APPLICATION OF ANALYSIS 59 



in total solids (about 16 per cent.), have a low acidity, 

 and are nearly clear, characteristics which permit of the 

 cause of curdling being established. 



Occasionally milk is alleged to be sour because it 

 turns blue litmus red ; all milk does this, as well as 

 turning red litmus blue, owing to the amphoteric 

 reaction of the phosphates and citrates of the milk. 



The three popular superstitions that milk can be kept 

 warm, i.e. a little above blood heat for several hours 

 without change, that custards can be heated to any 

 temperature, and that blue litmus can be used as a test 

 for sour milk, though all fallacious, are responsible for 

 many allegations of sour milk. The peptonisation of 

 milk with powders which are insufficiently alkaline or 

 which have been kept too long may cause it to curdle, 

 and to be thought to be sour. 



The Cause of Unusual Taste. Milk on boiling 

 acquires a taste, and the tests for heated milk (p. 54) and 

 soluble albumin (p. 35) will show this. Mixture with 

 dirty water may give an evil taste to milk, and usually 

 the establishment of the presence of water is all that 

 can be done to explain this ; if an alkali is added to 

 milk, the taste is soapy and the smell fishy, and an 

 increase in ash and its strongly marked alkalinity will 

 detect the cause. If the taste is due to a fermentation^ 

 other than the normal lactic one, or to the food of the 

 cattle, it is usually difficult to detect the cause by 

 chemical analysis. 



An unpleasant taste may be due to the presence of 

 urine, accidentally or wilfully added. 



Detection of Urine. Half fill a small test -tube 

 (2 X f is large enough) with sodium hypobromite 

 solution (see Appendix) ; carefully fill the tube with 

 milk so that the two liquids do not mix. Place the 

 thumb over the tube, and invert once or twice, and 

 then hold it, with the thumb still over the opening, 

 upside down. Milk causes practically no pressure on 

 the thumb, and gives not more lhan one -fifth of its 

 volume of gas ; if urine has been added, much pressure 



