132 Milk and Its Products 



steamed again for 20 minutes after the material has 

 reached the temperature of steam. The third day they 

 are steamed again for 20 minutes. 



Procedure for plating. Wipe off the desk or table 

 with 5 per cent carbolic acid solution. Shake thor- 

 oughly twenty-five times, the sample of milk. Unless 

 the bacterial count of the sample is nearly known, it 

 is well to make three dilutions, with two plates per 

 dilution (six plates in all). The dilution should be 

 large enough so that not more than one or two hun- 

 dred colonies will develop on the petri dish. Thus, in 

 plating fresh milk, a dilution of 1:100,1:200 and 

 1 : 1000 may be used. To make the 1 : 1000 dilution, 

 measure out with a sterile pipette (care should be taken 

 in drawing out the pipette from the case that the 

 point touches nothing but the milk, and, if many pi- 

 pettes are kept in a metal case, each pipette should be 

 passed through a flame before using it) 1 c. c. of 

 milk, and put it in a sterile bottle containing 99 c. c. 

 of the sterilized water. Shake this thoroughly. With 

 another pipette, place ,1 c. c. of the solution in a 

 petri dish, being careful to raise the cover of the petri 

 dish only high enough to introduce the mouth of the 

 pipette. Five-tenths of a cubic centimeter of this solu- 

 tion would give a dilution of 1 : 200, while .1 would be 

 1 : 1,000. If higher dilutions are needed, more bottles 

 may be used; for instance, if a dilution of 1 : 1,000,000 

 is wanted, arrange two bottles each with 99 c. c. of 

 water. Place 1 c. c. of milk in the first, shake thor- 

 oughly, take 1 c. c. of this dilution and place in the 

 second. This is then ready to transfer to the plate. 



