When wine has a thirst for research. © FOTOLIA, Fotolia

When wine has a thirst for research

These yeasts that can make good wine

Grapes turn into wine under the action of fermentation yeasts, micro-organisms whose role no longer needs to be proved.  They are to wine what breadmaking yeasts are to bread.  Thanks to them, almost all the sugars present in grapes are transformed into alcohol.  The diversity of yeast species and strains contributes to the sensory complexity of wines.  But how do they react to disturbances? How can these strains be improved?  These micro-organisms are the subject of particular attention from researchers.

Updated on 11/18/2013
Published on 11/18/2013

Yeasts and genes

In 2009, INRA teams working in collaboration with the National Sequencing Centre, were able to sequence the genome of a wine yeast: Saccharomyces cerevisiae.  The scientists showed how its genome has evolved in a spectacular manner by retrieving genes from other yeasts in wine.  This was the first time that the acquisition of eukaryote genes1 belonging to species distant from yeasts had been demonstrated in Saccharomyces cerevisiae.  The acquisition, maintenance and amplification of these foreign genes suggest that they may contribute to adapting these strains to their particular ecosystem, and understanding these molecular mechanisms facilitates the choice of yeasts for winemaking.  This has led to efforts to  improve Saccharomyces cerevisiae strains, for example by endowing them with a nutritional advantage at the end of fermentation.  These studies also enabled the development of a strain overproducing esters (compounds which participate in the bouquet of a wine, giving it a fruity note), and reducing the production of acetate (a compound that is undesirable at high levels).  Trials confirmed this strain's potential to produce aromatic, fruity wines, and it was put on the market in 2012.  A  strain producing low levels of sulphur dioxide was also selected, while another - producing little alcohol - is currently under development.  Finally, global mathematical modelling studies are ongoing to analyse the respective roles of the metabolism of carbon and nitrogen and aromas, which will help to understand and predict the behaviour of yeast during fermentation and at the production of aromas which play a key role in the organoleptic balance of wines.

Winemaking wasps!

The essential role of these insects was revealed for the first time in 2012 by INRA researchers, who showed how wasps are reservoirs and vectors for the yeasts present in grapes from year to year.  Their results demonstrated the close relationship between these microflora and the fauna associated with a vineyard, and suggested that any environmental change affecting insect biodiversity might create a major risk of diminishing the biodiversity of yeasts and hence have an impact on the quality of fermented products.

1 Living organisms with a nucleus containing DNA that is isolated from the cytoplasm by a membrane. Yeasts are single-cell eukaryotes.  On the other hand, bacteria are prokaryotes (their DNA is not bundled up in a nucleus).

Focus on aromas

Secondary aromas and aroma precursors

Wine tasting. © Fotolia
Wine tasting © Fotolia
By far the most abundant, secondary (or fermentation) aromas contribute fruity, floral or vegetable components.  These aromas are linked to the activity of yeasts and bacteria during alcoholic and malolactic fermentation processes. Some of these aromas are common to all types of wine, while others result from the release of varietal aroma precursors and are more typical.  While aromas suggestive of brioche, bread and beer are attributed to yeasts, milky and buttery aromas are a result of bacterial fermentation.

Driving aroma production during fermentation?  Fermentation aromas play an important role in the aromatic profile of young wines.  Several studies, performed using original approaches (post-genomics, on-line monitoring, etc.) have enabled major advances in our understanding of their synthesis by yeasts, and particularly that of esters and higher alcohols linked to nitrogen metabolism.  A study currently under way at INRA will try to propose management methods for fermentation that can drive the aromatic profile of wines.

Aromas as new, natural food ingredients! Working with their industrial partners, INRA researchers have developed a technique to capture the aromas that are released from fermentation vats.  These trapped aromas are extracted and can be added to drinks such as alcohol-free wines or sodas, contributing a fruity flavour, a fermented character and freshness; in other words, a new and natural ingredient for the food industry.

Aroma precursors: the case of volatile thiols. Some aromas exist initially in the form of precursors that are odourless in grapes.  Under the action of yeasts or bacteria, molecular transformations occur and these precursors then release their aromas.  This is the case of volatile thiols.  They are highly characteristic of Sauvignon blanc wines, whose principal descriptors are broom, blackcurrant buds, grapefruit, passion fruit, citrus notes and in some cases smokiness or roasted meat.  INRA researchers have developed methods that can determine different classes of volatile compounds and their precursors.  For example, they have identified the aromas specific to Syrah and Grenache  varieties, and varietal thiols in Sauvignon blanc.

The aromas of Riesling. Riesling and Sauvignon blanc are two historic varieties whose wines are much appreciated for their remarkable aromatic characteristics.  In collaboration with colleagues in Germany, a team from Université Bordeaux Segalen, involving INRA, has studied the aromas of Riesling wines by comparison with those of Sauvignon blanc. Citrus and yellow fruits are the descriptors associated with Riesling.  Furthermore, this wine contains high levels of 3 sulfanylhexanol, a highly odorant volatile thiol reminiscent of the odours of grapefruit and rhubarb.  In addition, studies have led to the unexpected identification of ethyl trans-cinnamate, a highly odorant compound with fruity and floral notes, which is present in Riesling wines at concentrations that are sufficiently high to contribute to their aroma.