PHYSICAL PROPERTIES OF MILK 95 



Effect of chemical agents 

 Acids. 



When milk or cream is allowed to stand under conditions that 

 favor bacterial development, the milk-sugar is gradually changed 

 into lactic acid. The increasing acidity due to this change 

 causes a gradual precipitation of the casein, at first in micro- 

 scopic clots . Indue time this finally forms a semi-solid coagulum 

 as is seen in loppered or sour milk. The formation of these 

 clots is accompanied by an increase in the consistency of the 

 milk even before it shows signs of curdling. Although they have 

 the same general effect in the milk, yet the formation of these 

 incipient curd clots has no connection with the normal grouping 

 of the fat globules. This explains undoubtedly the thickening 

 of cream during the ripening, also, in part, the heavier body of 

 gravity cream that is usually much older and more acid than 

 separator cream. 



This same result is reached when acid is artificially introduced 

 into milk or cream, the only difference being that in the addi- 

 tion of commercial acid, the inability thoroughly to distribute 

 it through the milk results in the formation of much larger curd 

 clots. These clots inclose large numbers of fat globules, and 

 have under the microscope a light yellow appearance which 

 distinguishes them from the normal fat aggregation in which 

 the matrix is transparent. Salts like magnesium sulfate that 

 have the power of coagulating casein exert a similar effect. 

 Alkalies. 



When caustic alkalies like sodium or potassium hydrate are 

 added to milk, the consistency is increased. This increase is 

 probably due to the formation of a soluble casein compound 

 that augments the viscosity of the milk and cream, although 

 at the same time the influence of the fat is somewhat diminished, 

 owing to dissolution of the fat aggregations. 



Under the microscope normal milk treated with dilute solu- 

 tions of these alkalies still shows the usual fatty aggregations, 



