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

When wine has a thirst for research

Grapes that are good enough to eat

The quality of a grape berry - the starting material for wine production – is essential to the final quality of the product.  The pulp mainly concentrates water (between 75% and 90%), sugars, acids (tartaric and malic) and minerals.  The skin of the grape contains the aromas and their precursors, as well as the polyphenols that are so important to the taste, stability and colour of the wine.  Finally, on the surface of the berry, a varied microflora of fungi, yeasts and bacteria develops and intervenes in development of the wine and its sensory characteristics.  For example, the taste of sweet white wines results from a filamentous fungus (Botrytis cinerea or noble rot).  Starting from this cocktail found in the grapes, and as soon as they are crushed, reactions are initiated.  They then continue throughout fermentation and ageing.  It is essential to fully understand the ingredients in this cocktail, which is why INRA researchers have been trying to identify the genes and structure of the compounds founds in the fruits, and also the mechanisms by which they form.

Updated on 11/19/2013
Published on 11/18/2013

The secret of Pinot gris lies in its skin

Mutants of Pinot gris evolving towards Pinot blanc because of their chimeric structure. © INRA, Gilles Pelsy
Mutants of Pinot gris evolving towards Pinot blanc because of their chimeric structure © INRA, Gilles Pelsy
The genome of the grape epidermis differs from that of the berries themselves: it is a natural chimera! The grey character of Pinot is due to the combination of the epidermis of a Pinot noir around a berry that has become Pinot blanc through naturally-occurring genetic mutations (also referred to as spontaneous mutations)!  Studies have revealed that the difference in colour between black and white varieties is due to the insertion of a mobile element in a gene that controls the synthesis of anthocyanins.

Polyphenols under the microscope

Polyphenols are major constituents in wine.  The principal types are anthocyanins (pigments contained in the skin of the berries) and tannins found in the skin, pips and stalk1. Anthocyanins, which are responsible for red coloration, are important participants in the quality of fruits, juices and red wines.  But the great majority of these molecules are methylated (CH3 groups are attached to them), which intensifies their colour and endows them with considerable stability.  INRA teams, working in collaboration with Italian researchers, have identified the gene responsible for this methylation of anthocyanins in grapes.  They have also evidenced other genes which trigger the transport of anthocyanins, and other reactions involving polyphenols.  In the future, these genes may serve as markers in genetic selection programmes.

Sugars and maths

Other key components in grapes are sugars (glucose and fructose). Genotype, environment, and viticultural management.... numerous parameters directly influence the sugar levels in these berries and their metabolism.  INRA researchers have developed a mathematical model that can predict the accumulation of sugars in different types of grapes and under different growth conditions.  This type of model could help winegrowers to choose or adapt their wines and practices in a given region.

Step by step cuisine: Wöhler sauce with polyphenols!

Thanks to an extraction and fractionation process developed at INRA, it is now possible to extract polyphenols from wine in the form of a powder called “provinol”.  In the kitchen, it has been suggested that this product could be used to make a sauce made solely from compounds (and not plant or animal tissues): the “Wöhler sauce”, named after the first chemist to synthesise an organic compound in 1828. The recipe: dissolve 100 grammes of glucose and 2 grammes of tartaric acid in 20 centilitres of water; then add 2 grammes of polyphenols, salt, and a little piperine (a pepper derivative), a sheet of gelatine and a spoonful of cornflower.  Bring to the boil and then form an emulsion with oil.  Some chefs use this sauce to accompany shellfish such as lobster.

1 The stalk is the green part of the bunch (without the grapes).

The vine genome deciphered...

In 2007, an international collaboration, which notably involved scientists from the Genoscope and INRA, obtained a high-quality sequence of Vitis vinifera.  Publication of the vine genome sequence was the starting point for a detailed characterisation of the functions of the genes in this plant.  More recently, in early 2012, INRA opened a phenotyping platform in Colmar that is unique in Europe.  This facility that enables the observation of vine behaviour is designed to encourage the creation of new varieties that will be sustainably resistant to the principal vine diseases (downy mildew and and powdery mildew).  These varieties will have to meet demands concerning limitations on pesticide use, while at the same time guaranteeing high-quality and competitive wine production.

INRA's seedless grapes

Three white varieties (Danuta, Exalta and Madina) and one black (Alvina) were created through crossing and selection.  Obtained in 1964 by crossing Dattier de Beyrouth and Sultana moscata, Danuta was the first seedless variety to be registered in the official French catalogue in 1990.

FOCUS ON AROMAS

Primary aromas

Citrus, blackcurrant, pepper.... are all examples of primary (or varietal) aromas.  Grapes from practically all varieties are little aromatic themselves, and contain very few so-called primary (or pre-fermentation) aromas.  These aromas, which give a wine its typicality, result from the metabolism of the grape during ripening.  They are characteristic of a variety or family of varieties.  They are present in the fruit either in an odorant state - when they participate in the aromatic palette of the wine without prior transformation - or in an odourless form called an "aroma precursor" (such as the terpenes in Gewürztraminer or the volatile thiols in Sauvignon blanc).  The odorant potential of these precursors will then be revealed during wine production (alcoholic fermentation, ageing).  

The commonality between a rose and a glass of Gewürztraminer? The floral aromas of this wine are due to odorant molecules that are also found in the perfumes of many flowers: terpenes.  It is the exceptionally high levels of terpenes found in Muscat and Gewürztraminer wines that give them their characteristic aromas.  Studies at INRA have enabled identification of the genetic origin of muscat aromas, linked to a point mutation located in a gene called DXS.  This discovery offers promising perspectives in biotechnology, notably for the production of terpenes of pharmacological interest.  

Molecules that taste of sweet pepper! While studying how grapes manufacture their aromas, researchers at the Institute for Vine and Wine Sciences in Bordeaux and INRA identified a gene responsible for the synthesis of methoxypyrazines in these berries.  The molecules, which are particularly important in Sauvignon grapes, give a sweet pepper aroma to our wines! Their effects are so powerful that a single drop would suffice to flavour an Olympic swimming pool with sweet pepper!