EXPERIMENT STATION BULLETINS. 



423 



Expt. rii. 



Comparison of Acid-Reduction in Mixed Cultures of Four Yeasts and 

 Four Lactic Bacteria in Millc and in Whey. 



Each of the yeasts used in the previous experiment was grown with 

 each of four different lactic bacteria, Strain 2, Strain 4, No. 53B2 and 

 Bad. hulgaricum. Duplicate cultures were made in milk and in whey. 

 These were titrated over a period of two months, test cultures being 

 made during and after this period to ascertain the activity of the lactic 

 in question. 



The following tables give the fluctuations in acidity in the separate 

 and combined cultures in milk and whey. All cultures were kept at 

 room temperature as the activity of the red yeast is greatly checked at 

 30° — 32°C., and entirely arrested at 37°C. 



TABLE VI. 



AcidUy of Mixed Cultures in MUk. 



♦Slimy. 



tReinoculated with 1 cc. of a 16 day culture of yeast LZ because the red color of the yeast had not appeared at this time. 



"Test cultures made at this time; also after titrations were no longer carried on. 



From this table, it will be noted that the types of the different lactic 

 bacteria are markedly differentiated by the quantity of acid and the 

 rapidity in which it is made. Bad. hulgaricum may seem atypical but 

 this strain has been grown at room temperature (21° — 25°C.) for 

 over three years and is therefore quite comparable with No. 53B2, the 

 other high-acid-producing organism. The active acid-reducing property 

 of the veasts other than the red veast LZ, is also marked. These char- 

 acteristics are more graphically illustrated by curves II, III, and IV. 



It will be remarked that in every case the lactic bacteria growing with 

 the yeast LZ continue to make acid, even at the end of 56 days. This 

 was attributed to the fact that this yeast apparently did not grow, as 

 no red color had developed at the end of 32 days; microscopical ex- 

 amination confirmed this theory. At this time, the delinquent cultures 



