MILK. 449 



Cheese is made either from pure milk, from skimmed milk, or from 

 a mixture of milk and cream, and accordingly varies considerably in 

 composition. Practically, cheese is made by causing milk to coagulate 

 (either by allowing it to stand or by the addition of rennet, acids, or 

 other substances), and separating the curd (casein and fat) from the 

 whey by mechanical means, such as filtering and pressing. The curd 

 is placed in suitable moulds and afterward allowed to stand or " ripen " 

 for a shorter or longer period. The process of ripening is a partial 

 decomposition (decay and putrefaction) of the casein, and the value of 

 cheese depends mainly upon the nature of the products formed during 

 this decomposition. 



Adulterations of milk. Of these, the most commonly practised 

 are removal of cream, addition of water, or both. Sometimes sodium 

 carbonate, sugar, and even chalk are added, but these latter adultera- 

 tions are fortunately but rarely practised by milk-dealers. The ques- 

 tion whether or not milk has been tampered with is generally decided 

 by ascertaining whether cream has been removed or water added. It 

 is, therefore, chiefly the quantity of total solids which has to be de- 

 termined in order to decide the purity of milk. But it has been shown 

 by the above tables of milk analyses that the quantity of these solids 

 varies considerably, and a minimum of total solids should, therefore, 

 be adopted legally. While no such minimum quantity is officially 

 recognized in many States of this country, it is safe to say that milk 

 containing less than 1 1 per cent, of total solids may be looked upon 

 as adulterated. (The above given lowest quantity of 9.8 per cent, 

 of total solids in cow's milk is very abnormal.) The methods for 

 detecting such fraud will now be considered. 



Testing- milk. There is, unfortunately, no instrument which will 

 indicate the purity or quality of milk directly. An instrument used 

 for that purpose, and known as the lactometer, is simply a hydrometer 

 which indicates the specific gravity of milk. There are, however, in 

 milk substances which have a tendency to increase the specific gravity, 

 such as lactose, salts, and casein, whilst there is at the same time one 

 substance, the fat, which is specifically lighter than water. The 

 specific gravity of milk ranges from 1.027 to 1.034, the average being 

 about 1.030. If water be added to milk, the specific gravity will 

 become lower, but the same effect may be obtained by adding fat or 

 cream. Again, if cream be removed, the specific gravity will be 

 higher, and in order to bring the milk back to the standard of 1.030, 



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