METHODS OF EXAMINING MILK 



249 



contain up to 1 to 2 mg. per litre from this source. Rievel 

 also states that a positive reaction may occur with milk 

 soiled with manure, but Tillmans has never found dirty 

 milk to give any indication of containing more nitrates 

 or nitrites than milk from vessels that had been washed 

 with water rich in nitrates or nitrites. A negative reac- 

 tion to the nitrate test is no evidence that the milk has not 

 been watered, as water free from these substances may 

 be used for dilution. According to Utz, milk to be 

 tested for nitrates should be boiled unless the test can 

 be made at once, because the action of the lactic acid- 

 forming bacteria may reduce the nitrates or cause them 

 to disappear entirely. 



DETECTION OF THE USUAL ADULTERATIONS 



When milk is adulterated, it is usually by partial 

 skimming, dilution with skimmed milk, or dilution with 

 water. The changes caused by these various forms of 

 adulteration are shown in the following table, which is 

 taken from Jensen's Milk Hygiene: 



