PROPERTIES OF MILK. 11 



conclusion, although of a very general nature, is nevertheless of 

 great practical importance. 



4. Properties of Milk. Milk, obtained under the usual condi- 

 tions, is a pure white fluid, which appears completely opaque when in 

 large quantities. In thin layers, however, it is slightly transparent. 

 It possesses a slight smell, similar to the exhalation from the skin 

 of the cow, and is of a mild, rich, slightly sweetish taste. It 

 exhibits a slight amphoteric (alkaline and acid) reaction, and can 

 be boiled without coagulating. If left standing undisturbed at the 

 usual temperature, a collection of microscopically minute globules 

 of fat rises to its surface, and forms a layer of cream. When kept 

 standing some time longer, the milk spontaneously coagulates. 

 Previous to coagulation the milk is in such a condition that, 

 although at ordinary temperatures it undergoes no change, yet on 

 boiling, or even on slightly heating it, the milk immediately 

 changes. Even at the ordinary temperature it is coagulated on 

 the addition of a minute quantity of a strong acid, or on the addi- 

 tion of carbonic acid. 



On milk standing at a temperature of over 50 C., a skin is 

 formed, consisting of coagulated albuminous matter, enclosing small 

 quantities of the other milk constituents. As often as this skin is 

 removed it renews itself. It is the formation of this skin on the 

 surface of the milk that causes it when it is boiled to froth over so 

 easily. Boiling imparts to the milk a peculiar taste and smell 

 (cooked taste). 



The chief constituents which milk contains are water, albuminous 

 bodies (proteids), butter-fat, milk-sugar, and mineral salts. Milk 

 has always a greater specific gravity than water. In the case of 

 the milk of single cows, or the milk from single milkings, its specific 

 gravity at 15 C. rarely exceeds the limits of 1*028 and 1*034, and 

 a mixture of the milk of five or more cows, or of two or three 

 milkings, rarely exceeds a specific gravity of 1'029 and 1*033. On 

 an average its specific gravity may be stated at 1*0312. The 

 specific gravity of the total solids of milk varies between 1*30 and 

 1*40, and that of the non-fatty solids is almost always constant, 

 and may be stated with approximate accuracy at 1*6 at 15 C. 



The opacity and colour of milk is due to the numberless fatty globules 

 suspended in it, as well as to a portion of its albuminoids and mineral 

 matter, which are also in a state of suspension. According to Soxhlet, 



