Fermentation can be carried out in a variety of ways, but all methods depend on removing the beans from the pods and heaping them together to allow micro-organisms to develop and initiate the fermentation of the pulp surrounding the beans.
On smallholdings, fermentation is usually done in heaps of beans enclosed by plantain or banana leaves. Heaps can be used to ferment any quantity from about 25kg to 2,500kg of cocoa beans. The fermentation usually lasts about five days and some farmers will mix the beans on the second or third day. Another smallholder method is to use baskets, lined and covered with leaves, to ferment the beans. Similarly, holes or small depressions in the ground can be used but this makes no provision for the juices to drain away.
In plantations or fermentaries, fermentation is normally carried out in large wooden boxes that typically hold 1 to 2 tonnes of beans. The boxes must have provision for the liquefied pulp to drain away and for entry of air. Boxes can measure 3ft to 5ft across and be 3ft deep, but shallow levels (10-20 inches) of beans are preferred to promote good aeration. The beans can be covered with banana leaves or sacking to conserve the heat generated during fermentation. Beans can be transferred from one box to another each day to ensure uniform fermentation and increase aeration. The boxes can be tiered to allow easy transfer of beans. Plantations usually ferment for a longer period than smallholders and 6 to 7 days is usual.
In some areas, where particularly acidic beans are produced, the beans are pressed prior to fermentation to reduce the amount of pulp and allow for better aeration of the beans and so reduce the acidity.
The fermentation process begins with the growth of micro-organisms. In particular, yeasts grow on the pulp surrounding the beans. Insects, such as the Drosophila melanogaster or vinegar-fly, are probably responsible for the transfer of micro-organisms to the heaps of beans. The yeasts convert the sugars in the pulp surrounding the beans to ethanol. Bacteria then start to oxidise the ethanol to acetic acid and then to carbon dioxide and water, producing more heat and raising the temperature. The pulp starts to break down and drain away during the second day. Lactic acid, which converts the alcohol to lactic acid in anaerobic conditions, is produced but, as the acetic acid more actively oxidises the alcohol to acetic acid, conditions become more aerobic and halt the activity of lactic acid. The temperature is raised to 40oC to 45oC during the first 48 hours of fermentation. In the remaining days, bacterial activity continues under increasing aeration conditions as the pulp drains away and the temperature is maintained. The process of turning or mixing the beans increases aeration and, consequently, bacterial activity. The acetic acid and high temperatures kill the cocoa bean by the second day. The death of the bean causes cell walls to break down and previously segregated substances to mix. This allows complex chemical changes to take place in the bean such as enzyme activity, oxidation and the breakdown of proteins into amino acids. These chemical reactions cause the chocolate flavour and colour to develop. The length of fermentation varies depending on the bean type, Forastero beans require about 5 days and Criollo beans 2-3 days.
Following fermentation the beans are dried. The oxidation reactions begun through fermentation continue during drying.
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Cook, L.R., Meursing, E.H., Chocolate production and use. Revised edition. Harcourt Brace Johanovic, 1982
Wood, G.A.R., Lass, R.A., Cocoa. 4th edition. Longman, 1985