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.

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