MILK BACTERIA 1 27 



brisk boil, oleomargarine and renovated butter will sputter very briskly and noisily 

 without foaming. Genuine butter boils less noisily and with abundant foam 

 formation. 



2. Fat Cohesion Test. — Fill a medium beaker about half full of sweet milk (pref- 

 erably skimmed) and heat to within near the boiling point. Add about 5 grams of 

 the sample and stir until completely melted. Remove from the fire and place beaker 

 in ice water. When the fat begins to congeal, stir with a small piece of stick. Fat or 

 oleomargarine will collect in one mass or lump at the end of the stick, whereas pure 

 butter granulates and will not adhere to the stick. This test is rot applicable to 

 renovated butter which behaves like unrenovated or fresh butter. 



As is generally known, milk is an excellent culture medium for 

 a great variety of bacteria. For a time after the milk is drawn, 

 bacterial development is checked by the bacterolytic properties 

 which all fresh milk is said to possess. These lysins, however, 

 gradually grow less and less until there is no longer any evidence 

 ofjtheir existence. 



Milk bacteria may be grouped into the acid formers, digesting 

 bacteria and those which appear to have but little effect on the 

 appearance of the milk. The acid-forming group is a large one and 

 includes the true lactic acid bacteria which are carried into the 

 milk from stable dust and other dirt in and about the stables and 

 elsewhere. The initial bacterial changes in the milk are, however, 

 not produced by the acid formers, but rather by those bacteria 

 which decompose proteids, to which belong the B. subtilis and its 

 aerobic allies. Streptococcus acidi lactici ferments both proteids 

 and lactose as does also B. coli communis and some of its allies. 

 In a short time, however, the true lactic acid bacteria multiply in 

 such large numbers as to crowd out or almost completely check the 

 development of the other species. They transform the lactose 

 into lactic acid. On longer exposure, Oidium lactis enters from the 

 atmosphere which fungus begins to decompose the lactic acid 

 and some of the remaining proteids, having the effect of lowering 

 the acidity which again encourages the renewed multiplication 

 of the lactic acid group. This alternating preponderance of 

 lactic acid bacteria and higher fungi continues until the proteids 

 and the milk sugar are almost completely used up. Butyric acid 



