Ask wine connoisseurs about their favorite vintage and they’ll probably mention the aroma from the uncorked bottle, the color in the glass, and the complex flavors. However, unwanted oxidation, discoloration, and microbial growth during production and after bottling can compromise all of these characteristics, putting revenues and reputations at risk.
To prevent these undesirable processes and extend product shelf life, winemakers commonly add preservatives in the form of sulfites—sulfur-containing compounds such as hydrogen sulfite (HSO3-), sulfite salts (SO32-), and sulfur dioxide (SO2)—that possess strong antioxidant and antimicrobial properties. Achieving the right balance of sulfites in wine is of utmost importance to protect product quality in line with stringent regulations. Increasingly, many wineries are recognizing the benefits of using automated titration systems that are capable of monitoring sulfite levels and delivering accurate and reliable results, quickly and cost-effectively.
The Importance of Monitoring Sulfites
Sulfites may be added at various stages of the wine production process, from the crushing of the grapes until just prior to bottling, depending on the type of wine being produced and the individual preferences of the winemaker. They may be present in wine as free sulfites (HSO3-, SO32- or SO2, depending on the pH) or bound to other wine components, such as phenols and carbonyl compounds.
For wineries, getting the level of sulfites right is of critical importance. If sulfite levels are too low, wine quality can be compromised, potentially resulting in the need to discard entire batches. Get sulfite levels too high, however, and wineries face a different set of challenges. Not only is the over-addition of sulfites costly, the presence of excess sulfites can delay key fermentation processes and have a detrimental impact on wine taste and aroma.
On top of this, sulfites are thought to cause allergic reactions in some people. Consumers who are particularly sensitive to sulfites may experience symptoms including skin rashes, stomach complaints, and breathing difficulties. Regulations around sulfite levels are in place to protect the public’s health, and wineries cannot sell wines that don’t meet these regulations.
Regulatory requirements for total sulfites (free and bound) in wine vary by region and product type. In the United States, wines cannot exceed total SO2 levels of 350 mg/L, and any wines containing more than 10 mg/L sulfites must be labeled with a warning. In the European Union, tighter controls around sulfite use are enforced, with different limits depending on the type of wine. These regulations limit total SO2 to 150 mg/L in most red wines and 200 mg/L in most white and rosé wines. Sparkling wines may contain up to 235 mg/L total SO2, while certain sweet wines may contain higher sulfite levels up to a maximum of 400 mg/L. Similar regulations around sulfite levels are in place in other countries.
Determine Sulfite Levels
A wide range of methods are available to monitor sulfite levels in wine. These include distillation followed by acid/base titration, iodometric titrations, and enzyme assays involving colorimetric or spectrophotometric detection techniques.
The Monier-Williams method and the Ripper iodometric titration are two of the more widely used methods for the determination of sulfites in wine. The Monier-Williams method is a multi-step process that first involves capturing SO2 in hydrogen peroxide by distillation. The sulfuric acid that’s generated from this step is then titrated with sodium hydroxide to determine the concentration of SO2. While the Monier-Williams method is a very precise technique for determining levels of sulfites in wine, the need to perform a distillation step often makes the use of this method for routine analysis applications impractical.
The Ripper titration is an alternative approach that enables sulfites to be measured directly, without the need for time-consuming distillation steps. Many wineries perform this iodometric titration manually, using starch as an indicator to monitor a color change end point. Levels of free SO2 can be determined by acidifying samples prior to titration, while total SO2 can be measured by first treating samples with sodium hydroxide, which releases the bound sulfites. After the bound sulfide is released, the titration proceeds as for the free SO2.