Term for a chemical reaction in which an atom or molecule accepts electrons (reduces) and another substance gives up these electrons (oxidises). In a narrower sense, in connection with oxygen, a reducing agent is the term used to describe the substance that removes oxygen from another, such as oxide (oxygen compound), and is itself oxidised in the process. Many substances can be both oxidised and reduced. Whether a substance is a reducing or an oxidising agent depends solely on the reaction partner. A very important reducing agent for winemaking is sulphur. As a reducing agent it can react with oxygen to form sulphur dioxide, but also as an oxidising agent it can convert metals (e.g. contacts of wine with metallic substances in the cellar) into their sulphides and thus cause a wine defect, which manifests itself as a metallic tone.
Reducing agents are substances that easily give off electrons. These include hydrogen, carbon and base metals such as iron, magnesium and sodium. The reducing agents are themselves oxidised. Oxidising agents are substances that can easily accept electrons, such as oxygen, chlorine and fluorine. The oxidising agents are themselves reduced. Reduction is therefore always associated with oxidation and vice versa. Both processes together are partial reactions of a redox reaction.
In winemaking, reduction takes place, among other things, in the final stage of fermentation, in which acetaldehyde (alcohol precursor) is converted into ethanol (alcohol) and carbon dioxide is produced (in the initial stage, a state of oxidation promotes the necessary rapid growth of the yeasts). For most white wines, reductive ageing is quite deliberate. This is necessary because, unlike red wines, they have fewer phenols that protect against oxidation. After pressing, the must is therefore usually sulphurised immediately and fermented in steel tanks under exclusion of air. This prevents undesirable oxidation processes. The effect is tangy, fruity and pure-toned wines. In the case of wine types such as sherry and port, however, oxidative ageing is deliberate.
Reduction must be carried out in a controlled manner, because wine kept too much under air exclusion can lead to the formation of hydrogen sulphide and thiols (mercaptans), resulting in reductive notes, which are called reduction aromas. At low levels, this may be acceptable. At high concentrations, this is a wine defect, the dreaded Böckser (sulphur boil). A wine in the bottle (or in an airtight container) is in a reductive state because the small amount of oxygen in the neck of the bottle is slowly consumed by reactive processes. See also under oxygen management and a complete list of all processes under winemaking.
In the past, you needed a wealth of encyclopaedias and specialist literature to keep up to date in your vinophile professional life. Today, Wine lexicon from wein.plus is one of my best helpers and can rightly be called the "bible of wine knowledge".
Prof. Dr. Walter Kutscher
Lehrgangsleiter Sommelierausbildung WIFI-Wien