52 BIOLOGY AND TECHNIQUE 



fermentation by the yeasts are extremely important, since, upon obser- 

 servance of these, depends much of the uniformity of result which is so 

 desirable in the industries mentioned above. The optimum concentra- 

 tion of sugar for the production of the highest quantity of alcohol is at 

 or about 25 per cent. The temperature favoring the process ranges 

 about 30 C. Under such conditions fermentation may continue until 

 the alcohol forms almost a 20-per-cent solution. Most of the fermenta- 

 tions important in the wine, beer, and spirit industries, take place under 

 anaerobic conditions, since the carbon dioxide which is formed soon 

 shuts out any excess of air. 



In the industrial employment of yeasts for fermentative purposes, it 

 is necessary to work with specific strains, and in scientifically conducted 

 vineyards, breweries, and distilleries the study and pure cultivation of 

 the yeasts form no unimportant part of the work. Certain races of yeasts 

 are more uniform in their fermentative powers than others, and the by- 

 products formed by some races differ sufficiently from those of other 

 races to cause material differences in the resulting substances. In the 

 wine industries, the yeasts differ much from one another according to 

 climatic and other environmental conditions. In vineyards, natural 

 inoculation of the grapes occurs by transportation of the yeast from 

 the soil to the surface of the grapes by wasps, bees, or other insects, 

 through whose alimentary canals the microorganisms pass uninjured. 

 In the autumn the yeast is returned to the soil by falling berries and 

 remains alive in the upper layers of the ground throughout the winter 

 months. In actual practice this natural yeast inoculation is not de- 

 pended upon, but pure cultures of artificially cultivated yeasts are 

 employed for inoculation. In some of the wine-growing countries these 

 are supplied by special government experiment stations. 



Denitrifying Bacteria. Nitrogen is most readily absorbed by plants 

 in the form of nitrates. These are furnished to the soil chiefly by the 

 proteid decomposition induced by the proteolytic bacterial enzymes. 

 It is self-evident, therefore, that any cleavage which reduces nitrog- 

 enous matter beyond the stage of nitrates, to nitrites and ammonia, 

 detracts from the value of the nitrogen as a food stuff for plants, 

 and the eventual setting free of nitrogen in the elementary state ren- 

 ders it entirely valueless for any but the leguminous plants. 



Nevertheless, this process of nitrogen waste or denitrification is 

 constantly going on in nature. In the course of ordinary decomposition, 

 there is a constant reduction of nitrogenous matter to nitrites and salts 

 of ammonia, actively taken part in by a host of bacteria, as many as 



