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Wine cellar in the Agronomy and Viticulture Experimentation Unit at the INRA center in Colmar. Opening of a cask used in a microvinification experiment. © INRA, MAITRE Christophe

Yeast: how wines get made

Wine-making wasps

INRA researchers in Montpellier, and their Italian colleagues, have for the first time demonstrated the essential role played by wasps in maintaining the presence of yeasts in grapes from year to year.  Because these yeasts are essential to wine fermentation, it is clearly important to clearly understand these fauna-flora relationships within the vine ecosystem.  These findings were published in 2012 in the journal PNAS.

Updated on 12/22/2016
Published on 04/09/2013

Germination of the spores of Saccharomyces cerevisiae yeast (electron microscopy). © INRA, SALMON Jean-Michel
Germination of the spores of Saccharomyces cerevisiae yeast (electron microscopy) © INRA, SALMON Jean-Michel

Without yeasts, there is no wine.  It is they – or principally Saccharomyces cerevisiae – which ensure the alcoholic fermentation of grape must.  Many winegrowers add yeasts with selected properties to their products, but some do not need to do so; sufficient yeasts are present in the grapes, cellars and fermentation tanks, even if they are cleaned between each fermentation cycle.

But one mystery remains: where do the yeasts present in grapes come from, insofar as they are only found in ripe grapes at the time of harvest, but very little in young berries?  And how do these yeasts – which therefore appear during grape ripening – remain in the ecosystem from year to year?

Interview with Jean-Luc Legras, from the INRA Sciences for Oenology Joint Research Unit (1) in Montpellier

Why did you focus on wasps?

Jean-Luc Legras: experiments at the beginning of the century, which involved covering sterile grapes with netting, showed that yeasts were not spread by the wind; the grapes remained sterile and did not ferment.  The airborne vector theory thus eliminated, research turned to animal vectors, and particularly insects.  Among these, the lifespan of drosophila is very short, and bees are devoid of the mouth parts that would allow them to drill into the grapes.  Wasps thus appeared to be much better candidates because they are capable of attacking the berries and, in the same way as bees, feed their larvae within their nests.

How were you able to demonstrate the role of wasps in the transmission of yeasts?

J-L. L.: We analysed the different populations of yeasts collected by our colleagues at the University of Florence from the digestive tracts of wasps.  We showed first of all that populations of Saccharomyces cerevisiae were present throughout the year in the intestinal flora of wasps.  Genetic analysis demonstrated that most of these strains were those found in wine.  Among the other strains present were those characteristic of bread.

How can you explain the continuity of transmission from year to year?

J-L. L.: The decisive experiment consisted in feeding the queens with a strain of S. cerevisiae labelled with green fluorescent protein (GFP), so that we could follow its fate.  We were thus able to prove that the queens harboured yeast cells from autumn to spring, the overwintering time during which they nourish larvae with regurgitated food.  They thus transmitted the yeasts to their progeny, providing a permanent reservoir of yeasts for the grapes, which they subsequently inoculated by biting into the berries.

What can you conclude from these findings?

J-L. L.: Wasps are probably not the only vectors of yeasts in vines, but they play a crucial role.  Other insects, such as flies or bees, can then take over to spread the yeasts from foci created by the wasps.  Birds also peck at the berries and may spread the yeasts present in their digestive tracts after they have ingested insects.  But they do not challenge wasps for their leading role in maintaining yeasts in the ecosystem, because the lifespan of yeasts in the digestive tracts of birds is very short.  Our study thus showed that vines constitute an ecosystem in which flora and fauna are closely associated.  Any disturbance – such as climatic conditions – that might affect insect populations will have effects on the maintenance of yeast biodiversity and consequently on wine quality.

(1) UMR1083 SPO: INRA/Montpellier SupAgro/Université de Montpellier I Sciences for Oenology Joint Research Unit.

Contact(s)
Scientific contact(s):

Associated Division(s):
Microbiology and the Food Chain , Science for Food and Bioproduct Engineering
Associated Centre(s):
Occitanie-Montpellier

Reference

Stefanini, I., Dapporto, L., Legras, J.-L., Calabretta, A., Di Paola, M., De Filippo, C., Viola, R., et al. 2012. Role of social wasps in Saccharomyces cerevisiae ecology and evolution. Proceedings of the National Academy of Sciences 109(33), 13398–403.