224 ALCOHOL, VINEGAR, SAUER KRAUT, TOBACCO, SILAGE, FLAX. 
longer than this. So long as it remains properly acid it will keep, 
and if kept cold will remain in this condition for many months. 
Eventually a scum appears on its surface. This scum proves to 
be made of microorganisms, chiefly a species called Oidium lactis, 
a very common species around farms. These organisms, growing 
in the scum, gradually absorb and destroy the lactic acid and, as 
they do so, the sauer kraut becomes less acid and is finally alkaline. 
After this has occurred the putrefactive bacteria that are sure to 
be present have an opportunity to grow, and the sauer kraut begins 
to decay, so that it rapidly spoils. This product is, thus, one that 
is at first prepared and preserved by certain kinds of microorganisms, 
but is eventually ruined by the growth of other species. 
Certain other vegetables are prepared in a similar way. Soured 
beans are prepared in certain countries, and the souring of cucum- 
bers to make dill-pickles is, apparently, an identical process. Soured 
beets and asparagus are also articles of diet. Bacteria similar to 
those found in sauer kraut are concerned both in the souring of 
these products and in the subsequent neutralization of the acid 
preparatory to the final spoiling of the product. 
THE CURING OF TOBACCO. 
Tobacco is a product whose value is almost wholly dependent 
upon the success of its curing and its final preparation for market. 
The green plant, as taken from the field, is in itself valueless, and 
many a crop is injured or perhaps ruined in the curing. The 
relation of the curing to microorganisms is not yet settled, but 
since the curing is undoubtedly a fermentation, it properly belongs 
to our subject. 
When the leaves are fully grown the crop is reaped and hung up 
in a shed or barn to undergo a partial drying. After the drying 
process has reached a desired stage the leaves are ready for the 
fermentation proper, the process upon which the value of the 
product largely depends. There are two quite different methods 
of bringing about this fermentation. In the first method the leaves 
are left hanging a long time, and are eventually packed closely in 
