Sulphur dioxide has been used since Roman times and remains the most common additive in winemaking. Due to its anti-microbial and anti-oxidative characteristics, it is challenging to substitute it with another single product having both characteristics.
Motivations for alternative products
A worldwide growing consumer demand for low or sulphite-free wines exists. Supporters of the “natural” wine concept believe that the terroir expression in wines requires limiting additives during winemaking. Both these factors are driving the action to look for sulphur dioxide (SO2) alternatives.
Anti-microbial alternative to SO2 – Metschnikowia pulcherrima
Worldwide interest exists regarding the biological control of spoilage organisms in food. This also applies to wine. Consumers prefer that additives like SO2 and preservatives that may have potential health consequences be used less or eliminated during winemaking. Biological control or biocontrol comprises the inoculation of products with a microorganism, which can inhibit or even kill the potential spoilage organism in a product. Specific wine yeasts can be used for that purpose. Metschnikowia pulcherrima, a non-fermenting, non-Saccharomyces yeast, is an example. It is available as a commercial product and can be used as a bio-protector for grapes and grape juice and a substitute for SO2 as an anti-microbial product for wine.
Metschnikowia pulcherrima does not inhibit the natural fermentation process, and undesirable aromas like volatile acid and ethyl acetate are not formed. Unwanted organisms are, however, inhibited. It can improve the aroma profile of wines because more esters, higher alcohols, and fruity and cultivar aromas are enhanced. Increased glycerol concentrations will also improve the taste profile of wines. For juice with a high undesirable microorganism population or a lower potential alcohol concentration, a low dosage of SO2 should still be added.
Anti-oxidative alternative to SO2 – oenological tannins
Botrytis cinerea infection of grapes can result in high levels of the laccase enzyme in the juice. This enzyme can cause oxidation and consequential browning in the presence of oxygen. High dosages of SO2 can effectively prevent this, and its legal limits for Botrytis wines are considerably higher than for still wines.
An alternative to SO2 is using specific oenological tannins to limit the effect of the laccase enzyme and consequently lower or eliminate the use of SO2. The use of oenological tannins during winemaking is a common practice approved by the OIV. Five to 100 g/hL dosages are used for juice and wine, depending on the commercial product used.
Oenological tannins are classified as hydrolysable or condensed tannins. Hydrolysable tannins consist of gallotannins and ellagitannins. The primary sources of gallotannins are nutgalls and tarapods, while oak and chestnut are the main sources of ellagitannins. Condensed tannins originate, amongst others, from grape skins and seeds.
The different tannin groups have different compositions and, consequently, different characteristics. Oenological tannins can be used as an antioxidant and to improve and stabilise the colour of red wines, enhancing the mouthfeel and removing reductive aromas. It can be added at any stage, depending on the potential oxidation during the crushing of grapes, after fermentation, during rackings or before bottling.
Ellagitannins are present in different forms of oak.
References
Archer, L.M., 2023. Natural alternatives for sulfur. Wine Business Monthly, June 2023: 22 – 30.
Di Gianrito, P., Englezos, V., Rantsiou, K. & Cocolin, L., 2022. Bioprotection strategies in winemaking. International Journal of Food Microbiology 364.
Vignault, A., Pascual, O., Jouraes, M., Moine, V., Fermaud, M., Roudet, J., Canals, J.M., Teissedre, P-L. & Zamora, F. 2019. Impact of enological tannins on laccase activity. OENO One 531.
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