ISO FERMENTATION IN MILK 



view, and proved facts in any of these directions threw light upon 

 the other allied subjects. 



In 1776 Spallanzani showed that organic liquids and materials 

 could be prevented from undergoing fermentation by being 

 thoroughly boiled and subsequently protected from the air by 

 sealing the flasks. If, however, the flasks were cracked, or, in any 

 other way, air even in small quantities gained admittance, then 

 almost invariably both organisms and fermentation appeared. This 

 phenomenon was interpreted as due to the entrance of the oxygen 

 of the air (Gay-Lussac). Schwann, the founder of the cell- theory, 

 and Schultze both showed, however, that if the air gaining access to 

 the flask were calcined or drawn through strong acid (or potash and 

 sulphuric acid), the result was the same as if no air entered, namely, 

 there were no organisms and there was no fermentation. It thus 

 became evident that the origin of the fermentation was not in the 

 atmosphere, nor yet in the oxygen of the atmosphere, but in some 

 fermenting agent borne into the flask by the unsterilised air. To 

 this primary contribution Schwann added two other new facts. 

 The first was the discovery of the yeast cell and its function. The 

 yeast itself was simultaneously seen by Cagniard-Latour. In point 

 of fact neither of these men was first in that particular observation, 

 for Leeuwenhoek had already seen yeast plants, but had mistaken 

 them for crystals. Schwann, however, advanced a great step forward, 

 for he not only rediscovered the yeast cell but learned its function. 

 He devised a simple but striking experiment to illustrate the direct 

 action of yeast on a solution of sugar, namely, splitting it up into 

 carbonic acid and alcohol. In a long tube he placed a solution of 

 sugar faintly tinged blue with litmus, and to this he added very 

 little yeast, so that the cells had time to subside as a deposit at 

 the foot of the tube. The red coloration of the litmus due to the 

 liberation of the carbonic acid then began to appear at the bottom 

 of the tube.^ 



The second new fact contributed by Schwann proved to be a 

 stage in the further establishment of the hypothesis that the fer- 

 menting agent was introduced by the air. To unsterilised air 

 he added an antiseptic substance which would destroy the fer- 

 menting agent. Arsenic was found by Schwann to possess this 

 germicidal property. From these facts Schwann drew the conclu- 

 sion that vinous fermentation was due to an agent borne in by 

 the air, and that this agent was yeast. If, however, the air, as the 



^ Some Apostles of Physiology, by William Stirling, M.D., D.Sc. (1902), 

 p. 98. 



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