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

When wine has a thirst for research

Innovations from harvest to the bottle

For the past twenty years, experimental studies initiated by INRA have produced technological tools which have been transferred to professionals in the sector.  A dynamic crusher, Flash détente, decanter centrifuge, cold maceration, etc., are all new practices that offer winemakers enhanced accuracy and operational control.  A look back at these technological innovations as they affect the process of transformation of grapes into wine.

Updated on 06/20/2017
Published on 11/08/2013

A new concept: "precision oenology"

Grape harvest at INRA Colmar. © INRA, MAITRE Christophe
Grape harvest at INRA Colmar © INRA, MAITRE Christophe
Destemming, pressing, fermentation, extraction, stabilisation, packaging... the concept of precision oenology can be applied to all stages in the production of a wine.  It involves not only new working practices but also the mastery of oxidation phenomena, and the control of all factors that determine wine quality (for example, aroma precursors, polyphenols, compounds that are produced during winemaking  or ageing). Finally, this concept also integrates eco-design and sustainability approaches.

Dynamic grape crushing

The first step in the journey towards a wine is often destemming (or stripping) that separates the berries from the stem, as the latter may give green aromas to the wine that are little appreciated.  Innovations have improved both this destemming step and mechanical sorting, both in the vineyard  during mechanical grape harvesting and in the winery.  After the grapes enter the winery, they fall into a crusher where the berries are crushed to release the juice (or must).  A new process to split open the berries, developed by INRA researchers in partnership with the company Pellenc, has been developed: the dynamic crusher.  This enables a more rapid and efficient extraction of the juice because the berries are fully opened.  In 2013, this technology moved to an industrial production stage.  For white wines, the next step in the process is pressing, followed by an essential settling step, which consists in clarifying the must by removing all particles in suspension.

A hot flash on the harvest!

Must. © Fotolia, Fotolia
Must © Fotolia, Fotolia
Throughout the world, winemakers frequently apply liquid phase production technologies that involve heating their harvest (thermovinification).  In the early 1990s, INRA researchers focused on this area and developed an unique technique: Flash détente. First of all, the grapes are heated rapidly (in an oxygen-free atmosphere) so that within a few minutes, a maximum temperature of 90°C is reached in the centre of the berries.  The heated grapes are then transferred to a vat under a vacuum, where an abrupt drop in pressure causes immediate cooling and the release of steam: the "détente" '(relaxation).  This technology causes mechanical destructuring of the cells in the grapes which favours the release of the compounds targeted.  Flash détente thus enables optimisation of the next step in winemaking, and produces wines with greater typicality.  This physical pre-treatment of the grapes is the starting point for numerous winemaking methods, and also the production of grape juice that is both more richly coloured and contains more polyphenols.

Continuous juice production

The emblematic stage of pressing, which may be mechanical or pneumatic (or in other words, intermittent, often for white and rosé wines), can be replaced by an alternative, continuous process if a decanter centrifuge is used.  This new tool - studied by INRA - enables the extraction of juice with a yield that is comparable to that of a wine press, and with a degree of clarification that is generally adapted to the winemaking process, notably in the case of white and rosé musts.

When the wine macerates...

Micro-winemaking vat, settling tank. © INRA, Christophe Maitre
Micro-winemaking vat, settling tank © INRA, Christophe Maitre
Maceration in fermentation vats consists in extracting constituents from the solid parts of the harvest (tannins, anthocyanins, colorants, aroma precursors).  This is a crucial stage in the production of a red wine, and there may be differences between wines; for example, those for laying down require a longer maceration.  Cold, pre-fermentation maceration (4 - 15°C for a few days) is a variant of traditional maceration: using ripe grapes, it can obtain red wines expressing more intense, fruity notes.  For white and rosé wines, maceration consists in the contact for just a few hours between the grape skins and the juice, thus allowing aromatic precursors and anthocyanins (for rosé wines) to leach into the latter.

Automated fermentation

This process allows the yeasts to gradually transform sugar into alcohol.  It is carried out in a vat (generally made of stainless steel) and lasts from a few days (for a dry wine) to several months (for a sweet wine).  Fermentation generates large quantities of carbon dioxide and a rise in temperature that may be harmful to both the yeasts and product quality.  INRA teams have proposed new control strategies for fermentation based on the on-line monitoring of its kinetics (measuring the rate of carbon dioxide emissions) and vat-by-vat optimisation of the temperature profile and nutrient supplementations (mainly assimilable nitrogen and oxygen).  These strategies are already enabling more reliable fermentations.  The next phase in this work will consist in using automation to drive the aromatic profile of wines.

Winemaking vats. © INRA, Yves Cadot
Winemaking vats © INRA, Yves Cadot
Wine remains unstable for as long as it is not protected by the addition of sulphites, and if malolactic fermentation has not yet occurred.  This malolactic fermentation is able to reduce the acidity of wines by facilitating the transformation of malic acid into lactic acid by lactic acid bacteria, at a temperature of around 20°C. Of particular value in red wines, this fermentation is not desirable for white wines.  The wines then continue to follow their path: racking consists in running the wine out of the vat by separating it from the cap of marc1 and the lees (deposit of dead yeasts).  This stage enables elimination of the carbon dioxide acquired during the alcoholic and malolactic fermentation processes.

The end of the road

During fermentation, racking and returning (delestage) consists in recovering all the must that has accumulated in the bottom of the vat and transferring it to a second vat.  It is then poured over the marc cap that has become compacted in the bottom of the maceration vat.  As for piégeage, this is an operation specific to red wine production during which the marc cap is punched downwards and crushed into the fermenting juice in order to encourage the dissemination of phenolic compounds and aromas.

 
The wine continues its voyage with the assembly of components from the different vats available.  Once assembled, the wine is aged in vats (under micro-oxygenation) or in barrels.  "Primeur" wines (in the sense of "new", such as Beaujolais nouveau) are not aged.  Topping up consists in ensuring the periodic filling of barrels to compensate for losses from evaporation.  Fining involves the addition of a protein-based substance.  Egg white was used in the past, but numerous other techniques and compounds are now available to clarify and stabilise wines by causing the precipitation of particles in suspension.  Finally, the wine may be filtered before bottling.

1 The marc cap designates the solid parts of the grapes (stems, pips, skins) that are macerated during fermentation and rise to the surface under the action of carbon dioxide release.

Carbonic maceration

As early as 1874, Pasteur expressed his intuition relative to this phenomenon, but it was only in 1934 that a scientist in the Wine Research Unit in Narbonne (which would later become an INRA unit) developed this process.  Carbonic maceration consists in placing whole clusters of unbroken grapes - harvested by hand - in an atmosphere that is depleted of oxygen and saturated with carbon dioxide, in order to exploit the spontaneous fermentation that occurs under such conditions.  The transformation then occurs inside the berries, and is due to endogenous metabolism of the fruit, thanks to the action of enzymes specific to grapes and without the intervention of micro-organisms.  The grapes are then subjected to "standard", microbial fermentation for 8 to 10 days.  This process enables smoother implementation of the different phases of winemaking that follow, and gives the wine a greater overall harmony.

The chemistry of sulphites

SO2, or sulphur dioxide, is used during winemaking for its antiseptic and antioxidant properties.  It prevents "bad" micro-organisms from turning the wine into vinegar, while at the same time allowing other micro-organisms to do their work, or in other words, turn grapes into wine.  The sulphiting of wine can protect it from oxidation.  In France, it is compulsory to indicate on a wine label whether sulphites have been used during its production.  Current research is trying to develop wines that contain less or even no sulphites but which retain their organoleptic characteristics.

The largest collection of vine varieties in the world is held by INRA!

Vinestock in the Collioure vineyard. © INRA, CARRERAS Florence
Vinestock in the Collioure vineyard © INRA, CARRERAS Florence
On the shores of the Mediterranean, between Sète and Marseillan-Plage (Hérault), INRA owns and manages the largest conservatory of vines in the world.  The Domaine de Vassal experimental farm groups more than 7500 accessions (elemental samples in a collection of varieties) grown on 19 hectares that are wholly dedicated to the conservation, characterisation and exploitation of vine biodiversity.  In Bordeaux, Colmar and Angers, INRA also holds other collections of varieties, accessions and rootstocks2.

2 Main underground part of a vine that is resistant to phylloxera (a vine pest), onto which the scion of a fruit-producing variety is grafted.