MILK FP:RMENTATIONS 75 



1. Presence of curd. 



2. Time required to curdle. 



3. Character of curd, acid or rennet, homogeneous or 



showing gas holes, hard or soft. 



4. Whey, amount, clear or turbid, reaction. 



5. Digestion of curd, partial, complete. 



6. Digestion without previous curdling. 



7. Odor. 



8. Gas. 



9. Change produced by boiling a few seconds. 

 10. Amount of acid produced. 



Plain milk may be used or that modified with litmus. 

 The reaction of milk or whey may be tested by removing a 

 loopful and placing on delicate litmus paper. Care must be 

 exercised sometimes in discriminating between a rennet and 

 an acid curd. Reliance cannot be placed on the chemical re- 

 action, as a milk curdled with a rennet-like enzyme is gen- 

 erally acid to some extent. The digestion of casein cannot 

 always be determined with certainty in tube cultures. The 

 curd may sometimes shrink in volume and cause extrusion 

 of the whey without \mdergoing digestive changes. In doubt- 

 ful cases digestion can be definitely determined by the use of 

 milk agar, which is prepared as follows : A tube of nutrient 

 agar is melted and allowed to cool to 50-55 C. With a 

 sterile pipette 10-15 per cent of sterile skwn milk is added 

 to it, the mixture being poured into a Petri dish and allowed 

 to solidify. The plate is inoculated with the culture to be 

 tested by making a streak culture with the loop. The action 

 of the organism on the casein is thus localized, and can be 

 noted even though it is very slight. The diffused proteid 

 (casein) is peptonized or rendered soluble by the action of 

 digestive ferments. The action of acid-producing organisms 



