FERMENTATION TEST 197 



The Fermentation Test in Milk Examination 



This test is performed by incubating the sample in sterile 

 vessels and observing the chemical and physical changes that 

 take place. 



The earliest experimental work in this connection was prob- 

 ably that of Walter, cantonal chemist at Soleure. This ob- 

 server kept milk at 98° F., and stated that " milk, if good, will 

 not curdle or undergo abnormal fermentation in ten to twelve 

 hours." A special apparatus was devised for this purpose by 

 Schaffer,^^ who recorded the amount of gas evolved in 100° F. 

 from a definite volume of milk. He found that good milk 

 formed no gas and remained fluid after twelve hours. This 

 test was chiefly used in connection with the suitabihty of milk 

 for cheese manufacture; milks that produced " heaving '^ 

 were detected by this test. 



The Wisconsin curd test ^^ was also evolved for cheese 

 manufacture and differs from the Swiss tests given above in the 

 use of rennet for the production of a definite curd which is 

 pressed and afterwards set aside for observation. 



The Gerber fermentation test consists in incubating tubes of 

 milk at 104° to 106° F. for six hours and then observing the 

 odour, taste, and appearance for abnormal qualities. The 

 heating is then continued for a second six-hour period and any 

 abnormal coagulations, such as gas holes, are then noted. 

 Gerber stated that coagulation in less than twelve hours is 

 abnormal, and that milk that does not curdle in twenty-four 

 hours to forty-eight hours is open to suspicion regarding 

 preservatives. 



According to Jensen,^^ the milk is heated to 30° to 35° C. 

 for eight to twelve hours and examined; replaced for a further 

 period and again examined. After the second period he found 

 that the clean samples are sour and curdled and form a homo- 

 geneous coagulum without much separation of curd and gas 

 formation. Frequently gas bubbles have split the coagulum 

 and considerable fluid has separated. This change, he states, 



