Sources and Impact of Sulfites in Beer

By Scott Bickham (Brewing Techniques)

Sulfur compounds are at the root of off-flavors as diverse as skunk, rubber, and vegetables, but they can also serve as antioxidants. This column focuses on the sources and control of sulfur flavors in beer.

The compounds responsible for sulfury off-flavors are ultimately derived from ingredients. The levels at which they appear in the final product are largely determined by the brewing process. Some of these flavors are desirable in continental lager styles, where they complement the malty aroma. On the other hand, some are considered defects by brewers of British ales, who select the malt, yeast, and fermentation method most likely to minimize the formation of volatile sulfur compounds. Though strain-dependent, lager yeasts generally produce a much larger variety of sulfur compounds during fermentation than ale yeasts; sulfury flavors are therefore one way of distinguishing between lagers and ales.

Not all sulfury flavors are formed during the normal brewing process; they can also result from bacterial contamination or poor handling. These antagonists transform sulfur compounds that otherwise are not flavor-active (such as the sulfur-containing amino acids methionine and cysteine) into undesirable off-flavors such as hydrogen sulfide and mercaptans.

By learning the characteristics of these flavors and their potential origin in raw materials and brewing processes we can gain the tools needed to control them.

Sulfitic Flavors

Sulfates and sulfites: One of the most important sources of sulfur in beer is the sulfate ion. Sulfate ions are derived from sulfuric acid or sulfate salts and consist of a sulfur atom covalently bonded to four oxygen atoms (SO₄^–). In nature, they are often found in combination with positively charged calcium and magnesium ions (which hydrate to form gypsum and epsom salts, respectively). The concentration of sulfate in regional brewing waters ranges from practically zero in Pilsen to over 400 ppm in Burton-on-Trent. Sulfate does not contribute any flavors at its typical level of 150 ppm, but at higher levels it can lend a dryness to well-hopped beers.

Sulfates share the sulfitic descriptor with sulfites. The sulfite ion is derived from salts of sulfurous acid and consists of a sulfur atom bonded to three oxygen atoms (SO₃^–). At a typical beer pH of 4, sulfite tends to go into solution as the bisulfite ion, which is a sulfite ion attached to a hydrogen ion (HSO₃^–). Sulfites and bisulfites are both strong reducing agents, capable of accepting oxygen atoms from other compounds to form sulfur dioxide and water. Most of the sulfites in beer are bound to carbonyl compounds, which diminishes both their flavor activity and their reducing capacity. A very small amount will remain free and eventually form sulfur dioxide. When present in high concentrations in beer (typically greater than 20 ppm), sulfur dioxide gives the aroma of a struck match.

Sulfitic flavors are rare in U.S. beer because sulfite additives are legally restricted (to 10 ppm without labelling), but they may be found in wine and cider in which potassium or sodium meta-bisulfite has been used as a preservative or in beers from other parts of the world. In Britain, for example, sulfites are legal beer additives (up to 40 ppm for cask ales). Hops may be another potential source of sulfites because growers often control mildew by dusting their hopyards with elemental sulfur or by burning rock sulfur in the oasthouse. Though 90% of this sulfur is eliminated during the boil, any remaining free sulfur is highly reactive and can form sulfites and other compounds. A small amount (0.5–20 ppm) of sulfur dioxide is also formed during fermentation (especially lager fermentations); however, as long as the level is subthreshold, its antioxidant power improves flavor stability with no detrimental effects. Remaining free molecules of sulfur dioxide in beer are generally scrubbed out during the fermentation.

Commercial examples. As mentioned already, sulfitic off-flavors are rare in beer. They are often detectable, however, in wine, cider, and mead products to which potassium or sodium metabisulfite has been added.

Doctoring tips. You can train yourself to detect these off-flavors in beer by doctoring a light lager (“reference beer”) with a mixture of sodium metabisulfite (available from most homebrew and winemaking retailers in the form of 0.5-g Campden tablets) and beer. Create the mixture by dissolving one Campden tablet as thoroughly as possible in 1 oz of beer, then add 1–2 tsp of this solution to a fresh 12-oz bottle of the reference beer (calibrate the precise amount needed by adding minute amounts of sodium metabisulfite to a smaller 3-oz sample until the off-flavor becomes perceptible). Warning: Samples containing sulfites should not be consumed by people with asthma or sulfite intolerance (the compound should still be detectable in the aroma).

Sulfidic Flavors

Sulfidic flavors are produced by hydrogen sulfide, thiols (or mercaptans), thioesters, and related compounds. All are undesirable in beer, and all become more offensive as their concentrations increase.

Hydrogen sulfide: Hydrogen sulfide is probably the best understood member of this group. It is a fermentation by-product with a low sensory threshold of only a few ppb. It makes its presence known by imparting to beer the aroma and taste of rotten eggs. Hydrogen sulfide undergoes several peaks in its concentration as the fermentation progresses, but most of it is purged by carbon dioxide during the primary fermentation and conditioning stages. Concentrations reach their highest level during the period of maximum yeast growth; the more trub and dissolved oxygen in the chilled wort, the higher the concentrations of hydrogen sulfide. The total amount of hydrogen sulfide produced depends on the yeast strain; under identical conditions, top-fermenting yeast produce less than lager strains.

Sources. Most of the sulfur in hydrogen sulfide production comes from barley and adjuncts, specifically from the amino acids cysteine and methionine and the proteins that contain them. Yeast need these amino acids for the synthesis of proteins, coenzymes, and vitamins. The yeast may use sulfate ions if organic sources (amino acids) are not available, but sulfate ions are energetically less favorable.

Most of the hydrogen sulfide that is carried over to the finished beer is bound to carbonyl compounds and is not flavor-active. It may, however, be produced by wort and beer spoilage organisms, particularly Gram-negative bacteria such as Zymomonas and Enterobacteriaceae. In such cases, the levels of hydrogen sulfide produced are too high to be scrubbed out by carbon dioxide or reduced by yeast.

Interestingly, it has been observed that the hydrogen sulfide level in filtered beer consistently doubles after pasteurization, which illustrates that the level is not static, but is affected by various redox reactions that take place in the packaged beer. The phenomenon also supports the perception that freshly pasteurized beer has a “green” flavor that decreases to normal levels as the hydrogen sulfide combines with other constituents. It has also been found that hydrogen sulfide may be completely removed by flowing finished beer through a copper electrolytic cell. Though this treatment increased the copper concentration in the beer used in the study, it could be speculated that the experiment lends some credibility to brewers who prefer using copper vessels in the production of quality beer.

Thiols: Thiols, also known as mercaptans, are closely related to hydrogen sulfide. The most relevant members of the thiol family in brewing are methyl-, ethyl-, and butyl-mercaptans. These compounds have aromas that may remind one of putrefied cabbage, garlic, onion, or egg. At very high concentrations they may be perceived as shrimplike.

Sources. The origins and fates of these compounds are very similar to those of hydrogen sulfide. Thiols are usually formed by yeast through the metabolism of sulfur-containing amino acids. A small amount may also come from hop oils, particularly in hops that have been treated with sulfur during farming. One of the most notorious thiols encountered in beer is 3-methyl-2-butene-1-thiol (MBT), which is the compound responsible for the “skunky” aroma detectable in light-struck beer (thiols are an important constituent in the odor emitted by skunks, hence a beer containing high levels of MBT is genuinely “skunked”).

The light-struck flavor is formed, just as the name implies, through a photochemical reaction triggered by light with wavelengths in the 350–500 nm range, which spans the blue to the near-ultraviolet portion of the electromagnetic spectrum. Light interacts with hop isohumulones almost instantaneously to produce MBT (which has an extremely low flavor threshold of less than 1 ppb).

Radiation from the sun and from fluorescent lighting (such as that used in grocery stores, unfortunately) is at the ideal wavelength to trigger this reaction, though it also occurs, more slowly, under incandescent light or diffuse daylight. Since the type of lighting that beer may be exposed to in supermarkets is beyond the control of the brewer, the best solution is to package the beer in brown or amber bottles, which are nearly opaque to the destructive wavelengths. Green and clear glass bottles, on the other hand, are nearly transparent in this region, which is why many imports are typically most susceptible to becoming skunked. (Some breweries have been able to sidestep the problem with clear bottles by using chemically modified isohumulones that are not susceptible to this type of photodegradation.) The problem is so prevalent among popular commercial beers that it could be speculated that some consumers have come to accept this particular off-flavor as part of the beers’ profile.

Other sulfidic flavors attributed to mercaptans have garlic, cabbage, or burnt rubber notes. Similar flavors are associated with diethyl sulfide, diethyl disulfide, and polysulfides. These diethyl compounds are generally regarded as equivalents to DMS with respect to their formation in the brewing process. This means that the levels of these compounds are correlated to the amount of DMS, so DMS can be used to estimate their concentrations. They contribute a pleasant, sulfury background to lagers at low levels, but are unpleasant at the high levels produced by bacterial spoilage. Polysulfides such as dimethyl trisulfide and dimethyl tetrasulfide are found in hop oils. Dimethyl trisulfide is usually destroyed by sulfur dioxide when sulfur is burnt in the oasthouse, but is slowly regenerated during storage. These compounds have extremely low taste thresholds and can therefore be important flavor constituents, particularly when large amounts of finishing hops or dry hops are used.

Doctoring tips. For those unfamiliar with the light-struck flavor, it is an easy task to prepare a sample for a doctored beer tasting session. Simply leave a bottle of the reference beer in bright sunlight for 1–3 days (of longer, if you have a dismal climate), depending on the desired flavor intensity and the color of the bottle. In practice, trained tasters may be able to identify other mercaptan flavors in addition to the skunky characteristic normally identified in light-struck beers.

A nice experiment that can help you learn the differences between the sulfidic compounds: prepare two glasses, and add a few milliliters of 1% solutions of copper sulfate to one and zinc sulfate to the other. Use a third glass as a control. In each glass, pour 4 oz of the beer being tested (either homebrew or a commercial beer), and swirl to mix the ingredients. Smell each sample and note the differences. Do not taste! These concoctions carry significant poisoning risk. Zinc sulfate releases a hydrogen sulfide aroma; copper sulfate releases both hydrogen sulfide and mercaptans. If you perceive no differences in your first attempt, repeat the experiment with a commercial light lager, which should naturally contain higher levels of sulfur compounds.

Yeasty flavors: Sulfidic flavors attributable to yeast comprise both healthy yeast flavors and unpleasant off-flavors resulting from yeast autolysis. Yeast autolysis produces an unmistakable rotten, rubbery, or shrimplike stench easily distinguishable from the meaty aroma of fresh yeast.

Source. These sulfidic off-flavors are produced by yeast autolysis, which is essentially a form of self-degradation. Autolysis occurs under stressed conditions such as high osmotic pressure (alcohol or sugars), extremely high temperatures, long-term storage, or sudden environmental changes. Under these conditions, the cell’s normally encapsulated digestive enzymes are released and destroy the cell from the inside. The contents of the dead cell are then released for other yeast to use as nutrients.

Doctoring tips. Autolyzed yeast-flavors may be found in very old bottle-conditioned beers. How old is “very old” depends on the temperature at which the bottle is stored (high temperatures speed yeast metabolism and hasten their demise) and the amount of yeast slurry present in the bottle — a typical homebrew will probably last two years at cool room temperatures.

Since this off-flavor is rarely encountered in filtered commercial beers, you may expedite the process by storing a portion of slurry from one of your recent brews at room temperature for several days. Periodically compare the slurry’s aroma to a sample that has been stored under a layer of fresh beer or sterilized water in the refrigerator. This experiment should acquaint you with both healthy and autolyzed yeast flavors. Keep in mind that the characteristics of each slurry would carry over into the flavor of any beers brewed with them.

Cooked Vegetable Flavors

DMS and related compounds: The final group of Class 7 off-flavors covered in this article are the cooked vegetable flavors associated with dialkyl sulfides. Dimethyl sulfide (DMS), is the most widely studied compound in this category because of its importance as a flavor constituent in lager beer styles. As noted earlier, DMS is rarely present in isolation, but is typically coupled with other compounds such as dimethyl disulfide (DMDS), dimethyl trisulfide (DMTS), and diethyl disulfide (DES). At typical concentrations, DMS has a cooked corn or cabbagelike flavor, whereas diethyl compounds have cooked onion and garliclike flavors. Beer takes on cooked patsnip or celery tones at the high levels that can result from wort infection.

The fates of several other compounds are closely related to that of DMS. This is why DMS is often used as a tracer to determine how levels of these related sulfur compounds evolve during the brewing process. Relative levels of each of these compounds will vary, which can lead to dramatic differences in the flavor of the beer, particularly when wort spoilage organisms are involved.

Sources. Almost all of the DMS in beer originates from malted barley, though a small amount is also found in raw barley and adjuncts. The level and character of the sulfury flavor will depend on both the source of the barley and on the malting procedure. Kilning plays a large role in determining how much of the DMS precursor s-methyl methionine (SMM) is broken down to DMS and other compounds because the reaction occurs more quickly at higher temperatures. Therefore, the more highly kilned the malt, the more SMM that has been reduced to DMS and subsequently driven off, and the less remains to carry through to the brewing process. This factor typically renders light-colored beers using lightly kilned malt more susceptible to DMS flavors (though predrying the malt can help). Lagers are thus known to have a DMS flavor profile (pale lager malts typically yield DMS precursor levels of 4 ppm or more, for example, whereas some ale malts can have SMM levels of less than 0.1 ppm).

SMM levels can also vary from variety to variety. Canadian barley varieties, for example, appear to have high DMS-producing potential, but the maltster can moderate the problem without inhibiting modification or extract levels by reducing germination growth. Also, continental two-row barley has been found to give a more refined sulfury flavor than midwestern six-row. Further, levels of SMM increase with the protein content of the malt (SMM often binds with peptides). Thus British malts, which are typically much lower in protein than continental malts, contribute negligible amounts of SMM (and hence, DMS) to the wort.

The boiling of a portion of the malt during a decoction mash may reduce a small amount of SMM to DMS, but nearly all of the DMS in the mash will evaporate during an open, rolling boil (the boiling point for DMS is 100 °F [38 °C]); if the kettle is not vented, DMS will condense back into the wort. An 8% evaporative loss in wort volume during the boil is probably the minimum needed to remove DMS. DMS production will slow during cooling (SMM converts to DMM at around 140 °F [60 °C], but it is important to reduce the temperature as quickly as possible because most of the DMS created at this point will carry through to the finished beer. Some commercial lager breweries even spray air or carbon dioxide gas through the cooling wort to maximize the rate of DMS evaporation.

In addition to being formed by SMM, DMS may also be formed through the reduction of dimethyl sulfoxide (DMSO). Opinions differ as to the importance of this mechanism in beer production. According to the authorities cited in reference 6, this appears to be of less importance generally than the amount of DMS contributed by DMS and SMM in the malt. On the other hand, a great deal of research has been done on the reduction of DMSO to DMS by brewing yeast, particularly at lager fermentation temperatures. Sulfur compounds produced during fermentation have been correlated to the yeast strain, wort oxygen concentration, yeast pitching rate, and fermentation temperature (9). Gram-negative bacteria are also able to carry out this reduction, producing extremely high levels of DMS, hydrogen sulfide, and mercaptans. This problem can be particularly troublesome in the microaerobic environment of a beer cask.

Commercial examples. Most tasters can detect DMS in midwestern and Canadian lagers and cream ales such as Little Kings, Old Milwaukee, and Carling Black Label. All of these products contain corn as an adjunct, which also gives a slight creamed corn flavor; in my experience, though, the cornlike flavor of DMS is distinguishably harsher and has a slight graininess.

To study some brewing experiments that illustrate the formation and reduction of sulfury flavors, see references 3 and 10.

Doctoring tips. Pure DMS is often used as a doctoring agent in flavor analysis courses at brewing schools and can be obtained on the market from the UK company FlavorActiV (Lingfield, Surrey). It is also possible (though maybe a bit tricky) to brew a test batch with elevated levels of DMS and related compounds by using midwestern six-row malt, covering the boil, and fermenting with lager yeast at low temperatures. Elevated levels are also associated with wort spoilage, often due to long lag times, but this is not a recommended method because other off-flavors such as acetaldehyde and lactic acid may also be produced.

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