LABORATORY WORK IN DAIRY BACTERIOLOGY 303 



soured, and describe any other notable change. After the plates 

 have had an opportunity to grow, count the number of bac- 

 teria. 



No. 52. Sterilization by Autoclav. Sterilize milk in a plugged 

 milk flask for 15 minutes in the autoclav at a pressure of 15 

 pounds. Allow to stand for several days and then make gelatin 

 plates from it and determine whether the milk was completely 

 sterilized or not. 



*No. 53. 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 temper- 

 ature 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 

 litmus gelatin and calculating the number and percentage of 

 acid colonies in the milk before and after pasteurizing. 



No. 54. Pasteurizing Machines. If a pasteurizing machine is 

 available, test its efficiency by several experiments. In each 

 case a thoroughly mixed sample of milk should be first care- 

 fully tested by gelatin plates, and corresponding sets of plates 

 should be made from the milk after pasteurization. Use the 

 machine under varying conditions of temperature and time of 

 pasteurizing, and determine the efficiency under the different 

 conditions. 



No. 55. Condensed Milk. Open a can of condensed milk and 

 remove with a sterile platinum loop, or a sterile knife, a quantity 

 of the condensed milk and dilute with sterile water. Make a 

 series of bacterial plates with a loopful of the diluted milk and 

 determine whether the condensed milk was sterile. If colonies 

 develop in the plates, examine with the microscope' and deter- 

 mine whether they are yeasts or bacteria. 



