New York Agricultural, Experiment Station. 899 



In the rapid examination of the samples from the 33 farms, which 

 were not studied in detail, only a few fields were examined on each 

 smear and if very few or no bacteria appeared on these fields the milk 

 was " passed " as of good sanitary quality. Is this a safe procedure? 

 Of the 225 samples thus examined 60 were passed; and the plate 

 counts of these 60 samples showed that 42 of them contained less 

 than 50,000 bacteria per cubic centimeter, eight were between 

 50,000 and 100,000, eight more less than 200,000 and two above 

 this figure. (One of these high counts was probably due to a con- 

 taminated plate.) 



Among the 120 samples examined more closely, 101 would have 

 been " passed " by the more rapid, commercial examination, of 

 which only two showed more than 100,000 bacteria per unit. The 

 average plate count of 161 samples where no bacteria would have 

 appeared on examination of a few fields was 29,000 per cubic centi- 

 meter; that is, practically all of the milk was of good or excellent 

 sanitary character. 



In other words, it seems safe to assume that practically all samples 

 passed by the microscope as having no bacteria present when several 

 fields are examined would yield a plate count of less than 100,000 

 per cubic centimeter. 



On the other hand, out of 450 samples examined there were 246 

 that gave plate counts below 100,000 per unit; and 67 of these gave 

 microscope smears in which bacteria could be readily found. Thus 

 the plate method passed 67 of the 246 samples as having less than 

 100,000 bacteria per cubic centimeter when the microscopic examina- 

 tion showed they had many more than this. 



Considering both comparisons and assuming all of the bacteria 

 to be active, we find the plate method passing 67 out of 246 samples 

 as below a certain limit when the microscope count showed them 

 to be above that limit — an apparent error of 23 per ct. ; whereas 

 the microscopical method passed erroneously only 9 or 10 of 60 

 samples cursorily examined, a 15 per ct. or 17 per ct. error, or two 

 samples of 101 carefully examined — a two per ct. error. Thus 

 when the microscope is used in this way it tests milk more severely 

 and probably more accurately. 



The comparison of the two methods has been made only on fresh, 

 unpasteurized milk, and the conclusions reached must be understood 

 as applying only to the use of the microscopical method in milk of 

 that character. Whether this method can be made applicable 

 in studies of milk from unknown sources, which may include some 

 that has been pasteurized, future studies must determine. In such 

 milk most of the bacteria are dead and presumably harmless; 

 but these dead germs appear under the microscope for a time, at 

 least. 



