The Role of Sulphur Compounds in Brewing: Production, Impact, Management and its use as a preservative.

Sulphur compounds play a significant role in brewing, contributing both positive and negative sensory attributes. While some sulphur compounds enhance aroma complexity, others lead to undesirable off-flavours. Understanding their origins, yeast metabolism pathways, control measures and key parameters when using it as a preservative are essential for brewers seeking to optimize beer quality.

Key Sulphur Compounds in Brewing

Sulphur compounds in beer arise from raw ingredients, yeast metabolism, and brewing processes. They can be categorized into desirable and undesirable compounds based on their sensory impact.

Classification of Sulphur Compounds in Brewing

How Sulphur Compounds Are Produced in Brewing

Sulphur compounds primarily come from three key areas in the brewing process: raw materials, yeast metabolism, and brewing processes. Each of these contributes different sulphur compounds depending on ingredient selection, fermentation conditions, and processing techniques.

Raw Materials

Malt: Contains sulphur-containing amino acids and precursors like S-methylmethionine (SMM), which contribute to DMS. Pilsner malt is particularly high in SMM, making it more prone to DMS production if not boiled sufficiently. Other lightly kilned malts, such as pale malt and some Vienna malts, also contain moderate levels of SMM. In contrast, darker malts (e.g., Munich malt, roasted malts) have lower SMM levels due to their higher kilning temperatures, which degrade sulphur precursors.

Hops: Contain polyfunctional thiols that can be released during fermentation, adding fruity aromas.

Water: The sulphate content of brewing water influences yeast metabolism and sulphur compound formation.

Yeast Metabolism

The sulphate assimilation pathway (SAP) is the primary metabolic route through which yeast converts sulphate into essential sulphur compounds required for cell function, such as cysteine, methionine, and glutathione. These compounds are crucial for protein synthesis, enzyme function, and antioxidant defence in yeast. However, during this metabolic process, intermediate sulphur compounds such as hydrogen sulphide (H₂S), sulphur dioxide (SO₂), and methanethiol can be produced as by-products

Why Does SAP Lead to H₂S, SO₂, and Methanethiol Production?

H₂S Production – Under normal conditions, H₂S is assimilated into cysteine and methionine via trans-sulfuration pathways. However, if yeast lacks essential nutrients (such as nitrogen, vitamins, or proper oxygenation) If cysteine and methionine synthesis are incomplete (due to limited nitrogen or excessive sulphur uptake), excess of unincorporated H₂S may accumulate and leak into beer. 

  • SO₂ Formation – Sulphite (SO₃²⁻), an intermediate in SAP, may not always fully reduce to sulphide and instead be released as sulphur dioxide (SO₂), particularly under yeast stress or oxygen-limited conditions.

  • Methanethiol Production – Degradation of methionine can result in methanethiol (CH₃SH), which contributes to unpleasant rotten cabbage aromas if not metabolized efficiently due to an imbalance in sulphur metabolism, insufficient yeast nutrition, or a lack of proper fermentation conditions.

This pathway is essential for yeast survival, but imbalances in sulphur metabolism can lead to excess volatile sulphur compounds, negatively impacting beer aroma and flavour.

Yeast stress, resulting from various environmental and nutritional factors, can significantly contribute to the excessive production of undesirable sulphur compounds.

The primary causes of yeast stress:

Nutrient Deficiencies: Insufficient nitrogen, B vitamins (such as biotin and thiamine), or essential minerals like zinc can impair yeast metabolism, leading to an accumulation of sulphur by-products such as H₂S and SO₂.

Fluctuating or High Fermentation Temperatures: Excessively high temperatures accelerate yeast metabolism, increasing the production of volatile sulphur compounds, while rapid temperature drops can shock yeast, slowing down sulphur assimilation.

Oxygen Imbalance: Poor wort aeration at the start of fermentation can limit yeast cell growth and sulphur incorporation, while excessive oxygen later in fermentation can lead to oxidative stress and increased SO₂ production.

High Alcohol and Osmotic Stress: High-gravity worts or high ethanol concentrations create osmotic stress on yeast cells, forcing them into survival modes that favour sulphur compound production.

Poor Yeast Handling and Repitching Practices: Using old, unhealthy, or improperly stored yeast strains can result in sluggish fermentation and excessive sulphur compound retention.

pH Imbalance and Water Chemistry: High sulphate levels in brewing water can push yeast metabolism toward the increased sulphur compound formation, while inappropriate pH levels can hinder enzymatic activity, leading to inefficient sulphur assimilation from various environmental and nutritional factors, can significantly contribute to the excessive production of undesirable sulphur compounds.

Sulphur Compounds Management

Brewers can control sulphur compounds to enhance desirable aromas while minimising off-flavours through:

  1. Yeast Management

    • Selecting yeast strains with low sulphur production.

    • Ensuring healthy yeast propagation and appropriate pitching rates.

    • Providing adequate yeast nutrients, particularly nitrogen and zinc, to support complete sulphur metabolism. Click here to look at Fermoplus Perfect-Brew Zn, our complete nutrient with yeast-derived zinc for superior bioavailability and ease of assimilation by yeast.

  2. Optimizing Brewing Techniques

    • Adjusting wort aeration to prevent excessive H₂S and SO₂ production. Proper aeration ensures yeast has enough oxygen during the early stages of fermentation to build sterols and unsaturated fatty acids, which are essential for membrane integrity and cell function. Inadequate aeration forces yeast to rely on anaerobic pathways that can lead to incomplete sulphur metabolism, increasing the accumulation of H₂S and SO₂. Conversely, over-aeration late in fermentation can cause oxidative stress, further exacerbating sulphur compound retention.

    • Extending the boil time to reduce residual DMS precursors. During the boil, S-methylmethionine (SMM), a precursor found in malt, is converted into dimethyl sulphide (DMS). However, DMS is highly volatile and escapes with steam during boiling. If the boil time is too short or lacks sufficient vigor, not enough DMS is expelled, leaving excess precursors that can later break down into DMS during fermentation or wort cooling. A longer, vigorous boil ensures maximum volatilization of DMS and its precursors, reducing its presence in the final beer.

    • Employing proper fermentation and conditioning techniques to allow sulphur compound reabsorption.

  3. Balancing Water Chemistry

    • Adjusting sulphate and chloride levels in brewing water to influence sulphur compound development.

    • Monitoring pH levels, which can affect yeast metabolism and sulphur compound production.

How Potassium Metabisulfite Works as an Oxygen Scavenger

When potassium metabisulfite (K₂S₂O₅) is added to beer, it releases SO₂. Sulphur dioxide acts as an oxygen scavenger because it reacts readily with oxygen, preventing oxidative reactions in beer. This process helps preserve freshness, and extend shelf life, The SO₂ also binds to oxidative compounds, preventing them from forming stale flavours. This is a summary of the main oxidative compounds in brewing and their impacts on beer attributes:

By targeting these oxidative compounds, potassium metabisulfite helps maintain beer freshness and stability over time.

Inhibition of Oxidative Enzymes

The SO₂ released from potassium metabisulfite inhibits oxidative enzymes such as polyphenol oxidase (PPO), which helps prevent unwanted haze formation and colour degradation in beer. Colour occurs through multiple mechanisms:

• Direct Enzyme Inhibition – SO₂ binds to the active sites of PPO enzymes, altering their structure and rendering them inactive. This prevents the oxidation of polyphenols, which is a primary cause of browning and haze in beer.

• Reduction of Quinones – SO₂ reacts with quinones, which are oxidation products of polyphenols, converting them back into their original phenolic forms. This reaction halts oxidative chain reactions that could negatively impact beer stability.

Antioxin SBT: its use in practice.

Antioxin SBT is an advanced antioxidizing agent designed to reduce oxidation during the brewing process. It contains potassium metabisulfite, ascorbic acid, and gallotannin. Click here to read the previous article we did evaluating the effectiveness of Antioxin SBT. 

While the recommended usage temperature is between 38°C and 42°C, it is important to note that ascorbic acid begins to degrade above 40°C, but its complete deactivation does not occur until temperatures exceed 80°C. Despite partial degradation, Antioxin SBT remains effective within the standard mashing temperature range of 62°C to 78°C for brewers who are unable to use temperature control in their mashing program, ensuring its antioxidative properties remain active throughout the process.

Brewers can evaluate the effectiveness of the product by assessing wort colour, either visually or through spectrophotometry. Another key parameter to measure is redox potential, which can be analysed using ITT.

When using Antioxin SBT, brewers will notice a paler wort colour and a reduction in dissolved oxygen (DO), as sulphur dioxide (SO₂) displaces oxygen, lowering oxidation levels. However, it is crucial to follow the recommended dosage to ensure compliance with permitted SO₂ limits. Excessive antioxidant concentrations may have a pro-oxidant effect, counteracting the intended benefits.

References

 

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  7. Narziss, L. (2005). Sulfur dioxide in beer production. Journal of the Institute of Brewing, 111(3), 169–177. https://doi.org/10.1002/j.2050-0416.2005.tb00701.x

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  10. Verstrepen, K. J., & Derdelinckx, G. (2003). Flavor formation by Saccharomyces cerevisiae: The role of carbon–sulfur lyases in the generation of volatile sulfur compounds. Applied Microbiology and Biotechnology, 63(2), 136–144. https://doi.org/10.1007/s00253-003-1420-1

Ana Victoria Vasquez de la Peña

ana@neumaker.com.au

21 February 2025