36o LIFE : OUTLINES OF GENERAL BIOLOGY 



FERMENTATION 



The Yeast-plants. — In connection with fermentation something 

 must be said of the common yeasts. One is usually told that yeasts 

 are fungi; but even if this is a technically accurate classification, 

 it is not very enlightening. Just as there is much to be said for 

 separating off bacteria from all other creatures as a line of life by 

 themselves, a distinct phylum or group, so the yeasts should be 

 separated out of the mob of fungi — moulds and mildews, toadstools 

 and mushrooms — and kept by themselves. They are minute uni- 

 cellular plants that have the power of setting up the process of 

 fermentation, which is so important in making both bread and beer. 



The use of yeast in changing sugars into alcohol and carbon 

 dioxide gas goes back for at least four thousand years, and some 

 people associate the discovery with the name of Noah. For many 

 centuries, at any rate, it has been well known that the "barm" or 

 "leaven" was indispensable in the brewing or the baking, yet its 

 rank as a living creature was not realised till less than a hundred 

 years ago. It is true that in 1680 Leeuwenhoek, a most extraordinary 

 observer, sent a paper to the Royal Society entitled "De Fermento 

 Cereviseai", in which he described the small ovoid globules of yeast, 

 each about one-three-thousandth of an inch in diameter; but he 

 did not recognise their plant nature. As Mr. A. Chaston Chapman 

 recently pointed out in his presidential address to the Royal 

 Microscopical Society, it was not till about 1837 that "the true 

 nature of the yeast organism was definitely and independently 

 discovered by three observers — Cagniard de Latour, Schwann, and 

 Kiitzing. These observers recognised that yeast is composed of a 

 vast number of small transparent globules which reproduce by 

 budding, and which consist of a cell wall with granular contents." 

 About two years afterwards Schwann observed that the yeast- 

 plant can also multiply by forming internal spores. By and by came 

 the detection of vacuoles and nucleus, granules and threads, in the 

 well-equipped microcosmic laboratory. 



The oldest notion of yeast fermentation was that the yeast is a 

 very active something with particles in vigorous motion, and that 

 this mobility is somehow imparted to the more sluggish particles 

 of the substance that is fermented. Second, there was Liebig's 

 chemical theory (1839) that oxidation of nitrogenous matter in the 

 yeast brings about contact-decomposition in the molecules of the 

 fermentable matter in whjch the yeast is working. Thirdly, this was 

 modified by Pasteur's insistence (1856) that the formation of the 

 various fermentation products is directly associated with the life 

 activity of the yeast-plant. 



But fourthly, there came in 1897 Biichner's important discovery 



