294 



THE CHEMISTRY OF DAIRYING 



white liquid, but it often exhibits a yellow or a bluish tint. A 

 yellow colour is popularly regarded as an indication that the 

 milk is rich in fat, and the dairymen, in towns, generally add 

 annatto to intensify the yellow appearance. As a matter of 

 fact, the yellow colour, if natural, is due to the 

 fat; but, even when very pronounced, it does 

 not necessarily indicate a large percentage of 

 that constituent, nor does the absence of such 

 colour show the contrary. Samples of milk 

 rich in fat are often quite white. 



When the fat is removed, the blue tint is 

 generally more apparent, and the opacity is 

 reduced. Feser's lactoscope (Fig. 22), for the 

 estimation of fat in milk, is based on this 

 principle, but it does not give accurate results. 

 The characteristic odour of fresh, warm 

 milk is also due to the fat. When the milk is 

 cold and fresh, the odour is barely perceptible. 

 The whole of the sugar and part of the 

 mineral matter can be separated from the milk 

 by dialysis. Trapes of nitrogenous matter also 

 diffuse through the membrane. 



The capacity for heat of milk solids is less 

 than that of water. Consequently, those sam- 

 ples of milk which contain the largest per- 

 centages of solids have the lowest capacity for 

 heat. The average, according to Fleischmann, 

 is about 0*847 (water = i). 



Like most other substances, milk expands 

 when heated, and contracts on cooling. The co- 

 efficient of expansion varies with the tempera- 

 ture and the percentage of total solids. Milk 

 does not attain its maximum density at 4 C. 

 From that temperature up to 15 C. it expands more rapidly 

 than water, but at higher temperatures not quite so rapidly. 



The specific gravity of a body varies directly as its density, 

 and the two terms are used, colloquially, as if they were 

 synonymous. Specific gravity is often defined as the mass of a 



FIG. 22. 



Feser's 

 Lactoscope. 



