MICROBIOLOGY OF ALCOHOL AND ALCOHOLIC PRODUCTS. 423 



on injured or carelessly handled grapes sufficiently to interfere with fer- 

 mentation and seriously impair the quality of the wine. 



THE MICROORGANISMS FOUND IN WINE. 



Wine microorganisms may be conveniently divided into two groups: 

 those which grow only in the presence of notable supplies of free 

 oxygen, and those which require or grow better in the absence of free 

 oxygen. 



AEROBIC ORGANISMS. Mycoderma. If a normal wine, especially 

 one strong in alcohol, is left with its surface exposed to the air, it will 

 usually, in a few days, be covered with a whitish film, thin and smooth 

 at first but gradually becoming thicker and finally rough and plicate. 

 This is what is known to wine-makers as "wine flowers." This film con- 

 sists of yeast-like cells, somewhat longer and more cylindrical than S. el- 

 lipsoideus, reproducing by budding and forming large aggregations. 



Pure cultures show that there are many varieties of this organism 

 differing in the color and texture of the film, in the cloudiness of the 

 liquid and in the character of the deposit. They are called collectively 

 Mycoderma vini, though one form which has been found to produce 

 endospores has been called S. anomalous. 



These organisms are strongly aerobic and can develop only on the 

 surface in full contact with the air. They are a serious enemy to the 

 wine, rendering it insipid and cloudy. They attack the extract, fixed 

 acids, and alcohol, producing at first volatile acids and finally causing 

 complete combustion of the organic matters to carbon dioxide and 

 water, destroying the wine completely. 



Acetic Bacteria. The film formed on wines exposed to the air, 

 especially on those of low alcoholic content, will often differ from that 

 due to Mycoderma vini. It will be thinner, smoother and consist of 

 bacteria. These are the vinegar bacteria described on page 448. They 

 grow not only on the wine at the expense of the alcohol, but on crushed 

 grapes and must at the expense of the sugar, producing acetic acid in 

 both cases. 



Acetic acid in small amounts is produced by the yeast and is a normal 

 constituent of wine. Unless in excess its effect is not injurious. There 

 may be present from .09 g. in 100 g. in light white wine to .14 g. in a 

 heavy red wine without deterioration of quality. In sweet wines, even 

 a somewhat larger amount may be present without causing injury. 



