No. 6. 



DEPARTMENT OF AGRICULTURE. 



655 



91. The Use of Lactometers iu Testing Milk. 



(1.) The Specific Gravity of Milk. — By ihe specific gravity of milk, 

 we mean the weight of a given bulk of milk as compared with the 

 weight of an equal bulk of water at the same temperature. For 

 illustration, suppose we have a vat which, when just full of water, 

 contains exactly 1,000 pounds of water. Now, if we fill such a vat 

 full of milk, this amount of milk will weigh about 1,032 pounds, be- 

 cause the milk contains beside the water iu it several solid sub- 

 stances heavier than water. Hence, we say the specific gravity of 

 average milk is 1.032. Since the specific gravity of milk depends 

 upon the amount of these solids in it heavier than water, then 

 specific gravity will be found to vary, because we know that the 

 amount of solids in milk varies considerably. So, we find some milk 

 with specific gravity below 1.030, while that of other milk is above 

 J. 035. The casein, albumin and milk-sugar are heavier than water. 

 Since milk-fat is lighter than water, the more milk-fat we have 

 in milk in proportion to the other solids, the lower is its specific 

 gravity. By adding cream to milk, we make its specific gravity 

 less than that of normal milk; on the other hand, by removing fat 

 from milk, we increase the specific gravity, because we remove what 

 it lighter and leave what is heavier than water. The addition of 

 water to normal milk lowers the specific gravity. Thus, it is easily 

 possible by removing cream from normal milk to increase the specific 

 gravity and then, by adding water in right amounts 

 lower the specific gravity back to that of the norraal 

 milk. The addition of sugar or salt to milk increases 

 its specific gravity. Since water used to be the most 

 common adulterant of milk, it was thought that adul- 

 teration could readily be detected by ascertaining the 

 specific gravity. 



(2.) Quevenne Lactometer. — A lactometer is an in- 

 strument used for measuring the specific gravity of 

 milk. The Quevenne lactometer has a scale divided 

 into 25 equal parts, going from 15 to 40. Each divi- 

 sion is called a degree. These divisions correspond to 

 those on an ordinary hydrometer, ranging from 1.015 

 to 1.040. The Quevenne lactometer is graduated so as 

 to give correct readings at 60 degrees F. For other 

 temperatures the reading must be corrected by adding 

 .1 for each degree above GO degrees F., or by substract- 

 ing .1 for each degree below 60 degrees F. So, when, 



Fig. 25. 



(Dii ft vp.n Tiff's 



this lactometer is used, the milk should be at 60 de- lactometer and 

 grees F., or else the correction must be made. If the "^coTbineT'" 



