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 the SO2 content (bound and free) in beer and juices
This method is suitable for analyzing beer and other beverages.
Sulfite (sulfurous acid) is oxidized in the presence of oxygen through sulfite oxidase (SO2-OD) to sulfate [1].
SO22- + O2 + H2O \(^{\underrightarrow {\text{SO}_2 {\text{-OD}}}}\) SO42- + H2O2
The hydrogen peroxide produced by this reaction is reduced in the presence of reduced nicotinamide adenine dinucleotide (NADH) by the enzyme NADH peroxidase (NADH-POD).
H2O2 + NADH + H+ \(^{\underrightarrow {\text{NADH-POD}}}\)2 H2O + NAD+
The amount of NADH consumed in reaction (2) is equivalent to the quantity of sulfites, including the quantity of sulfites chemically bound to aldehydes. The NADH is the substance being measured and is determined photometrically based on its absorption at 340 nm.