DETERMINATION OF BACTERIA IN MILK 581 



of peptone will not be exactly the same and the peptone of one 

 manufacturer may vary from time to time. Even meat extract 

 is not constant in composition. In view of these facts it may 

 be well to eliminate peptone and meat extract from standard 

 media and so discount any possible effect of variations in their 

 composition. With this thought in mind a medium was devised 

 consisting of skimmed-milk powder, yeast extract and washed 

 agar. Yeast extracts (Ayers and Rupp, 1920) have been used 

 extensively in these laboratories and found to be very satis- 

 factory for many purposes. 



Ingredients for 1 liter of medium 



f 5 grams skimmed-milk powder ] T ,. 



(a) \ 1 gram sodium dibasic phosphate. (Sorensen's > . . „ , 



, , , v tilled water 



[_ phosphate) J 



(b) 10 grams pure dry fresh yeast. In 250 cc. distilled water 



Mix (a) and (b) and add 500 cc. of double strength (3 per cent) 

 washed-agar solution. 



DIRECTIONS FOR PREPARATION OF MEDIUM 



The milk-powder solution should be prepared as previously 

 described. 



The yeast extract is made from dry fresh (not autolyzed) yeast 

 which contains no added fillers. To facilitate filtering, the yeast 

 should be dried at a temperature of from 105° to 110°C. for five 

 hours. The extract is made by steaming 10 grams of yeast in 

 100 cc. distilled water for 45 minutes in the Arnold, then filtering 

 until the filtrate is clear and brilliant. When most of the extract 

 has been filtered, 100 cc. of distilled water should be poured on 

 the filter to wash out the remainder. Make up to 250 cc. with 

 distilled water. The filter papers previously mentioned for 

 filtering the milk-powder solution give good results with yeast 

 extracts. In order to obtain a clear, brilliant filtrate, it is usually 

 necessary to pour the filtrate back into the filter a few times 

 until it clogs slightly. 



The milk-powder solution and yeast extract are then mixed 

 and 500 cc. of double strength (3 per cent) washed agar is added. 



