72 DAIRY BACTERIOLOGY 



While many of the above mentioned defects, however dangerous 

 they may be, will not be readily detected and indeed are only to be 

 demonstrated with great difficulty, the following defects will hardly 

 escape observation, and it is accordingly these which are commonly 

 alluded to in speaking of milk defects. We will begin with those 

 which are visible and then pass on to defects of taste and smell. 



Fermenting and Gassy Milk. Milk may be described as fer- 

 menting if it shows copious evolution of gas even before coagula- 

 tion sets in, and gassy if this only occurs after keeping for some 

 time at a high temperature (see the fermenting test). Both 

 defects are due to coli and aerogenes bacteria (more seldom to 

 yeast), and the difference between them is only determined by the 

 degree of infection. Fermentation may arise through direct 

 infection from fermenting fodder or when the cows are suffering 

 from violent diarrhoea or aerogenes mastitis. Both fermenting 

 and gassy milk should be avoided in cheese making. Fermenting 

 cream may give trouble in churning. 



Prematurely Coagulating and Cheesy Milk. Badly-cooled milk 

 may coagulate when only a few hours old ; as the milk will not be 

 appreciably sour in such cases the phenomenon must be ascribed 

 to an action resembling that of rennet ; it is always due to excep- 

 tionally active development of peptonising lactic acid bacteria 

 (Streptococcus liquefaciens or Tetracocci), which will probably 

 have started to secrete coagulating enzymes in the udder. The 

 well-known phenomenon of milk being specially liable to curdle 

 in thundery weather does not seem to be ascribable to any other 

 reason than the high temperature which usually precedes a 

 thunderstorm. Milk which has been coagulated by rennet is 

 generally described as cheesy ; it can easily be distinguished from 

 sour milk by the separation of clear whey and the contraction of 

 the coagulum. If gas-producing enzymes are present as well, the 

 curd will become flaky or lumpy owing to the disturbance of the 

 milk by rising bubbles. The same effect may be produced in 

 practice in presence of coagulating bacteria alone, if the milk is 

 stirred or shaken. 



Slimy and Ropy Milk. Milk may become slimy on standing, and 

 sometimes the effect is so pronounced that the milk may be 

 pulled out into threads a metre long. The change is most marked 

 at 18 to 20 C. At higher temperatures the slimy organisms may 

 be suppressed by the lactic acid, but even in pure cultures they 

 form more slime at the temperatures mentioned. According to 

 Gillebeau, one of the * commonest slime-producing organisms is 

 Micrococcus Freudenreichii, a large coccus 1 2 ^ thick, which 



1 " Landwirtschaftliches Jahrbuch der Schweiz," 1891, p. 135, and 

 1902, p. 342. 



