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winemaking are those in which there is a transfer of electrons from one compound to another, altering the chemical properties of both compounds. Oxidation and reduction are therefore mutually dependent reactions. Oxidation refers to a loss of electrons by an atom or molecule, while reduction is the gain of electrons by an atom or molecule. The compound that loses electrons is in an oxidized state, and the compound that gains electrons is in a reduced state. "Redox" is a term that derives from a combination of the words "reduction" and "oxidation." Molecular oxygen is a good oxidizing agent on its own (Figure 2), as long as there is a catalyst, in addition to serving as a substrate in many enzymatic reactions that take advantage of its oxidant activity. Non-enzymatic oxidation in juice and wine refers to direct exchanges of electrons between chemicals. Such exchanges are usually facilitated by the participation of catalysts such as iron, either chemically or in enzymes, and generally these electron movements are a cascade involving a series of intermediates by way of 1-electron transfers. Although The Fenton Reaction and Acetaldehyde F Figure 3. oxidation is commonly thought of as being dependent upon exposure of the juice or wine to molecuFenton Reaction lar oxygen, oxidation reduction reactions mostly occur in wine without directly involving participation of molecular oxygen. Oxygen attracts electrons from other molecular species, changing the chemical nature of those compounds. OH O OH O2 O + H 2O 2 Fe+2 HO · H2O 2 Fe+2 CH3CH2OH HO · CH3 C ·HOH Hydroxyl radical CH3 C ·HOH Ethoxyl radical CH3CHO Fe+3 Acetaldehyde Figure 3: The Fenton reaction and acetaldehyde formation. In chemical oxidation reactions, intermediates are produced that have a higher affinity for electrons than molecular oxygen, for instance the superoxide radical (Figure 2). Many oxidation reactions do not involve oxygen in any way, but involve transfers of electrons between other atoms moving electrons from a compound with low affinity for electrons to one with a higher affinity. Hydrogen peroxide is an early intermediate in the chemical reduction of molecular oxygen, and it is a stronger oxidizing agent than molecular oxygen, meaning it has a higher affinity for electrons. Without iron (acting as the catalyst) hydrogen peroxide is not very reactive, although it will react with good reducing agents such as SO2. The reduction potential of a compound is a measurement of its ability to gain electrons. In complex solutions such as juice and wine, many compounds can participate in oxidation/reduction reactions. In such a mixed solution, an equilibrium is reached with compounds having highest affinity for electrons being in their reduced (gained electrons) state. Those with the weakest hold on electrons are in their oxidized state (carbon, for example), having lost electrons to those compounds 66 p racti c al w i ne ry & v i ne yard O CTO BER 20 13