Today we are going on a slightly deep dive on another source of faults, though confusingly, also a potential source of good and more complex flavours in a finished cider or perry. We are going to look at volatile sulfur compounds, with a look at mercaptans, thiols, disulfides and other goodies along on the way.

I recently had occasion to explore sulfur-related cider faults in more detail, as when I see words like polyfunctional thiols, I want to know what that actually means. Cue a series of rabbit holes, but I’ll try to spare the gory details in this piece and just take in the bigger picture so you can recognise what is going on in your glass when something seems a bit stinky, and what you can do about it if it hasn’t gone too far.

But let’s start with a simple and most common sulfur-related fault, H₂S, or hydrogen sulfide.

Hydrogen sulfide is a pretty strong volatile compound that smells like sulfury, rotten eggs or in extreme cases can be reminiscent of sewage. It is produced by practically all yeast, so is nearly always present during fermentation, the levels depending on a range of environmental and genetic factors. Yeasts working in a low nutrient (low nitrogen) juice will tend to produce more hydrogen sulfide. Without free nitrogen in the juice, the yeast will degrade sulpher-containing amino acids to get at the nitrogen, even in their own amino acids, thus releasing H₂S and other sulfur compounds as by-products. Stressed yeast are also more prone to producing hydrogen sulfide

But the key here is that it is volatile, so in most cases it will simply dissipate with oxygen contact. This is why you often hear the term “reductive” or “reduced” being used to describe a cider or wine with such sulfury traits. In this case, reductive simply means that the cider was fermented in a reductive, or reduced oxygen environment, a process choice of the maker. This is as opposed to an oxidative process, where air contact is more frequent. Think hermetically sealed stainless steel tanks rather than the micro-oxygenation found using wooden barrels.

Reduced oxygen contact during fermentation can help preserve the fruity characters of a wine or cider, but on the downside it can sometimes lead to the accumulation of these stinky H₂S side-aromas. Carefully aerating a cider at some stage in the process will generally get rid of this fault, or even better, giving the yeast a little love and care during fermentation can prevent it becoming a major issue in the first place. Of course, too much oxygen is also a bad thing for cider, so makers need to delicately balance this. But if still present in the bottle, giving it a little time to breath in the glass, or decanting it, will usually dispel these unwelcome “reduced” aromas.

All clear so far? Good. The problem is that this relatively harmless and volatile hydrogen sulfide can be a precursor to things more insidious and much harder to get rid of. And this is where we come to mercaptans/thiols.

What are mercaptans and thiols?

Let’s get the slightly complicated stuff out of the way first, including the various names under which these compounds lie. In organic chemistry, a thiol is any organosulfur compound that is essentially the sulfur-based version of an alcohol. In other words, sulfur takes the place of the oxygen part of the alcohol compounds, so chemically they have SH instead of an OH group, becoming sulfhydryls instead of hydroxyls – the ethyl alcohol produced by yeast fermenting cider, wine, beer etc… is a hydroxyl. This relationship between the sulfur compounds and alcohol is well encapsulated in the word thiol, as it is a portmanteau of “theion“, the Greek for sulfur, and “alcohol”.

In 1832, Danish chemist William Christopher Zeise coined the term ‘mercaptan’ for these same compounds, coming from the Latin mercurio captāns , ‘capturing mercury’, because they bind easily with mercury. So, lots of words for the same thing, and generally interchangeable in texts. For this piece I will stick to using thiol.

But what have thiols got to do with cider and hydrogen sulfide? Well, H₂S readily reacts and combines with other components and sulphides in fermenting cider to form longer chain thiols (and disulphides).  So, if hydrogen sulfide is not taken care of, it can result in the development of these even less pleasant and harder to deal with sulfide-based thiols. These can have aromas ranging from vegetal elements like cabbage, onion, garlic and tinned corn, through to struck match, rubber and rotting flesh. Thiols are also used to add an aroma to natural gas for safety reasons, so the stink will let you know there is a leak. But generally, and thankfully, usually they manifest in tainted ciders as cabbage, rubber and struck match!

Clearly, the presence of any of these characteristics renders a cider faulty, as volatile sulfur compounds will tend to dominate the original, often less intense primary aromas of the fruit itself. However, in some cases, a tiny amount might lend a cider more complexity, like a pinch of seasoning, so paraphrasing Paracelsus, the dose makes the poison.

What are the bad ones we associate with cider?

The two most common thiols found in cider that cause issue are methanethiol (aka methyl mercaptan) and ethanethiol (aka ethyl mercaptan). They are formed directly as a result of yeast activity but can also sometimes happen after a cider is bottled by breakdown of precursors in the bottle (methyl thiocetate and ethyl thiocetate if you really want to know).

Ethanethiol has same chemical structure as our friend ethanol (CH₃CH₂OH) but the oxygen atoms are replaced by sulfur (CH₃CH₂SH, see what it did there?). This can bring aromas of onion, burnt match, sulfidy, earthy or rubbery aromas to the cider. More often perceived as a negative, in small doses it might add a little spice to the drink.

Methanethiol (CH₃SH),  is an organosulfur found in the blood, brains and faeces of animals. It’s also a by-product in your urine after eating asparagus! Wonderful that yeasts in cider can produce it too. It is the analogue of methanol (CH₃OH), the alcohol that can turn you blind in badly distilled spirits, and which is present in low levels in ciders. Methanethiol will manifest in cider as a more putrid aroma of rotten cabbage, cooked cabbage, burnt rubber and stagnant water.  It sounds really unpleasant and thankfully I have yet to try a cider with such a fault. Methanol is also objectionable, but I kind of like the smell of it.

Oxidation of thiols to disuphides

Of course the lifecycle of volatile sulfur compounds doesn’t stop with ethanethiol and methanethiol. Both can oxidise to produce so-called symmetrical or asymmetrical disulfides, diethyldisulfide (DEDS) and dimethyldisulfide (DMDS).

Methanethiol can convert into dimethyl sulfide which shows up in high levels as cooked vegetables, cooked corn or tinned tomatoes, but at low levels can present as blackcurrant concentrate, truffles or quince. Doesn’t sound too bad. Or dimethyl disulfide which is more reminiscent of cabbage or intense onions at high levels.

Ethanethiol can oxidise into diethyl sulfide which is more or less rubbery on the nose, or diethyl disulfide that apparently smells like bad garlic, onion and burnt rubber

There are more, but I don’t want this article to just turn into a list of unpleasant aromas and chemical names nobody will remember.

What about good thiols?

To be fair, it would be a mistake to simply brand all thiols as bad, as there are many that can add complexity and richer flavours to a cider. Indeed, as a formed brewer, I knew that thiols are often sought out in certain hop varieties, as they can add fruity flavours to finished beers.

There are some sulfur-originating thiols that can also have beneficial properties for cider, so-called polyfunctional thiols that are often found in wines like Sauvignon Blanc, or blackcurrant aromas in some red wines.

4-mercapto-4-methylpentan-2-one (4MMP) that presents as blackcurrant aromas, 3-mercaptohexan-1-ol (3MH) that is reminiscent of citrus, grapefruit aromas, and 3-mercaptohexyl acetate (3MHA) that can present as tropical/passion fruit at low levels, but sadly as cat’s urine at higher levels (making me think of simcoe hops). Again, the dose makes the poison. And again, chemical names nobody can remember.

How to deal with volatile sulfuric compounds.

Makers have some tools at their disposal to stop all of this happening, or deal with it before it gets too far. Good yeast husbandry can really go a long way to head off the problem in the first place (something I know from experience). As already mentioned, careful use of aeration can dispel hydrogen sulfide once it is present, and before it gets truly problematic, or use of copper added near the end of fermentation can also resolve it if aerating is not an option.

Makers also have tests at their disposal to determine exactly what kind of volatile sulfur compound is causing the problems (using copper, cadmium and ascorbic acid in samples to see what makes the off-aroma dissipate), but disulfides and dimethyl sulfides are pretty much there to stay one they are in the cider.

But more importantly, as a drinker, if you encounter hydrogen sulfide/rotten egg aromas in your cider you can usually just let it breath, so it wafts away. You can speed this up by decanting, or one trick told to me by a maker was to throw a copper object into the cider, but generally it’ll just go away.

If it doesn’t go away, then you are likely into thiol or disulfide territory and, basically, you probably have a drain pour in your hands if the levels are over the acceptable threshold. The maker should have caught it at the hydrogen sulfide stage and corrected it before bottling, and I for one would like to be told if any of my ciders had such a fault.

The aroma threshold for hydrogen sulfide, methanthiol and ethanthiol are all rather low, so a little will go a long way. But the disulfides generally (though not all) have a higher threshold to cross before detection.

So there you go. I think you are almost guaranteed to hit a “reductive” cider showing an eggy, sulfury hydrogen sulfide aroma at some point in your cider travels, but I hope this shows that it doesn’t have to be a disaster. But I also hope that you don’t have to meet the successors of that mostly harmless compound!