20 DAIRY BACTERIOLOGY 



several dilutions to achieve the desired result. A number of 

 100 c.c. Freudenreich flasks sterilised in the autoclave with 50 c.c. 

 of water should be prepared in readiness, and also some dry 

 sterilised plugged pipettes, graduated in quarters of a cubic 

 centimetre. When making counts in milk, the following dilutions 

 may be made : 



Dilution 1. 1 c.c. milk in 49 c.c. water, of which 0-5 c.c. in Petri 



dish No. 1. 

 2. 0-5 c.c. of Dilution 1 in 49*5 c.c. water, of which 



0-5 c.c. in Petri dish No. 2. 

 3. 0-5 c.c. of Dilution 2 in 49'5 c.c. ^water, of which 



0-5 c.c. in Petri dish No. 3. 



The number of organisms per cubic centimetre is obtained 

 by multiplying the number of colonies on plates 1, 2 or 3 by 

 100, 10,000 or 1,000,000, respectively. Butter is measured by 

 means of a rounded platinum spoon, holding exactly 0-25 c.c. 

 The spoon and the knife used for scraping off the surface of the 

 butter must be sterilised by passing through a Bunsen flame 

 before use. If the water used for dilution is warmed to about 

 40 C., the butter will readily melt from the spoon and become 

 distributed on shaking. Cheese gives more trouble, as it must be 

 weighed and comminuted ; about 1 gram of cheese may be 

 accurately weighed into a Freudenreich flask containing 50 c.c. 

 of water and ground up by repeatedly pressing it out in the water 

 by means of a flamed glass rod. An alternative method, resembling 

 that for determining the soluble nitrogenous matter, is to grind 

 10 grams of the cheese with sterile water at 40 to 50 C. in a 

 sterile mortar, pouring the emulsion into a sterilised 250-c.c. 

 measuring flask and making up to the mark with sterile water. 

 This method is to be preferred as a more representative sample 

 is obtained, a matter of some importance as the organisms are 

 usually very unevenly distributed in cheese. Further, the cheese 

 is more easily ground up in the mortar than in the flask, while the 

 number of organisms introduced by infection from the air will be 

 negligible compared with the large number present in the cheese. 



In all cases uniform working methods should be adopted in 

 order that the results may be comparable ; the colonies should 

 be counted after incubating for a definite time, say seven days at 

 20 C. The counting is facilitated by placing the plate on a 

 squared glass plate resting on a black surface. 



For use when travelling, Petruschky flasks (Fig. 19), sterilised 

 with the proper amount of nutrient gelatine, are more convenient 

 than Petri dishes ; they are generally used in water examinations, 

 which are best carried out on the spot, as an increase in the 



