Reduction and Reductive Conditions
Technically speaking, reduction is a chemical reaction in which a molecule, atom or ion gains electrons. Reduction is a half reaction which cannot occur on its own. It must always be accompanied by oxidation, which is a chemical reaction in which a molecule, atom or ion loses electrons. Together they form what is known as a reduction-oxidation, or redox, reaction. Hence, reduction and oxidation are complementary processes: as one compound is oxidised, another is reduced. These reactions happen in wine all the time, both during winemaking and also during maturation.
In the early stages of winemaking, oxygen is helpful, and indeed necessary for fermentation to proceed. After this, however, its impact is generally negative (see oxidation), and development occurs best in the relative absence of oxygen, that is, in “reductive” conditions. Unfortunately, reductive conditions can also favour the development of foul smelling volatile sulphur compounds, and what is gained by avoiding one set of faults is lost by the acquisition of another.