From her own experience, Julie Lestang knows what happens when undesirable microorganisms penetrate the cacao fermentation process. Before coming to Zurich in November 2021, she worked for two years in Ivory Coast for the cacao division of an international food company. “Mould is a big problem,” Lestang says. “Sometimes, a ship’s entire cacao cargo has to be scrapped because it arrives mouldy.”
Mould not only affects the taste of the cocoa, but it can also be a health hazard, as the fungi produce dangerous mycotoxins. “That’s why, in our research project, we’re looking for microorganisms that inhibit fungal attack and lead to controlled and optimal fermentation of cacao beans,” Lestang explains. The French agronomist is currently pursuing a doctorate in the Laboratory of Food Biotechnology at ETH Zurich’s Department of Health Sciences and Technology. When she was younger, she first wanted to be a chocolatier after visiting a chocolate factory with her school. She ultimately chose another path, but during her agronomy studies in Montpellier, France, where she specialised in food production in Mediterranean and tropical countries, Lestang’s interest in the cacao fruit grew.
Improved control of fermentation
By the time a bar of chocolate reaches the grocery store shelf, the cacao it contains has not only travelled a long way, but has also undergone a certain amount of processing. A core part of this is fermentation, a natural process that begins as soon as the green to reddish-yellow cacao fruit is harvested, broken open, and the beans laid out in wooden boxes to dry. Naturally occurring microorganisms found on tools, on the banana leaves used to cover the beans, or on the farmers’ hands break down the sugars in the pulp that surrounds the beans. The process heats up the beans and makes them more acidic, destroying their cells. “This prevents the seeds from germinating; otherwise, you’d soon have lots of little cacao trees,” Lestang says with a laugh. During fermentation is also when the beans acquire their cocoa-brown colour and the bitter, earthy taste so typical of unsweetened chocolate.
The quality and taste of the cacao depend largely on which microorganisms are involved in this fermentation process. “If we treat the cacao beans with the right mix of microorganisms at the start of fermentation, we reduce health risks, quality loss and food waste,” Lestang explains.
Food technologists use yeasts and lactic acid bacteria as fungus-inhibiting fermentation starters. However, not all cacao beans react the same way to the microorganisms, because the beans’ chemical profiles and properties vary depending on the variety of cacao and where it was cultivated. That’s why in her project, Lestang is developing a method to determine the chemical fingerprint of cacao beans. Her work is making clear which starters best protect a bean from fungal attack.
For her analysis, Lestang uses rapid evaporative ionisation mass spectroscopy (REIMS), a method primarily used for microbiological tests in medicine; it is rarely used in the food sector. “The advantage of REIMS is that both sample preparation and evaluation are much less involved and time‑consuming than conventional test methods.”
