100 EXPERIMENTAL DAIRY BACTERIOLOGY 



incubation, may be inoculated cultures in order to determine 

 the ability of the organism to produce acid. Into a series of 

 flasks containing a definite amount of sterile milk are inocu- 

 lated varying amounts of the fresh twenty-four-hour-old milk 

 culture (1/100 cc., 1/1000 cc., 1/10,000 cc. for 100 cc. of 

 milk). The flasks should be kept at 25-30 C. With a cul- 

 ture of high fermentative power 1 part to 1000 of milk 

 should produce curdling in twenty -four hours at 30 C. 



The amount of acid in the curdled milk should be de- 

 termined by titration with N/20 sodium hydroxide. 



Preservation of properties on propagation. Many lactic- 

 acid cultures when first isolated from milk show desirable 

 properties for use in a starter, but on continued propagation 

 lose them more or less rapidly. They may cease to produce 

 sufficient acid, or they may form it very slowly. Manifestly, 

 such organisms are not well adapted for starter propagation. 

 Their flavor-producing properties may deteriorate, or they may 

 commence to produce a slimy or ropy fermentation in the 

 milk. The ability of an organism to preserve its original 

 properties can only be determined by continued propagation 

 under practical conditions, that is, in the same manner as 

 would be done in a creamery in the carrying on of mother 

 starters in small quantities. The propagation should be carried 

 out under definite conditions as to temperature of incubation 

 and the degree of acidity developed in the starters. 



Flavor-producing properties. The ability of the organism 

 to produce a desirable flavor in the starter and in the butter 

 may be determined by frequent examinations of the starters 

 as propagated. The students should be drilled in the judging 

 of starters. 



Exercise. Each student will make a detailed study of a commer- 

 cial starter, as to its purity, vitality, flavor-producing properties, etc. 



