976 



EXPERIMENT STATION RECORD. 



A comparison of practical methods for determining' the bacterial content 

 of milk, r. (J. IlKiNKMANN ;uul T. 11. Cii.KN'N {.lour. Jiifict. Discii.scN, 5 ilHO.S), 

 No. J/, 1)1). 'iL! 4-'"' cliiiit I). — ExiH'iiiiu'iits were uiidcitaki'ii in urdiT to jiro- 

 mote iniiforuiity in IalK)rat<>ry protvdiuv for conipariiif^ the various methods 

 now in use. Many workers favor an iiuMihation temperature of 87° C. because 

 it saves time, but the authors believe that incubation at 20° is superior because 

 a higher count and a better differential count are obtained. At 37° acid 

 formers, which are chiefly of fecal orijiin, are favored at the expense of the 

 nonacid formers. 



" Some milk bacteria of the liactciiuin airogoic^ type form red colonies at 

 first and then later these colonies assume the blue color again. This phenom- 

 enon was not observed in dextrose agar. We conclude from this that dextrose 

 is to be preferred as an addition to the medium to lactose. 



"Official bacterial standards for milk should include a statement of the 

 methods by which the bacteriological control is to be obtained. Sinct* milk is 

 usually consumed before the results of a bacteriological examination can possi- 

 bly be reported, a gain of one or even two days is innnaterial. The l>acteriologi- 

 cal and chemical examination of milk ought to be carried on chiefly with the 

 object of improving the whole milk supply of a commonwealth rather than 

 liunishing individual offenders. To this end the most accurate and scientific 

 method of examination is the preferable one." 



A study of the determination of bacteria in milk in relation to the 

 composition of the media, Z. Northrup and Bell Farrand (Michigan Sta. 

 Rl)t. 1908, pp. 130-152). — To determine the media most favorable to the growth 

 of germs in milk, media were prepared of agar, peptone, and salt and divided 

 into four portions, adjusting them to 5°, 10°, 15°, and 20° acid, respectively, 

 with normal sodium hydroxid. Milk from to hours old was used for plating 

 and the accounts were kept at 21° and 37° C. A bacterial count was taken at 

 intervals to ascertain the degree of acidity, the percentage of lactose, and the 

 percentage of i)eptone most conducive to the growth of the milk germs. The 

 results are sunuiiarized in the following table : 



Pcrccntagi' of minplcs groicing best under varioua conditions at 21° and 37° C. 



As regards temperature. 03 per cent of the samples were found to grow best 

 at 21°, 9 per cent at 37°. and the remainder indeterminate. 



"To check up the results obtained by plating the miscellaneous milk organ- 

 isms, pure cultures of dairy germs (taken from butter), were grown on whey, 

 ordinary and 4 per cent lactose agar. 



" Four per cent lactose agar was used in this experiment as it was found to 

 support the growth of the milk germs better than the ordinary (1 per cent) 

 lactose agar, at 37° C. The explanation for this is that the media, in the per- 

 centage of lactose, reaches more nearly the composition of milk. 



