ALKALI-FOKMING BACTERIA FOUND IK MILK. 



27 



In order to show whether the alkaline reaction was due to the 

 production of carbonates or bicarbonates, or both, Table 12 has been 

 prepared, using the organic acids which were utilized as test sub- 

 stances in this investigation. Here again attention is called to the 

 fact that the essential process in the production of an alkaline reaction 

 is the replacement of the relatively strong organic acids by the 

 relatively weak carbonic acid. The figures in the table are based 

 on the complete oxidation to the carbonate or bicarbonate of 100 c. c. 

 of a 0.1 per cent solution of the acid. In the next to the last column 

 of the table the tenth-normal equivalent is given in cubic centimeters 

 for both the carbonate and bicarbonate. This table should be 

 studied in connection with the plot in figure 1 which represents the 

 change of any of the 

 test acid salt media 

 from the initial Ph 6.9. 

 The resulting hydro- 

 gen-ion concentration 

 obtained when known 

 amounts of tenth-nor- 

 mal NaHCOg were 

 added to the medium 

 above is represented 

 by one curve. The 

 other curve was ob- 

 tained similarly by 

 plotting the resulting 

 hydrogen-ion concen- 

 tration when known 

 amounts of tenth-nor- 

 mal NagCOg were 

 added to the same 

 synthetic medium. It 

 will be noted that the large quantity of alkali generated by some 

 of the fermentations could not possibly be due to the complete 

 oxidation of the acid to bicarbonate. Let us take, for example, 

 propionic acid; on complete oxidation this acid yields an equivalent 

 of 1.3 cubic centimeters of tenth-normal alkali. From the curve it 

 is seen that 1.3 cubic centimeters of tenth-normal alkali gives a hy- 

 drogen-ion concentration of Ph 7.9 while the average hydrogen-ion 

 concentration of the alkali group of bacteria which fermented pro- 

 pionic acid was Ph 7.4. 



From this it is evident that the entire amount of alkali produced 

 from this organic-acid salt might be accounted for by the production 

 of sodium bicarbonate. On the contrary, if we examine the figures for 

 glyceric acid it will be found that this acid on complete oxidation yields 

 an equivalent of 0.94 cubic centimeters tenth-normal alkali,wliich corre- 





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Cubic ceniimeters of j^ a/lta/i 



Fig. 1.— Curve showing efEect of addition of bicarbonate and sodium 

 carbonate on the reaction of the sodium-ammonium-phosphate 

 medium. 



