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The Use of Sulphur Dioxide (SO2) in Winemaking

by Charles Plant

When Nature ferments grapes, or any other fruit for that matter, wine is not the end product. Instead, unpleasant concoctions containing vinegars, mercaptans and other substances are formed, with the final end being water and assorted solids and gases.

Although most good winemaking involves interfering with Nature as little as possible nonetheless we need to steer her a bit, and in fact completely stop some natural processes at just the right moment.

An indispensable ally of the winemaker in achieving these things is sulphur dioxide—frequently referred to by its chemical formula, SO2

In this article we will be investigating how to use SO2 to do the following things for us: inhibit wild and spoilage yeasts and unwanted bacteria; help prevent oxidation; preserve flavour and freshness in wine.

Sourcing SO2

SO2 is a pungent, choking gas which is somewhat soluble. The most practical source for the home winemaker is the salt, potassium metabisulphite, which is 57% SO2. Since you can detect SO2 when you smell a sample of potassium metabisulphite it is evident that the solid decomposes easily. This happens on contact with carbon dioxide and moisture in the air. Keep your container of potassium metabisulphite tightly closed to minimize this problem. In any case, you probably shouldn't keep the stuff around for more than a year before buying fresh.

"Campden" tablets are made of potassium metabisulphite. Each tablet, when fresh, contains 0.44 grams of it. However, if they are old, a lot of the SO2 will have been lost and their effects will be unreliable. You're better off to use bulk potassium metabisulphite. It's cheaper too.

Sodium metabisulphite is also a source of SO2 but should be used only for sterilizing purposes, not in must or wine.

Sometimes an old fashioned winery will burn a sulphur stick in empty barrels to keep them sterile. Under no circumstances should the home winemaker ever do this. The presence of any elemental sulphur, such as might drip into the barrel will lead inevitably to the formation of the dreaded hydrogen sulphide. In the winemaking business, -ite sulphur compounds are friendly, -ides are deadly enemies.

Under a very few circumstances, solid potassium metabisulphite may be used directly. For instance if you decide to add SO2 to red grapes before crushing, a scant ¼ teaspoon sprinkled on a 36 pound lug of grapes will give you about what you need—somewhere around 40 parts per million SO2.

Don't do this with white grapes or when using red grapes to make a rosé. When you press, the SO2 will wash off into the juice in uncontrollable amounts and you will likely have far too much in the free run, and next to none in the pressed portion.

Make a 10% Solution

A much better way to get your SO2 is from a 10% solution of potassium metabisulphite in water.

For instance, you could add water to 1 pound of potassium metabisulphite to make a total volume of 1 imperial gallon. Or, if you prefer metric, add enough water to 100 grams of potassium metabisulphite to make up a total volume of 1.00 litres.

Fresh 10% solution is 5.7% SO2.

The most commonly used unit of measurement for SO2 in must or wine is "parts per million" or "ppm". Incidentally, 1 ppm is the same as 1 milligram per litre. We will use ppm.

For example, if you add 10.0 millilitres of 10% potassium metabisulphite solution to 19.1 litres of wine (this happens to be a 5 U. S. gallon carboy) you will be adding 30 ppm SO2. A convenient table can be made up showing commonly used additions one might wish to make.

Putting SO2 to good use

You might hear a commercial winemaker tell you that she—quote—doesn't use any SO2 at all until after the primary ferment is complete, particularly with white wines—unquote. Such a winemaker knows the complete history of her grapes—exactly where they came from and how they were handled. She undoubtedly also has elaborate handling equipment—must coolers, inert gas covered tanks and all the rest. You should know a lot about what you're doing before you decide to postpone adding SO2 until some middle point in the winemaking process.

Let's start with red grapes. You need to suppress any bacteria and wild yeasts they may have picked up, prior to inoculating with a selected yeast culture. If you try to depend on wild yeasts, they will likely die before all the sugar is fermented out, leaving you with a sticky problem or worse. Vinegar bacteria can produce an undesirable amount of ethyl acetate in the early part of the fermentation if not checked.

You probably bought the grapes by the pound and can assume around 5 litres of finished wine from each 20 pounds. Addition of 3.5 millilitres of 10% potassium metabisulphite solution works out to 40 ppm S2. If the grapes are in reasonable shape, this should do the job for you.

If you are planning to have a malolactic ferment, or ML, happen at the same time as the sugar ferment, don't add the ML culture until the sugar ferment is well underway. By that time enough of the S2 will have gone so that the ML bacteria can multiply and flourish. Alternatively keep your S2 addition down to, say, 20 ppm. We'll talk more about ML when we discuss white wines.

If you are concerned about excessive mould, possibly accompanied by traces of vinegary smells, increase the SO2 addition to 50 or 60 ppm or in extreme cases even more.

The SO2 you add will also lead to production of small quantities of glycerol in the early part of the ferment. This is generally desirable.

When you make white or rosé wine the situation is a bit different. Grape skins contain phenols. These add flavour and colour to wine. They can also contribute astringency, bitterness and browning. These things are of more concern in whites and rosés than in red wines.

SO2 can contribute to phenol extraction from the skins and thus should not be added until after the pressing has been done. Further, once you have pressed, you have an accurate measure of your yield and can thus calculate the S2 addition more precisely.

How much SO2?

How much to add depends on a number of factors. What was the condition of the grapes? What is the pH? (We shall see later, how SO2 is more effective at lower pH). Are you planning on putting the wine through a malolactic ferment? Is the juice intended for making a champagne method sparkling wine?

30 ppm SO2 for juice from sound fruit with a pH of 3.4 or so, and destined for a regular wine should be fine.

If you hope to have a malolactic ferment happen along with the sugar ferment, you likely have a higher acid Chardonnay, or something, say around pH 3.2. Smaller SO2 additions are okay here—say 20 ppm. Malolactic bacteria won't work at level higher than 15 ppm, but by the time you add an ML culture, a bunch of SO2 will have been used up.

Juice destined for Champagne method wine will probably have a low pH, close to 3.0, say. You are going to want to have a malolactic ferment occur. If the grapes were perfect, you might get away with no SO2 at all until the first racking. This however is a bit nerve-wracking, like having a tooth filled without anesthetic. Better to go with 10 ppm SO2 or so.

At the other end of the scale, juice from grapes with a lot of mould, possibly with some vinegary smells, should have 50 to 60 ppm SO2 or even more added. Who knows—maybe you have lucked on to some botrytised Riesling or Semillon and plan a serious dessert wine.

How about frozen or sterile packaged musts? With white or rosé juice, you can either trust the shipper to tell you how much SO2 was added, or you can test and make additions accordingly.

We will discuss testing for SO2 later on.

It is difficult to test reds for SO2, so you pretty well have to trust the information on the shipping label.

The next time you are going to consider adding SO2 to the wine is at the first racking. In most cases, this will be after the sugar fermentation is complete and the new wine is dry.

If you want to stop active fermentation to retain residual sugar, don't try to use SO2 as your main tool. A vigorous ferment of a strong yeast will laugh at you and carry right on to the end. Selected combinations of racking, fining, chilling and filtering are the way to go. SO2 will be involved, but only as it would be normally used in conjunction with these other processes.

Stifling Oxidation

An important reason for adding SO2 when you rack is to avoid oxidation. It does this in three main ways.

When you smell a wine that is oxidized, the chemical you are smelling is acetaldehyde. SO2 combines with acetaldehyde to form a stable compound.

When there is oxygen around, SO2 itself becomes oxidized before phenol compounds in the wine do, and so acts as an oxygen scavenger.

SO2 suppresses the activity of enzymes that cause browning and other problems.

So, when you add SO2 it doesn't all hang around. Lots of it gets used up doing these various jobs for you and becomes bound. Before we discuss how much SO2 to add when racking, this is a good time to talk about measuring how much there is left free from previous additions. The free SO2 is, of course, the total amount you've added to date, minus the amount that has become bound.

Testing for SO2

The test procedure that follows works well only for white or rosé wines. Some of the colouring matter in red wines reacts to the test chemicals in the same way as SO2 making the results pretty well meaningless.

It should be noted that SO2 testing kits may be available at your local winemaking supply shop. Since they will contain all the necessary ingredients, instructions and measuring vessels, you will save yourself substantial effort by buying one. What follows assumes you can’t, or don’t want to, find such a kit.

You will need the following chemicals, which you might need some help with. The chemistry teacher at your local high school might be receptive to a contribution to his or her science department's petty cash fund.

0.02 molar iodine solution: Accurately weigh out 2.54 grams of iodine. Roughly weigh 5 grams of potassium iodide. Add a few millilitres of distilled water, enough to cover the chemicals, and agitate until the iodine is completely dissolved. This may take a bit of time. Finally, add enough distilled water to make an accurately measured 1.00 litres of solution.

Dilute sulphuric acid: Add about 250 millilitres of concentrated sulphuric acid to about 750 millilitres of water. Unless you have previous experience handling sulphuric acid, don't even think of doing this dilution yourself.

Starch solution: Add about 1 gram of starch to about 100 millilitres of water. Stir and bring to a boil then cool.

To do the test, first fill a clean dry syringe with the iodine solution and note the reading carefully. Next, accurately measure out 50 millilitres of the wine to be tested. Add 1 or 2 millilitres of starch solution. Add about 10 millilitres of dilute sulphuric acid.

Now immediately start adding iodine solution to the sample, swirling as you go. You will note a purple-black patch which disappears as you swirl. As soon as the purple colour persists, stop adding iodine, and note how much is left in the syringe. Subtract from the starting amount and multiply the result by 12.8. This will give you the number of ppm of free SO2 in the wine

Adding SO2 at racking time

When racking red wines, depending on pH, the addition of from 20 to 30 ppm SO2 each time should do the trick nicely. For the first couple of rackings, when the total SO2 added since the beginning is less than 50 ppm or so, about half of what you add immediately gets bound, leaving half as free. Above about 60 ppm total most of your additional SO2 remains as free.

Be sure to pour the SO2 solution into the bottom of the receiving container first and then rack the wine. This way the SO2 is around all the time to suck up unwanted oxygen.

If you have started a malolactic ferment as well and you are not certain it has completed, you could go with less SO2 at racking—maybe 15 ppm, maybe only 10. In this case, your pH is likely to be pretty low anyway and as we're going to see later, that makes the SO2 much more effective.

You are, of course, keeping a good record of your SO2 additions, aren't you? A reasonable rule of thumb for red wines is to keep the total addition of SO2 from crush to bottling at less than 150 ppm.

With white or rosé wines, test before racking, and add enough SO2 to bring the free up to 20 or 30 ppm.

Once again, if there is a malolactic ferment involved and/or you are going to do a bottle ferment later, for champagne method sparkling wine, you want to keep the SO2 down. Since under these conditions, the pH is going to be low, you are probably okay adding only 10 ppm or so.

A reminder about racking techniques is in order here. Always make sure your siphon tube is down to the bottom of the receiving container. Don't splash the wine. If you are trying to get away with minimal SO2 and you have a carbon dioxide cylinder, purge the receiving container of air with CO2 before adding SO2 and racking.

SO2 and pH

We have made several references to the connection between the effectiveness of SO2 and pH. It is about time to explain how this works.

What is really protecting your wine is called molecular SO2. The level at which molecular SO2 can be detected by the senses is about 2 ppm. This is also the level needed for maximum protection, although in practice, you’re safe with levels of less than half that. The amount of molecular SO2 depends on the amount of free SO2 and the pH. For instance, at a pH of 3.0, around 36 ppm of free SO2 gets you 2 ppm of molecular. At pH 3.6, the amount of free needed for the same job, jumps to 137 ppm.

2 ppm molecular SO2—As a function of pH and Free SO2

pHFree SO2 necessary for 2 ppm Molecular SO2
2.8 23
3.0 36
3.2 56
3.4 87
3.6 137
3.8 216

Of course you are not going to need to use amounts like this—half or even a bit less will probably do you well—but it does underline the desirability of starting off with a must of lower, rather than higher pH.

And now sorbate

Often one wishes to finish a wine with some residual sugar left—Riesling, Gewürztraminer and Chenin Blanc are some grapes that lend themselves well to this. In order to prevent renewed fermentation after ferment has been stopped, potassium sorbate is often used. To get close to the same effectiveness from a given dose of potassium sorbate would require 50 ppm of free SO2 at pH 3.6 as opposed to only 20 ppm at pH 3.2.

Incidentally, it is essential when using sorbate to have effective SO2 levels high enough to prevent a malolactic ferment from happening. If ML occurs in the presence of sorbate, a peculiarly revolting geranium-like smell is produced for which, alas, there is no remedy. The wine is a goner.

Wrapping things up

In this article you have read why you as a winemaker need to use sulphur dioxide (SO2) in your winemaking. You have been told about the best source of SO2, how much to use, relationships with pH, and test methods for checking how much you have present in the free form.

The use of SO2 started with the Romans, and I’m sure there isn’t a self respecting winery in the world today that gets away without it. Go consider.

For further information including useful tables, check the rest of the VAWA website.