PI. v DECOMPOSITION PRODUCTS 217 



acid bacteria cease to develop, but they begin again if the lactic 

 acid which they make is neutralised, e.g. by the addition of 

 chalk. If milk is left undisturbed for some time and plenty of 

 air can have access, then decomposition by moulds, such as the 

 ordinary milk mould, O'idium lactis, as well as Penicillium, 

 Aspcrgillus, Mucor, etc., continues. The moulds oxidise, that is 

 burn up, the acids that have been formed, and prepare the way 

 for proteoclastic bacteria, which generally prefer a neutral or 

 faintly alkaline nutritive medium. It may further be mentioned 

 that the moulds themselves attack the casein of milk. The 

 proteoclastic bacteria, chief amongst which are the hay bacteria 

 (B. mbtilis), decompose casein with the formation of albumoses 

 and peptones. Afterwards the decomposition mainly follows 

 the lines already sketched for the ripening of cheese (p. 182). 

 In addition to the aerobic hay bacilli are putrefactive bacteria, 

 both aerobic and anaerobic, and these actively destroy the 

 protein. Amongst these bacteria are Bacillus faecalis alcali- 

 genes (Petruschky), Bacillus putrificus (Bienstock), etc. 



The splitting-up of the milk fat into glycerine and free fatty 

 acid is accomplished much more slowly than in the case of the 

 milk sugar or the protein. Bacillus fluorescens liquefaciens is 

 the most active of this group. 



It takes a very long time, at least several months, for milk 

 to completely decompose. The slowness of the operation is 

 chiefly caused by the large amount of acid which is formed in 

 the early stages, for this naturally has a strong inhibitory 

 influence upon bacterial life. 



In pasteurised milk the spontaneous decomposition takes 

 another direction owing to the lactic acid bacilli having been 

 destroyed. If the milk is not infected afresh by lactic acid 

 bacteria none of this acid can be produced, and the first 

 decomposition-changes of normal unheated milk do not occur. 

 Instead of that the rnilk undergoes putrefaction through the 

 spore-forming hay and putrefaction bacteria that have sur- 

 vived the pasteurisation. 



When pasteurised milk is kept at a higher temperature, say 

 about 40 C., under anaerobic conditions a vigorous butyric acid 

 fermentation sets in which causes the glass bottle (if the milk 

 is in one) to burst. The inciters of the fermentation are chiefly 

 the anaerobic bacteria: Granulobacillus saccharobutyricus im- 



