Sulfite: Difference between revisions

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 Sulfites are NOT directly responsible for the sulfurous/rotten egg/burnt match aroma, as is commonly mistaken — [[hydrogen sulfide]] and/or [[mercaptans|ethyl mercaptan]] are the sources of that off-flavor.<ref>https://www.therealreview.com/2018/07/17/understanding-hydrogen-sulphide-and-sulphur-dioxide/</ref><ref>Williamson, B. [https://williamsonwines.com/recognizing-wine-flaws "Recognizing Wine Flaws."] Williamson Wines. Accessed online March 2020.</ref><ref>Mansfield, AK. [https://cpb-us-e1.wpmucdn.com/blogs.cornell.edu/dist/0/7265/files/2016/11/SulfurOffOdor-1vp1vm4.pdf "Kicking up a Stink: Treatment for Sulfur Off-Odors."] ''Cellar Dweller.'' Cornell University - NYSAES. April 2010.</ref> Sulfites should not be confused with [[sulfate]] or [[hydrogen sulfide|sulfide]].
+Sulfur dioxide (SO2) is a gas that is 85 g L−1 soluble in water at 25 °C and has a boiling point of −10 °C.170 In solution, it undergoes equilibrium reactions with SO2.nH2O, the bisulfite ion (HSO3 −), and the sulfite ion (SO3 −). At beer pH, which is generally 3.8−4.4, the predominant form is the bisulfite ion.158,171 Because all of these species can be converted to, measured as, and reported in terms of SO2, they are often generalized under “SO2” or “sulfites”.<ref name=baedec>Baert JJ, De Clippeleer J, Hughes PS, De Cooman L, Aerts G. [https://www.themodernbrewhouse.com/wp-content/uploads/2017/03/Baert-Aldehyden.pdf On the origin of free and bound staling aldehydes in beer.] ''J Agric Food Chem.'' 2012;60(46):11449–11472.</ref>
 ==Sources of Sulfite==
+the major source of sulfite in beer is the reduction of sulfate in water and grist by the yeast metabolism (endogenous SO2). The SO2 content is also increased by the addition of sulfiting agents (exogenous SO2) such as SO2 (E220), Na2SO3 (E221), NaHSO3 (E222), Na2S2O5 (E223), K2S2O5 (E224), CaSO3 (E226), Ca(HSO3)2, (E227), and KHSO3 (E228) before beer packaging.115,158,171−173 According to Johannesen et al.,175 no difference could be noticed between the (E)-2-nonenal concentrations of forced-aged beer with sulfite derived from endogenous or exogenous origin.175<ref name=baedec/>
 ===Products===
 Sulfite is available in powdered form as sodium metabisulfite and potassium metabisulfite, and also in tablet form with the brand name Campden. These products are not entirely interchangeable, so it's important to note their differences when selecting a product. Neither potassium nor sodium affect the action of the sulfite, but they can have other effects.
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 ===Natural Sulfite===
+Sulfite comprises an intermediate product of cysteine and methionine biosynthesis, and its excretion by yeast proceeds in four stages22,171,175,188,189 (Figure 20). In stage 1, methionine and threonine present in wort inhibit and repress certain enzymes, preventing sulfite excretion. During the second stage, the pathway is switched on, but sulfite excretion remains low due to a high demand for sulfur-containing amino acids. In stage 3, yeast growth ceases, which lowers this amino acid demand. However, extract, and thus energy, is still available, which favors sulfite production. Sulfite excretion commences due to an oversupply in the metabolism. The alcohol level at this moment is about 1.5% w/w.75,190 In the fourth stage, the extract is depleted, sulfate reduction stops, and sulfite excretion stops accordingly.189 The extent of sulfite excretion depends on the yeast strain used; lager strains often produce more SO2 than ale strains, for example.191 It has been found that beer produced with a yeast strain with augmented sulfite secretion shows better flavor stability.192 Furthermore, higher sulfate supply to the yeast, higher original wort gravity, higher wort clarity, higher fermentation temperature, lower pitching rate, and lower wort oxygenation all result in higher SO2 contents.158,171,189,190 In general, sulfite secretion is inversely proportional to yeast growth, independent of the applied parameters.189<ref name=baedec/>
 Sulfite is produced naturally by yeast during fermentation, and may be present at the end of fermentation in some amount, usually less than 30ppm although some strains can produce vastly higher amounts.<ref name="Rotter">Rotter, Ben. [http://www.brsquared.org/wine/Articles/SO2/SO2.htm "Sulphur Dioxide."] ''Improved Winemaking.'' 2011.</ref><ref name="Werner">Werner, M., Rauhut, D., Cottereau, P. [https://www.infowine.com/intranet/libretti/libretto7646-01-1.pdf "Yeasts and Natural Production of Sulphites."] ''Internet Journal of Enology and Viticulture.'' 2009 N12/3</ref><ref name=lundmn>Lund MN, Andersen ML. [https://www.tandfonline.com/doi/abs/10.1094/ASBCJ-2011-0620-01 Detection of Thiol Groups in Beer and Their Correlation with Oxidative Stability.] ''J Am Soc Brew Chem.'' 2011;69(3):163–169.</ref> Yeast produce sulfite by reducing [[sulfate]], although the concentration of sulfate may have only a minor effect on the amount of sulfite produced, depending on the yeast strain.
 Yeast also produce compounds during fermentation that bind to sulfite, decreasing the proportion of free SO<sub>2</sub>.
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 Sulfur dioxide (SO2 ) is a powerful antioxidant, though the levels in beer permitted by regulation (10 mg/l in the USA) are generally insufficient to confer flavor stability; it forms addition compounds with aldehydes and so in beer is mostly bound SO2 . SO2 arises in beer by yeast action and is a variable depending on yeast strain, or arises by addition of KMS in the kettle or post- fermentation. It is an effective flavor preservative at levels somewhat below its flavor threshold of about 25 mg/l.<ref name=lewbam>Lewis MJ, Bamforth CW. Chapter 12: Oxygen. In: Lewis MJ, Bamforth CW, eds. [[Library|''Essays in Brewing Science.'']] Springer; 2006:131–142.</ref>
+It is generally accepted that sulfites protect beer from staling in two different ways.23,43,158,171,176−178 First, they can act as antioxidants, improving beer flavor stability by inhibiting oxidative chain reactions through radical scavenging of both ROS and other radicals. Sulfite seems to interact with peroxides in a two-electron nonradical producing reaction, preventing the formation of staling aldehydes and many other undesired products.21,179 Second, they have a role as carbonyl-binding agents through the formation of aldehyde−bisulfite adducts, the so-called hydroxysulfonates (Figure 18). As an illustration, the addition of sulfite to fresh beers strongly delayed the appearance of cardboard flavor during beer aging, and the level of free flavor-active (E)-2-nonenal lowered upon addition. Free SO2 disappears from beer over time, with a very low, but nonzero rate, at 0 °C, and faster with increasing temperature, following first-order kinetics. These rates are barely affected by the initial SO2 content.183 Free SO2 is most likely lost as an antioxidant pool, but likely also as a pool for binding de novo formed aldehydes or aldehydes released from, for example, imine adducts,38,176,183 as well as reversible or irreversible interaction with a whole range of other components such as reducing sugars, Maillard intermediates (thus inhibiting the Maillard cascade), cysteine residues, thiamins, quinones, and polyphenols. As acetaldehyde represents >95% of all aldehydes in beer, the majority of carbonyl-reacted SO2 will be associated with this compound. It has been suggested that most of (E)-2-nonenal is bound as a sulfite adduct as long as the total amount of SO2 in aging beer exceeds 2 mg/L. For the total carbonyl content, a maximum of 40% appears to be bound when 5−10 mg L−1 sulfite is added, which has been mentioned by Bushnell et al.180 as the optimal sulfite concentration in beer. Kaneda et al.177 found a similar optimal sulfite content in packaged beer, being 8−9 mg L−1 . From the above, it is clear that the precise role of SO2 in beer flavor stability is complex and that additional research is required. For instance, it has been mentioned that acetaldehyde−bisulfite adducts still show antioxidant activity in aging beer, protecting other compounds from oxidation,177,188 and it has even been proposed by Kaneda et al.188 that this activity may be more important than the actual carbonyl scavenging ability of sulfite.<ref name=baedec/>
 *Martinez-Periñan E, Hernández-Artiga MP, Palacios-Santander JM, El Kaoutita M, Naranjo-Rodriguez I and Bellido-Milla D, Estimation of beer stability by sulphur dioxide and polyphenol determination. Evaluation of a Laccase–Sonogel–Carbon biosensor. Food Chem 127:234–239 (2011).