The smell of rotten eggs or cooked vegetables like cabbage or broccoli are two of the common sulfur (sulphur for my British friends) related odors faults that can be found in cider. The culprit is generally Hydrogen Sulfide (H2S), though other sulfur compounds like diethyl sulfide can contribute. Humans are very good at detecting even small amounts of H2S as the odor threshold is only 1.6 ug/l(1). That makes it easy to notice. It’s also pretty easy to address, which is good as it can be formed in a number of situations.
- Elemental Sulfur: H2S can form from elemental sulfur found on the fruit. This could occur if the orchard treated the trees with sulfur. The elemental sulfur can be converted to H2S during fermentation. It is probably not as common for apples and cider as grapes and wine but, could still occur.
- Lacking Nitrogen: Inorganic nitrogen is yeast’s preferred source but if that is lacking, it will breakdown organic nitrogen sources. The most common is amino acids. This can create many precursor compounds that in later pathways form aromas. One of those precursors is sulfur. While many aromas are positive contributions, sulfur isn’t generally one of them. Luckily, the low sugar level of apples compared to grapes means it normally doesn’t lack sufficient nitrogen. Note that chaptalizing the juice with sugar (adding sugar to increase %ABV) may create issues.
- Aggressive Fermentation: A second common cause is when the fermentation is too fast. This is usually when too much inorganic nitrogen is available and/or the fermentation temperature is too warm. We will discuss temperature later but, just like not having enough nitrogen can cause H2S to form, having too much can also create H2S. This occurs because the yeast population grows too fast, using too many of the nutrients and not allowing enough for later needs. Now you are back to needing more and having to breakdown other compounds to get it.
- Yeast Type: All yeast produce hydrogen sulfide but some produce more than others. There is a belief that non-Saccharomyces strains produce excessive amounts of sulfur but, research is showing that H2S production is strain dependent. Many commercial Saccharomyces strains produce higher levels of H2S than many non-Saccharomyces strains. If you find your fermentations are producing sulfur smells, a different yeast may be worth trying or you need to consider the needs of your yeasts and add nutrients when appropriate.
- Fermentation Temperature: As noted above, high temperature tends to increase the fermentation rate, this can lead to stress and sulfur/H2S production. However, cold can also impact the production of H2S. Vigorous fermentation will naturally help purge H2S from a cider. The CO2 basically scrubs the H2S from the cider as it is released. The opposite is true for colder ferments since the cold slows the production of CO2 and increases the suspension of CO2 in your cider.
Ultimately, the best way to avoid hydrogen sulfide production is to prevent it. That doesn’t mean add yeast nutrients to every batch but, it does mean you should either understand the amount of assimulatable nitrogen your yeast needs or monitor your fermentations for that rotten egg or cooked vegetable aroma. If you notice it, add some inorganic nitrogen, like DAP, to address it. Just avoid adding to much as it can create other issues during aging. For yeast that need more nitrogen, considering adding some organic nitrogen sources to the juice at pressing. However, if you find your cider is has sulfur aromas, consider aeration and/or adding some sulfite (Campden tablets) to oxidize it.
(1) M. Ugliano and Associates, Occurrence of hydrogen sulfide in wine and in fermentation: influence of yeast strain and supplementation of yeast available nitrogen, J Ind Microbiol Biotechnol, 38, 423–429, 2011
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