XV. MILK AND MILK PEODUCTS 315 



with cotton cake commenced. 1 Sesame-oil cake, almond cake and 

 cocoanut-oil cake used as food for cows alter the iodine value and 

 percentage of volatile fatty acids cf the butter fat. 2 Reference to the 

 change in composition of butter fat with advancing lactation has already 

 been made. 



The fat exists in the milk as minute globules of diameters varying 

 from -0016 to '010 millimetie. The number of globules in milk is 

 astonishingly great, being estimated by different observers at from 1'52 

 to 11 '4 millions in the cubic millimetre. The globules vary greatly in 

 size in any particular sample, but certain breeds of cows are remark- 

 able for the preponderance of large-sized or of small-sized globules. 

 It has been suggested that the fat globules are surrounded by an al- 

 buminous membrane, but this theory does not receive much support 

 at present, and the generally accepted view is that the fat is in the 

 form of a true emulsion, each globule being surrounded by a layer 

 of liquid, held in position by surface attraction. 



Rancidity. When butter fat becomes rancid, the chief change is 

 probably the hydrolysis of a portion of the fat into free acids and 

 glycerol, e.g., C 3 H 5 (C 4 H 7 2 ) 3 + 3H 2 O = C 3 H 5 (OH) 3 + 3HC 4 H 7 0. 2 . 

 The glycerol probably oxidises to acrolein, C 3 H 4 O, or acrylic acid, 

 C 3 H 4 (X. The fatty acids remain free, and those which are volatile, 

 e.g., butyric acid, give rise to the odour of rancid butter. Oleic acid 

 and other unsaturated acids are oxidised, yielding substances some of 

 which are soluble in water, and which cause butter which has become 

 rancid to give a brown colouration when dissolved in warm alkali. 



Albuminoids. Much work has been done in connection with the 

 detection and separation of the proteids present in milk, and very 

 different views as to their number and nature are held by various 

 investigators. 



Duclaux affirms that casein is the only proteid present, but that it 

 exists in three forms casein in suspension, colloidal casein and casein 

 in solution. The latter is found in the nitrate when milk is passed 

 through a porous earthenware cell, while the other two are retained. 

 The amount of the soluble casein is about one-eighth of the total pro- 

 teid. The colloidal casein is that found in whey after the rennet has 

 precipitated the suspended casein. He quotes the numbers 3 on the 

 following page as the results of an examination of milk and the whey 

 formed from it. 



Hammarsten (1872-1877) describes two albuminoids casein and 

 albumin ; Halliburton 4 also gives two caseinogen and albumin. 



Eugling and Sebelien 5 in addition found globulin. Danilewsky 

 and Eadenhausen (1880) described at least five proteids as present in 

 milk. The presence of casein or caseinogen, albumin and globulin is 



1 Thorpe, Jour. Chem. Soc., 1900, Abstracts, ii. 237. 



2 Baumert and Falke, Zeitschrift Untersuch. d. Nahrungs- & Genussm., 1898, 



665. 



<Compt. Bend., 98, 438 ; Jour. Chem. Soc., 1884, Abstracts, 762. 

 4 Chemical Physiology. r > Jour. Soc. Chem. Ind., 1886, 387. 



