v DECOMPOSITION PRODUCTS 219 



made strongly acid with phosphoric acid and shaken out with 

 ether in a separating funnel. After the ether has been distilled 

 off, a yellow, syrupy residue is left, which is dissolved in water 

 and filtered. The filtrate is now boiled with zinc carbonate, 

 again filtered, and the filtrate evaporated, at first on the water- 

 bath and afterwards in vacuum over sulphuric acid. The zinc 

 lactate which crystallises out is dried between filter-paper, dis- 

 solved in a small quantity of water, and allowed to crystallise 

 over sulphuric acid. The recrystallised zinc salt, which is 

 usually sufficiently pure for the following examination, is dried 

 again between filter-paper and then over concentrated sulphuric 

 acid to free it from the last traces of moisture. The thoroughly 

 air-dried zinc lactate is analysed, and the percentage of water 

 of crystallisation and of zinc determined, as well as its behaviour 

 towards polarised light. 



Air-dry inactive zinc lactate loses three molecules of water 

 of crystallisation =1818 per cent, when heated to 110, whilst 

 air-dry active zinc lactate loses only two molecules =12*9 per 

 cent. 



The amount of zinc oxide in anhydrous zinc lactate is 33*33 

 per cent. For the polarimetric examination, the solution of the 

 zinc salt must be as free as possible from colour, because the 

 rotation is not very powerful. 



2. Volatile Fatty Acids. 



When milk decomposes, there is always found a greater or 

 less quantity of fatty acids, due to the action of different 

 micro-organisms. In the lactic acid fermentation, in so far as 

 this is carried on by bacteria other than Bacterium lactis acidi, 

 small quantities of acetic and formic acids are also formed. 

 Butyric acid can arise both directly from milk sugar by the 

 action of the specific butyric acid bacteria mentioned in the 

 introduction to this chapter, and, secondarily, from calcium 

 lactate, as shown by Pasteur. Fitz has also proved that caproic, 

 valeric, and, under certain conditions, propionic and acetic acids 

 are formed, finally, volatile fatty acids can also arise from the 

 action of certain micro-organisms upon the casein. 



