Reduction (see there) is a complex chemical process. In winemaking, reductive vinification is understood to mean the complete exclusion of the oxygen required for the metabolism of microorganisms and thus the prevention of undesired oxidation. This requirement is met by the shortest possible transport routes and processing times for the grapes, must and wine with cool ambient conditions during pressing, must treatment, fermentation, ageing in hermetically sealed containers (steel tanks) and bottling.
This is also supported by measures such as sulphurisation, fining and filtration. Through a reductive ageing process, the colour, as well as the smell and taste, i.e. the entire aroma of a wine is strongly influenced. The wines are characterised by a typical tangy, fruity and pure taste with pronounced aromas (primary aromas) and a white to pale yellow colour (the ferrous pigments are oxidisable and give a darker colour). These taste and smell impressions are called reductive notes in the positive sense.
Reductive ageing is mainly used for white wines and sparkling wines. In Germany and Austria this is the rule. Exceptions are mainly in more southern regions like Italy and Greece. However, the process must be controlled, as too rigorous, strong oxygen cut-off can lead to the formation of volatile sulphur compounds such as hydrogen sulphide and thiols (mercaptans) with reductive notes. To a small extent, this may still be accepted as a so-called "stinker". At higher concentrations, however, these are negative taste directions such as musty tones or the dreaded "Böckser" (Schwefelböckser) and represent a wine defect. By aeration or carafing, a wine can possibly be brought back into balance (see also reduction aromas).
The complex, chemical changes during the bottle ripening or ageing of a wine are sometimes also called reductive ageing or reeductive process (see also under micro-oxygenation). In contrast to the reductive process, the oxidative ageing process involves a dosed contact with oxygen. This usually takes place with red wines with frequent barrel ageing.
Complete lists of the numerous vinification measures and cellar techniques, as well as the various types of wine, sparkling wine and distillate regulated by wine law are included under the keyword vinification. Comprehensive information on wine law can be found under the keyword wine law.