To set the new state of the tobacco leaf, another procedure called fermentation is necessary.
Insofar as the cell is living, microorganisms (all possible bacteria, mold and so on) parasitize it, but they do not interfere with its vital activity, and coexist peacefully. As soon as the cell dies and ceases to supply the "parasites" with everything needed, they themselves begin to obtain subsistence under the action of enzymes. Microorganisms are anaerobic, and they take the oxygen necessary for the life from the molecular chains of complex chemical compounds by splitting them. This is a description of the process of fermentation.
The first fermentation occurs in the same drying houses: at this stage the leaves are vulnerable, insufficiently elastic, and can be damaged while transport. Furthermore, too high a humidity or rain at the time of transportation can provoke uncontrollable fermentation and spoil tobacco.
In principle, it would be theoretically possible to begin and finish fermentation in the same drying houses. But state production in Cuba requires that this important stage be conducted at special plants (centro de beneficio), under the control of "persons in charge". Specifically, this explains the fact that fermentation is conducted in two stages, and the second stage is primary.
When some non-Cuban producers of cigars speak about the fifth or sixth fermentations their leaves allegedly undergo before their cigars are twisted, they are slyly deceiving. Fermentation is a unified process that can be broken into as many parts as convenient. Even if a finished cigar were moistened with water, fermentation would begin.
In the tobacco house (la casa del tabaco) the leaves are removed from the posts and are packed into large heaps - bales - very tightly so that air will not reach the inside. Otherwise microorganisms would take oxygen from the air, and they have to obtain it by splitting the molecular chains of chemical elements.
Humidity is another important factor that facilitates fermentation. Water causes fermentation. Microorganisms move more easily in water, and without it they perish.
The third factor is temperature (the splitting of complex chemical elements always educes the energy that was spent with their creation). Thermometers are placed in the bales with the aid of which the temperature is monitored so that the tobacco does not get overheated and burn. If it gets too high, then the leaves are thrown apart and gathered up again in order to stop the dynamics of combustion: thus the leaves get mixed - those that were outside may wind up in the middle and vice-versa. This can happen several times.
At the factories, leaves are sorted before the second fermentation, depending on their texture, density, size, and colors; the wrapper and binder leaves are sorted separately. This way fermentation is produced more precisely: indeed the oily, dense leaves from the upper part of a plant require a more prolonged process.
It is very complicated to determine when fermentation should be stopped. The process can be tracked by temperature: when it stops rising, the fermentation is approaching the end. Another method is to measure the level of the ammonia released: if it is reduced, the end is near. An experienced master determines when the fermentation is over immediately according to several indices, including the smell and external look of the leaves.
The duration of basic fermentation is 1.5 - 2 months. Excessive fermentation kills the taste and aroma of the tobacco: it simply rots. But if by mistake the time is stopped earlier, it is not terrible. At some factories (for example, El Laguito) the raw material undergoes one more, short, fermentation for one to three days: this is some kind of a last touch, although the third fermentation has practically no effect on the quality of the product.