Of the nature of the process we as yet know very little.  That it is a fermentation is certain, and it has been proved that it is produced by a definite species of bacterium which occurs on the indigo leaves.  If the sterilized leaves are placed in sterile water no fermentation occurs and no indigo is formed.  If, however, some of the specific bacteria are added to the mass the fermentation soon begins and the blue colour of the indigo makes its appearance.  It is plain, therefore, that indigo is a product of bacterial fermentation, and commonly due to a single definite species of bacterium.  Of the details of the formation, however, we as yet know little, and no practical application of the facts have yet been made.

Bacteria in tobacco curing.

A fermentative process of quite a different nature, but of immense commercial value, is found in the preparation of tobacco.  The process by which tobacco is prepared is a long and somewhat complicated one, consisting of a number of different stages.  The tobacco, after being first dried in a careful manner, is subsequently allowed to absorb moisture from the atmosphere, and is then placed in large heaps to undergo a further change.  This process appears to be a fermentation, for the temperature of the mass rises rapidly, and every indication of a fermentative action is seen.  The tobacco in these heaps is changed occasionally, the heap being thrown down and built up again in such a way that the portion which was first at the bottom comes to the top, and in this way all parts of the heap may become equally affected by the process.  After this process the tobacco is sent to the different manufacturers, who finish the process of curing.  The further treatment it receives varies widely according to the desired product, whether for smoking or for snuff, etc.  In all cases, however, fermentations play a prominent part.  Sometimes the leaves are directly inoculated with fermenting material.  In the preparation of snuff the details of the process are more complicated than in the preparation of smoking tobacco.  The tobacco, after being ground and mixed with certain ingredients, is allowed to undergo a fermentation which lasts for weeks, and indeed for months.  In the different methods of preparing snuff the fermentations take place in different ways, and sometimes the tobacco is subjected to two or three different fermentative actions.  The result of the whole is the slow preparation of the commercial product.  It is during the final fermentative processes that the peculiar colour and flavour of the snuff are developed, and it is during the fermentation of the leaves of the smoking tobacco—­either the original fermentation or the subsequent ones—­ that the special flavours and aromas of tobacco are produced.