Potassium metabisulphite in wine: how much, when and how

Even though sulphites have had a bad reputation since the 2000s, their use goes back to antiquity: around AD 77, Pliny the Elder wrote that burning sulphur had a preserving effect on wine. Today, sulphites in their various forms are synthesized from sulphur found in nature through modern chemistry. What I’m about to say won’t please everyone… but sulphites are also naturally present in grapes, and even today no single substitute is recognized that has the same antiseptic, antioxidant and aroma- and colour-preserving properties for wine. So this article presents my 5 practical tips for succeeding at a minimal sulphiting, one that aims to preserve the originality and the artisanal character of your homemade wine.

Know the difference between the types of sulphites

Sulphites are really an umbrella term covering several chemical compounds that, when added to wine, release sulphur dioxide (SO₂). Below are the 3 most common forms, in terms of how easy they are to use in a home winemaking context.

Potassium metabisulphite (K₂S₂O₅)

It’s a white, crystalline salt that has to be dissolved in water before being added to wine. It’s also the most widespread and most used form of sulphite, because the potassium it contains won’t alter the taste of the finished wine, unlike sodium metabisulphite, which, as its name says, contains sodium. I recommend Elo’s Pretium potassium metabisulphite, which comes in a 1 lb (454 g) aluminum bag that reseals and closes tightly: it is essential to keep sulphites away from ambient (and humid) air, as explained in the next paragraph.

Sodium metabisulphite (Na₂S₂O₅)

It, too, is a white, crystalline salt that has to be dissolved in water first. It’s less widely used because the sodium it contains can alter the taste of the wine in two ways: by accentuating the “salty” side of the wine and, at high doses, by adding metallic and bitter notes. For these reasons it is used much more rarely and, when it is, it’s rather to disinfect equipment by spraying it as sulphite water than by adding it directly to the grape must.

Sulphur dioxide (SO₂) gas

It’s impossible to get in home winemaking, since this gas is regulated because of its toxicity. What is possible, however, is to get sulphur wicks that, when burned, release sulphur dioxide gas. What for? To preserve barrels when they don’t contain wine: a wick burned for 5 minutes, hung from a wire inside the barrel, will preserve it for a month.

Understand the different sulphite measurements

Where it gets complicated is that sulphites partly bind to the wine after being added to it. That “bound” portion becomes inactive and so no longer provides the benefits you’re after. To tell them apart, the following formula is used:

Total sulphites = Free sulphites + Bound sulphites

Total sulphites

It’s the total sum of all the sulphites in the wine, regardless of their form (free/bound) or their origin (artificial/natural).

It’s also the total sulphites measurement that the Canadian regulatory authorities use to determine whether a wine must carry the mention “Contains sulphites.”

Free sulphites

These are the sulphites that haven’t combined and that are considered, for all practical purposes, “free.” An equivalent term is “active,” since they are the ones that act immediately to prevent oxidation and micro-organisms. Over time, they bind with the sugar, alcohol and acid molecules in the wine and then become bound sulphites.

Bound sulphites

It’s the portion that has combined with the wine’s components and has, by that very fact, become “inactive.” In natural grape juice, most of the sulphites present are bound sulphites and only a very small proportion is free, hence the value of adding artificial sulphites.

Know how to add sulphites to your wine

Before adding sulphites to wine, the amateur winemaker will want to measure the amount of free sulphites (not total!) in the must. In practice, that’s very hard to do, and the kit available in winemaking shops is only reliable for white wine (ideally dry), not red.

That’s when grape selection takes on its full importance: organic grapes, which haven’t been treated with sulphites, will have a very low natural concentration, letting you add sulphites without worry. Conversely, non-organic grapes or grapes treated with sulphites may already contain a large amount of free sulphites, and adding more then carries a non-zero probability of making them detectable by taste, something considered a very serious fault in a wine.

Note also that the concentration of sulphites (total, free or bound) is expressed in milligrams per litre, or mg/L. Another, somewhat less popular unit is parts per million, or ppm. To go from one scale to the other:

1 mg/L = 1 ppm

Prepare a 10% solution to sulphite your wine

The most commonly used method is called the “10% solution,” which gets its name from the fact that it contains about 10% of its volume in releasable sulphites (so, free once the solution has been added to the wine). Don’t forget that 1 g of K₂S₂O₅ gives, when dissolved in water, about 0.5 g of SO₂, in other words 0.5 g of free sulphites. To prepare the solution, you therefore need to:

1. Measure 20 grams of potassium metabisulphite. You can use a teaspoon 4 times, considering that 5 mL = 5 g. That’s not very precise, though; I recommend using a precise kitchen scale accurate to a tenth of a gram instead.
2. Dissolve completely (until the powder is no longer visible) in 100 mL of cold and, ideally, demineralized water. The cold water can be measured with kitchen tools such as spoons or cups.

There you go, the 10% solution is ready to use! After a month of storage in an airtight plastic container, it should be thrown out and made again.

Determine the volume of 10% solution to add to your wine

Using the good old formula from high school, C₁V₁ = C₂V₂, it becomes possible to calculate the volume of 10% solution to draw:

• C₁ = fixed at 100,000, that is, the concentration in mg/L of free sulphites in the 10% solution
• V₁ = the value you’re looking for, that is, the volume of 10% solution to draw
• C₂ = the desired sulphiting level, which is normally between 10 mg/L and 100 mg/L
• V₂ = the volume of the cuvée in litres that needs to be sulphited

If I want my 20 L of must to be sulphited to 50 mg/L, I’ll need to add exactly 10 mL of 10% solution:

C₁V₁ = C₂V₂ → V₁ = C₂V₂ / C₁ → 50 [mg/L] × 20 [L] / 100,000 [mg/L] = 0.01 L = 10 mL

Of course, this calculation is somewhat tedious and prone to human error: that’s why I’ve gathered in the table below the volumes in millilitres (mL) of 10% solution to draw according to the volume of the must and the desired concentration of free sulphites.

The degree of precision can also be hard to achieve with measuring spoons: I recommend using a small 10 mL graduated cylinder instead, which is accurate to 0.2 mL. By drawing 9 mL in that cylinder, you get the nearly exact volume (9.04 mL) for a 3 US gallon carboy that needs to be sulphited to 80 mg/L. For a volume above 10 mL, just fill the cylinder more than once.

Choose carefully when to add sulphites

Although good practice says it’s wise to split the addition of sulphites across several moments rather than adding one big quantity at once, that’s impractical in home winemaking. It’s hard to get optimal results from a second or even third sulphiting because:

1. Free sulphites tend to decrease over time, so the more time passes, the more sulphites you have to add to get the desired result; and
2. It’s hard to measure free sulphites accurately at home, so the risk of sulphiting too much or not enough is very real.

That’s why it’s better to choose your moment or moments for adding sulphites carefully.

At harvest and crushing

If the sanitary condition of the grapes isn’t ideal, for example if they have started to rot, or if past fermentations have had problems with Brettanomyces yeasts (“Brett”), then it would be wise to sulphite at harvest. For minor problems, sulphiting at 50 mg/L will be enough. For serious problems with rot and mould, sulphiting up to 100 mg/L will prove necessary depending on the severity observed.

You’ll of course need to crush the grapes before you can add sulphites. Be careful to watch that fermentation starts properly if no commercial yeast has been added, because sulphiting is known to harm natural yeasts. If the harvest is perfectly healthy, there’s no need to sulphite.

During or after alcoholic fermentation

In my previous article on alcoholic fermentation, I paint a picture of the problems that can come up during this critical stage. The symptoms to watch for are listed, along with the sulphite concentrations to add.

Another scenario after fermentation ends is wanting to prevent malolactic fermentation (MLF) from starting. Sulphiting at that point will also protect the wine against acetic and lactic bacteria, the lactic ones being responsible for MLF. If alcoholic fermentation went well and MLF is desired, there’s no need to sulphite.

After malolactic fermentation and before aging

The wine is ready to go into a carboy or barrel to be aged and stabilized. The smaller the container used, the higher the risk of oxidation. In a home winemaking context, planning a long aging period is discouraged, since many problems (made worse by the small size of the carboys used in artisanal winemaking…) can happen during that time.

If everything has gone well so far and an aging of several months is wanted, a sulphiting at 50 mg/L could be wise. If there is no aging period, there’s no need to sulphite.

After aging but before bottling

Aging went well (or simply didn’t happen!) and it’s now time to bottle the wine. If no sulphite has been used so far because the wine was made naturally, a risk of “refermentation” in the bottle remains. Such a refermentation, very undesirable since it can push the cork out, can happen when:

• The temperature rises (during transport, for example), reactivating dormant yeasts at the same time; and/or
• The wine contained special yeasts such as Brett that can keep fermenting in conditions unfavourable to the regular yeasts of alcoholic fermentation.

To reduce this risk, sulphite between 20 mg/L and 50 mg/L, depending on the wine’s acidity. A very acidic wine requires few sulphites whereas a low-acid wine will need the maximum dose of 50 mg/L.

Use sulphites with caution and moderation

To use sulphites properly, some precautions are in order.

• Don’t exceed 50 mg/L of free sulphites at consumption: Adding too many sulphites has undesirable effects, even affecting the taste of the wine. The smell threshold at which a professional taster will detect them is between 20 and 40 mg/L of free sulphites and depends on the wine’s acidity and pH. Beyond 50 mg/L of free sulphites, a sharp, acrid taste appears, considered a serious fault in a wine. Now, if a sulphiting at 100 mg/L was done to control rotten grapes, is the wine doomed? Not at all! Don’t forget that free sulphites decrease over time: up to 50% will become inactive (so, bound sulphites) after only 6 months according to a study by Inter Rhône (PDF in French).
• Only clean carefully chosen equipment: Sulphites will corrode metal and are off-limits on any stainless steel vat, for example. The 10% solution being very concentrated, any contact with skin and eyes must be avoided. Gloves and safety glasses greatly reduce the risk when handling. Likewise, its fumes are irritating and very unpleasant to breathe. In any case, there are more effective products for cleaning and sterilizing winemaking equipment, such as Oxy-San and One Step.
• Don’t treat sulphites as the ultimate protection: Sulphites oxidize on contact with air. That’s, incidentally, the method for reducing the amount of free sulphites if the taste is ever affected: stir and pour the wine from one container to another to incorporate oxygen, force the oxidation of the sulphites and lower their concentration. Sulphites protect against oxidation but won’t prevent it. For a lightly sulphited white wine, the usual precautions will still apply to prevent its oxidation and, consequently, the appearance of a fault.

That’s what I call a complete article on sulphites!

I also want to express my gratitude to Véronique Lemieux, who gave me an enormous helping hand by reviewing the winemaking concepts presented here. For those interested in her wines, you’ll find them on Instagram (@les_cuvees_de_veronique).