MICRO-ORGANISMS. 449 



does not develop till the milk has stood for some time, and appears 

 to be anaerobic. Some organisms produce acetic or propionic 

 acids as well as butyric. 



Alcoholic Fermentation. This does not readily occur in 

 milk. As already mentioned, small quantities of alcohol are 

 produced as bye-products by some organisms ; ordinary yeasts 

 Saccharomyces cervisice, etc., do not cause fermentation of milk- 

 sugar, but one species of Saccharomyces is known which converts 

 the bulk of the milk-sugar into alcohol ; this is found in kephir 

 grains, together with organisms producing lactic acid and others 

 acting on the proteins. 



Curdling Organisms. These organisms act on the casein 

 by the secretion of an enzyme, which resembles rennet in its 

 action ; these are usually bacilli, which readily form spores and 

 are difficult to kill by heating. 



Organisms which Curdle without Acidity and Redissolve 

 the Curd. This class is a very large one ; the organisms act by 

 the secretion of enzymes having proteolytic functions analogous 

 to pepsin and trypsin. Many of the organisms producing butyric 

 acid belong to this class ; among the most noticeable of which 

 are the hay- and potato-bacilli. 



Organisms which Peptonise the Milk without Curdling. 

 This class is probably more numerous than has been described ; 

 they also act by the secretion of a proteolytic enzyme.. It is 

 rare to find milk which shows their characteristic behaviour, as 

 there are generally other organisms present which curdle the 

 milk. When cultivated in sterile milk, no action is at first 

 apparent, but the milk gradually becomes more and more trans- 

 parent till it assumes an appearance like a liquid jelly. The 

 author has separated an organism of this class from " mazoum," 

 an Armenian preparation. 



Organisms acting on the Proteins with Production of 

 Evil-smelling Sulphur Compounds. There is a class of strict 

 anaerobes which peptonise the milk with some curdling, and 

 produce volatile sulphur compounds, including sulphuretted 

 hydrogen. These form spores, which are very difficult to destroy, 

 and are the most frequent cause of trouble in so-called sterilised 

 milk, in which the sterilisation has not been efficient. 



Golding and Fyleman have described a class of organisms 

 that, in the presence of small quantities of copper, dissolved 

 from an imperfectly tinned copper milk cooler, produced an 

 objectionable flavour in milk. 



Chromogenic Organisms Milk " out of Condition." 

 Several organisms have the property of producing coloured 

 substances ; these, and one or twp other classes, are the chief 

 causes of milk being " out of condition." 



29 



