254 CLEAN MILK 



care being taken to close the upper end of the pipette with the 

 finger so that no milk enters until after the mixing; or the pipette 

 may be emptied after stirring before the sample is taken. 



5. For certified milk samples it is recommended that, on ar- 

 rival at the laboratory, the bottle be opened with aseptic precau- 

 tions and the milk thoroughly mixed by pouring back and forth 

 between the original bottle and a sterile bottle. Another method 

 is to mix as thoroughly as possible by agitation for five minutes, 

 then burn through the pasteboard stopper with a hot iron and re- 

 move the desired amount of milk with a sterile pipette. 



THE INTERVAL BETWEEN COLLECTIONS AND ANALYSIS. 



Generally speaking, the shorter the time between the collection 

 and examination of milk samples the more accurate will be the 

 results. For routine work the attempt should be made to plate 

 within four hours of the time of collection. 



Too much stress cannot be laid on keeping the samples properly- 

 iced during this interval. They should be kept below 40 F., but 

 care should be taken that they are not frozen. 



DILUTIONS. 



Ordinary potable water, sterilized, may be used for dilu- 

 tions. Occasionally spore forms are found in such water which 

 resist ordinary autoclave sterilization; in such cases distilled water 

 may be used or the autoclave pressure increased. With dilution 

 water in eight-ounce bottles calibrated for ninety-nine cubic centi- 

 meters and in test tubes calibrated for nine cubic centimeters, all the 

 necessary dilutions can be made. 



Short, wide-mouthed " Blakes " or wide-mouthed French 

 square bottles are more easily handled and more economical of space 

 than other forms of bottles or flasks. 



