[HARRISON] PRESIDENTIAL ADDRESS 3 



Schroder and Dusch who gave to bacteriology the cotton wool 

 plug and who made important contributions to the study of fermenta- 

 tion, fell into error regarding milk, owing, as we know now, to the 

 difficulty of sterilizing it. They attempted to preserve milk by boiling 

 it and then filtering all the air that subsequently was brought into 

 contact with it, but as milk so treated soured they concluded that the 

 souring of milk was a spontaneous change. 



In the course of his experiments on spontaneous generation, 

 Pasteur, between the years 1850-1860 approached the subject of the 

 fermentation of milk. He showed that organisms were always present 

 in fermented milk and in 1857 described a "yeast" which he found 

 in sour milk. Undoubtedly he was not working with a pure culture, 

 as the milk acted upon by his "yeast" formed products which did not 

 result from the growth of lactic acid organisms. In 1858 he succeeded 

 in separating alcoholic from lactic fermentation and made a more 

 careful examination of the organism producing the latter. Pasteur 

 shewed that the two fermentations were distinct and whenever lactic 

 acid was formed he could always find his lactic yeast. 



Here, then, was the first real conception of a number of distinct 

 species of organisms in milk, each producing characteristic changes 

 or a distinct kind of fermentation. 



In 1860, Pasteur published his results dealing with the changes 

 occurring in milk which had been boiled and which had subsequently 

 fermented spontaneously. Such milk did not become sour but bitter, 

 and contained butyric and not lactic acid. Microscopic examina- 

 tion revealed the presence of numerous organisms different 

 from those of sour milk. He regarded these organisms as Infusoria 

 because they were motile. As they had withstood the temperature of 

 boiling he though that they were introduced from outside. He named 

 this organism Vibrio hutyricus. As this organism had withstood the 

 temperature of boiling, Pasteur tried the effect of exposing it to a 

 higher temperature. He heated milk under a pressure of two and a 

 half atmospheres and obtained a temperature of 110 to 112°C. After 

 such heating, milk if kept out of contact with air, would remain sweet 

 indefinitely, but it would undergo fermentation if inoculated with 

 micro-organisms. 



Thus, the butyric and lactic fermentations were separated, each 

 fermentation was associated with a particular type of micro-organism, 

 and milk was shown to be a liquid which might undergo a variety of 

 fermentative changes if acted upon by different organisms. 



Naturally, these results of Pasteur were challenged. 



