18 BULLETIN 642, U. S. DEPAETMENT OF AGEICULTUEE. 



almost always a lower count than that from the unwashed udders 

 The results again lead to the conclusion, as in Experiment No. 3, thai 

 three simple factors are necessary for the production of milk practi- 

 cally free from visible dirt and with a low bacterial content when 

 drawn, namely, sterilized utensils, clean cows (especially the uddei 

 and teats) , and the small-top pail. 



During Experiment Xo. 4 the udders of two cows were washed and 

 two left unwashed, and frequently the practice was reversed. The 

 change was made in order to eliminate the bacterial variation due 

 to a difference in number of bacteria in the milk from the udcler 

 of each cow. 



Occasionally the bacterial content of the middle milk drawn di- 

 rectly from the cow was determined, as in Experiment Xo. 3. The 

 average bacterial content of such milk drawn directly from the un- 

 washed udders was 757 per cubic centimeter, and from the washed 

 udders 739. Subtracting these numbers from the averages shown 

 in Table 4, there remain differences of 3,767 bacteria per cubic 

 centimeter in the case of the unwashed udders and 1,415 in the case 

 of the washed udders, which represent the external contamination. 

 However, as already pointed out in connection with the previous 

 experiment, there is probably less external contamination than the 

 figures show. 



The small additions of bacteria in milk from both washed and 

 unwashed udders must be kept in mind, as they represent the total 

 external contamination caused by factors formerly regarded as 

 important but which have not been considered in these experiments 

 in the production of low-count milk, which also was relatively free 

 from visible sediment. 



In order to show further that the fresh milk produced under such 

 conditions was of high quality from a bacteriological standpoint 

 and compared closely with the middle milk as drawn from the 

 udder, the bacterial groups in a number of samples were determined 

 by what is designated as the milk-tube method. This consisted of 

 picking off each colony on a plate and inoculating into litmus-milk 

 tubes. Tubes were incubated for 14 days at 30° C. (86° F.) . and the 

 bacteria from the plate were then divided into groups according to 

 the reactions produced. By that method it was possible to divide 

 the bacteria developing on a plate into 6 groups, namely, the acid- 

 forming, coagulating-acid forming, the inert (which produce no 

 change in milk), the alkali-forming, the peptonizing, and the acid- 

 coagulating peptonizing groups. A number of samples of milk were 

 studied by that method, and as often as possible samples were taken 

 directly from the udcler at the same milking. The results of the 





