314 MILK FAT 



is mainly made up of compound and not of simple triglycerides. 

 Saigfeld l has shown that alcohol does not extract tributyrin (easily 

 soluble in alcohol) from butter fat and therefore comes to the same 

 conclusion. Caldwell and Hurtley 2 also failed to find any tributyrin 

 in butter fat. 



All the acids, with the exception of the oleic acid, are saturated 

 compounds of the general formula C n H 2n+1 .COOH. Acids of low 

 molecular weight are liquids, soluble in water and volatile in steam, 

 such as butyric acid, C 3 H 7 COOH, eaproic acid, C 5 H n COOH, and 

 caprylic acid, C 7 H 15 COOH, whilst capric and lauric acids are very 

 slightly soluble and volatile. 



The acids of higher molecular weight are solid, insoluble in water, 

 and non-volatile. 



Milk fat, in addition to the above, contains traces of cholesterol, 

 C. 6 H 43 OH, lecithin, C 3 H 5 .(C 18 H 35 O 2 ),.[HPO 4 .N(CH 3 ) 3 C : ,H 4 (OH)] (not 

 more than O5 per cent of the fat), and a colouring substance of un- 

 known composition, which has been called " lacto-chrome ". Milk fat 

 is a variable mixture of chemical compounds and therefore liable to 

 considerable variation in properties. Moreover, its physical constants, 

 e.g., melting-point, are not sharply defined. It is insoluble in water, 

 though capable of dissolving about ^^ of its weight of water. It is 

 non-volatile at 100, but in contact with air, absorbs oxygen and thus 

 increases in weight ; this, no doubt, is because of the unsaturated fatty 

 acid (oleic acid) present. It melts between 29'5 and 33 C. (Rich- 

 mond) and is therefore liquid in the animal. Its specific gravity 

 varies, but is usually 0*930 at 15, compared with water at the same 



'_>7'8 39 '5 



temperature ; at ^|g (liquid) = 0'9118; at ^-^ = 0'9113. 



QI'O ocTo 



Solid fat is heavier, volume for volume, than the liquid form at the 

 same temperature (Richmond) ; so that, evidently, contraction occurs 

 at the moment of solidification. By very slow cooling of melted 

 butter fat, a partial separation of the various glyceryl salts occurs, the 

 portion solidifying first being characterised by containing less volatile 

 acids and less oleic acid or other unsaturated acids than the portion 

 remaining liquid. 



The index of refraction of milk fat varies from 1*4550 to 1*4586 at 

 35; the heat of combustion of 1 gramme is 9231 '3 calories. 3 It is 

 soluble in hydrocarbons, in ether, carbon disulphide, acetone, nitro- 

 benzene and in warm amyl alcohol. 



The composition of butter fat is liable to considerable variation, 

 being affected by the food, period of lactation, and other conditions 

 affecting the cows. It has been observed that large quantities of 

 cotton cake have a marked effect upon the butter and cause it to be- 

 come harder and whiter, and to give the reactions for cotton-seed oil. 

 This effect has been noticed within twenty-four hours after feeding 



1 Milchw. Zentr., 1910, 6, 122 ; Jour. Chem. Soc., 1910, Abstracts, ii. 327. 



2 Jour. Chem. Soc., 1909, Trans., 853. 



3 Stohmann & Langbein, Jour. Chem. Soc., 1891, Abstracts, 11. 



