710 BACTERIA IN AIR, SOIL, WATER, AND MILK 



of 1 : 100 is made, quantities ranging from 1 c.c. to 0.1 c.c. of this will 

 furnish 0.01 c.c. to 0.001 c.c. of the milk, respectively. Inoculation of 

 properly cooled tubes of melted neutral agar and gelatin, with varying 

 quantities of these dilutions, are then made and plates poured. After 

 twenty-four to forty-eight hours at room temperature or in the in- 

 cubator, colony counting is done as described upon page 161, and the 

 proper calculation is made. In samples in which few bacteria are 

 expected, direct transference of 1/20 or 1/40 of a c.c. of the whole 

 milk into the agar may be made. This saves time but is less accurate 

 than the method given above. 



Bacteria and Butter. — Butter is made from cream separated from 

 milk either by standing or by centrifugalization. After this, the cream 

 is agitated by churning, which brings the small fat-globules into mutual 

 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 " wUl 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 bacilh is present and these, as in milk, outgrow other species 

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



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

 cultures 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, dehvered 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 



> Conn, "Agricultural Bacteriology," Pliila., 1901. 



