BACTERIA IN MILK ' 711 



most cases, these so-called " starters " are not pure cultures, but mixtures 

 of three or more species derived from the original cream. 



Adverse accidents in the course of butter-making, such as " souring " 

 or "bittering" of butter, are due to the presence of contaminating, 

 probably proteolytic, microorganisms in the cream during the process of 

 "ripening." 



As a means of transmitting infectious disease, butter is of importance 

 only in relation to tuberculosis. Obermiiller,^ Rabinowitch,^ Boyce,' 

 and others, have repeatedly found tubercle bacilli in market butter, and 

 Mohler,* Washburn, and Rogers have recently shown that these bacilli 

 could remain alive and virulent for as long as five months in butter kept 

 at refrigerator temperature. The acid-fast butter bacillus, described by 

 Rabinowitch as similar to the true Bacillus tuberculosis, shows decided 

 cultural and morphological differences from the latter. 



Bacteria and Cheese. — ^The conversion of milk products into cheese 

 consists in a process of proteid decomposition which, by its end products, 

 leucin, tyrosin, and ammonia compounds, largely determines the cheese- 

 flavors. The production of cheese, therefore, is due to the action of 

 proteolytic bacterial enzymes '' and the variety of a cheese is largely 

 determined by the microorganisms which are present and by the cultural 

 conditions prevailing. The sterilization of cream, or the addition of 

 antiseptics, absolutely prevents cheese production. 



The organisms which are concerned in such processes have been 

 extensively studied and attempts have been made, with moderate 

 success, to produce a definite flavor with pure cultures. 



In the production of cheese the two varieties, hard and soft cheeses, 

 depend not so much upon the bacterial varieties as upon the differ- 

 ences in the treatment of the curds before bacterial action has begun. 

 In the former case, a complete freeing of the curds from the whey 

 furnishes a culture medium which is comparatively dry and of almost 

 exclusively proteid composition; in the latter, retention of whey gives 

 rise to cultural conditions in which more rapid and complete bacterial 

 action may take place. The holes, which are so often observed in some 

 of the hard cheeses, are due to gas production during the process of 

 "ripening." 



1 Obermuller, Hyg. Rundschau, 14, 1897. 



2 Rabinovntch, Zeit. f. Hyg., xxvi, 1897. 



3 Boyce and Woodhead, Brit. Med. Jour., 2, 1897. 



*Mohler, U. S. P. H. and Mar. Hasp. Serv. Bull. 41, 1908. . 

 ^ Freudenreich, Koch's Jahresbericht, etc., 135, 1891. 



