Definition - What does Oxygen (O2) mean?
Oxygen is a key element to the wine-making process. While it is key to the fermentation process, in excess, it can be an element of spoilage and lead to the pre-maturation of wine. As oxygen (O2) is the normal atmospheric oxygen, it carries a -2 charge; it has the ability to attach itself to other elements for equilibrium. In large amounts, O2 during fermentation can oxidize the must (juice) and thus spoil the wine. Throughout winemaking, it is important to keep O2 in check and at a minimum to avoid the oxidizing of wine in barrels and other storage vats. This is done by bubbling the wine and gassing vessels with nitrogen or argon.
O3, also known as ozone, is an oxygen molecule which is formed via the action of UV light and atmospheric electrical charges. In the winery, the formation of O3 is created through a special machine and used for sterilizing and cleaning tanks and vessels, i.e. barrels.
WineFrog explains Oxygen (O2)
Oxygen is an element which is vital in many winemaking processes. However, as in life, excess oxygen is undesirable and even toxic. Oxygen in the winery must be regulated and avoided in excess. During fermentation, it is necessary to have O2 present in order for the yeasts to survive and complete fermentation of the wine. However, prior to fermentation, excess oxygen can spoil the must (grape juice) and thus create undesirable aging, spoilage and off-aromas.
Further on in the winemaking process, wine is often aged in vats and vessels. It is important to gas and fill such vessels to keep as little headspace (empty air space) as possible, which occurs during the wine's evaporation. These vessels must also be gassed from time to time with a less-binding element such as argon or less-harmful nitrogen. In defense of oxygen, when a bottle is removed from its storage for consumption, oxygen plays a great role in the aeration of wine allowing for aromas and flavors to showcase themselves and make the wine more enjoyable.
O3, while found or present in our natural atmospheric environment, is often used via an ozone machine which converts O2 to O3 via an electrical charge. The O3 molecule is used to sanitize tanks and especially oak vessels where chemical cleansers cannot be used. In its natural form, O3 is the molecule which falls to the earth with rain during thunderstorms.