Thank you everyone for the plethora of great responses last week on Cool Climate Viticulture! Take a peek if you missed it!
This week: Residual Gas
Name and describe some of the reasons a still white wine may visually contain residual gas
Blanketing attempts to maintain a gas layer above the wine surface in the hopes of minimizing wine-air contact. Both nitrogen and CO2 are used for the purpose, although nitrogen is preferred because it has a very low solubility in wine, 14mg/L. This is a principal reason why it is also an effective sparging gas. In order to prevent growth of aerobic microorganisms on the wine surface, the O2 concentration must be reduced from the 29.9% O2 found in air to 0.5% or less at the wine surface.
Sparging involves the introduction of very fine gas bubbles to help remove dissolved oxygen or CO2 or occasionally to add CO2. The solubility of a gas in a liquid is proportional to the partial pressure of that gas in the gaseous atmosphere in contact with that liquid. When fine bubbles are dispersed, a partial pressure develops between the sparging gas (usually N2) and the dissolved gas (usually O2). The difference in partial pressures causes the dissolved gas to leave the wine. The effectiveness of this process is dependent on the wine, temperature, time, gas volume, and bubble size.
This one is more of a wine flaw--but if the wine undergoes malolactic fermentation in the bottle, carbon dioxide is released as a bi-product of the conversion of malic to lactic acid.
As an additive CO2 can keep oxygen away from the must and the wine and may be added to give it a fresh, crisper mouth-feel. However, you can lose flavor intensity of the wine, and make the wine thinner making it hard to pick up on aromas. CO2 can just be a byproduct of fermentation produced by yeast and perhaps the levels were too elevated in the finished product. Either way this could leave some visual gas in the bottle.
Along this line, many bottles that are bottled soon after fermentation will have risidual dissolved CO2 from said fermentation. It's pretty common for young whites not aged in oak.