PREPARATION OF NUTRIENT BROTH 29 



on account of the resistant spore-forming organisms which 

 are almost universally present in the chopped meat; economy 

 of time also is a consideration. Unless sterilized immedi- 

 ately, meat infusion decomposes quickly owing to the 

 abundance and diversity of the microflora acquired during 

 the various processes of preparation for market. 



Infusion made from freshly chopped lean beef will vary 

 in acidity between +15 and +25 Fuller's scale. If the 

 reaction is markedly lower or higher, microbial action is 

 taking place, which is, or may be, injurious to the food 

 value of the medium in which the meat infusion is 

 used. 



The infusion contains very little albuminous matter and 

 consists chiefly of the soluble salts of the muscle, certain 

 extractives, and altered coloring matters along with slight 

 traces of protein not coagulated by heat. 



EXERCISE 7. PREPARATION OF NUTRIENT BROTH 



Nutrient broth is the standard liquid employed for cul- 

 tivating microorganisms. It is practically a beef tea con- 

 taining peptone. Peptone, a soluble protein not coagulable 

 by heat, is added to replace the coagulated albuminous 

 substances which precipitate when the meat infusion is 

 sterilized. Salt is added to take the place of the phosphates 

 and carbonates, some of which are precipitated on adjusting 

 the acidity of the medium by sodium hydroxide. 



The reaction of ordinary nutrient media is adjusted to 

 about +15 with phenolphthalein as indicator, as it is found 

 that most microorganisms grow best on a medium neutral 

 or slightly alkaline to litmus. 



When it is required that nutrient media be clear, egg 

 albumen reduced to a smooth paste with water (or the well- 

 beaten white of an egg) is added. By coagulation, the egg 

 albumen removes mechanically all small particles in suspen- 

 sion which otherwise would pass through the filter paper. 



