Sulphur Dioxide in Winemaking

The Use of Sulphur Dioxide (SO2) in Winemaking

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. We will refer to it 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 (this can include the malolactic bacteria at sufficiently high SO2 levels); help prevent oxidation and preserve fruity 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 probably should be used only for equipment sterilizing purposes, not in must or wine. For one thing, many people avoid sodium in their diets, for another, the presence of potassium ions in wine is more useful than sodium.

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 30 to 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.

The 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.

A commonly used unit of measurement for SO2 in must or wine is “parts per million” or “ppm”. 1 ppm is the same as 1 milligram per litre. I will use ppm.

For example, if you add 2.4 millilitres of 10% potassium metabisulphite solution to 1.0 imperial gallons of wine you will be adding 30 ppm SO2. If you have a 19.2 litre carboy to which  you wish to add 20 ppm SO2, multiply 0.35 by 19.2 to get an SO2 addition of 6.7 mL of 10% solution. Consider making up your own spreadsheet giving SO2 additions for your own sizes of barrels and carboys.

Putting SO2 to good use

You might hear a commercial winemaker tell you that she “doesn’t use any SO2 at all until after the primary ferment is complete, particularly with white wines.” 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 2.7 millilitres of 10% potassium metabisulphite solution for each 20 pounds works out to 30 ppm SO2. If the grapes are in reasonable shape, this should do the job for you. If your grapes are in perfect shape and the pH is low enough, you can do with less. We will deal with pH considerations later.

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 SO2 will have gone so that the ML bacteria can multiply and flourish. Alternatively keep your SO2 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 this is another reason it shouldn’t  be added  to a white or rosé until after the pressing has been done. However, the addition should be made promptly since white must quality suffers from oxygen absorption from the air. As soon as you have pressed, you have an accurate measure of your yield and can thus calculate the SO2 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 levels higher than around 15 ppm, but by the time you add an ML culture, much of the 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 anaesthetic. The danger of some undesirable oxidation of the must is there, so 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. Botrytised grapes may require 100 ppm SO2 or even more for adequate protection.

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.

It is difficult to test reds for SO2, because the red colouring matter interferes with the chemical reaction involved in the test and also makes it difficult to see the colour change involved. You pretty well have to trust the information on the shipping label. The fact that testing reds for free SO2 is difficult makes it imperative that you keep an accurate record of all SO2 additions in order to be able to estimate the situation at any given time.

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.

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

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

3. 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”. The remainder remains “free”. The bound portion consists of two parts. One part is made up of irrevocably bound compounds with aldehydes and proteins. The other part is made up of less stable compounds. These can partly turn back to the free form when the existing amount of free is lowered, or even if temperature is increased. This free portion also consists of two parts: one is relatively inactive bisulphite and the other, smallest of all the segments, as shown in the accompanying chart is molecular  This is the crucial active portion and its size depends both on pH and the total amount of free SO2.

It is worth noting at this point that in the early stages of a wine, when the total SO2 additions are less than 50 ppm or so, roughly half of further additions remains free and half immediately becomes bound. Later, when total additions are above about 60 ppm, most of any further addition remains as free. This knowledge gives us further reason to keep good records of SO2 additions, particularly in the case of reds, where direct measurement of free SO2 is not reliable.

Testing for Free 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 wish to put together your own 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, barely 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 6 or 10 mL syringe with the iodine solution. Next, accurately measure out 50 mL of the wine to be tested. Add 1 mL or so of starch solution and about 10 mL of dilute sulphuric acid.

Immediately start adding iodine solution to the sample, swirling it 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 many mL you’ve used. Multiply this by 12.8 to give you the number of ppm of free SO2 in the wine.

Testing for Total SO2

You will need  some 10% sodium hydroxide solution in addition to the chemicals required for the free SO2 test. To make this up, add enough water to 10 g of solid sodium hydroxide to bring the volume up to 100 mL. Great accuracy here isn’t necessary. Mix thoroughly.

To do the test: Accurately measure 20 mL of wine and put it in a narrow necked container such as an erlenmeyer flask. Add roughly 25 mL of 10% sodium hydroxide solution. Immediately cover the container and allow to sit for 15 minutes. Fill a clean dry 6 or 10  mL syringe with 0.02 molar iodine solution. At the end of the 15 minutes, add 10 mL dilute sulphuric acid along with  about 1 mL of starch solution to the sample and immediately start adding iodine solution. Stop when the purple colour persists. Note the volume of iodine solution used in mL, and multiply by 32.  This is your total SO2 in ppm.

Stability of Chemicals.

The dilute sulphuric acid and 10% sodium hydroxide solutions are very stable and will last for years. The sodium hydroxide should be stored in high density plastic in preference to glass. The starch solution will get mouldy. It should be replaced as soon as the slightest bit of discolouration occurs. Iodine is highly volatile. The iodine solution should be in as small a glass container as is convenient, and kepttightly closed and in a cool place. An alternative method of managing the iodine is to make up a 0.20 molar stock solution (10 times working strength). From time to time make up as much working strength (0.02 molar) solution as you will need for a month or so by diluting one volume of the 0.20 molar stock solution with 9 times that volume of distilled water.

Adding SO2 at racking

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. After your total additions over the life of the wine add up to around  60 ppm or more,  most of any  additional SO2you add 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 syphon tube is down to the bottom of the receiving container. Don’t splash the wine. If you 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

I 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 molecular SO2.

When you add SO2, depending on circumstances, some of it immediately becomes bound. What remains is called “free” and is in two parts. The larger, and relatively ineffective free part is “bisulphite” (HSO3-). The smaller part of the free is the active  molecular SO2. The amount of molecular SO2  in your wine depends both on the level of free SO2 present as well as pH. For instance at pH 3.2 the amount of free SO2 for 0.8 ppm molecular SO2 is 22 ppm. At pH 3.5, you will need 43 ppm free – essentially double. In most situations, 0.8 ppm molecular SO2 during bulk storage and at bottling will provide you with adequate protection from oxidation and bacterial action. This includes prevention of ML bacteria as well—important if you’ve blended ML affected wine with non-ML affected and require stability. It is important to remember that the amount of free SO2 in the wine depends on three things: how much you added, how much was present before the addition and how much of your addition promptly becomes bound. In the case of whites and rosés, the best thing to do is a free SO2 check. In the case of reds, you need to do some good estimating, based on previous SO2 additions as mentioned elsewhere in the article.

The level at which molecular SO2 can be detected by the human senses is about 2.0 ppm. This is also the level which is needed for maximum protection of your wine. This is particularly true in the case of sweet, and most notably, botrytised wines

Using Potassium Sorbate

Sometimes one wishes to finish a wine with some residual sugar left—Riesling, Gewürztraminer, Muscat Canelli and Chenin Blanc are among the grapes that lend themselves particularly well to this. In order to prevent renewed fermentation after ferment has been stopped,00 to 250 ppm potassium sorbate is often used. The effectiveness of potassium sorbate is pH dependent. To get close to the same effectiveness from a given dose of potassium sorbate would require around 55 ppm of free SO2 at pH 3.6 as opposed to only 28 ppm at pH 3.3.

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.

Bottle Rinsing with SO2

I often find it useful to use an SO2 bottle rinse when I am bottling. The rinse solution is 50 mL of 10% potassium metabisulphite solution made up with water, to about 750 mL in a winebottle. I have tested the effect of this several ways, and consistently find that after rinsing, and draining the bottles for about a minute, the free SO2 added is close to 8 ppm. Curiously, this is true for both 750 mL and 375 mL bottles. This is a useful way of adding a touch of SO2 at bottling time, particularly if the carboy you’re bottling has a bit of sediment and you don’t wish to stir it , or subject it to one more racking.

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