THE ACTION OF HEAT OX MILK. 191 



wtich do not usually interfere with the analysis. The cream 

 rises and clots on the surface, and it is not easy to obtain an 

 average sample. 



Cold Storage. — Samples may be frozen and kept in a cold 

 chamber, if one is available ; they keep for an indefinite period 

 thus, but require carefully remelting and remixing. This 

 method, which is not always available, is superior to all others, 

 and should be resorted to in those dairies which possess a freezing 

 plant and cold storage room. 



The Action of Heat on Milk. — When milk is heated the following 

 changes occur : — At about 70° C. a change takes place in the 

 albumin ; it is not precipitated, but is converted into a form 

 which is precipitated by acids, magnesium sulphate, and other 

 precipitants of casein. 



At about 80° C. certain organised principles, the nature of 

 which is not fully known, undergo a change. The presence of 

 these principles in an unchanged form is shown by the following 

 reactions ; — They cause an evolution of gas from hydrogen per- 

 oxide in the cold and give a blue colour with para-phenylene- 

 diamine (para-di-amino-benzene), and hydrogen peroxide. Other 

 substances may be substituted for the para-phenylene-diamine, 

 but, according to Leffmann, this substance is the most charac- 

 teristic. Rosier, working in the author's laboratory, has found 

 that meta-phenylene-diamine gives very characteristic results, 

 if a small quantity of amyl alcohol is added to dissolve and 

 concentrate the light blue colouring-matter formed. Saul re- 

 commends " ortol," which is a mixture of quinol with ortho- 

 methyl-amino-phenol. 



Near 100° C. calciimi salts in small amount are deposited, and, 

 by keeping at this temperature for some time, slight oxidation 

 sets in, with the production of traces of formic acid and a marked 

 reduction of the rotatory power of the milk-sugar ; a brown 

 colour is produced at the same time. A deposition of salts, and 

 perhaps, also of albimiLn takes place on the fat globules, which 

 increases their mean density, causing them to rise slowly to the 

 surface, when the tnilk is afterwards cooled ; during the heating 

 the fat globules are expanded and may somewhat coalesce. 



If the surface of the milk is freely exposed to the air, a skin 

 forms at temperatures exceeding 60° C. This has been stated 

 to consist of casein, but has not the properties of this substance ; 

 it is partly of a protein character, and there is some reason to 

 suppose that it is an oxidation product. It contains all the con- 

 stituents of the milk in a concentrated form. The taste and 

 smell of milk are changed by heating to above 70"" C. 



It is not known how far the action of heat on milk aSects its 

 di2;estive qualities. Milk which has been heated is curdled less 



