458 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



The reaction of milk to litmus is amphoteric, but it is generally 

 alkaline to red HtmXis paper. 



The Effect of Heat. No coagulation of proteins occurs on boiling 

 milk, but a scum forms on hot milk on standing. This is due to 

 evaporation on the surface, a layer of the dried constituents forming 

 there and separating out It forms each time milk is boiled and allowed 

 to cool. The coagulated proteins will be present in the first scum. 

 . Differentiation between Fresh and Boiled Milk. 



Fresh milk gives the Guaiac reaction. Some tincture of guaiacum 

 is added to a little fresh milk and a little hydrogen peroxide. On 

 mixing and allowing to stand, it gradually becomes blue owing to the 

 oxidation of the guaiacol in the tincture by an oxidising enzyme, 

 or oxidase in the milk. Instead of hydrogen peroxide, old oil of 

 turpentine, which contains a peroxide, may be used. The reaction 

 generally succeeds better with this than with the hydrogen peroxide. 



Boiled milk does not give this reaction, as the oxidase is destroyed 

 by heating. 



Separation of the Fat. 



The fat of milk gradually rises to the surface on standing, forming 

 cream, but it is generally separated by centrifugalisation. The fat may 

 also be extracted by shaking some milk with twice its volume of ether. 

 The ether deposits the fat on evaporation. After removal of the fat 

 by ether the opacity of milk is hardly altered and is due to the opal- 

 escence of the calcium salt of caseinogen. 



If milk be treated with a little caustic soda and shaken with ether, 

 the aqueous solution is translucent as the sodium salt of caseinogen is 

 formed, which does not give an opalescent solution. 



Butter. 



Butter is obtained from cream by churning. It consists of the 

 triglycerides of palmitic, stearic and oleic acids like other fats, but 

 contains in addition the triglycerides of butyric and caproic acids 

 (tributyrin, tricaproin). Recent work by Caldwell and Hurtley points 

 to the absence of tributyrin in butter. When it becomes rancid by 

 the action of bacteria which contain the enzyme lipase the smell of 

 butyric acid and caproic acid is noticeable. The presence of these 

 lower volatile fatty acids constitutes the chief difference between 

 butter and ordinary fat. The butyric acid probably arises from the 

 glutamic acid of the caseinogen which is converted by bacteria into 

 butyric acid. Butter usually contains some milk and therefore also 

 caseinogen ; hence the origin of the butyric acid. 



Margarine is prepared from animal fat, or from vegetable fats. 

 These are generally oils, so that they are thickened. This is effected by 

 hydrogenisation, i.e. reducing unsaturated fatty acid to the saturated. 

 Butter is frequently added to margarine. 



