There are two chemical forms of sulfur dioxide: ‘free’ and ‘bound’. The free form is the pH-dependent balance of molecular SO2 and bisulfite. This is the critical form from a winemaking perspective because molecular SO2 is the antimicrobial or preservative form. The bound form consists of bisulfite attached to aldehydes and ketones. Acetaldehyde is a common binding compound. Together, the free and bound forms give the ‘total sulfur dioxide’ concentration: this is the legislative value in winemaking countries around the world.
From a consumer perspective, the issue with SO2 is its perceived allergic reaction. This requires mandatory labelling indicating that ‘sulphites’ have been added in excess of 10 mg/kg. The ‘added’ is important here because some sulfite may be produced during the fermentation. The wine industry seems reluctant to move from the use of ‘sulphites’, despite the IUPAC nomenclature rule change and the 1999 decision of the Australian Therapeutic Goods Administration approved terminology of ‘sulfites’ for medicines. Well, wine is sometimes regarded as medicine!
Whether the reaction to sulfite in wine is an allergic response or not is still not certain. In 2021, the ‘Food Safety Experts’ of the International Organisation of Vine and Wine (OIV) published a detailed review on the safety assessment of sulfite in wine (bit.ly/3vxG2SJ). The report authors concluded that ‘Most sulfite sensitivities are not true allergic reactions’. There is a lack of clarity about what causes sulfite intolerance. It would appear that the intolerance is not just to sulfite itself, and the OIV review concludes that it is ‘likely due to various biological reactions, depending on the individual genetic background’.
A replacement for SO2 needs to address its dual role of antimicrobial and antioxidant activity. Lysozyme has been extensively studied (Compr. Rev. Food Sci. Food Saf. 2014, vol. 13, pp. 1062–73), although it has several limitations. Its common source is egg white and in some jurisdictions its use requires allergen labelling. It is most effective against Gram-positive organisms but has a very limited half-life in wine. Professor Leigh Schmidtke of Charles Sturt University advised that ‘I’m not immediately aware of any winemakers using it regularly – maybe this reflects changes in the wine production syllabus over the past 15 or so years, ensuring good wine conservation measures are emphasised so there is not the need’.
Recently, chitooligosaccharide has been proposed as a possible replacement for SO2 (Appl. Sci. 2020, vol. 10(2), p. 578). This oligosaccharide is claimed to show antibacterial, antifungal and antioxidant behaviour, among other things. Approval for its use has not been obtained.
Ascorbic acid is used as an antioxidant in white wine. However, because the oxidation of ascorbic acid produces hydrogen peroxide, some SO2 is required to scavenge the peroxide. In addition, some ascorbic acid degradation products can react with wine phenolic compounds, leading to a brown coloration, an unattractive aspect of white wine (Aust. J. Grape Wine Res. vol. 22, pp. 169–81). Ascorbic acid is not effective in red wine.
Glutathione is found in small amounts in juice or must after grape processing and in wine post-fermentation. It is known to scavenge some phenolic compounds and it is capable of protecting aroma compounds through its antioxidant behaviour. It is thus a potential replacement for SO2, but it is not an approved additive. Some yeasts release glutathione and this might be a potential source.
A detailed review on glutathione identified several deficiencies in our knowledge of its behaviour in wine (J. Agric. Food Chem. 2013, vol. 61, pp. 269−77). One major deficiency was the lack of experiments carried out under wine-like conditions, especially longer-term storage. Glutathione is known to contribute to hydrogen sulfide production and this would detract from the wine’s acceptance.
The Australian Wine Research Institute has recently reported the outcomes of a longer-term glutathione trial using small-scale wine production trials (bit.ly/3oOF6FO). The results questioned the value of adding glutathione because low added concentrations showed little effect, while higher concentrations led to ‘undesirable sensory outcomes’.
Finding replacements to reduce the amount of SO2 remains a challenge. The 2021 OIV review suggested a need to perform winemaking with a lower SO2 concentration than is presently common practice. The OIV published in 2020 a ‘Review of practices for the reduction of SO2 used in winemaking’ (bit.ly/3PQmQrr) that sets out management principles for each winemaking step from the vineyard through to the finished wine in bottle.
Using less SO2 is now increasingly common here in Australia and elsewhere. In France, where I am more familiar with the practice, many wines are now available that have no added sulfite at any stage of the winemaking process. This is sometimes a deliberate decision due to sulfite sensitivity: Drappier sans soufre champagne is a classic example. Or it may be a response to producing so-called natural wines, where no or minimal additions are made. This has been driven by the ‘Raw Wine’ movement. Check out Isabelle Legeron’s book Natural wine (3rd edn), for a detailed discussion. In France, two optional labels for natural wines are available – one for zero sulfite addition and another for a maximum of 30 mg/L sulfite addition.