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.
Despite this, using the manual Ripper titration to measure SO2 can be challenging for a number of reasons. Given the need to monitor the color change associated with this titration method by eye, it can be problematic to accurately determine end points in red wines, as the dark color of the sample can make it difficult to identify the onset of the color change. This limitation means that measurements can often be inconsistent and unreliable, putting the quality and regulatory compliance of the end product at risk. Moreover, as operators must be fully engaged with the titration throughout the experiment, manual titrations can be very resource intensive. For wineries with limited resources or those deciding to scale up production, the need for a dedicated, trained operator, or team of operators (for large productions), to perform manual Ripper titrations can prove to be a bottleneck.
Using Automated Titrators to Measure Sulfites
Given the importance of monitoring sulfite levels to protect the quality of wine and extend product shelf life, winemakers are increasingly using automated titration systems to generate results faster and more efficiently. As automated Ripper titrations use electrodes to monitor potentiometric end points, rather than subjective color changes, they provide precise results regardless of which operator performs the test. Moreover, by generating accurate results that are right the first time, these systems are able to support rapid and more informed decision-making.
Modern automated titration platforms are also capable of performing testing with no manual intervention except for the initiation of tests with the push of a button, enabling wineries to undertake sulfite testing more efficiently. In addition, this ease of use frees up operators to work on other tasks, such as additional safety or quality tests, and gives wineries the flexibility and capacity to quickly scale up sulfite testing activities without having to significantly expand their teams. The latest automated platforms for sulfite testing extend beyond data collection to processing and analysis, enabling wineries to automatically calculate and store results in line with regulatory requirements, while avoiding the risk of transcription errors that can occur using manual workflows.
Additionally, some of the latest titration platforms enable wineries to program and save frequently used method details in the system for routine use by operators. These convenient and intuitive systems can help wineries work more efficiently by eliminating the time required to set up the relevant conditions before each test. More advanced platforms will allow system administrators to lock the pre-programmed tests, preventing them from being changed by unauthorized users. For laboratories with large workflows, these features can be highly beneficial in increasing productivity and delivering more consistent results.
The robustness of sulfite testing workflows is a key priority for many wineries, especially those with high-volume testing requirements. Recent improvements in the operational resilience of automated titration systems are helping to minimize maintenance requirements and simplify upkeep. Some modern automated titrators will even diagnose performance issues and guide operators through recalibration and maintenance steps using clear on-screen instructions. As sulfite testing is often undertaken by operators without any in-depth technical knowledge, the improved simplicity and ease of use of these systems can allow wineries to extend the interval between maintenance operations and get titrators back in action more quickly when issues do arise. These innovative features improve operational efficiency and productivity and help get wineries back to what they’re good at: ensuring great wine makes the journey from vineyard to wine glass.
Gleichauf is an applications lab manager at Thermo Fisher Scientific. Reach her at email@example.com.