What does fermentation do to tobacco?

It's imperative that the wrapper, binder and filler are properly processed before rolling a cigar. After the cigar is made, fermentation ceases.

Tobacco fermentation means laying the leaves into huge "bulks", the centers of which develop heat. The heat in the center of a bulk should not be allowed to exceed about 115-130° Fahrenheit, depending on the type of tobacco, otherwise it will be ruined or “burned out.” When it gets up to that temperature, and it will do so in its own time depending on the leaf and its condition, the bulk gets turned inside out and the heat build up (fermentation) begins again. When the heat levels off, the fermentation is complete. This could occur after four turns or eight turns, referred to as "sweats". Over-fermentation will ruin the leaf, cause it to become "spent" and lose its flavor and aroma.

During "sweating", the fermentation process causes the emission of nitrogen compounds and other chemical compounds and somewhat reduces the nicotine content. After fermentation, further aging in bales helps to settle the leaf and enhances flavor and burning quality. Manufacturers who can't afford to wait or who just don't care to wait until this process is completed, produce inferior cigars.

If when you smoke your box of 25 cigars and find the following telltale signs, chances are the leaf has not been fully fermented or aged:

Harshness or bitterness on the tongue, lips and in the mouth. A feeling something like heartburn in the chest cavity. The cigar keeps going out easily. If this happens with a few cigars in your box, the manufacturer is not consistent in the use of his tobaccos. If this happens with a majority of them, he is not making the investment in fully-aged leaf and is using the tobacco before it’s ready.

One last point is once the cigar is made, it is impossible to ferment the tobacco further. How would it be possible it get the temperature up to 115° Fahrenheit (45+° Celsius.) in order to do the job? Some cigar people say properly stored cigars will "mature" and become mellower. Maybe, but, if unfermented or "raw" tobacco has been used, no amount of aging or maturing in the box will cure it.