LABORATORY WORK 307 



temperature of an ordinary room and compare the time of souring 

 and curdling in the two cases. 



30. The Effect of Careful Milking. — Compare the effect 

 •of milking, for six successive days, with and without precautions. 

 The first three days use open milk pails and make no special attempt 

 at cleanliness. The second three days use closed milk pails, groom 

 the cows, wash the udders and perform the milking out-of-doors in- 

 stead of in the closed barn. In each case make a quantitative analy- 

 sis of the number of bacteria in the milk and compare the results. 



31. Sterilization by Boiling. — Make a set of bacterial 

 plates from a sample of milk gelatin. Then boil the milk for five 

 minutes in a flask closed with cotton and make another set of 

 plates, diluting by 10. Allow the boiled milk to stand for 24 hours 

 and make another set of bacterial plates, diluting by 100. Allow 

 the rest of the boiled milk to stand in the laboratory until it shows 

 signs of fermentation. Determine whether it has soured, and de- 

 scribe any other notable change. After the plates have had an op- 

 portunity to grow, count the number of bacteria. 



32. Pasteurizing Milk. — Obtain some milk that is from 

 12 to 20 hours old. Make a set of plates on agar, diluting by 1,000. 

 Divide the milk into two lots. Heat one lot to a temperature of 

 140 for one-half hour, stirring frequently. Heat the other lot 

 to 160 for 10 minutes. Allow both to cool and make agar plates 

 from each, diluting by 100. After plates have grown, calculate the 

 number of bacteria before and after pasteurizing at the different 

 temperatures. 



a. Repeat the last experiment, making plates, however, in lit- 

 mus gelatin and calculating the number and percentage of acid 

 colonies in the milk before and after pasteurizing. 



33. Quantitative Analysis of Miscellaneous Milk Samples. 

 — Obtain milk samples from several milkmen, learning, so far 

 as possible, the age of each sample of milk. These should be col- 

 lected in sterile bottles and kept on ice until they can be experimented. 



