166 DAIRY BACTERIOLOGY 



highly objectionable blown types 6 2 , ^3 an d s 3 on the one hand, 

 and all the remaining types on the other. In examining the 

 fermentation types the following should also be watched for : 

 sediment (and possibly pus), sliminess, and evil-smelling milk. 

 Alkaline milk keeps fluid for a long time, and often putrefies before 

 going sour. 



The combined rennet and fermentation test suggested by 

 Fr. Jos. Herz is a special form of the fermentation test, 2 c.c. of 

 the rennet solution mentioned above being added to each test 

 tube. Milk which coagulates badly will have separated compara- 

 tively little whey and formed a soft and non-coherent coagulum 

 after twelve hours. On examination after twenty to twenty -four 

 hours the coagulum should have assumed the form of a smooth 

 cjdinder, which only shows small holes in longitudinal section. 

 If the coagulum contains large holes, and especially if it forms a 

 screw-shaped sponge floating on the surface of the whey, the milk 

 must be considered unsuitable for cheese making. 



In the Emmental dairies, where a home-made rennet is used 

 which, at the same time, is also a culture of the more important 

 ripening bacteria, but which not infrequently contains colon and 

 aerogenes bacteria, it is of the greatest importance to test the milk, 

 both with and without the addition of rennet. If a bad result 

 is obtained without rennet and a good result with rennet, the milk 

 is certainly bad, though the lactic acid bacteria in the home-made 

 rennet will be able to counteract the defect. The milk is only 

 unsuitable beyond doubt if the test with rennet turns out badly. 

 If the last-mentioned result is obtained in spite of the fact of the 

 milk being good, then the rennet will be known to be unfit for use. 



The tubes and caps used in the fermentation test must be 

 cleaned immediately after use by rinsing carefully with hot soda 

 solution, after which they should be placed in the stands and 

 covered completely with water into which steam is then passed 

 for about fifteen minutes. Finally, they are to be dried in a warm 

 place. 



THE REDUCTASE TEST 



All living cells, including microorganisms, have reducing 

 properties, which are well illustrated by their behaviour towards 

 methylene blue. The rate at which milk decolorises methylene 

 blue will, therefore, depend on the number of microorganisms in 

 it. The reductase test devised by Barthel x and the author 2 is 

 based on this fact. 



1 *' Kungl. Landtbruks-Akademiens Handlingar och Tidskrift," No. 6, 

 1907. 



2 " Maelkeritidende," 1909, p. 359. 



