BACTERIA IN MILK 711 



contact, allows them to adhere to each other and form clumps of butter. 

 It has been a matter of common experience, however, that unless the 

 cream is allowed to "ripen" for a considerable period before churning, 

 the resulting butter lacks the particular quality of flavor which gives it 

 its market value. The interval of ripening, at first a necessity upon 

 small farms where cream must be collected and allowed to accumulate, 

 has now been recognized as an essential for the production of the best 

 grades of butter, and it has been shown that the changes taking place 

 in the cream during this period are referable to the action of bacteria. 

 Cream, which before the ripening process contains but 50,000 bacteria 

 to each cubic centimeter, at the end of a period of "ripening" will often 

 contain many millions of microorganisms. At the same time, the cream 

 becomes thick and often sour. 



The species of bacteria which take part in this process and which, 

 therefore, must determine to a large extent the quality of the end prod- 

 uct, are various and, as yet, incompletely known. Usually some variety 

 of lactic-acid bacilli is present and these, as in milk, outgrow other species 

 and, according to Conn, 1 are probably essential for "ripening." 



It would be of great practical value, therefore, if definite pure cul- 

 tures of the bacteria which favor the production of agreeable flavors 

 could be distributed among dairies. In Denmark this has been attempt- 

 ed by first pasteurizing the cream and then adding a culture of bacteria 

 isolated from "favorable" cream. These cultures, delivered to the 

 dairyman, are planted in sterilized milk, in order to increase their quan- 

 tity, and this culture is then poured into the pasteurized cream. In 

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

 tures 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, 2 Rabinowitch, 3 Boyce, 4 

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

 Mohler, 5 Washburn, and Rogers have recently shown that these bacilli 



1 Conn, "Agricultural Bacteriology," Phila., 1901. 



2 Obermuller, Hyg. Rundschau, 14, 1897. 



3 Rabinomtch, Zeit. f. Hyg., xxvi, 1897. 



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



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



