milk.- — If two pipets of equal size and cali- 

 ber are filled — the one with water and the other with milk — it 

 turns out that it takes a greater length of time to discharge the 

 milk than the water. This is due to the viscosity and adhesion 

 of the milk. Viscosity depends upon the milk solids, especially 

 the casein. The greater the quantity of solids and the lower the 

 temperature, the more pronounced is the viscosity. The viscosity 

 is an obstacle to the rising of fat. Dilution of milk reduces the 

 viscosity and creaming proceeds more readily in diluted than in 

 undiluted milk. Butter makers have discovered this fact and 

 frequently take advantage of it. 



6. The Size of the Fat Globules. — One of the most important 

 factors which influence creaming is the size of the fat globules, 

 since large globules naturally rise more rapidly than small ones. 

 Large globules, therefore, rise first and crowd to the surface, 

 while small ones rise more slowly and remain in the deeper laj'-ers 

 of the cream. The smallest globules do not rise at all and remain 

 in the skimmed milk. Milk from some breeds of cows has globules 

 of larger average size than milk from others. Jersey milk, for 

 example, creams more readily than Holstein milk because of the 

 difference in the size of the globules. Furthermore, the milk 

 derived from a cov/ in early stages of lactation creams better than 

 that drawn during later stages, due to the gradually decreasing 

 size of the globules as lactation progresses. Fat globules are 

 relatively large in colostrum milk, but diminish when normal 

 milk is secreted. This diminution goes on progressively to the 

 close of the lactation period. At this time the fat globules are so 

 §mall that creaming is slow and butter making becomes difficult. 



It has been previously stated that the last milk from the 

 udder — the strippings — contains more fat than other portions of 

 the milk and that the fat globules are larger than during earher 



