Non-flammable, colourless and odourless, acid gas (also carbon dioxide) with the empirical formula CO2. Colloquially, carbon dioxide is very often mistakenly called carbonic acid. The gas is produced during all combustion processes and also during the respiration of animal and human beings. It also occurs in volcanic rocks and at great depths of the earth. In the atmosphere it makes up only a small proportion of 0.039 percent by volume, but this tiny amount is essential for all life, because plants need it in photosynthesis, which is how oxygen is produced. Since industrialisation, this proportion has increased by around 25% due to the so-called greenhouse effect, which is directly related to climate change. The proportion is larger (probably due to the larger land mass) in the northern hemisphere.
Carbon dioxide dissolves in liquids as well as in wine depending on pressure and temperature. In the solution, carbon dioxide is also produced by reaction with water in the smallest amount of only 0.2%. However, by far the largest part with over 99% is the carbon dioxide dissolved in the wine (which does not yet produce carbonic acid), which is combined with the actual carbonic acid as "free carbonic acid". When the vine is growing, carbon dioxide and water are used to form the sugar in the grapes. During fermentation, the sugar is then converted into carbon dioxide and alcohol. At 1.98 g/l, carbon dioxide is about 1.5 times heavier than air. There is therefore a danger of suffocation in the fermentation cellar if the gas is not removed by means of exhausters. The formation of carbon dioxide is promoted by yeast storage, as well as by expansion in stainless steel tanks. Still wines may contain a maximum of 3 g/l (corresponds to 1 bar at 20 °C). A sparkling wine must have at least 3.5 bar overpressure (corresponds to 6.5 g/l at 20 °C). The recommended or prescribed quantities are (measured at 20 °C):
Although carbon dioxide is not an acid, it enhances an acidic sensation and has a great influence on the sensory properties. In white and rosé wines it has a refreshing and lively effect. If the concentration is too low, especially in white wine, a hollow (empty) taste impression can develop. In red wine, on the other hand, it is usually perceived as disturbing, because it increases the astringent effect of the tannins, thus creating a sharp taste impression and masking (covering) aromas. In the case of red wines intended for quick enjoyment, such as Beaujolais Nouveau, the method of vinification results in a higher proportion and is also desired there. For sparkling wine, wine legislation stipulates a minimum quantity, whereby particularly fine bubbles or high pearlability are important quality criteria.
Refreshing is useful for white wine below 0.7 g/l and for red wine below 0.4 g/l. In order to achieve a desired level of carbon dioxide in the wine, it must be measured regularly and regulated if necessary. An increase or reduction is achieved by means of a fumigation fitting. For the pressure measurement there are different methods. Titration is recommended by the OIV, but this requires appropriate laboratory equipment. The simplest method for still wine is the Veitshöchheimer shaking cylinder, which consists of a riser tube, thermometer and conversion table. In the closed measuring cylinder, the carbon dioxide is "shaken out" of 100 ml of wine, displacing a corresponding amount of wine from the cylinder depending on its content. Special measuring instruments are required for sparkling wine. The pressure in the bottle is determined by an aphrometer.
In addition, there are other applications or uses of carbon dioxide in winemaking. In the so-called impregnation process it is used to produce sparkling wine. If maceration is used in red wine making, the empty space of the container is filled with it in order to intensify the extraction of the colourings and tannins (see also Macération carbonique). By displacing air (oxygen) in containers and bottles, an oxidation protection is achieved. In addition, carbon dioxide is used with the aid of a candle frit to remove off-flavors such as light Böckser (Schwefelböckser). And last but not least, carbon dioxide is also used to neutralize alkaline cleaning solutions (as used in the cleaning of bottles and containers) with regard to cellar hygiene. See a list of all wine ingredients under total extract.
Picture: From © Nevit Dilmen, CC BY-SA 3.0, Link
edited by N. Tischelmayer - January 2019