214 



OUTLINES OF BACTERIOLOGY 



Brewery, Copenhagen. This species had crept into the stock yeast of 

 this brewery, imparting to the beer a disagreeable bitter flavour and a 

 smoky smell (Fig. 120). The cells have a strong tendency, as will be 

 seen by the diagram, to run into the sausage rather than the normal 

 globular or oval shape. As an example of a culture yeast, we may 

 take Saccharomyces cerevisiae I., isolated from a top-fermentation 

 brewery in Edinburgh (Fig. 118). 



Not only " wild " yeasts, but also bacteria, compete with the culture 

 yeasts for the mastery of the wort. There are, however, only a few 

 species which can do harm to any appreciable extent, because the 

 bitterness of the wort inhibits the growth of most bacteria, whilst the 

 introduction of the pure culture system renders more certain the success 

 of the culture yeast. Bacteria cause mucilage formation, decolorisa- 

 tion, turbidity, acid formation, and disagreeable smells and tastes. 



A common complaint in beer is ropiness. When thus affected, the 

 beer becomes thick, mucilaginous, and capable of being drawn out into 



threads. Two of the bacteria causing 

 this disease have been isolated from 

 Belgian ropy beers, and named respec- 

 tively Bacillus viscosus I. and Bacillus 

 viscosus II. These bacteria form rods 

 about 0'8 p broad and l'6-2'4 //, long. 

 They differ in that beer affected with 

 the first shows yellowish-white viscous 

 patches on the surface, which are want- 

 ing in beers infected by the second. 

 A third, called Bacillus viscosus III., 

 has been isolated from British ropy 

 beers. The ropiness is due to a 

 change in the cell-wall of the bacteria, 

 which becomes mucilaginous, and not 

 to any compound formed in the beer. 

 Finally, ropiness may be caused by the 

 ravages of a member of the Mould 

 Fungi, called Dematium pullulans. 

 This fungus has a branched, thread- 

 like structure (Fig. 12 la), and also forms a number of yeast-like cells 

 (Fig. 121ft). Another defect in beer is the production of turbidity, 

 which may be caused in several ways. It may be due to the precipita- 

 tion of albuminoids (gluten turbidity), or to the presence of unsaccharified 

 starch (starch turbidity), or to a high content of yeast-cells (yeast 



FIG. 121. Dematium pullulans. (a) 

 Tubular cells, forming conidia ; (I) cells 

 dividing up to form short swollen cells. 

 (After Zopf.) 



