Gasses dissolve in liquids. The gas molecules occupy space between the liquid molecules.
But, when all of the space has been used, the liquid becomes saturated. No more gas can be dissolved once a liquid becomes saturated. Air contains about 21 percent oxygen, and some oxygen dissolves in wine anytime wine is exposed to air.
Depending on exposed area, temperature, etc., oxygen dissolves quickly until wine becomes saturated. The dissolved oxygen then reacts with some of the other materials in the wine, and the dissolved oxygen disappears from the wine. However, many wine oxidation reactions take place slowly and considerable time may be required for the dissolved oxygen to completely disappear from wine. Depending on storage conditions, several days to several weeks may be required before all of the dissolved oxygen is gone.
Several winemaking operations depend on how gasses dissolve and saturate liquids, so understanding dissolved gases is of value to winemakers. Three examples are discussed below.
Yeast needs oxygen to multiply and produce new cells, and fermentations can be sluggish or even stick when not enough oxygen is present in the juice. Popular strains of wine yeast need from 5 to 15 milligrams of oxygen per liter of juice to produce the large cell population needed for a vigorous fermentation. (Sometimes, just racking and splashing the juice to introduce a new supply of oxygen can restart stuck fermentations).
Grape juice becomes saturated with oxygen during normal crush operations, and saturated juice contains roughly 10 milligrams of dissolved oxygen per liter. If enough sulfur dioxide is added to deactivate oxidative enzymes, this dissolved oxygen will remain in the juice for some time. Little extra oxygen is needed by the yeast to complete fermentation because much of the needed oxygen is already in the saturated juice. This is why fermentations can be done in large, closed tanks even though the yeast requires oxygen early in the fermentation process.
Professional winemakers prefer to use copper to remove the stench of hydrogen sulfide from wine. On the other hand, most home winemakers prefer to splash wine vigorously to remove hydrogen sulfide gas. The major disadvantages to splashing are the possibility of oxidizing the wine and the possibility of oxidizing mercaptan into disulfide. However, the amount of oxygen entering the wine can be reduced considerably if the splashing is done properly.
Splashed wine quickly becomes saturated with oxygen, and once saturated, little more oxygen can enter the wine. The saturated wine can then be splashed for an extended time to remove the hydrogen sulfide gas. Because of the saturation effect, one large exposure to air produces much less oxidation than several splashing treatments applied over an extended time.
Novice winemakers often panic when they discover a bung on the floor because they are afraid the wine in the open barrel will be oxidized. Depending on storage conditions, wine in an open barrel becomes saturated with oxygen quickly, but once the wine is saturated little more oxygen can enter. Some of the dissolved oxygen reacts slowly with sulfur dioxide or other materials in the wine. The dissolved oxygen disappears slowly, so the wine in the open barrel remains saturated or nearly saturated.
Once the bung is replaced, sulfur dioxide continues to scavenge oxygen. Consequently, a barrel of wine may be open to the air for a day or so, but the saturation effect often prevents excessive wine damage. Of course, catastrophic oxidation can occur if the sulfur dioxide content of the wine is low or a barrel remains open for an extended time.