PROPERTIES OF MILK. 33 



only be obtained at one given temperature, for, as the tempera- 

 ture of the substance becomes higher, the density of it grows less, 

 and consequently the specific gravity will be less. The tempera- 

 ture at which the lactometers are standardized is 60 F. 



The variations in the specific gravity of milk will also vary 

 according to the relative variation in amounts of the different 

 components of milk. 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. If the sample of 

 milk is rich in fat, as, for instance, in cream, the specific gravity 

 will be less. 



By adding water to milk, the specific gravity of it is lessened. 

 Owing to this fact it was first thought that adulteration of milk 

 with water could be detected by testing its specific gravity. 

 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 of milk 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 milk is above or 

 below, corrections must be made. The amount of correction 

 which will give approximate results is .1 of a degree added to 

 the lactometer reading for every degree Fahrenheit of tempera- 

 ture the milk is above 60 F., and also .1 of a degree subtracted 

 from the lactometer reading for every degree of temperature the 

 milk is 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 



