NORMAL AND ABNORMAL MICROFLORA OF MILK 65 



examination after forty-eight hours showed the total number to 

 have increased. Chilling to C. will therefore not prevent bacterial 

 development indefinitely. At temperatures above 10 C., bacterial 

 multiplication is rapid ; thus in the present example, after twenty- 

 four hours at 12C., the number of organisms had increased one 

 hundred fold, and at 30 C., five thousand fold. Thirty degrees to 

 thirty -five degrees is the optimum for most milk bacteria ; already 

 at 38 C., many of them cease to grow and, moreover, the inhibitory 

 effect of the acid which is formed increases with the temperature, 

 as is shown in the table, particularly in respect to the liquefying 

 organisms. In order to make bacterial counts of samples kept at 

 the higher temperatures, it was necessary to employ agar, prefer- 

 ably in Bum's tubes (see p. 19) at 40 C., as organisms would be 

 present which would not grow at ordinary temperatures. A 

 comparison between gelatine and agar plates from milk kept for 

 forty-eight hours at 38 C. showed that the original flora was being 

 suppressed, while a new flora consisting entirely of rod-shaped 

 lactic acid bacteria was making its appearance. At 45 C., a 

 temperature unfavourable for the development of the common 

 milk bacteria, this change proceeds much more rapidly. The 

 lactic acid rods which develop at this temperature are mostly 

 thermobacteria producing Isevo lactic acid. The results of 

 investigations of the flora of milk at different temperatures, by 

 Conn and Esten 1 9 Arthur Wolff, Luxwolda 2 and the author, are as 

 follows : 



1. Under 5 C. the fluorescent bacteria predominate. 



2. Between 5 C. and 10 C., in addition to fluorescent bacteria, 

 proteus bacteria, micrococci, alkali producing rods, and, as 

 Beijerinck has shown, also some rods which produce an aroma of 

 fruit. 



3. Between 10 C. and 15 C. betacocci, streptococci, and some 

 species of aerogenes bacteria in addition to the above. 



4. Between 15 C. and 30 C. the streptococci, especially Sc. lactis, 

 predominate. 



5. Between 30 C. and 40 C. coli and aerogenes bacteria and lactic 

 acid forming rods in addition to streptococci. 



6. Above 40 C. lactic acid forming rod bacteria and Saccharo- 

 mycetes, which ferment lactose, predominate. The streptococci 

 which are found are now chiefly Sc. thermophilus and Sc. fcecium. 



It is well known that the lower the temperature to which milk 

 is cooled the better it keeps ; in this connection, some results 

 obtained by Kjaergaard Jensen may be quoted ; they repre- 

 sent the bacterial counts of the same milk kept for eighteen 



1 Ann. Rep. Storr's Exp. Station, 1904. 

 " Centralblatt f. Bakt.," 2 Abt., 1911, Bd. XXXI., p. 129. 



P,B. 5 



