TECHNICAL FERMENTATION 123 



sewer contents is able to ferment filter paper (pure cellulose). The paper 

 becomes soft and transparent, and is finally completely dissolved. Methane 

 bacteria are at work also in the intestines of herbivores and of man, and 

 cause distention by the gases they produce. 



Mucilaginous Fermentation (96). 



Wine, beer, and milk sometimes become ' ropy,' slimy or ' stringy ' ; 

 boiled vegetables, too, sometimes undergo the same change. This is due to 

 the growth of bacteria, which ferment the carbohydrate constituents of the 

 liquid, and whose chief product is mucilage, carbon dioxide, free hydrogen, 

 and the inevitable fatty acids arising as by-products. In some cases the 

 nascent hydrogen combines with dextrose to form mannite. The mucilage 

 is a gum-like carbohydrate allied to vegetable gums, and having the same 

 composition as cellulose (C 6 H 10 O 5 ) n . It is not a fermentation product, 

 however, in the true sense ; that is to say, it is not a direct result of the 

 protoplasmic metabolism, but arises through the imbibition of water by the 

 outer layers of the cell-membrane. It is, in fact, similar to the jelly that 

 holds together the cells in a zoogloea. A useful task would be the investi- 

 gation of the nature of the unaltered inner layers of the cell-wall, which 

 consist perhaps of a cellulose-like carbohydrate. 



A number of mucilaginous bacteria have been described, and the species 

 are said to be different for different kinds of sugars. B. viscosus sacchari is 

 said to flourish only in fluids containing cane sugar, another only in grape 

 sugar (wine), and a third (B. viscosus lactici) in milk sugar. 



Fermentations in technical processes. 



Wherever fermentable material is handled on a large scale there is 

 danger of the invasion of ferment bacteria. And in many processes that 

 ' go on of themselves ' we must assume that bacteria are at work. Places 

 where such changes occur, and where the bacteriologist must seek for them, 

 will occur to every one. Some cases, such as the souring of bark liquor in 

 tannery, have already been tentatively examined. It will be worth while 

 to mention a few others. 



The ' Retting ' of Vegetable Fibres (97), such as flax and hemp, is also the 

 work of bacteria. The tissues are allowed to soak for a long time in water, 

 and fermentation sets in, the fibres being freed from the enclosing cells by 

 the solution of the middle layer of the cell- walls. This inter-cellular cement- 

 substance consists of so-called pectine compounds (pectine salts of calcium) 

 related to carbohydrates, which are dissolved by the bacteria. The fibres 

 can then be separated by mechanical means. Up to the present we know 

 only one of the micro-organisms concerned, an anaerobic Plectridium 

 by o-8/x), which can use ammonia as a source of nitrogen and 



