34 PROPERTIES OF MILK 



The specific gravity of milk will also vary according to the 

 relative variation in amounts of the different components. If a 

 sample of milk is rich in solids not fat, as, for instance, skimmed 

 milk, the specific gravity will be high and usually between 1.033 

 and 1.037. K the sample of milk is rich in fat, as, for instance, in 

 cream, the specific gravity will be less. 



The specific gravity of milk is lessened by the addition of 

 water. Owing to this fact it was first thought that adulteration 

 of milk with water could be detected by testing its specific grav- 

 ity. But this method was soon found to be erroneous, as it is 

 possible to take cream away and add water in such a proportion 

 as not to alter the specific gravity of the sample. A low specific 

 gravity may, however, cause the suspicion that the milk has 

 been adulterated, and the test for water adulteration can be 

 supplemented by testing it for fat. 



As has been mentioned before, the lactometer reading should 

 be taken at 60 F. If the temperature of the milk is above or 

 below, corrections must be made. The amount of correction 

 which will give approximate results is . i of a degree added to the 

 lactometer reading for every degree Fahrenheit of tempera- 

 ture above 60 F., and .1 of a degree subtracted from the lactom- 

 eter reading for every degree of temperature below 60 F. The 

 temperature of milk when tested for lactometer reading should 

 never go any lower than 10 below 60, nor any higher than 10 

 above 60. This would leave the range of temperature between 

 50 and 70 F. 



In chemical laboratories, the specific gravity of milk is usually 

 determined by the use of a picnometer. 



In practice there are three instruments in general use for 

 determination of lactometer reading, or specific gravity, viz., 

 Quevenne lactometer, New York Board of Health lactometer 

 and the ordinary hydrometer. The Quevenne lactometer is the 

 one that is used chiefly in creameries. The graduation of each 

 one of them is given in the accompanying diagram. It may be 

 seen from the figures that in order to change the Quevenne 

 lactometer reading into specific gravity, all that is necessary is 

 to add 1000 and divide the sum by 1000. In changing the 



