Definition: Wine production

To illustrate the transformation of grapes into wine, the winemaking process that follows the harvest, we are pleased to present the work of two talented young students from Burier High School, whom we would like to thank:

Éléonore Margot

Morgane Huber

Gymnase de Burier - April 2011

Introduction

Wine is a product that comes in many varieties and has been part of everyday life for a very long time. The first trace of wine dates back 7,000 years and was discovered in Iran. Despite the fact that it is widely consumed, few people know how it is made. What's more, the process is complicated because winemakers must use different techniques for each grape variety. We therefore found it interesting to look at the winemaking process for white wine and red wine, which are the two most widely consumed types of wine.

Production

When it comes to wine production, and especially winemaking, we do not talk about manufacturing but rather production, as it is simply the chemical transformation of one product into another.

White wine

White wine is produced by alcoholic fermentation. With a wide variety of grape varieties and winemaking methods, no two production processes are the same.

Once the grapes have been harvested, they are destemmed and pressed, and only the juice, called must, is collected. It is then placed in vats to begin alcoholic fermentation with the help of yeasts.

What is alcoholic fermentation?

Alcoholic fermentation only occurs in certain situations. The temperature must be between 15°C and 25°C, as if it drops too low, the process will slow down, and if it rises too high, the yeast will die. Next, care must be taken to ensure there is enough oxygen, because although fermentation is a product of the anaerobic (oxygen-free) metabolism of yeast, yeast still needs oxygen to reproduce and secrete a molecule that allows it to resist alcohol. In addition, care must be taken to ensure the presence of assimilable nitrogen, as a deficiency can have negative consequences because this nitrogen is necessary for yeast reproduction.

Thanks to the yeast, the sugar contained in the must begins to transform into ethyl alcohol and releases carbon dioxide, according to the following chemical equation:

C6H12O6 + 2 ADP ? 2CH3CH2OH + 2CO2 + 2ATP

In addition, care must be taken with the alcohol content, as concentrations above 14° to 16° kill the yeast.

After the must has been fermented, it requires a little care before it can be drunk. Sometimes, depending on the grape variety and the winemaker's preference, the wine may undergo malolactic fermentation. This second fermentation is carried out by simple bacteria, which deacidify the wine. Chemically speaking, it transforms malic acid, which has two carboxyl groups, into lactic acid, which has only one.

The first stage is called 'ageing'. Ageing takes place in vats and lasts until the liquid clarifies, is ready for bottling and allows the fine lees of the wine, which contain dead yeast, to settle.

Next comes what is known as "stirring". Stirring is the process of agitating the lees at the bottom of the barrel using a stick, a technique that must be mastered as there is a risk of obtaining a reduced taste as a result of stirring the sulphites in the yeast.

Next comes clarification, which involves removing the last elements suspended in the wine and preserving their solubility. Winemakers can use "oenological fining agents" such as tannic acid C76H52O4.

Towards the end of the winemaking process, care must be taken to ensure that the wine is stable, and there are several ways to achieve this.

Metatartaric acid can be added to the wine to prevent the crystals from growing.

Or, electro dialysis can be used, passing electricity between two plates: this attracts the ions in the wine and can eliminate them. However, this also risks altering its taste.

During stabilisation, some wines may show traces of rosé colouring (wine with slight pink hues). This is simply the presence of polyphenol, which is normally colourless but has turned pink as a result of oxidation. Traces of rosé colouring disappear when oenological fining agents are used.

The final chemical process used in white wine is SO2, which is simply sulphur dioxide. It protects the wine from excessive oxidation and an excess of bacteria and yeasts.

The doses are prescribed according to the percentage of sugar in the wine, because if there is too little, it may start fermenting a second time.

Depending on the residues, the wine may also be filtered. It should be noted that filtration does not only remove undesirable elements from the wine. Over-filtration can also contribute to its impoverishment.

The final stage, packaging, simply consists of bottling the wine and labelling the bottle so that the label provides the customer with all the mandatory information.

Red wine

Red wine is produced by fermenting the must of black grapes in the presence of their skins and seeds, because the flesh of the grape is white (with the exception of a few grape varieties known as "teinturier" with red juice), and the skins will release their pigments into the must.

Stored in large vats, the must will begin its alcoholic fermentation, which is almost identical to that of white grape varieties.

It is at this point that tannins, red pigments, are released into the must.

The pulp and skin mass float and form a "cap". The winemaker then stirs the vat to submerge the cap and enhance the colour of the wine.

The winemaker then ends the maceration by draining the wine, separating it from the cap. The cap is then pressed to extract the juice it contains.

For red wines, malolactic fermentation is common. The process is the same as for white wine, except that for red wine, this second fermentation eliminates all aggressive aspects and makes it supple and stable.

Next, the wine must be aged, which is done in the same way as for white grape varieties.

All red grape varieties require ageing, which can take place in oak casks or barrels.

To remove the sediment from a wine, the winemaker will use tannic acid for fining.

Finally, before bottling, the wine is stabilised and the temperature of the vats is lowered significantly so that tartar crystals form before bottling.

Conclusion

During our research on these wines, we learned about the changes that chemical processes have on their taste and appearance. The flavour of a grape variety can change completely depending on the duration and method of vinification, which is why all wines are different. Each winemaker follows their own process.

We also noticed that winemaking is very complex, as it is largely a matter of chemistry. It is a profession that must be mastered, where the slightest mistake can have serious consequences.

If we had wanted to highlight all the chemical processes used in winemaking, we would have had to do a great deal of research into the techniques used around the world to see the many differences between sweet, full-bodied, dry, fruity, sparkling wines, etc.

Bibliography

• DEFAYES Guy, MARGOT Antoine, SALAMIN Jean-Yves. La qualité et la conservation du vin (The quality and preservation of wine). EPFL technical environment project. Lausanne. 1987

• DUSSERT-GERBER, Patrick. Dussert-Gerber Patrick: Defend ethics or nothing. France: Dussert-Gerber Patrick.

Acknowledgements

• To all the specialists and friends consulted who provided us with valuable support.

The illustrated PDF of the full article is available for download below.