792 VII. LIPID DISTRIBUTION IN SPECIFIC TISSUES 



proportion of these acids may be related to variations in species or to die- 

 tary differences. Hydroxy palmitic and cerotic acids are two other un- 

 usual fatty acids present in milk fats. 468469 As much as 0.5% of elaeo- 

 stearic acid has been reported in the milk fat of cows after the feeding of 

 tungoil. 470 



b. Phospholipids in Milk Fat. Phospholipids occur in milk fat to the 

 extent of 0.03 to 0.1%. Apparently there is no relationship between the 

 phospholipid content and the proportion of fat in the milk. Although 

 Bloor 49 considers that the phospholipids are adventitious products in this 

 fluid, it is possible that they may function in promoting emulsification of 

 the fat, since they combine with protein, to form a protective covering 

 around the fat particles in cream. 471 Koch 472 reported values of 0.036 

 to 0.049%, for lecithin, and 0.072 to 0.086% for cephalin in cow's milk. 

 Chapman 473 published figures for total phospholipid as follows: whole 

 milk, 0.045%; cream, 0.20%; skim milk, 0.016%; and buttermilk, 

 0.13%. According to Hess and Helman, 474 the phospholipid content of 

 cow's milk is twice that of human milk, while the total amount of phos- 

 phorus in cow's milk is four times the level obtained in human milk, and 

 that in goat's milk is six times that of human milk. The lecithin and 

 cephalin fatty acids of cow's milk are stearic, palmitic, and oleic acids. 475 

 None of the lower fatty acids are present. 476477 



c. Cholesterol in Milk Fat. Cholesterol is known to be an invariable 

 component of milk from various species of animals. In 1913, Meigs and 

 Marsh 478 recorded an average value of 0.021% for the cholesterol content of 

 whole cow's milk. However, reports of various workers since that time 

 have consistently given lower values. The results of a number of workers 

 are summarized in Table 25. 



In the extensive investigations of the Keys' group, 479 it was found that 

 the average cholesterol content of milk was 11.4 ± 1.50 milligram per cent 

 in the winter and approximately the same in the summer (11.3 ± 1.56 



«» A. W. Bosworth and G. E. Helz, /. Biol. Chem., 112, 489-492 (1936). 



469 G. E. Helz and A. W. Bosworth, J. Biol. Chem., 116, 203-208 (1936). 



470 J. Houston, A. G. Cotton, and S. K. Kon, Biochem. J., 33, 1626-1629 (1939). 



471 L. S. Palmer and H. F. Wiese, J. Dairy Sci., 16, 41-57 (1933). 



472 W. Koch, Z. physiol. Chem., 47, 327-330 (1906). 



473 O. W. Chapman, J. Dairy Sci., 11, 429-435 (1928). 



474 A. F. Hess and F. D. Helman, /. Biol. Chem., 64, 781-796 (1925). 



476 W. Diemair, B. Bleyer, and M. Ott, Biochem. Z., 272, 119-132 (1934). 



476 F. E. Kurtz, G. S. Jamieson, and G. E. Holm, /. Biol. Chem., 106, 717-724 (1934). 



477 T. P. Hilditch and L. Maddison, Biochem. J., 35, 24-30 (1941). 



478 E. B. Meigs and H. L. Marsh, /. Biol. Chem., 16, 147-168 (1913). 



479 B. Nataf, O. Mickelsen, A. Keys, and W. E. Petersen, /. Nutrition, 36, 495-506 

 (1948). 



