Fermentation plays a critical role in the transformation of cacao beans to finished chocolate. Before fermentation, cacao beans taste almost nothing like chocolate. The principal effect of fermentation is to eliminate or drastically reduce astringency and to increase the cacao's flavor complexity.
Astringency is the drying sensation in the mouth. This is the effect you get when chewing on grape skins or eating a bit of the interior skin of an underripe banana. Astringency should not be confused with bitterness which is associated more with the caffeine-like chemical in chocolate called "theobromine".
The principal effect of fermentation is to eliminate or drastically reduce astringency and to develop the full cacao flavor.
Fermentation must be carried out on or near the cacao farm shortly after harvest. It involves removing the cacao seeds and surrounding pulp from the protective pod, then placing them in a wooden bin covered with banana leaves. Yeast settles on the pulp and ferments the natural sugars to alcohol, at which point naturally-occurring bacteria take over. The pH of the seeds decreases while the temperature increases to as high as 140degrees F.
The entire fermentation process takes four to seven days. During this time, the farmer's knowledge and experience dictates the timing of critical steps, such as turning the cacao beans for aeration.
After fermentation, the beans are dried in the sun for up to seven days. Sun drying preserves the beans' natural flavors, and reduces the danger of exterior hardening before the bean's interior moisture can evaporate.