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Even the biggest chocolate lovers, though, might not recognise what this ancient food has in common with kimchi and kombucha: its flavours are due to fermentation.
That familiar chocolate taste is thanks to microorganisms that help transform chocolate’s raw ingredients into the much-beloved rich, complex final product.
After decades of testing, researchers have solved many of the mysteries that govern cacao fermentation, including which microorganisms participate and how this step governs chocolate flavour and quality.
From seed pod to chocolate bar
The food you know as chocolate starts its life as the seeds of football-shaped pods of fruit growing directly from the trunk of the Theobroma cacao tree. But as long as 3900 years ago the Olmecs of Central America had figured out how to transform these giant seed pods into an edible treat.
First, workers crack the brightly coloured fruit open and scoop out the seeds and pulp. The seeds, now called beans, cure and drain over the course of 3-10 days before drying in the sun. The dry beans are roasted, then crushed with sugar and sometimes dried milk. At this point, the chocolate is ready to be fashioned into bars, chips or confectionery.
It’s during the curing stage that fermentation occurs. Fermentation is the process of improving the qualities of a food through the controlled activity of microbes, and it allows the bitter, otherwise tasteless cacao seeds to develop the rich flavours associated with chocolate.
Microorganisms at work
Cacao fermentation is a multi-step process. Any compound microorganisms produced along the way that changes the taste of the beans will also change the taste of the final chocolate.
The first fermentation step may be familiar to home brewers, because it involves yeasts. Just like the yeast in your favourite brew, yeast in a cacao fermentation produces alcohol by digesting the sugary pulp that clings to the beans.
This process generates fruity-tasting molecules called esters and floral-tasting fusel alcohols. These compounds soak into the beans and are later present in the finished chocolate.
As the pulp breaks down, oxygen enters the fermenting mass and the yeast population declines as oxygen-loving bacteria take over. These bacteria are known as acetic acid bacteria because they convert the alcohol generated by the yeast into acetic acid.
The acid soaks into the beans, causing biochemical changes. The sprouting plant dies. Fats agglomerate. Some enzymes break proteins down into smaller peptides, which become very "chocolatey"-smelling during the subsequent roasting stage. Other enzymes break apart the antioxidant polyphenol molecules, for which chocolate has gained renown as a super-food. As a result, contrary to its reputation, most chocolate contains very few polyphenols, if any.
All the reactions kicked off by acetic acid bacteria have a major impact on flavour. These acids encourage the degradation of heavily astringent, deep purple polyphenol molecules into milder-tasting, brown-coloured chemicals called o-quinones. Here is where cacao beans turn from bitter-tasting to rich and nutty. This transformation is accompanied by a colour shift from reddish-purple to brown.
Finally, as acid slowly evaporates and sugars are used up, other species - including filamentous fungi and spore-forming Bacillus bacteria - take over.
Terroir of a place and its microbes
Cacao is a wild fermentation - farmers rely on natural microbes in the environment to create unique, local flavours.
This phenomenon is known as "terroir": the characteristic flair imparted by a place. In the same way that grapes take on regional terroir, these wild microbes, combined with each farmer’s particular process, confer terroir on beans fermented in each location.
A bar from Madagascar may be reminiscent of raspberries and apricots, while Peruvian chocolate can taste like it’s been soaked in sauvignon blanc. Yet in both cases, the bars contain nothing except cacao beans and some sugar.
This is the power of fermentation: to change, convert, transform. It takes the usual and make it unusual - thanks to the magic of microbes.
• Caitlin Clark is a Ph.D candidate in food science at Colorado State University.