More on cacao fermentation

Nov 12 2019

Here, I’d like to treat the topic of cacao fermentation in more depth. Let’s talk first about how it is carried out and how it progresses biochemically, then we’ll talk about the importance of fermentation. Normally I cite all my references, but this post will be a little different because I’ve had so many casual conversations, emails, and text messages with cacao farmers to accumulate anecdotal information that I won’t be able to cite those. So, if I make an uncited statement in the following paragraphs, it is drawn from one of these conversations.

Cacao fermentation looks different around the world. In Western and Central Africa, where more than 80% of cacao is grown by smallholders whose farms do not exceed 4 hectares in size (Duguma, Gockowski, & Bakala, 2001) harvests are generally very small and so there the heap fermentation method is used, although it is rare elsewhere. Heap fermentation consists of scooping the cacao seeds and mucilage (pulp) into a pile on top of banana leaves on the forest floor (Daniel et al., 2009). The pile is left to ferment for a few days, covered in banana leaves, until the smell seems right and the beans have turned a dark brown color, at which point the piles are gathered up and brought to a more central location to be dried.

A related method is basket fermentation, in which small volumes (say, an individual family’s harvest) of cacao beans and pulp are placed in a woven basket to ferment (Hatmi, Kobarsih, & Cahyaningrum, 2015). One mostly hears of this method being used in Asia, and it is applied to shockingly small quantities—I spoke with one cacao farmer using this method whose baskets held only 3kg of beans! Frankly I was amazed this was enough volume to reach the necessary heat requirements for cacao fermentation [a ferment must reach 45-50C to kill the embryo of the seed (Schwan & Wheals, 2004)] .

Recently in Central Africa some producers have begun attempting tray fermentations, in which cacao beans are spread out on bamboo trays about 10cm in height which may be stacked on top of one another. The stacked layers allow for more air circulation and eliminate the need for stirring. Thus, fermentation proceeds more quickly than with heap or basket fermentation. However, it is likely that this method is not appropriate for fine flavor cacao beans.

A much more common method is box fermentation, which is usually practiced in Latin America, and requires a larger harvest (Papalexandratou, Vrancken, de Bruyne, Vandamme, & de Vuyst, 2011). At its most simple, a box fermentation is simply a collection of beans and mucilage in a wooden box (usually called “single box fermentation”) but most box fermentation methods involve a network of tiered boxes that are poured from one to the other as a mechanism of turning or stirring the beans. Three or even four tiers are typical. Upon harvest, beans and mucilage will be placed into the uppermost box, then covered with banana leaves (which serve as both an inoculum, because the waxy underside of the banana leaf is covered with fermentative microorganisms, and insulation to help maintain a warm anaerobic environment). It is left alone there for 24-48 hours. Then, panels on the front of the box are removed, and the contents of the first box are scooped into the second box, where the process is repeated (Papalexandratou et al., 2011). This pouring from one box to the next aerates the beans, making an environment conducive to acetic acid bacteria, but it also helps homogenize the temperature and break up clumps. When the process has been carried out properly, the beans reach the bottom box at the same time that fermentation is finishing, and then they are spread out and dried.

Regardless of the method used, the microbial succession proceeds in about the same way. First, it enters the yeast stage. This stage is anaerobic; the organisms are inhibited by oxygen. However, there is not much oxygen in the mass at this stage, because the pulp creates one gooey, sticky blob, and when the top of the mass is covered with something (usually banana leaves) this insulates further from airflow. The organisms active at this stage, yeast, are different in every cacao fermentation, but tend to fall into the genuses of Hanseniasporia, Saccharomyces, Pichia, Candida, Torulaspora, and Kluyveromyces (Mota-Gutierrez et al., 2018). Contrary to popular belief, it is not the beans themselves that ferment per se but rather the cocoa pulp. It is rich in sugars (glucose, fructose, and sucrose) and polysaccharides, both of which are metabolized by the yeasts. The simple sugars are metabolized into alcohol, and the polysaccharides (structural sugars like pectin) are broken down in a way that helps to liquify the pulp (De Vuyst & Weckx, 2016). As it drains away, more oxygen enters the environment. Together, over the course of the first day or two of fermentation, these mechanisms gradually make the environment less suitable for the yeasts, which die off and are replaced by acetic acid bacteria (Ozturk & Young, 2017). The acetic acid bacteria (AAB) thrive in the aerobic stage that follows, converting the alcohol produced by the yeast into acetic acid and generating a great deal of heat. This combination of acid and heat (and some diffusion by ethanol) kills the embryonic seed of the cacao bean, which catalyzes a number of internal biochemical reactions that can greatly affect the final flavor of the chocolate (Schwan & Wheals, 2004). Organisms in the Acetobacter and Gluconobacter genuses are both very active in this stage. The exhaustion of the substrate (pulp) is indicated by a sharp drop in temperature, because the AAB no longer have a reactant for their metabolism, so they begin to die off. If fermentation has been carried out properly, this should coincide with the death of the germ and a stabilization in bean pH of approximately 5.0 (the specific pH will depend on the variety of the bean and the conditions of the fermentation). De Vuyst (2016) claims that this should happen after no more than four days of fermentation, but I find this number crazy—the length of the fermentation is very dependent on the bean variety, geographical origin, fermentation mass volume and method, etc. One of the best beans I know is fermented for twelve days! If the farmer told anyone he fermented for 12 days, no one would buy his beans—but he doesn’t tell them, and they buy his beans by the metric ton because they are delicious. There are all types of fermentations out there.

It is worth mentioning that “behind the scenes” while yeast and AAB have been running the show, a third category of organisms has been quietly working; these are the lactic acid bacteria (LAB) such as Lactobacillus spp. along with some Enterobacteriaceae and Tatumella (Papalexandratou et al., 2019). The role of these organisms is still being studied, but we know they are present during an entire cacao fermentation—throughout both the yeast and AAB phases (they are good cohabitators!). Some authors have speculated that they serve to balance pH through the production of a number of organic acids like succinic and malic acid (Mota-Gutierrez et al., 2018). It is also known that they can cause a slight rise in pulp pH at the very beginning of fermentation due to the assimilation of pulp citric acids; this may be significant, but so far its biochemical effects are unknown (Papalexandratou et al., 2019).

If fermentation goes on too long, or if beans are dried too slowly, gram-positive spore-formers in the Bacillus genus can take over the fermentation. This defect can be identified by an off-flavor that reminds me of old hot dogs or the smell of dog food, although it is probably characterized by tetramethyl pyrazine and short-chain fatty acids (Schwan & Wheals, 2004). Filamentous fungi (molds) may also play a role in extended (or poorly dried) cacao fermentations (Schwan & Wheals, 2004). They are associated with earthy, mushroomy, dirt-like off-flavors that not everyone finds entirely unpleasant.

Here, I have discussed the very minimum information about the microbiome of a cacao fermentation and their most basic inputs and outputs. The reality, of course, is much more complicated, and I hope to get into it in a later post. Each individual species of microorganism produces many hundreds or thousands of metabolites (byproducts), every single one of which may be flavor-active, aroma-active, chemosensory, or act as a precursor for additional flavor reactions. For example, enzymes produced by microorganisms break down the proteins in the cacao bean. At first, these don’t have much flavor, but when the cacao bean is roasted, they undergo Maillard reactions to become all the wonderful flavor compounds we recognize as chocolate. Without fermentation, the precursor compounds would not be available to undergo this reaction—a roasted, unfermented cacao bean would not taste like chocolate.

No two cacao fermentations ever measured have been exactly the same. Each fermentation has its own unique properties autochthonous to its growing region, season, and handlers. In the wine world, this would be known as terroir. Chocolate’s terroir comes from its fermentation step. Nat Bletter of Madre Cacao claims that fermentation gives chocolate 75% of its final flavor, and most in the industry are inclined to agree. Fermentation is the most important contributing factor to the final taste and quality of chocolate.

And yet, most of the people who perform this step don’t know much at all about how to do it. Most farmers ferment the way their parents, grandparents, uncles, or community members taught them. It is rare that a cacao farmer has any science to back up their manufacturing choices. How does a farmer decide when fermentation is finished? How does a farmer know if fermentation was carried out properly? How can a farmer assess if his or her beans are of good quality, or know if the beans will make good chocolate? A few farmers have had some training that allow them to make informed judgements of this type, but many more are essentially operating blind, in many cases without ever even having been exposed to the finished product (chocolate).

In the next post, I will discuss some of the metrics that I think might be of interest in determining fermentation quality and developing good manufacturing practices on the farm level.

Works Cited

Daniel, H. M., Vrancken, G., Takrama, J. F., Camu, N., De Vos, P., & De Vuyst, L. (2009). Yeast diversity of Ghanaian cocoa bean heap fermentations. FEMS Yeast Research, 9(5), 774–783.

De Vuyst, L., & Weckx, S. (2016, July). The cocoa bean fermentation process: from ecosystem analysis to starter culture development. Journal of Applied Microbiology, Vol. 121, pp. 5–17.

Duguma, B., Gockowski, J., & Bakala, J. (2001). Smallholder cacao (Theobroma cacao Linn.) cultivation in agroforestry systems of West and Central africa: Challenges and opportunities. Agroforestry Systems, 51(3), 177–188.

Hatmi, R. U., Kobarsih, M., & Cahyaningrum, N. (2015). Fungi Level Analysis of Cocoa Beans Based on Fermentation Box Type and Duration. Procedia Food Science, 3, 371–382.

Mota-Gutierrez, J., Botta, C., Ferrocino, I., Giordano, M., Bertolino, M., Dolci, P., … Cocolin, L. (2018). Dynamics and biodiversity of bacterial and yeast communities during fermentation of cocoa beans. Applied and Environmental Microbiology, 84(19), e01164-18.

Ozturk, G., & Young, G. M. (2017, May 1). Food Evolution: The Impact of Society and Science on the Fermentation of Cocoa Beans. Comprehensive Reviews in Food Science and Food Safety, Vol. 16, pp. 431–455.

Papalexandratou, Z., Kaasik, K., Kauffmann, L. V., Skorstengaard, A., Bouillon, G., Espensen, J. L., … Nielsen, D. S. (2019). Linking cocoa varietals and microbial diversity of Nicaraguan fine cocoa bean fermentations and their impact on final cocoa quality appreciation. International Journal of Food Microbiology, 304, 106–118.

Papalexandratou, Z., Vrancken, G., de Bruyne, K., Vandamme, P., & de Vuyst, L. (2011). Spontaneous organic cocoa bean box fermentations in Brazil are characterized by a restricted species diversity of lactic acid bacteria and acetic acid bacteria. Food Microbiology, 28(7), 1326–1338.

Schwan, R. F., & Wheals, A. E. (2004). The microbiology of cocoa fermentation and its role in chocolate quality. Critical Reviews in Food Science and Nutrition, 44(4), 205–221.