THE RELATION OF MICKOORGANISMS TO MILK 383 



non-volatile and volatile acids are far different, lactic acid comprising 

 less than 30 per cent of the total acid formed, while volatile acids, such 

 as acetic and formic, make up the remainder. Traces of succinic acid 

 (C2H4(COOH2)) and alcohol have also been found. The lactic acid is 

 of the laevo-form. 



Milk is usually curdled, although some members of the group do 

 not produce enough acid to cause curdling. Depending on the amount 

 of gas formed, the curd may be almost perfectly homogeneous or it 

 may be very spongy. In all cases the curd shrinks to a greater or less 

 extent and thus becomes so firm that it is difficult or impossible to 

 emulsify it again. The odor of the fermented milk is offensive and the 

 taste disagreeable and sharp. The organisms of this group are to be 

 classed as undesirable and the fermentation produced by them cannot 

 correctly be called a lactic fermentation. 



Representatives of these two great groups of acid-forming 

 bacteria are to be found in every sample of market milk in varying 

 proportions. Both find in milk favorable conditions for growth, and 

 the normal souring is produced conjointly by them, each producing 

 its own specific products, the relative amounts of which are largely 

 dependent on the number of each group that is originally introduced 

 into the milk. The value of milk for butter and cheese is determined 

 by the relative amounts of the products of the desirable and the 

 undesirable acid-forming bacteria. 



The difference in taste and odor between milk fermented by pure 

 cultures of Bact. lactis acidi, and that which has sour,ed spontaneously, 

 emphasizes the difference in the products of the fermentations produced 

 by the two groups of acid-forming bacteria. 



Characteristics of the Bact. Bidgaricum Group* — The organisms of 

 this group are to be classed as true lactic bacteria, since they produce 

 almost exclusively lactic acid from the sugar fermented and only small 

 quantities of other acids as formic, acetic, and propionic. They vary, 

 widely in form and size; but are usually large rods, 2/t to 3^ long and 

 o.5;u to i/x wide. There is a tendency to form long threads. They 

 are Gram-positive and when stained with methylene blue often show 

 distinct granules in the cells; with Neisser's stain the appearance of 

 some cultures is similar to that of the diphtheria bacterium. They 

 are non-motile and do not form spores; capsules are seldom noted. The 



• Prepared by E. G. Hastings. 



