Winetech Technical Yearbook 2022

SEPTEMBER

THE COMPLEXITIES OF SULPHUR DIOXIDE

WINEMAKERS KNOW THE SULPHUR DIOXIDE TERMINOLOGIES LIKE FREE SULPHUR, TOTAL SULPHUR, MOLECULAR SULPHUR AND ITS ANTI-OXIDATIVE AND ANTI-MICROBIAL CHARACTERISTICS, BUT THE LINKS BETWEEN THEM ARE NOT ALWAYS VERY CLEAR.

BY CHARL THERON

ti-enzymatic activities of SO 2 . Winemakers usually aim to maintain a free SO 2 -concentration of 20 to 40 mg/L to prevent oxidation or the release of bound oxidative aromas. Anti-microbial activities are attributed to molec ular SO 2 and a 0.4 to 1.0 mg/L molecular SO 2 con centration is the aim, depending on the sweetness of the wine. The fermentation risk of sweetish wines is higher and requires a higher concentration. As mentioned above the percentage of molecular SO 2 is influenced by pH, so the concentrations of the two forms must be considered separately. BOUND SULPHUR DIOXIDE Some of the bisulphite will combine with other wine components to form bound SO 2 . These include strongly binding acetaldehyde (with a prominent bruised apple aroma) and weaker binding aldehydes such as methional, as well as sugars, anthocyanins and other components, which do not contribute to an oxidative character. The bound and free SO 2 are in equilibrium and if some free is used in oxidation reactions, the bound SO 2 will dissociate to restore the equilibrium. Con trary to the near-instantaneous restoration of the molecular and bisulphite equilibrium, the equilibri um between free and bound SO 2 may require hours or days. TOTAL SULPHUR DIOXIDE The sum of free and bound SO 2 is expressed as the to tal SO 2 , which is the concentration legally prescribed in different countries. In South Africa it is 160 mg/L depending on different conditions.

THE ROLE OF SULPHUR DIOXIDE as a microbial preservative can be substituted to quite an extent by other preservatives, but its ability to limit oxidative aromas and colour browning remains dif ficult to substitute. The role of sulphur dioxide in redox reactions must be understood better to do more research about it. FREE SULPHUR DIOXIDE Sulphite or SO 2 is often used as a blanket term for the pungent gas in compressed gas cylinders and several other sources like potassium metabisulphite or aqueous solutions of sulphur diox ide. The preservative properties of sulphur dioxide can primarily be attributed to the so-called free SO 2 , which consists of neutral molecular SO 2 and its corresponding base bisulphite (HSO 3 - ). These two exist in a rapid equilibrium. At a wine pH of 3 to 4, the bisulphite is more than 95% of the free SO 2 , with even higher proportions at higher pHs. Bisulphite is the primary form responsible for the anti-oxidative and an

Research has confirmed that bisulphite (HSO 3 - ) is the antioxi dant form of sulphur dioxide. It does not bind the oxygen as such, but reacts with different wine oxidation products. Two wine oxi dation pathways, namely the iron-phenolic and copper-sulphydryl exist. The first pathway seems to be the major oxygen consump tion pathway. Its key steps can be summarised as follows: • Oxygen reacts rapidly with the reduced ferro form of iron (Fe +2 ) to form its oxidised ferric form (Fe +3 ) and hydrogen peroxide. Fe +3 then oxidises the phenolic compounds to quinones and is returned to the Fe +2 form. Iron thus functions as a recycling oxidation catalyst enabling oxygen to act as an oxidant. • Quinones can react with different desirable thiols. Bisulphites can however reduce quinones back to their phenolic form and prevent their further oxidation reactions. • Hydrogen peroxide (H 2 O 2 ) may undergo the so-called Fenton reaction in the presence of Fe +2 to form hydroxyl free radicals.

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WINETECH TECHNICAL YEARBOOK 2022