This post was prompted by finding visible CO2 gas (just CO2 herefater) in artisanal – aren’t they all, Swiss pinot noir, with an otherwise clear and bright appearance and clean nose and palate. This is not a common occurrence, so was it faulty? Or is there another explanation?
An explanation, and some things worth knowing about CO2 in wine …
- Still reds – CO2 may be retained/added to a still red wine, to an EU limit , and is used eg where a low sulphur philosophy is followed. It is either retained towards the end of fermentation and maintained by reductive handling (not racking, zero storage ullage) thereafter, or introduced at bottling. Both methods serve to keep the head space purged of oxygen, at least in the short-term. This protects against premature oxidation. The CO2 content diminishes with time by gas exchange through the cork or semi-permeable screw cap seal, and gentle oxidative ageing takes over. Contemporary experience includes:
- Still whites – again CO2 may be retained to protect against premature oxidation in a low intervetionist wine making regime eg
- A low intervention white Sancerre.
- Carbonating whites and reds – CO2 is typically introduced on the bottling line eg to create some NZ sparking sauvignon blanc, much Portuguese white Vinho Verde, cheap South African sparkling wine for their domestic market, and perhaps some inexpensive sparkling shiraz.
- Low level CO2 can add freshness as well as a little perlage, eg in vinho verde, slightly elevating acidity, since CO2 in solution in water, and wine is largely water, is carbonic acid, and in the process enhancing flavour intensity a little.
- BUT it can blunt the nose, and amplify tannins in a red .
- Champagne reserve wine – a similar(ish) approach is taken by some champagne producers eg Pierre Gimmonet, who add a little yeast and sugar to their still reserve wines held in magnum or bottle, the CO2 generated protects the wine from oxidation and premature ageing.
It should come as no surprise that the CO2 pressure in sparkling wines, such as finished champagne, similarly diminishes, albeit much more slowly with time. This is down to the mechanical properties of the cork, which though much more efficient at gas retention than a standard cork, becomes less flexible, and more rigid over time. This reduces its sealant properties, see Fig.1 below. Indeed  reports a champagne example loss of 30% equivalent CO2 pressure over a 75 month period under cork.
Figure 1. Evolution of a full pressure sparkling wine cork … from left to right: unused, courtesy of Ployez-Jacquemart, then becoming less able to recover the original shape, becoming less pliant and unyielding, particularly the cork discs at the base of the three corks to the right. It is this loss of pliability that enables an easier cork extraction with a well-aged sparkling wine, and contributes to loss of CO2 pressure with age. What is remarkable is how the glued together parts remain as one.
Figure 2. Same corks as in Fig.1, base view, 4th from left is a Cava interloper, still, it is a full pressure sparkling wine, made with second fermentation in bottle.
- Easton, S. (2009) Role of carbon dioxide in still wines. [online]. Accessed 14/09/16.
- Liger-Belair, G., Villaume, S. Losses of dissolved CO2 through the cork stopper during champagne ageing: toward a multi-parameter modelling. Journal of Agricultural and Food Chemistry, Vol.59, pp.4051-4056, 2011.