Sulfur-containing compounds, such as sulfite salts (SO32-), sulfur dioxide (SO2) and hydrogen sulfite (HSO3-), are widely used as preservatives in the food and beverage industry. Thanks to their powerful antimicrobial and antioxidant properties, they are commonly added to a broad range of products to prevent unwanted oxidation, discoloration, and microbial growth, ultimately protecting the quality of the product and extending its shelf life.
Although sulfites are used as preservatives in the manufacture of a wide array of products, including prepared soups, dried fruits, and French fries, they are perhaps best known for their role in winemaking, where they have long been used to control the growth of undesirable yeast and bacteria. Indeed, it is often claimed that even the Romans and Egyptians used sulfur to improve the quality of their wines, although the evidence for this is questioned by some.1 Today, sulfites are added at various stages of the wine production process to enhance the taste and appearance of the final product, and may be present in wine as free sulfites (HSO3-, SO32- or SO2 depending on the pH) or bound following reaction with other molecules in wine such as phenols or carbonyl compounds.2 These free sulfites are available to react and provide the desired antimicrobial and antioxidant properties, and are of particular interest during production when deciding how much to add. The total sulfites (the free sulfites and the bound sulfites) in the final product are of interest for labeling and regulatory compliance purposes since many countries have guidelines on the maximum permitted amounts of sulfites.
Ensuring sulfites are present in sufficient quantities is essential to safeguard product quality and stability. Produce prepared without the addition of appropriate levels of sulfites can ultimately lead to entire batches being compromised. However, it is equally important that sulfite levels are not too high, and a careful balance is struck. The excess addition of sulfites is not only costly and unnecessary for producers – large quantities of sulfites can also delay some types of fermentation processes and adversely impact on the flavor and aroma of products. As a result, the use of other chemicals may be required to mitigate these effects, at additional cost.
Furthermore, the sulfites added to food and beverage products have been linked to allergic reactions in some people. Individuals who are particularly sensitive to sulfites may experience a range of symptoms upon exposure, including skin rashes, stomach complaints and breathing difficulties.3 Given these effects, maximum levels of sulfites permitted in foods and beverages are carefully regulated. In the United States (US), the upper limit for total SO2 in wines is 350 mg/L.4 The European Union (EU) has stricter controls around sulfite use,5 and limits total SO2 to 150 mg/L in most red wines, and 200 mg/L in most white and rosé wines. Other limits are enforced for sweet and sparkling wines, with the upper limit for some particularly sweet wines being 400 mg/L. In both the US and EU, a sulfite warning label must be present on all wines with levels of sulfites greater than 10 mg/L.
Since food and beverage manufacturers need to carefully control the sulfite levels in their products to meet quality and regulatory requirements while also making the most efficient use of resources, the accurate monitoring of free and total SO2 throughout production processes is essential. When it comes to winemaking, SO2 testing is widely conducted before or after primary fermentation (when opening vessels for tasting, testing, blending, fining or topping up), prior to bottling to enable adjustment for aging, and with the end product to ensure compliance with regulations for specific markets.
The challenge of determining SO2 levels in food and beverage products
An extensive selection of methods for monitoring free and total SO2 in food and beverage products are available, including distillation followed by acid or base titration, iodometric titration, as well as enzyme assays with colorimetric or spectrophotometric detection.
One of the most widely used methods for SO2 determination is the Monier-Williams method. The approach involves an initial sample distillation step to capture SO2 in hydrogen peroxide. This produces sulfuric acid, which can be titrated with sodium hydroxide to calculate the concentration of SO2. However, a major limitation of this method is the need for time-intensive distillation steps, which limits its use for routine analysis.
An alternative and less time-consuming approach, commonly employed in the wine industry, is the Ripper iodometric titration. Unlike the Monier-Williams method, the Ripper titration measures SO2 directly, eliminating the need for lengthy distillation steps. When performed manually, the Ripper titration involves monitoring a color change end-point using starch as the indicator. Alternatively, the titration may be performed faster and more efficiently using automated titrations systems, which make use of a potentiometric end-point determined using an electrode. To determine levels of free SO2, samples under analysis are typically acidified prior to titration, while levels of total SO2 are determined by treating samples with sodium hydroxide to release the bound SO2 prior to analysis.
Despite its usefulness, the manual Ripper titration method suffers from a number of limitations. Determining end-points by eye can be challenging for deeply colored foods and beverages, such as dark red wines, where the original sample color can result in poor measurement accuracy and repeatability. Additionally, the need to monitor a color change also requires the operator to be fully engaged for the duration of the test, meaning that team members cannot walk away to work on other tasks or run other tests simultaneously. For food and beverage producers looking to scale-up production and therefore expand their sulfite testing capabilities, workflows based on manual Ripper titrations necessitate a dedicated operator, or team of operators for very large productions.
Given that SO2 determination is such a critical analysis in many food and beverage manufacturing processes, producers need reliable SO2 monitoring methods that deliver accurate and reliable results, quickly and cost-effectively, to meet their demands for high-throughput testing.
Streamlining SO2 testing workflows with automated titration technology
Thanks to improvements in compact instrument designs, dispense accuracy and digital data reporting, modern automated titration systems are helping food and beverage manufacturers overcome the challenges associated with using time and resource-intensive manual methods for SO2 monitoring. By measuring potentiometric end-points using electrodes, rather than ambiguous color changes, automated titrators offer improved accuracy and repeatability, no matter which operator collects the data. By delivering the right result the first time around, automated titration systems are able to achieve faster analyses and more reproducible results with which to support faster and more confident decision-making.
Modern automated titrators also significantly reduce the amount of manual effort required to set up and perform an analysis, helping food and beverage manufacturers work more efficiently with the same resources. The latest fully-automated systems eliminate the requirement for operator intervention during testing, allowing team members to walk away and work on other tasks. These efficiency savings give producers the flexibility and capacity to rapidly scale-up sulfite monitoring workflows without the need to invest in training additional operators. The benefits of automated workflows also extend to data processing and analysis steps, helping food and beverage producers to quickly calculate and record data in a regulatory compliant manner while minimizing the potential for errors that are commonly associated with transcribing data manually.
Many food and beverage manufacturers with multiple production processes operate several different testing protocols beyond SO2 determination. Some automated titration systems allow producers to create and store frequently-used method settings using the platform’s intuitive user interface, eliminating the need to set-up conditions each time. The latest systems will even lock these pre-programmed tests to prevent them from being accidentally modified by other users. These features are helping to boost productivity and ensure greater operator-to-operator consistency, factors that are particularly important for busy laboratories handling multiple products.
Further advances in SO2 testing workflow efficiency are being achieved by simplifying the day-to-day maintenance and upkeep of titration equipment. The latest automated titrators are able to guide operators through recalibration and maintenance steps using clear and instructive on-screen prompts. With many SO2 monitoring workflows operated by non-technical experts, the simplicity and ease of use afforded by these systems can reduce downtime and boost output by keeping equipment working effectively for longer, and solving maintenance issues faster should they arise.
Conclusion
Determining free and total SO2 levels in food and beverage product lines is an essential part of safeguarding quality characteristics and ensuring compliance with the latest regulatory standards. However, traditional manual approaches to sulfite monitoring can be time and resource-intensive. Thanks to the latest advances in electrochemistry, automated titration systems are helping manufacturers more accurately, consistently and cost-effectively control sulfite levels during production, to deliver regulatory-compliant products that meet consumer expectations.
Gayle Gleichauf is an applications lab manager at Thermo Fisher Scientific.
References
- Gross, L. Making Sense of Sulfites, Wines & Vines, 2011. https://winesvinesanalytics.com/features/article/82324/Making-Sense-of-Sulfites (Accessed August 2019)
- Barbe, J.; Revel, G.; et al. Role of carbonyl compounds in SO2 binding phenomena in musts and wines from botrytized grapes. Journal of Agricultural and Food Chemistry. 2000, 48, 3413–3419.
- Guerrero, R.F.; Cantos-Villar, E.; Demonstrating the efficiency of Sulphur dioxide replacements in wine: A parameter review, 2015, 42(1), 27–43.
- Alcohol and Tobacco Tax and Trade Bureau, CFR: Title 27. Alcohol, Tobacco Products and Firearms, 2014.
- European Commission, Commission Regulation (EC) No 606/2009, Official Journal of the European Union, 2009.