INTRODUCTION 



T^he development of dairy bacteriology is constantly clearing up more and 

 more of the problems which confront the practical dairyman in his daily work. We 

 know now that, in the great majority of cases, it is the true lactic acid bacteria which 

 give dairy products their good qualities, while, the aerobic and anaerobic spore-formers 

 (DucLAUx' Tyrolhrix species) which were formerly regarded as being of great imi)ortance 

 in the ripening process of cheese, not to mention the pseudo lactic acid bacteria (the Coli- 

 Aerogenes family) are the worst enemies of the dairy industry. 



We are still, however, far from having arrived at a complete elucidation of all the 

 questions involved. It is particularly difficult to understand how various sorts of hard 

 cheese, apparently containing the same microflora, should each have its own characteristic 

 taste and smell. There can hardly be any doubt that these sorts of cheese in reality contain 

 different species of bacteria, only we are unable to distinguish them by the methods 

 hitherto employed. The object of the present work is primarily to meet this want by 

 describing the useful bacteria of the dairy industry, so thoroughly that it may be possible 

 in the future to identify the strains encountered. 



A point of particular interest to the Danish dairy industry is the study of the i)acteria 

 which occur in Danish "dairy cheese" (Mejeriost), and these have accordingly been 

 subjected to particularly detailed treatment. The material used as a starting point consisted 

 of prize-winning exhibition cheeses from both rasv and pasteurised milk (noted in the tab- 

 les as R and P respectively). And in order properly to investigate the influence of pas- 

 teurisation upon the microflora, we have in certain cases had cheeses made at the dairies 

 from the same milk in raw and pasteurised state (in the tables, such cheeses are indicated 

 by the same numbers, e. g. S R and 8 P). 



On commencing the study of the lactic acid bacteria of milk and dairy produce, 

 however, it will soon be realised that the work will become onesided unless it be extended 

 to all lactic acid bacteria. For what we have to do is to ascertain whence the different spe- 

 cies are derived, and how they can be found when wanted. It need hardly be said that 

 the lactic acid bacteria of the dairy industry do not make up a complete whole in them- 

 selves. They are introduced into the milk to agreat extent through the cowdung, the bacteria 

 of which, again, are to an essential degree derived from the fodder, and it was therefore 

 necessary to study the lactic acid bacteria both of the animals and the plants. We have 

 therefore isolated and investigated the lactic acid bacteria most frequently met with in 

 the excrements of cows and calves, as also those of human beings, adults and children, 



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