Yeasts potential interactions influencing the formation of fine aromas during cocoa fermentation process

The first post-harvest treatment of cocoa beans relies on the appropriate fermentation of the cocoa pulp. This process is now considered as a key step in the quality of the final chocolate. Recent researches showed that aromas of fine chocolate involving fruity and/or floral notes might be partly linked to the surrounding environment of cocoa beans. Among microorganisms involved in the fermentation process, yeasts represent the most interesting producer of aroma compounds, either by specific enzymatic, or by their own metabolism with the production of higher alcohols, and esters. Some researchers showed that specific aroma from cocoa fruit introduced into the fermentation process could be further recovered in the cocoa liquor, supporting the idea that aroma production in the surrounding environment of coca bean could help in adding beneficial traits to the final product. These findings show the significance of an efficient yeast starters selection strategy able to enhance and harmonize the production of fine chocolate. To date, the interactions between indigenous yeasts of cocoa fermentation have been poorly investigated. Studying these interactions could help understand why fermentation leads to higher or lower quality chocolate. In this study, 90 strains were isolated from fermentation batches which gave chocolate of standard and high quality. Among these strains, 56 different species were identified by sequencing ITS region. The majority of isolates were identified as Saccharomyces cerevisiae (22), and Pichia kudriavzevii (20), the others being shared within other genera such as Candida, Pichia Hanseniaspora, or Torulaspora. Intraspecific variability within the two major species (i.e. S. cerevisiae and P. kudriavzevii) was performed using GTG-5 fingerprinting and a minimal medium relevant for cocoa pulp environment was designed to screen fermentation performances (sugar consumption/ethanol production) and aroma production profiles. S. cerevisiae strains exhibited faster fermentation kinetics than P. kudriavzevii. Yeast species were typically clustered according to their aroma profile, with some intraspecific variability regarding their production capacities. Killer phenotypes of all strains were screened against main isolates and were subsequently cocultured with their target to evaluate the impact on aroma production. When grown with indigenous killer species, fine aroma compound production by high producers like P. kudriavzevii was drastically limited. These findings showed that fine cocoa harmonization process not only relies on the presence of beneficial yeasts but can also be influenced by the presence of competitors that could decrease the production of fine aroma during the fermentation process.