Breaking down the microbiology world one bite at a time
Improving wine with a cocktail of microorganisms
Microorganisms live and interact in various ecosystems such as fermented foods. Wine, for example, is home to a large diversity of single-celled fungi, the yeasts. This population of microorganisms is usually composed of several genera, species and even several different strains. The majority of wines are produced by the fermentation of industrially produced strains of Saccharomyces cerevisiae. The S. cerevisiae species ensures quality wine making because it is particularly efficient at transforming sugar into alcohol during fermentation. When this transformation is complete, the fermentation is said to be finished. Recently, the addition of a second species of yeast of a different genus is being explored in order to improve the aromatic complexity of the wine and control itsthe alcohol content. Some species allow for a reduction in ethanol concentration, while others bring floral, smoky, fruity or nutty aromatic notes.
What are the optimal conditions for these mixed fermentations? To answer this question, a study conducted by a team of researchers from Montpellier provides insight into the interactions between yeasts under oenological fermentation conditions.
The experiment involved four new yeasts: two species of the genus Hanseniaspora and two species of the genus Metchnikowia. They were each fermented alone (monocultures) and also in association with S. cerevisiae (mixed cultures), also conducted in monoculture. Fermentation is followed for 300h and the parameters analyzed including the consumption of sugars, nitrogen and the production of metabolites were analyzed.
Only the monoculture of S. cerevisiae finishes the fermentation leaving only 0.1% of the initial sugar concentration after 220h. It is not surprising that it outperforms the other monocultures. The latter, unable to complete the fermentation, left between 45% and 71% of the initial sugars. The mixed cultures did not finish fermentation at 300h, but various indications suggest that they would be able to do so with more time. The consumption of assimilable nitrogen, essential for their multiplication, follows the same dynamics as that of sugars. The mortality of cells in mixed cultures is also higher than in monocultures. One hypothesis to explain these results is that one of the two species produces metabolites that affect the other. For example, ethanol produced by S. cerevisiae is not well tolerated in high concentrations by the other species. Indeed, the amount of ethanol produced in mixed cultures is similar to that produced by S. cerevisiae, whereas it is lower with the other monocultures. The production of acetic acid (vinegar), which we are trying to limit in wine, seems to depend on the genus. Hanseniaspora yeasts produce more acetic acid than the others, and it is the opposite for Metchnikowia yeasts. Finally, the mixed cultures produced more glycerol than all the other cultures, which does not give the wine more taste but does give it a smoother texture. This last result proves the ability of a mixed crop to outperform monocultures through positive interactions.
In conclusion, S. cerevisiae dominates the fermentation confirming its better efficiency and the fermentation parameters of the mixed cultures reach an intermediate level compared to the two corresponding monocultures. Nevertheless, for the production of some metabolites, mixed cultures are more efficient than the two corresponding monocultures.
There is no shortage of ideas for going further, and other associations involving the interaction of strains of the same yeast species or fermentations without S. cerevisiae are tracked to be tested. The continuation of this work promises the discovery of intensely aromatic wines.
Link to the original post: O. Harlé, J. Legrand, C. Tesnière, M. Pradal, J.-R. Mouret, & T. Nidelet: Investigations of the mechanisms of interactions between four non-conventional species with Saccharomyces cerevisiae in oenological conditions, PLOS ONE, vol. 15, no 5, p. e0233285, may 2020, doi: 10.1371/journal.pone.0233285
Featured image: https://www.flickr.com/photos/benmacaskill/43835494530