i6o FERMENTATION IN MILK 



organism. For Pasteur, when he discovered it, discovered also the 

 principle of anaerobic life. It was in November i860 that, return- 

 ing to his studies on fermentation in general and lactic fermentation 

 in particular, Pasteur met with some complications which hampered 

 the purity and progress of his lactic fermentation. In short, he found 

 a secondary fermentation proceeding, namely, butyric. But, as 

 previously in the lactic acid investigation, he found that in addition 

 to the butyric acid being present there was also present a new 

 kind of organism. " The most constantly repeated tests," he 

 wrote in February 1861, "have convinced me that the transforma- 

 tion of sugar, mannite, and lactic acid into butyric acid is due 

 exclusively to those infusories, and they must be considered as the 

 real butyric ferment." Further, in studying under the microscope 

 a drop of butyric fermenting fluid, he observed that the organisms 

 at the edge of the drop were motionless and apparently dead, whilst 

 in the central portion of the drop the bacilli were executing those 

 active movements which are characteristic of their vitality. To 

 Pasteur's mind this at once suggested, what he was able later to 

 demonstrate, namely, that these organisms at the periphery were 

 paralysed by contact with the oxygen of the air. When he passed 

 a stream of air through a flask containing a liquid in butyric fer- 

 mentation, he observed that the process slackened and eventually 

 ceased. " Those infusory animalcula," he wrote, " live and multiply 

 indefinitely, without requiring the least quantity of air. And not 

 only do they live without air, but air actually kills them. It is 

 sufficient to send a current of atmospheric air during an hour or 

 two through the liquor where these vibriones were multiplying, to 

 cause them all to perish, and thus arrest the butyric fermentation, 

 whilst a current of pure carbonic acid gas passing through the 

 same liquor hindered them in no way. Thence this double 

 proposition," concluded Pasteur : " the butyric ferment is an 

 infusory ; that infusory lives without free oxygen." Thus was 

 discovered what Pasteur afterwards termed the anaerobies} It will 

 now be readily understood that the aerobic ferments give rise 

 to oxidation of certain products of decomposition ; the anaerobic 

 organisms, on the other hand, commence to grow when the 

 aerobic have used up the available oxygen. Thus in such fer- 

 mentation certain bodies (carbohydrates, fatty acids, etc.) undergo 

 decomposition, and by oxidation carbonic acid gas is produced ; 

 the remaining bodies are "reduced" products of the whole pro- 

 cess. Hence sometimes this is termed fermentation by reduction. 

 ^ For methods of cultivation of anaerobic bacteria, see p. 78. 



