420 MICROBIOLOGY OF SPECIAL INDUSTRIES. 



destruction of a certain amount of material, and a consequent loss of quan- 

 tity, which is, in certain circumstances, more than counterbalanced by an 

 increase in quality (wines of the Rhine, Sauternes) ; again, an oxydase 

 is produced which tends to destroy the color, brightness and flavor of the 

 wine. This can be counteracted by the judicious use of sulphurous acid. 



YEASTS. The true yeasts occur much less abundantly on grapes than 

 the molds. Until the grapes are ripe they are practically absent, as first 

 shown by Pasteur. Later, they gradually increase in number and on very 

 ripe grapes often become abundant. In all cases and at all seasons, 

 however, their numbers are much inferior to those of the molds and pseudo- 

 yeasts. The cause of this seems to be that in the vineyard the common 

 molds find conditions favorable to their development at nearly all seasons 

 of the year, but yeasts only during the vintage season. 



Investigations of Hansen, Wortmann and others show that yeasts 

 exist in the soil of the vineyard at all times, but hi very varying amounts. 

 For a month or two following the vintage, a particle of soil added to a 

 nutritive solution contains so much yeast that it acts like a leaven. For 

 the next few months, the amount of yeast present decreases until a little 

 before the vintage, when the soil must be carefully examined to find any 

 yeast at all. As soon as the grapes are ripe, however, any rupture of the 

 skin of the fruit will offer a favorable nidus for the development and in- 

 crease of any yeast cells which reach it. Where these first cells come from 

 has not been determined, but as there are still a few yeast cells in the soil, 

 they may be brought by the wind, or bees and wasps may carry them from 

 other fruits or from their hives and nests. 



The increase of the amount of yeast present on the ripe grapes is often 

 very rapid and seems to have (according to Wortmann) a direct relation to 

 the abundance of wasps. These insects, passing from vine to vine, crawl- 

 ing over the bunches to feed on the juice of ruptured berries, soon inocu- 

 late all exposed juice and pulp. New yeast cultures are thus produced, 

 and the resulting yeast cells quickly disseminated over the skins and other 

 surfaces visited. 



The more unsound or broken grapes present, and the more honey -dew 

 or dust adhering to the skin, the larger the amount of yeast will be. The 

 same is true, however, also of molds and other organisms. 



In the older wine-making districts, much of the yeast present on the 

 grapes will consist of the true wine yeast, S. ellipsoideus. The race or 

 variety of this yeast will differ, however, in different districts. Usually 



