308 



CLEAN MILK 



with in the laboratory. Dilute each sample of milk 1,000 times and' 

 make agar plates from the final dilution; incubate at 98 . After 

 24 hours count the number of bacteria in each plate and compare 

 the samples of milk. 



34. Qualitative Analysis of Miscellaneous Milk Samples — 

 With the specimens above collected it will be useful to attempt a 

 qualitative analysis, though this will be more difficult. The dilu- 

 tion of the milk must be varied according to its age and temper- 

 Fig- 74- 



?fK 



m 



Flasks and vials for quantitative bacteriological analysis. 



ature. If it is fairly fresh — only a few hours old — a dilution of a 

 thousand times is satisfactory. If it is 12 hours old, a dilution 

 should be, at least, 10,000; and if 24 hours old, it should be as high 

 as 100,000, or higher. The amount of dilution necessary may be 

 determined by a direct microscopic study. Make and stain a slide 

 from each sample of milk, as directed in No. 3. Count the number 

 of bacteria' per field. If the average number per field is less than 

 10, dilute the milk 100 times. If it is as high as 100, dilute 1,000 

 times ; and if still higher, dilute in the same proportion. 



