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.
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:
- Sulphur dioxide – gas molecules in solutions
- Sulphite ions – as described at the start of this post
- Bisulphite ions – like sulphite but with a hydrogen, and less charge
- Unstable compounds – formed by bonding with various other chemicals
- 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
- Actual sulphur dioxide molecules,
- Free sulphur dioxide,
- Bound sulphur dioxide (unlikely perhaps),
- 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.