by Ângela Carvalho

This friendship discovered by researchers in Belgium promises to revolutionize the way we see and consume chocolate. You may be wondering what yeast and chocolate have in common or the reason behind this special friendship.

Chocolate is produced from the seeds of the Theobroma cacao tree fruit, which literally means “food of the gods”.Cocoa trees are native to the tropical regions of Central and South America and their seeds, cocoa beans, are the primary ingredient used for chocolate production. Most of the flavors that we recognize in chocolate only start to develop during the fermentation of the beans. This process involves leaving the cocoa beans and cocoa pulp together for several days. The cocoa pulp is rich in sugars and contains several microorganisms, like yeasts, that are responsible for the fermentation and subsequently the final flavor profile of chocolate. Cocoa fermentation is not only important to give chocolate the best flavour but also to remove the tannins (a bitter plant polyphenolic compound that gives an astringent flavour to chocolate) present in cacao beans.

After harvesting, cocoa beans are collected and placed in wooden boxes or piled on the ground. For this reason, there is no control over what microorganisms are present during the fermentation. This makes the process variable which affects both the flavor and quality of the chocolate.

The microorganisms present in the coca pulp are very diverse and different microorganisms are responsible for distinct chocolate flavor properties.  The main microorganisms present in cocoa pulp are lactic acid bacteria, acetic acid bacteria and yeasts. Lactic acid bacteria produce several organic acids, mainly lactic and acetic acid but also other compounds such as carbon dioxide, ethanol, mannitol and glycerol. Acetic acid bacteria, who are mainly present at later stages of the fermentation, produce acetic acid that will contribute to the disruption of the cocoa beans. Finally, yeasts are also present during the fermentation in its entirety. Apart from the main metabolites ethanol and carbon dioxide (CO2), yeasts are also responsible for the production of several secondary flavor-active metabolites that add different fruity aromas to chocolate. These fruity volatile compounds include aldehydes, carbonyl compounds, esters, fatty acids, higher alcohols, organic acids, phenols and sulphur-containing compounds.

In collaboration with the world´s largest chocolate producer, Barry Callebaut, researchers at the University of Leuven, created yeast hybrids for a more efficient fermentation of cocoa pulp with a high production of volatile flavor esters.

The researchers bred novel yeast hybrids that combine robustness with strong flavor production. Not only Saccharomyces cerevisiae was tested, two different yeasts species (Pichia kluyveri and Cyberlindnera fabianii) very well known to produce a large amount of fruity esters were also used in this study.

Aromatic volatile producing parents were selected out of a large pool of industrial S. cerevisiae strains used for the production of beer,  sake, bread, bioethanol and wine. The strains were selected based on high production of flavor and aromatic compounds like isoamyl acetate (banana-like flavor) or longer-chain acetate esters (apple-like flavor) (1). Robust strains were selected based on their capacity to efficiently and consistently ferment cocoa pulp (2-3). The yeast hybrids were generated by crossing yeast parents with individuals showing robust fermentation and aromatic properties. These hybrids were then selected based on improvements in robustness and aromatic ester production when compared to the parental strains.

The researchers then applied these new hybrids in the fermentation of cocoa pulp in farms. Sensory analysis by an expert panel confirmed differences in the aroma of chocolates produced with different starting cultures. Like beer brewing and wine making, yeast can be used in cocoa fermentation to improve chocolate flavor properties and add a new collection of aromas and taste to chocolate. This is great news for all the chocolate lovers around the world.

References:

(1) Steensels J., Meersman E., Snock T., Saels V., Verstrepen KJ. Large-scale selection and breeding to generate industrial yeasts with superior aroma production. Applied and Environmental Microbiology 80:6965-6975. 2014.

(2) Meersman E., Steensels J., Paulus T., Struyf N., Saels V., et al. Breeding strategy to generate robust yeast starter cultures for cocoa pulp fermentation. Applied and Environmental Microbiology 81. 2015.

(3) Meersman E., Steensels J., Paulus T., Struyf N., Saels V., et al. Application of a breeding strategy to generate robust yeast starter cultures for cocoa pulp fermentation. Applied and Environmental Microbiology. 2015. DOI:10.1128/AEM.00133-15