Sulfur Dioxide in Brewing
Beer is a complex mixture of more than 3,000 different compounds and water, in a precarious balance. Unlike more stable distilled alcoholic beverages, beer properties can change rapidly or over longer periods of time, not always for the best. Nowadays, many of these instabilities such as the growth of microorganisms in beer are largely under good control, thanks to novel processes and technological advances. However, flavor instability, which represents any change in fresh beer aroma and/or taste, has risen to be the most challenging quality problem that modern brewers at any production scale encounter. Sensory attributes of fresh beers, such as higher fruitiness, malt aroma, fullness, acidity, freshness, and bitterness decrease during aging, giving place to undesirable notes, consequently reducing beer complexity and overall quality.
Bottled beer is not in a state of chemical equilibrium as chemical reactions continue to take place during beer storage. Therefore the “staling” must be avoided as much as possible, or at least prolonged. Stale beer flavors range from wet cardboard in pale beers to caramel, soy sauce, and potato-like characteristics in other styles. These flavors are said to be attributed to aldehydes, in particular (trans-)2-nonenal, a flavor-active volatile compound that has been proven to rise beyond its flavor-threshold level of 0.1 mg/L during beer aging. Staling is not only characterized by an increase of undesired aging flavors; the loss of positive flavor attributes, such as floral, fruity, and estery aromas, plays an important part as well, particularly in pale beers that often have more delicate aroma profiles. Beer aging is a very complex phenomenon connected to changes in chemical constituents.
Alchemy of flavor stability
Over the years, knowledge and understanding of beer flavor stability has improved substantially. The flavor instability and oxygen concentration of beer have been closely connected because many of the unwanted off-flavors are in fact generated through oxidative processes such as Strecker degradation, lipid oxidation, and Maillard reactions happening during and after beer manufacturing —85% and 15% respectively. While it is unavoidable that chemical alterations will take place in beer over time, the focus should be avoiding the development of more negative flavor-active components after the desired product has been achieved. Consequently, it is best practice to adopt procedures that reduce oxygen as well as maximize antioxidant components, such as phenolic compounds and sulfur dioxide (SO2), to inhibit oxygen effect by capturing reactive oxygen species (ROS) and free radicals. Free radicals are unstable molecules that rapidly oxidize other components of the beer, creating a chain reaction that results in the “cardboard-like” flavor (ROS are a subset of this group that contain oxygen). Minimizing the formation and activity of ROS must be a first step for improving beer flavor stability and prolonging its shelf life.
Modern equipment, such as bottle/can fillers, has helped keep oxygen below the maximum recommended concentration of 50 ppb, but tends to be costly and with too big of a footprint for nanobrewers and homebrewers. To mitigate these adverse effects of oxidation, brewers started using deaerated water and inert gases to displace air in the various equipment, prevent further ingress through leaking valve and pump seats, and being extra vigilant during the filling process. Alongside these methods, which are necessary for a better beer taste and longer shelf life, using chemicals known to scrub oxygen from beer before it has detrimental effects could come in handy in delivering a longer-lasting beer. Indeed, oxygen scavenging by bioactive compounds offer a cheap and easily accessible support to minimize the formation and activity of ROS in beer and wort. While polyphenols — natural antioxidant molecules present in hops and malt — have shown to have a bigger impact on preserving flavor stability in the mashing and wort boiling steps, in finished beer, SO2 has proven to be the best compound that can delay the cold-side oxidation by preventing formation of ROS. The use of other antioxidants, such as ascorbic acid, is often contradicted despite their capabilities, because of the poor stability in beer storage conditions.
The fate of sulfites during fermentation and storage of beer
SO2 is a natural component of beer. The raw materials used in brewing may contain different amounts of it, but most of it disappears during wort boiling. Therefore, the SO2 concentration in the final beer is quite low, around 2.5–4 mg/L, and depends mostly on the yeast strain and the fermentation condition. MET10 gene in brewer’s yeast has been linked to sulfite production and reducing or eliminating its activity resulted in higher concentration of sulfites. Aside from the yeast genetic differences, SO2 increases with higher wort clarity and reduced wort oxygenation, pitching rate, and fermentation temperature. Beer produced with such yeasts and fermentation parameters showed to have quite satisfactory flavor stability, but brewers have contradictory opinions on the efficiency of naturally produced bisulfite due to incredibly small differences in final SO2 concentration between the different conditions.
SO2 in beer is not only derived from yeast, though. Brewers can also add it in various forms. These include sulfur dioxide (E220), potassium bisulfite (E228), potassium metabisulfite (E224), sodium bisulfite (E222), sodium metabisulfite (E223), sodium sulfite (E221), calcium sulfite (E226), and calcium hydrogen sulfite (E227). SO2 dissolves rapidly in beer to give a rather complicated reaction mixture, strongly dependent on concentration, temperature, and pH. At the usual pH of beers (3.8–4.4), most of the SO2 is present as the bisulfite (SO3-) anion (Figure 1).
As bisulfite, it is beneficial to beer flavor stability due to its remarkably high affinity for oxygen but has little to no antimicrobial actions. Once dissolved, it reacts very fast with oxygen in solution to form sulfates (SO42-), thus preventing other compounds from oxidizing. In addition, sulfites act as important mask agents for stale flavor by reacting with already present staling compounds to form adducts, which are non-volatile and flavorless. Therefore, bisulfite has a dualistic mode of action in counteracting the staling process. Firstly, as an antioxidant it diminishes the formation of aldehydes. Secondly, its addition to stale beer lowers the actual free aldehydes concentration, potentially removing the cardboard flavor. However, this binding is not lasting forever; over time, the bisulfite tends to be oxidized to sulfate, thus releasing the aldehydes again and restoring the beer’s stale flavor character. Sulfate formation is inevitable, and it means that SO2 is progressively lost from beer at a constant rate. To slow down the reaction and minimize the loss, a brewer should keep the storage temperature as close to 32 °F (0 °C) as possible. This measure will minimize the loss of SO2 and prolong the overall flavor quality of the final product over time. Because SO2 added to beers is rapidly bound up with components other than aldehydes and oxygen, the required final concentration varies between 1–10 ppm.
Usage and final remarks
Sulfur dioxide has been recognized to prolong the shelf life of beer by brewers, but it has a negative perception with some alcohol consumers. Even if reactions to sulfites are extremely rare, some people have sensitivity to SO2, with symptoms varying from headaches to nausea and vomiting. For this reason, it’s considered an allergen and some countries have defined limits that specify the maximum permissible concentration in commercially produced food and beverages. In the European Union, for example, the sulfur dioxide content may not exceed the limit of 20 ppm SO2 in beer or 100–200 ppm in wine. Concentrations exceeding 10 mg/L SO2 must be indicated by the label “Contains sulfites.” While in the U.S. the upper limit is 350 ppm for alcoholic beverages, the same mandatory label declaration applies. Even homebrewers should be careful to label their beverages if they share bottles with others or enter them into competition so nobody is caught off guard that the beer was made with a sulfite addition.
On the market, sodium and potassium metabisulfite (KMBS) are the readily available options to add sulfites to beer and function identically, with the latter being preferred because it does not contribute any dietary sodium. In this case, based on the atomic weight, the addition of 1 g of KMBS releases around 0.57 g of SO2. To exploit the most effective role of SO2 in delaying flavor staling of beer while considering the health effects and legal requirements, adding 0.5–1 g KMBS per 100 L (26.5 gallons) of beer (2.8–5.6 ppm SO2) is advised, preferably immediately after fermentation or prior to bottling, without overlooking optimal bottling and storage conditions to extend its lasting effects.
For homebrewers, the preparation of a stock solution is strongly recommended to be accurate with your measurements. Start by adding 1 g of KMBS to 1 L of water to have a 1,000 ppm concentration, then add 2.8–5.6 mL for every liter of beer (53–106 mL for 5 gallons).
References:
1 Ilet, D. R. “Aspects of the analysis, role, and fate of sulphur dioxide in beer–a review.” Technical quarterly (1995).
2 Guido, Luis F. “Sulfites in beer: reviewing regulation, analysis and role.” Scientia Agricola 73 (2016): 189-197.
3 Wietstock, Philip C., Thomas Kunz, and Frank-Jürgen Methner. “Relevance of oxygen for the formation of Strecker aldehydes during beer production and storage.” Journal of agricultural and food chemistry 64.42 (2016): 8035-8044.
4 Lehnhardt, Florian, et al. “A Comprehensive Evaluation of Flavor Instability of Beer (Part 1): Influence of Release of Bound State Aldehydes.” Foods 10.10 (2021): 2432.
5 Vanderhaegen, Bart, et al. “The chemistry of beer aging–a critical review.” Food Chemistry 95.3 (2006): 357-381.
