.EXPERIMENT STATION BULLETINS. 359 



than facilitation of the lactic micro-organisms in growth and in the produc- 

 tion of its enzyme. 



The gradual increase of lactic bacteria would support this view. Inas- 

 much as micro-organisms producing acid and alkali are both capable of this 

 incentive action, it follows that in all eases, it can not be the neutralization 

 of acid formed; nor since in some cases the acidity increases and in some 

 the alkalinity, can it be due to any stimulating action upon the lactic bac- 

 teria of an increased acid reaction of the nutrient medium. All workers 

 are aware that the casein in milk does neutralize the acid formed in such 

 a manner as to render possible the continued growth of lactic bacteria within 

 limitations, but this would in no way elucidate the influence at work when 

 the micro-organisms are cultivated together. Thus it is apparent that 

 the acid or alkali added by the associate germ cannot account for the in- 

 fluences. 



In all cases it cannot be the immediate influences of association, as the 

 consumption of oxygen, because in some instances the associate germ 

 may be cultivated, the culture sterilized, the lactic bacteria inoculated 

 into the sterilizing culture and the same associate results obtained. 



Bearing in mind that enzymes would be destroyed by sterilization, that a 

 combination of the associate micro-organisms in sterilized culture with 

 the lactic micro-organisms A\dll in some instances yield the same results, 

 that no set of influences as oxygen, temperature, et cetera, is common to 

 all cultures, the immediate agent causing the hastening of the action of lac- 

 tic bacteria is indicated at least by a change in the culture which yields a 

 more suitable food for the lactic bacteria. 



PERSISTENT TYPES IN ASSOCIATION. 



The well-known fact is apparent in these associational studies that nearly 

 all filth micro-organisms disappear with the increased production of lactic 

 acid. It is not so well recognized that there are types which can and do 

 persist along with the lactic micro-organisms, and some of these are capable 

 of causing a greater degree of acidity than may be yielded by the lactic 

 micro-organisms in pure culture. Besides producing a greater amount of 

 acid, they are frequently accountable for "off flavors" and "off aromas" — 

 all of which are so troublesome to dairy workers. 



])OES THE SPECIES OF BACTERIA PRESENT IN SAMPLES OF MILK HAVE ANY- 

 THING TO DO WITH THE RAPIDITY OP SOURING? 



Inasmuch as the associate micro-organisms exert a direct influence upon 

 the lactic micro-organisms, it follows that the above question would be 

 logical and pertinent. To answer it satisfactorily calls for the handling 

 of a larger number of milk samples than have been studied in this article. 

 However, we append a summary of counts and acidities which will con- 

 vey some light. 



SAMPLE I. 



Number of Acidity 

 Time after inilking. micro-organisms, of milk. 



lirs 530 16" 



20i lirs 49.500 16° 



28i hrs i.'75,000 16° 



40J hrs 69,000,000 16° 



60§ hrs 2,000,000,000 87° 



