The most prevalent volatile sulfur compound generally found in beer in the highest concentration is sulfur dioxide. It reacts with carbonyl compounds and for this reason is found primarily in bound form in beer. Sulfur dioxide is a metabolic product of yeast and in beer is in its active form. In many countries, sulfur dioxide is added to beer as a preservative in order to increase flavor stability. According to the regulations governing additives in Germany dating to 29 January 1998, the maximum SO2 content permissible in beer is 20 mg/l (50 mg/l for barrel, cask, keg-conditioned beer). According to EU no. 1169/2011, supplements of sulfur dioxide or sulfites must be declared in concentrations above 10 mg/kg or 10 mg/l as SO2.
Determination of total SO2 content (bound and free) in beer and juices
This method is suitable for analyzing beer and other beverages.
SO2 is primarily present in beer or wine in a bound form (mostly on aldehydes, ketones and sugars). Sulfur dioxide is liberated through the addition of 2 M sulfuric acid and subsequent heating to 95 °C. In an acidic medium, this reaction forms SO2 ⋅ H2O from carbonyl sulfite adducts, which can no longer act as a functional nucleophile. Additional heating reduces the solubility of gases, causing SO2 to diffuse through a Teflon membrane where it is dialyzed in a 1% sulfuric acid solution. It then reacts with formaldehyde to form a carbonyl sulfite adduct (HO-CH2-SO3H). This adduct binds at a temperature of 45 °C to para-rosaniline forming a red complex; the absorbance is measured at 560 nm with a UV-VIS detector. When performing the analysis, care should be taken to only use the Teflon membranes listed below; otherwise, other Maillard reaction products can cause incorrect blank values.