Comparison Between DTNB and p-Rosaniline Methods to Quantify Total SO2 in Beer

Maria E. Moutsoglou. Research and Development, Sierra Nevada Brewing Company, Chico, CA, U.S.A.
Food products containing sulfites (SO2) above 10 mg/L are legally required to have “contains sulfites” clearly displayed on packaging. Accurately quantifying SO2 in beer is necessary to ensure sulfites are below the 10 mg/L limit. One common method of quantifying total SO2 (free and complexed) is using a segmented flow analyzer (SFA) with p-rosaniline (PRA) as a chromophore. The accuracy of this method was assessed in comparison to a discrete flow analyzer (DFA) using 5-5′-dithio-bis(2-nitrobenzoic acid) (DTNB) as a chromophore. SO2 was supplemented into beer and was quantified using the DFA and SFA. Each method accurately quantified the amount of supplemented SO2; however, there was a ≈3.6 mg/L difference in total SO2. To determine if this difference was due to the ability of the dyes to bind complexed SO2, SO2 was supplemented in a buffered solution and in lager and ale beers (to measure free SO2), and in an acetaldehyde-saturated buffered solution and lager beer (to measure complexed SO2). Although PRA was consistently less accurate than DTNB in quantifying supplemented bound and free SO2 in both a buffered solution and beer, both methods remained within approximately ±3 mg/L of the supplemented SO2 target. In unsupplemented beer, however, an up to 5.6 mg/L difference was observed in total SO2 between the two methods, in which PRA consistently quantified lower total SO2 than DTNB. This difference corresponded to a maximum observed percent difference of 127.3% for lager beers. Lagers from other breweries were tested, and the percent difference between DTNB- and PRA-derived SO2 decreased linearly as the concentration of SO2 increased. This suggests PRA has a larger limit of detection for SO2 in beer produced during fermentation than DTNB, and it should not be used to quantify total SO2 in beer.
Brewing Resources

Ask the Brewmasters Technical Quarterly MBAA Webinars MBAA Podcasts Food Safety Brewery Safety Vendor Search