Winetech Technical Yearbook 2022

These radicals can oxidise alcohols to form compounds with oxidation aromas or contribute to browning. The oxidation of ethanol by the Fenton reaction leads for example to acetalde hyde, which causes the so-called bruised apple character. Bisulphite can consequently prevent oxidation in three different ways: • It reacts with quinones and hydrogen peroxide to prevent their further oxidative reactions with wine components. • The rapid reaction of bisulphite with hydrogen peroxide and quinones accelerates the oxygen consumption of wine, which limits the participation of oxygen in other pathways and leads to thiol losses. • Bisulphites can bind acetaldehyde and other oxidation aromas resulting in non-odorous aromas. The loss of free bisulphite will be replenished by the dissociation of bound sulphur dioxide to restore the equilibrium between the forms. OXIDISED AROMAS The oxidised aromas of wines can mainly be attributed to free aldehydes, which are not bound to sulphur dioxide. It consists of acetaldehyde (bruised apple aroma), phenylacetaldehyde (honey character) and methional (baked potato character). The increase in the aldehyde concentrations can occur via two different path ways: • As a result of the Fenton reaction, aldehydes can be formed through iron catalysed oxidation of alcohols by hydrogen per oxide. • Following the loss of bisulphite, bound sulphur dioxide will release acetaldehyde that was bound during alcoholic fermen

tation, to restore the equilibrium between the two sulphur dioxide forms. Little evidence proves that the Fenton reaction plays a role in the aldehyde formation of real wines. The following conclusions can be made with the recent available data: • Trace levels of free sulphur dioxide are required to react rapidly with hydrogen peroxide preventing the Fenton reaction. • The rate of bound SO 2 dissociation from acetaldehyde or antho cyanin adducts is quicker than the ingress of oxygen to wine. Aldehyde aromas are rather the result of dissociation from bound forms than formed during wine storage due to oxygen ingress. • Aldehydes are formed during alcoholic fermentation through amino acids formation, and if SO 2 is added to wines after fer mentation these aldehydes are bound, which can be liberated by dissociation at a later stage. The aldehyde formation can be limited by lees contact (“sur lie”) after fermentation. • By applying micro-oxygenation (MOX) during red wine mak ing, it is believed wines are softened due to the polymerisation of tannins and anthocyanins. This can also be reconsidered by rather utilising the acetaldehyde liberated from the dissociated bound form with sulphur dioxide. • Sulphur dioxide can also be consumed by other wine com ponents in the absence of oxidation, implying that such wine can smell oxidised if bound acetaldehyde is released due to dissociation. REFERENCE https://www.wineland.co.za/sulphur-dioxide-understanding complexities/

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WINETECH TECHNICAL YEARBOOK 2022