Sulphites in wine – clearing up a few points

I studied a bit of chemistry at university so I know what a sulphite is, I thought. It’s an ion that has a single sulphur atom, three oxygens and a double negative charge. And I most definitely would not make the common error of confusing sulphite with sulphate, which is spelled with an a rather than an i, and has four oxygen atoms – a totally different beast.

The usual story with sulphites is that they are added to wine to work against oxidation, and kill yeast and bacteria, but the downside is that some people are allergic to them, and in the US and EU they require a warning label if present at 10 ppm (parts per million – very similar to mg/l) or more. They are also anecdotally associated with headaches, and thought by some to be a Bad Thing simply because they are not Natural. However, at the next level of sophistication you might also be aware that some sulphite content actually is natural, as it is a fermentation product, and thus present in every wine even if not added artificially.

So far so good, but when you start poking around a bit more, asking what sulphite concentrations actually mean, and how many of us are allergic to sulphites, it suddenly gets rather more murky. What people call sulphites are not necessarily the sulphites a chemistry undergraduate is confident about, and it is not clear precisely what people are allergic to. I don’t claim to have a complete overview of all these issues, but will try here to clarify what I can.

The sulphite ion (top left), sulphur dioxide molecule (top right), and the two tautomers of the bisulphite ion (bottom)

Let me start by talking through some of the points made on Ben Rotter’s excellent webpage on sulphur dioxide. Sulphur dioxide can be introduced into wine using any of a number of approved additives but, regardless of which one is used, the same set of sulphur-related entities will result:

  1. Sulphur dioxide – gas molecules in solutions
  2. Sulphite ions – as described at the start of this post
  3. Bisulphite ions – like sulphite but with a hydrogen, and less charge
  4. Unstable compounds – formed by bonding with various other chemicals
  5. Stable compounds – notably the product of reacting with acetaldehyde

Even though only number 1 in the list is actually sulphur dioxide, 1-3 are often referred to as free sulphur dioxide, and 4 and 5 as bound sulphur dioxide. The total sulphur dioxide includes both the free and the bound forms. Any protective properties are largely lost in the bound forms, with free sulphur dioxide doing the vast majority of the good work. In fact, it is the molecular sulphur dioxide that is most effective form but, depending on the pH, there may not be much of it present as a fraction of the total.

You may have noticed that from talking about sulphites we quietly slid into the subject of sulphur dioxide. You will find that is quite common in discussions of this subject, and the two are often seen as being practically synonymous. It does not help that when someone says sulphur dioxide they could be referring to

  1. Actual sulphur dioxide molecules,
  2. Free sulphur dioxide,
  3. Bound sulphur dioxide (unlikely perhaps),
  4. Total sulphur dioxide

In fact you could add a fifth, which is the sulphur dioxide measured by a specific procedure. More on this later, but let’s first take a look at sulphur compound allergies.

The first point to make on the subject of allergies is that people can have adverse reactions to several different sulphur compounds, and just because you have problems with one does not mean you need to avoid contact with all of them. In the food and drink industry it is common to talk about adding sulphites, and the product as containing sulphites. Accordingly, from a medical point of view, what is of interest to us here is usually referred to as a sulphite allergy or intolerance, as it is a reaction to food or drink that contains these sulphites. Also, my impression is that people take the practical view that all sulphited products finish up containing the same range of chemicals, as listed above, and do not bother to distinguish between the individual chemicals when gathering data about allergies.

There are many possible reactions to sulphites, both true allergies (due to an over-sensitive immune response) and other intolerances. The most severe response is anaphylaxis, which is rare but very serious and possibly lethal, while the most common is the worsening of any asthma symptoms, also potentially lethal in some cases. Other less serious possible reactions are hives and allergic rhinitis. But as far as I know there is no hard evidence that sulphites cause headaches. Estimates of the numbers of people affected by adverse reactions vary a lot. One estimate is that 1% of the population is affected, of which 5% also suffer from asthma; while another source says 5% of asthmatics are sensitive to sulphites, compared with 1% of the rest of the populations; and a third source claims that in America less than 0.05% of the whole population is affected.

Speaking of inconsistency in terminology, you might also note that although food scientists and medical people tend to talk about sulphites, and the warning text on wine labels reads “contains sulphites”, the threshold level for warnings is defined in terms of total sulphur dioxide content, a label warning being required for 10 ppm or more. In fact a specific procedure (or one that gives similar results) is mandated for measuring sulphur dioxide: the optimised Monier-Williams distillation-titration procedure.

Looking at methods similar to optimised Monier-Williams (here and here), it seems that the first step is to acidify, which converts all the free sulphur dioxide, and also a fixed proportion of bound sulphur dioxide, to the molecular form of sulphur dioxide, and then the sulphur dioxide gas content is measured. So it seems likely that the concentration levels determined for comparison with the 10 ppm limit, are expressed in terms of molecular sulphur dioxide gas, rather than the ionic forms. Another conclusion is that, as not all the bound sulphur dioxide is converted to the molecular form for measurement, the procedure will not actually give the true total sulphur dioxide content – it will be somewhat less, but greater than the amount of free sulphur dioxide.

Why is the limit set to 10 ppm? I could not find a definitive answer, but 10 ppm is referred to as being a detectable amount, so it seems that it is linked to the practicalities of measurement, rather than the level at which some people might react badly. As the mandated procedure under-measures, it would anyway seem silly to start fretting too much about the science behind the limit of 10 ppm. Perhaps the best that can be said about the regulations is that they are capable of being enforced consistently, and offer a degree of protection to those who are allergic.

I can only assume that the sulphite concentrations often bandied about by natural wine advocates also refer to sulphur dioxide as measured by the optimised Monier-Williams method, but I have never seen it stated. You will typically see sulphite concentrations quoted if a low-intervention producer choses to add sulphites, but otherwise an informal description of a low-intervention wine will often only say “no added sulphites”. Just remember that there will still be sulphites in a no-added-sulphites wines, and quite likely more than 10 ppm. This may be fine if you are mainly concerned about the natural-credentials of the wine, but is definitely something to bear in mind if you have a serious sulphite allergy.

So that is my best shot at explaining more precisely what sulphites actually are, how they are regulated, and to what extent they are responsible for allergies. I have done my best to stick to facts to the extent I could establish them, and keep my opinions to myself, but I might be more indulgent in future posts now I know a bit more what I am talking about. If I’ve got something wrong, or could have explained something better, please let me know, and I will try to correct or improve what I have written.

About Steve Slatcher

Wine enthusiast
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5 Responses to Sulphites in wine – clearing up a few points

  1. Paul Howard says:

    This is great Steve. For me, a really great follow up to this would be about the maximum amounts allowed (as measured) for different styles of wine, how whites often have more as they have a lack of tannins, and how organic and biodynamic regimes have stricter lower maxima. I’m particularly interested in how poorly made and often cheap wines often use high or max quantities because it’s much cheaper and low effort compared with careful testing and managing stability in the winery – i.e. the commercial aspects. As for headaches, anecdotally I think there’s a potential link, probably from exposure to high concentrations, though of course alcohol and tannins are also doubtless responsible as well, and difficult to differentiate between them. Certainly where I have persuaded a few white wine drinkers to trade up to better quality wine they have reported fewer headaches

  2. Thanks Paul.

    Yes, there is a lot to potentially follow up on. I thought the follow-up would largely be opinion – hopefully informed opinion more or less – but you are right that there is quite a bit of factual regulatory stuff to cover too.

    Do you know of any sources for measured sulphite levels in cheaper wines? We all have our suspicions, but I don’t recall seeing numbers.

  3. Paul Howard says:

    Hi Steve, thanks for this. I am convinced that cheaper wines have a tendency to employ indiscriminate levels but I have not found any numerical research on this. However, I will take another look to see if there are now any published materials. However, my opinion is based on “smoking gun” evidence: palettes of the stuff left lying around in wineries, the smell of struck-match sulphur on many cheaper wines, especially those from tank or barrel samples, (even dry reds) and the occasional technical sheet that has included a figure (and many more that omit any references to it while giving other technical info). I look forward to reading the follow-up, as its a real regulatory mish-mash. best wishes, P

  4. Thanks again, Paul. You may have noticed I published a follow-up yesterday, describing the regs and (where possible) actual levels. And I still have not yet mentioned the bits I find most interesting

  5. Paul Howard says:

    I hadn’t at that point, but have now…and added a couple of thoughts there!

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