Understanding and using sulphur dioxide for better mead

Introduction

The use of sulphur dioxide, or its solid compound potassium metabisulphite, is nearly routine for making wines, but is often thought of as just an antimicrobial agent and an optional one at that. In fact, sulphur dioxide plays a number of roles, some of which are of special interest to mead makers. I have tasted a few great meads that were made without sulphites, but I have tasted many more that would have benefited from its use. Is your honey aroma less than you hoped for? Is there a lingering harsh aftertaste? These are common problems that can be solved with the disciplined use of sulphur dioxide.

To those who are not chemists, I apologize for casually mixing up the words sulphur dioxide, which is a gas that can dissolve in water, with metabisulphite and sulphite, which are a family of solid compounds formed from sulphur dioxide (note that these solid compound names end in -ITE, not -ATE or -IDE). When speaking generally, these words are often used as equivalent as the compounds inter-convert to one another once dissolved, but the differences matter when calculating additions. I avoid the use of “ppm” below as the number is the same in mg/L, which makes calculating your additions clearer.

The uses of sulphur dioxide

The range of food packaging problems solved with sulphur dioxide is astonishing. If someone tried to sell you a drug that cured as many conditions, you would think they’re a fraud. Sulphur dioxide (SO₂) has been used in wine making since Roman times, originally produced by burning sulphur inside casks before filling. Today this compound is used with a wide range of foods as it:

• prevents oxidation by combining with free oxygen (SO₂ + ½O₂ + H₂O = H₂SO₄)
• remedies oxidation by combining with aldehydes to eliminate harsh taste
• maintains colour and flavour by combining with aldehydes to suppress Maillard reactions during fermentation and aging
• maintains balance and colour by inhibiting enzymatic browning (like a cut apple) and enzymatic oxidation of tannins
• inhibits growth of most bacteria of concern
• inhibits growth of most wild yeast since after 2000 years of exposure, domestic brewing yeast have essentially been selected for SO₂ tolerance.

In practice, homebrewers will be exclusively using potassium metabisulphite as some worry that sodium metabisulphite can affect flavour. The typical wine maker’s working solution (5-10%) is an effective residual sanitizer, and a quick rinse of fermenters or bottles with working solution is a traditional way adding a few ppm of sulphite to the wine. 

The antioxidant action is the main interest to makers of mead and fruit wines as it preserves delicate aromas, protects tannins, and prevents off-flavours from aldehydes or volatile acids. For wines in general it allows control of malolactic fermentation (MLF). Finally, for sweet wines, it is vital when using potassium sorbate as MLF bacteria can turn this stabilizer into nasty off-flavours. All of these functions are pH-dependent which is why potassium metabisulphite is not of much use in brewing beer (pH = 3.8 to 4.5) except for dechlorinating water.

Risks in using sulphur dioxide

There are risks in using sulphur dioxide carelessly. The working solution smells evil and tastes horrible. Some yeast will convert it to hydrogen sulphide (H₂S—sulphide with a letter D) which smells and tastes even worse. An overdose can knock out your yeast, and the standard dosage is enough to prevent malolactic fermentation when you need it. If you must use sodium metabisulphite, the sodium can emphasise bitter flavours.  But the main concern is that some people can have a life-threatening anaphylactic reaction to the compound. I would say that no wine is safe for these friends unless it is specifically sulphite-free, and homemade wines are probably quite risky unless the maker has full control by using fresh fruit and other ingredients as opposed to concentrates and prepared additives.

How to use sulphur dioxide

To use sulphite with precision requires a quick chemistry refresher.  You will be using potassium metabisulphite which can break down to yield sulphur dioxide:

                                    K₂S₂O₅ + 2H⁺ = 2K⁺ + 2SO₂ + H₂O

Such that 1 g pure potassium metabisulphite theoretically gives 0.574 g of SO₂. However, metabisulphite degrades on storage and SO₂ binds to many compounds in your must, so the actual yield will be lower. On top of this, when we add potassium metabisulphite to a wine, it dissolves to produce a range of SO₂ related compounds which have different actions. To obtain the desired protection, the concentrations of some main compounds must be estimated.

First, there is “molecular SO₂” meaning actual SO₂ molecules floating in the liquid. This is most relevant to anti-microbial activity, including MLF control. A typical target is 0.8 mg/L (or ppm, if you prefer) which protects against bacterial contaminants and will prevent MLF. The taste threshold is about 2.0 mg/L which makes for a narrow target range. Unfortunately, there is no practical test for home use so some guesswork will be required.

Second, there is “free SO₂” meaning the equivalent of SO₂ plus S₂O₅^2- (dissolved metabisulphite) plus HSO₃^– (dissolved sulphite) etc. These compounds are most relevant to oxidation prevention and control. They can be lumped into a single number as removing any one compound (e.g. degassing SO₂) simply causes the others to react just enough to restore the original proportions. This concentration can be measured with reasonably simple tests, at least for white wines. The desired concentration typically runs from 10 mg/L (low) to 100 mg/L (high). With this free SO₂ number and the pH we can estimate the molecular SO₂ level. When in doubt, 50 mg/L free SO₂ is typical for a wine, so try 0.1 g potassium metabisulphite per Litre (which is 1 Campden table per imperial gallon).

Finally, there is “total SO₂” which is all of the added SO₂ that hasn’t reacted with oxygen to become H₂SO₄. This determines the maximum level in the final product. Track all additions and try to keep the total under 200 mg/L in finished product.

Table 1:  Minimum addition to reach 0.8 mg SO₂ per Litre

pH	Free SO2 (mg/L)
3.0	14
3.1	18
3.2	22
3.3	28
3.4	35
3.5	44
3.6	55
3.7	69
3.8	87
3.9	109

To make a precise addition, you know you want between 0.8 and 2.0 mg/L SO₂. You measure the pH of your wine, let’s say it’s pH 3.4.  Looking at Table 1 for 0.8 mg/L, you find you need a minimum of 35 mg/L free SO₂. Table 2 for 2.0 mg/L tells you the maximum amount is 87 mg/L free SO₂. Based on this you decide to add 50 mg/L. If you add 0.1 g of potassium metabisulphite per Litre, that should be 57 mg/L free SO₂, it might come out lower but not higher so that is the dose to try.

Table 2:  Maximum addition to reach 2.0 mg SO₂ per Litre

pH	Free SO2 (mg/L)
3.0	36
3.1	45
3.2	56
3.3	70
3.4	87
3.5	109
3.6	137
3.7	172
3.8	216
3.9	272

If you have a test kit, take a sample after racking and mixing and see how close you came. If in the above example you expected 50 mg/L but measured only 33 mg/L, you divide 50 by 33 to get 1.5, in other words 50% more, so you add another 0.05 mg/L to get the desired concentration.

For me, protecting the delicate honey aroma is the first objective coupled with the goal of minimizing aldehyde formation. Unless you’re making a cyser, you probably don’t want malolactic fermentation reducing the little acidity you have so you don’t need to allow bacterial growth. For bottling a sweet mead stabilized with potassium sorbate, you will also need an anti-bacterial level of sulphites.

You need to think ahead to stay below a total SO₂ of 200 mg/L (Canadian regulations say 350 mg/L). You know you will add potassium metabisulphite at bottling and every time you rack. Therefore plan for a maximum three rackings, and hopefully only two. I usually use hot water to dissolve the honey which should knock down wild yeasts enough that pitching a strong fermentation strain is enough to out-grow any contaminants. The yeast will eliminate oxygen pretty quickly, so adding SO₂ before pitching is optional.  Oxygen breaks down SO₂, so if you decide to add some at this stage, don’t aerate until just before pitching the yeast.

Before each racking, test for free SO₂. It’s usually close to zero so the amateur practice of blindly adding 0.1 g/L metabisulphite is OK. However, you don’t know how fresh your supply is. You want SO₂ present during the racking, so weigh out the first addition, dissolve in a little hot water, and add it to the receiving carboy. After racking, mix the wine gently then re-test and estimate how much more is necessary to reach 50 ppm.

Practical test methods for free SO₂ depend on a colour change. There really isn’t a good test for dark wines. The professional “Titrets” kit is available, gives better results, but gets expensive when you have to re-test. The more amateur kit I use gives decently reproducible results. For best results, be fussy about volume measurements, handle sample gently to prevent aeration, and be quick. To prevent inadvertent dilution, rinse containers with a small bit of the sample before starting.

The instructions will be specific to the kit you use and must be followed exactly. Be aware that the reagents and their by-products are mildly toxic (typically iodine is produced). Plan your work to keep potable and non-potable samples and glasses separate. If you use domestic glassware, wash glasses with detergent and hot water.

To finish, an aside on ciders where you have some extra complications. You might be using ugly apples which make you worry about wild microorganisms. Sulphites preserve tannins, but prevent the formation of an attractive pink colour, and will prevent MLF which is often desirable for cider. I weigh out potassium metabisulpite based on 50 mg/L at my expected juice volume, but I add nearly all of it to the crabapples at crushing to protect the tannins and let the pulp of the sweet apples sit for a while to oxidize for colour development. By the time primary fermentation is over, the total SO₂ should be well below 20 mg/L and I won’t add any sulphites at the first racking so as not to inhibit MLF. I also don’t add metabisulphite at bottling if I plan to carbonate by sugar addition.

For more details:
http://www.bcawa.ca/winemaking/so2use.htm
Wikipedia
Winemakers’ sites in general
Pambianchi, Daniel, Techniques in Home Winemaking, Véhicule Press, Montréal, 2002.