390 THE ADaLTERATION OP MILK 



curately used at once, but the milk should be held for about 

 two or three hours to permit it to become of normal weight per 

 volume. 



Influence of Preservatives. — To keep milk test-samples from 

 souring various chemicals are added to kill the bacteria and 

 thereby preserve the sample. Corrosive sublimate or mercuric 

 chloride is such heavy stuff that a single small tablet in one 

 quart of milk will raise the lactometer reading higher than 

 possible for even skim milk. A preservative such as formalde- 

 hyde is so nearly of the same weight as milk that it is prefer- 

 able where specific gravity determinations are to be made. 



The Use of the Lactometer (Fig. 125). — It has been found 

 that the quantity of solids-not-fat is very nearly one-fourth the 

 variable portion of the specific gravity figures. For example, 

 the specific gravity of milk at 60 degrees varies from 1.029 

 with a naturally watery milk, to as high as 1.033 with a milk of 

 a medium-testing Jersey, the average being about 1.032. It 

 will be noted that the 1.0 portion of the figures is constant, 

 while the 29-33 varies. Milk carries from 8 to 8.6 per cent 

 solids-not-fat. The percentage amount of the non-fatty portion 

 of milk is then but little more than one-fourth these variable 

 figures. These figures, called lactometer reading, increase, 

 therefore, four times as rapidly as the per cent of non-fatty 

 solids. The fat in the milk also may b& determined almost 

 wholly by physical means, the Babcock test, rather than by true 

 chemical anlysis. 



Since fat has a specific gravity of 0.90 and skim milk or 

 milk serum a specific gravity of 1.036 it follows that the less 

 fat any given milk contains the heavier it becomes, and vice 

 versa. Also since water weighs only 1.000 against milk 1.032, 

 it follows that any addition of water to milk will lower its 

 weight or specific gravity. Therefore, by combining the two, 

 skim milk and water, it is possible to dilute a milk and still 

 retain its natural specific gravity. 



For this reason it is occasionally necessary to combine the 

 use of fat test and the lactometer in order to detect adultera- 

 tion. Thus, if the lactometer reading is higher than normal and 



