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80 WINES&VINES November 2018 WINEMAKING PRACTICAL WINERY & VINEYARD W inemakers routinely add sulfur dioxide (SO 2 ) to wines to prevent microbial and oxidative spoilage. In red wines, a large portion of SO 2 is bound to anthocyanins (red pigments). Stan- dard approaches to measuring SO 2 in wines in- volve an initial acidification step that releases this anthocyanin-bound SO 2 , but it was unknown if this affected the precision of SO 2 measurements for the purposes of predicting microbial stability. Some authors have proposed that anthocyanin- bound SO 2 is also antimicrobial, but this work refutes that claim and shows that standard ap- proaches do not accurately describe the antimi- crobial activity of SO 2 in red wines. Background: SO 2 terminology review In this article, "molecular AO " and "free AO " SO 2 indicate values that would be measured by conventional SO 2 analysis methods that rely on an initial acidification step, such as aera- tion-oxidation (AO), Ripper titration and flow injection analysis (FIA). "Molecular GDT " and "free GDT " indicate measurements that do not require an acidification step such as head- space gas detection tubes (HS-GDT). SO 2 can exist in several forms in wine: molecular SO 2 , bisulfite (HSO 3 -) and bound SO 2 . (See Figure 1: SO 2 measurement methods yield different proportions of SO 2 species.) The sum of molecular SO 2 and bisulfite is referred to as free SO 2 . Protection against microbial spoilage in wines comes from molecular SO 2 . The relative amount of molecular SO 2 to bisul- fite in a wine varies with pH. Only a small portion of the free SO 2 in wine exists in the molecular form–usually less than 5% at ordi- nary wine pH values. Typical recommended concentrations of molecular SO 2 range from 0.5 to 0.8 milligrams per liter (mg/L), depend- ing on a wine's risk for microbial spoilage. Protection against chemical oxidation in wines comes from bisulfite. Bisulfite is the most abundant free SO 2 form at wine pH, and free SO 2 recommendations are typically more than 30 mg/L. Bisulfite can scavenge initial wine oxidation products (quinones and hydro- gen peroxide). It can also form bound SO 2 complexes by reacting with other oxidation products, such as aldehydes. In red wines, bisulfite may also form complexes with antho- cyanins ("anthocyanin-bound SO 2 "), particu- larly in young, highly pigmented red wines. Background: SO 2 measurements Conventional industry methods for measuring free SO 2 in wine include aeration-oxidation (AO), iodometric titration (the Ripper method) and flow injection analysis (FIA). The initial acidification step in these methods disrupts anthocyanin-SO 2 complexes in red wine. (See Figure 1: SO 2 measurement methods yield different proportions of SO 2 species.) We de- scribe these methods as measuring free AO SO 2 , which is a sum of free SO 2 and SO 2 released from anthocyanin-bound SO 2 . Since molecular SO 2 is generally calculated using this free SO 2 value and the pH, the conventional measure- ment methods mentioned above will report molecular AO SO 2 values higher than the actual molecular SO 2 , particularly in red wines. The Sacks lab at Cornell University in Ithaca, N.Y., recently described a new approach to mea- suring molecular SO 2 , termed "head-space gas detection tube" (HS-GDT, see Figure 2 the online version of this article has a link to more informa- tion here) which does not disrupt anthocyanin- SO 2 complexes. The work showed that HS-GDT measurements of molecular GDT SO 2 in red wine average only 32% of the molecular AO SO 2 levels. In other words, AO typically overestimates actual free and molecular SO 2 by a factor of 3. In con- trast, HS-GDT measurements on white and rosé wines averaged 86% of the molecular AO SO 2 . Objectives It was previously established that molecular AO SO 2 is significantly overestimated using con- ventional measurement methods in compari- son to HS-GDT. The objectives of this work were to: 1. Determine if anthocyanin-bound SO 2 has antimicrobial activity, Sulfur Dioxide Measurements in the Winery Overestimating antimicrobial activity in red wine By Raquel Kallas, Gavin Sacks, Patricia Howe and Randy Worobo Hypothetical data showing the distribution of SO 2 species in a red wine with a total SO 2 of 80 mg/L mea- sured by two analytical approaches, AO (left) and HS-GDT (right). During the initial acidification step of AO, anthocyanin-bound SO 2 dissociates, and the SO 2 released is included in as part of free AO SO 2 . In HS-GDT measurements, there is no acidification step, and anthocyanin-bound SO 2 is not measured as part of free GDT SO 2 . The discrepancy between the two methods for a given wine will depend on anthocyanin concentration, among other factors. FIGURE 1: SO 2 MEASUREMENT METHODS YIELD DIFFERENT PROPORTIONS OF SO 2 SPECIES 35 mg/L n Bound n Free n Bisulfite n Molecular SO 2 BY CONVENTIONAL METHODS 75 mg/L SO 2 BY HS-GDT 44 mg/L 1.5 mg/L 5 mg/L 0.15 mg/L