Sulfur dioxide (SO2) is a preservative widely used in foods and beverages. It is used as an additive in various forms, most commonly as sulfites in the wine, cider, beer, and beverage and food industries, where it acts primarily as an antimicrobial and antioxidant preservative.
In beer, sulfur dioxide originates from yeast metabolism and reacts with carbonyl compounds to form hydroxysulfonates, which react with the carbonyl compounds that produce a stale, unwanted flavor. Sulfur dioxide also plays an important role as an antioxidant and is known to exert antimicrobial properties at high concentrations. Its concentration is controlled at the end of beer production to ensure beer quality.
In winemaking, sulfur dioxide is added to control the process and serves many useful functions. For example, it acts as an enzyme inhibitor in musts to prevent juice from browning. As a microbiological control agent, sulfur dioxide is added to the winemaking process to prevent oxidation in the finished product.
Why accurate and reliable sulfite measurements are important
Sulfur dioxide can be found in wine in its free forms, SO2 (gas) and bisulfate ion (HSO3–), or bound to compounds that incorporate a carbonyl group, such as acetaldehyde. Free forms of SO2 are pH and temperature-dependent, and because of the acidic nature of wines, SO2 is usually present and measured as bisulfate ion (HSO3-). Results are reported as SO2 .
Due to the increased awareness of the adverse effects of sulfites and the prevalence of sulfite intolerance in some individuals, sulfite levels in foods and drinks are strictly regulated by various governing bodies. As a result, accurate determination of sulfite levels in foods, beverages and wines is required. The presence of total sulfur dioxide (both free and bound) is regulated and — because sulfite is considered an allergen — a warning statement is required on wine labels. The European Union established a maximum permitted level of total SO2 in wine varying from 150 to 500 mg/L, which is dependent upon the sugar level of the product. In the U.S., the maximum level of total SO2 permitted is 350 mg/L. In the EU, levels above 10 ppm (total) must be labeled. That is why it is important to measure also the low level in beverages such as beers. Sulfur dioxide is typically measured by the European Brewery Commission (EBC) Method 9.25.3 or by the similar American Society of Brewing Chemists (ASBC) Beer-21 para-rosaniline (p-rosaniline) method.
Analytical methods for free and total SO2
Auto titration or ion chromatography are the most frequently used methods for sulfite analysis but are best suitable only for total sulfur dioxide. Spectrophotometric or discrete analyzers are suitable for both free and total sulfur dioxide.
The most common method used for total sulfur dioxide is aeration/oxidation (AO) , and is the basis of the approved OIV (International Organisation of Vine and Wine) method. The AO method is relatively robust in terms of interference from other wine components. The equipment and reagents required are, however, somewhat specialized and need to be carefully used and maintained. Each analysis takes around 20 minutes and requires the constant attention of a trained technician. Nevertheless, the method is highly robust and repeatable, which is why it is the mainstay of sulfur dioxide analysis in most wineries.
Free and total sulfur dioxide using a discrete analyzer
The discrete analyzer mimics the operation sequence of lab chemists such as dispense sample, mixing reagent, waiting for the reaction to complete, followed by photometric measurement, to provide fast and reproducible results.
Method principle to analyze total SO2: The method is based on the reaction between sulfur dioxide and 5,5’-dithiobis nitrobenzoic acid (DTNB) in basic conditions. The method is performed at 37 °C using a 405 nm filter and for side wavelength 700 or 750 nm filter.
Method principle to analyze free SO2: The method is based on the reaction between SO2 , p-rosaniline hydrochloride, and formaldehyde and is performed at 37 °C, using a 575 nm filter and a 700/750 nm filter for side wavelength. The purpose of measuring a side wavelength is to remove the effect of bubbles that may appear in cuvettes. The side wavelength is determined from the spectrum area where no reaction occurs.
An automated discrete analysis method, which compares well with the reference AO method, is fast, simple to use, and carried out on equipment used for other wine, beer, cider, food and beverage analysis — proving itself to be a significant benefit. The automated sulfur dioxide-free and total methods are quick and easy to use. Analysis of 60 samples takes only 35 minutes and allows simultaneous analysis of various sugars and organic acids.
The discrete analyzer with ready-to-use reagents offer:
- Accurate and reproducible results
- Ease of use with walkway efficiency
- Reduced waste disposal and overall cost per analysis
- Reduced sample and reagent consumption
- Real-time reagent consumption
3. Practical measurement of total SO2 in wine by Eric Wilkes, AWRI