294 



STATE BOARD OF AGRICULTURE. 



From the above table it will be interesting to note how some of the germs which 

 seemed the least resistant to carbon dioxide are most resistant to the acid. No. 123 is 

 a good illustration of this. In some of the other cases there is a correspondingly 

 apparent inhibition due to the acid as was previously due to the carbon dioxide. Con- 

 sequently some attention must be given to the direct bearing of the acidity upon the 

 growth of micro-organisms in milk; accordingly a study of carbon dioxide in its 

 action upon the indicator (phenolphthalein) should be made before considering the 

 direct bearing the acidity has upon aeration, for it is very evident that the acidity 

 of milk may be most erroneously estimated if the indicator does not satisfactorily 

 respond. The action of carbon dioxide upon phenolphthalein is well known. 



XXIII. HOW CARBON 



DIOXIDE MAY INCREASE THE ACIDITY OF MILK TO 

 PHENOLPHTHALEIN. 



Two Hesse flasks containing milk were sterilized, one was allowed to remain in con- 

 tact with the air and the other had passed into it carbon dioxide to complete substitu- 

 tion. The following brief statement gives the results: 



Flask of milk sterilized and exposed to air gave in acidity 16° (.144 per cent). 

 Same milk sterilized in Hesse flask and carbon dioxide substituted for air gave 

 37° (.333 per cent) acidity. 



Both flasks were allowed to stand 48 hours before testing acidity, using phenol- 

 phthalein as indicator in both cases. 

 This study is purposely made as explanatory to what follows. Phenolphthalein is 

 the most satisfactory indicator for measuring the acidity, yet the influence of carbon 

 dioxide and of acid phosphates upon it should be constantly regarded in the considera- 

 tion of this work, otherwise misinterpretations may follow. In the case above there has 

 been practically an increase of over .1 per cent acidity, yet further work indicates 

 that there is a greater difference in the amount of acidity produced in carbon dioxide 

 cultures as compared with that produced in air cultures than can be accounted for by 

 the influence of carbon dioxide upon the indicator. 



XXIV. HOW CARBON DIOXIDE MAY INCREASE THE ACIDITY OF MILK CULTURES 



OF SPECIFIC MICRO-ORGANISMS. 



Bearing in mind that carbon dioxide and acid phosphates may in and of themselves 

 increase the acidity from .2 per cent to .3 per cent, yet the table which is appended 

 indicates in many instances an advanced increase beyond this limit; in other words, 

 the acidity is greater than that which can be accounted for by carbon dioxide alone, the 

 acid phosphate being equal in both instances. However, care should be taken to allow 

 in each case, at least .2 per cent for the possible increase due to carbon dioxide present 

 in cultures. Perhaps other indicators might be found which would avoid the trouble 

 experienced by the use of phenolphthalein, still cochineal, methyl orange, rosolic acid, 

 and litmus were tried and the results were even more unsatisfactory. The table below 

 in several instances will give such an increase in the acidity, that to account for it by 

 the presence of carbon dioxide would apparently be fallacious. We have not been 

 able to increase the acidity of milk .5 per cent simply by saturation with carbon dioxide 

 as was done in the Hesse flasks in which the acidity was increased .189 per cent. 



XXI. 



HOW CARBON DIOXIDE MAY INCREASE THE ACIDITY 



SPECIFIC MICRO-ORGANISMS. 



OF MILK CULTURES OF 



No. of micro-organism. 



Acidity of air cultures. . 

 Acidity of COs cultures. 



126. 



.137 

 .436 



No. of micro-organism. 



Acidity of air culture. . 

 Acidity of COs culture. 



127. 



128. 



.641 

 .659 



129. 130. 131. 132. 133 



.173 

 .432 



.094 

 .486 



.580 

 .731 



Neut. 

 .522 



.036 

 .367 



134. 



.670 



.720 



