After more than 30 years of making wine, I've seen a lot of things come and go. Centrifuges were all the rage in the late 1970s. Rotary tanks had their run. There has been a long-lasting, ongoing debate over whether native or commercial yeast contribute your best chance for making fine wine. Change and evolution seem standard in winemaking, but one thing is so ubiquitous, so fixed a routine that we almost overlook it. Sulfur dioxide (SO2) has been, is being and will continue to be used by winemakers all over the world.
Lauded for its anti-oxidative and anti-microbial properties, SO2 has been proven to be one of the most effective agents the winemaker has in his winemaking arsenal. If you have spent any time making wine, then your nose has twitched with the acrid pinch of SO2. Your throat has been scraped by its prickly, irritating scratch. You have spent hours analyzing free and total sulfur dioxide and have bemoaned an inopportune bacterial problem when inattention allowed levels to get too low.
We decided it might be interesting to get some experts around our table and delve into the current uses for SO2. We wanted to know the most effective way to use it, to learn about any safety issues that might arise and to see if there was a way to make good wine without it. The group we gathered was experienced, contentious and up for a good discussion.
Pat Howe has two degrees from the University of California at Davis. She worked at Domaine Chandon and Mumm Napa Valley and overseas at Moët in France and for Domaine Chandon in Australia. After running her own sensory analysis and chemical testing business, she worked for large corporate wineries until opening Patricia Howe Wine 16019.
Zoran Ljepovic studied enology and viticulture in Croatia. He worked in Croatia as a winemaker until he came to California to pursue a master's degree in enology at Fresno State and then worked for Gallo for two years. In 2000 he went to work as quality control enologist for Franciscan Winery. Currently he is QC/QA director for Icon Estates of Constellation Wines.
John Katchmer left the entertainment industry to enter the master's program at UC Davis. Upon graduating, he decided he didn't want to make wine, so he ended up going to work for the private wine laboratory, Vinquiry. He has worked there as an enologist for 16 years.
What is SO2 and why do we use it?
Pat: It is both an anti-oxidant and anti-microbial agent that works well at wine pH, has a long history of successful use, is legal, generally regarded as safe, inexpensive and ubiquitous.
Zoran: And it is also produced by yeast.
John: Sulfur dioxide, when it goes into wine, becomes sulfurous acid (H2SO3), and it associates into three different species: the molecular, which would be SO2; then you have HSO3-1 (bisulfite) and SO3-2 , which would be sulfite. Most people use SO2 primarily as an anti-microbial, and the species that is the anti-microbial form is the molecular form. The amount available in molecular form under wine pH conditions starts at 6.1 percent at 3.0 pH but drops to 0.6 percent at 4.0 pH. That becomes very important when you are trying to use SO2 as an anti-microbial.
It's not so much the amount that is present; it's the amount that is necessary to effectively kill the microorganisms. So you have to know how much is going to be in that particular species at your particular pH and how much free SO2 is available; and out of that free SO2, how much of that is going to be molecular. Based on that, you can tell if you have enough to kill what you are trying to kill or not. There's a lot of debate over how much molecular SO2 is needed to effectively control microorganisms. Most people think you need .82 to.9 molecular.
Pat: I've seen published French work, specifically regarding acetic acid bacteria, that .5 to .8 molecular SO2 had been effective.
John: I've heard that acetic acid bacteria are somewhat resistant to SO2. But when it comes to Saccharomyces and Brettanomyces, then you need .825 to be effective.
Zoran: It is most desirable to be at .8 ppm molecular SO2; but from the practical point of view, in the winery we regularly pick something in the range of .6 to .8 ppm. If we know that Brettanomyces is present, then we will definitely go to highest possible molecular SO2 and the lowest possible temperature, which means storage conditions like 55°F or below.
If we find out that Brett is present, the ideal situation would be to rack the wine off lees, chill it, add SO2 to .8 ppm or about 55 ppm free SO2 because you don't want to go much higher, filter it and then put it back into treated barrels. That would be ideal, but very often winemaking doesn't accept that. In that case, the first suggestion would be to bump up the SO2. Usually, winemakers would rack the wine off the lees, adjust the SO2 and rack the wine to clean barrels. If it is close to final blending, the winemaker might rack the infected lots to tanks and keep them there at elevated free SO2 levels until blending.
Pat: To get back to the original question, anti-microbial is obviously an important part. The other thing that SO2 is useful for is preventing enzymatic browning in juice. It has multiple uses for multiple issues in winemaking. It has been one of our most flexible and least offensive agents since before Prohibition.
Zoran: Yes. Anti-oxidant activity definitely is important at grape receival and for bottling. It helps inhibit polypholoxidase in juice and wild microflora in grapes.
Tell me how you would use SO2 starting from when grapes arrive at the winery.
John: With red wine you are going to be using it mainly as an anti-microbial because most people nowadays use cold soaks so you have the risk of native yeast and bacteria trying to go off and do their thing and producing VA or ethyl acetate. So SO2 is used to control that. Lots of people don't have good control of temperatures during cold soak. If you cold soak at 60°F versus, say, 48°F, that's a big difference. You'd use SO2 there mainly to keep everything down while you are doing your cold soak extraction, then you'd let it come up to temperature, and ferment on its own or inoculate with a yeast.
Pat: How much SO2 you add depends on the cleanliness of your fruit and the history you have with that fruit. If you have bird-damaged or sun-damaged fruit, which you suspect is going to have a high load of yeast or bacteria, then your levels will need to be higher than when you have clean fruit with no history of contamination. For clean red grapes, I'd use 30 to 50 ppm.
Zoran: SO2 additions would be made to red grapes at the crusher and to white grapes at the press pan after pressing. For sound white grapes we think 30 ppm would be right and 50 ppm for white grapes with damaged fruit. For cold soak red grapes from sound fruit we'd use 50 ppm. If the fruit is not completely sound, it could go to 75 ppm or even 100 ppm.
Pat: This brings up another point. If we know that once we make that addition it is all pretty much in the bound form, then what is the basis for the anti-microbial activity in that situation?
John: The general thinking is that only this free molecular form of SO2 is anti-microbial, that any bound SO2 has no anti-microbial effect. What I found during my master's thesis research was that there is an equilibrium that exists between the bound SO2 and the molecular free. Depending on which compound it was binding to, the amount that was needed to kill was significantly different. So there would seem to be some effect from the bound part.
My research tended to show that there was a pH effect when using lower levels of molecular, and that the higher the pH, the less molecular that was needed. Of course, at higher pHs, more free is needed to achieve a particular level of molecular, but you may need less molecular than you thought.
Zoran: I would agree with that because producers of malolactic bacteria warn that, if your total SO2 goes over 35 ppm milligrams per liter, you will have a hard time conducting malolactic fermentation.
John: With Chardonnay that you are going to barrel-ferment, a lot of people will just crush, settle, put it into a tank to get fermentation going, put it into the barrel and let it go. They don't add any SO2 because they want it to brown and then have those brown compounds drop out. Then they have a stable color. That has been very popular for a number of years.
Zoran: My opinion is that you should use SO2 as soon as you can, and you shouldn't try to do any extra oxidation to get rid of browning pigments.
Pat: You need to know what you want to do with that fruit long before it comes into the winery. For a stainless steel-fermented Chardonnay from a bird-damaged vineyard, I'd be adding sulfur. If I was looking at clean fruit that was gong to get malolactic and barrel fermentation, I'd consider using no sulfur at all. If I was making sparkling wine out of whole cluster, heavy press Pinot Noir, then I would consider using no SO2 plus oxidizing.
Zoran: Let's mention that this process of forcing oxidation in white juice precipitates the oxidative compounds out of the juice and eventually leads to a more stable, lighter color. It also means you don't have to fine later on. If you do "green" winemaking, which means low input of oxygen, you will be putting SO2 on the grapes early and then again in the press pan. It's not called "green" because it is organic but because of the color of the juice.
John: It's like that with Sauvignon Blanc where the aromatics are the prize, and you don't want those destroyed. Oxidizing could disrupt that because you are trying to preserve as much of the aromatics as you can.
Pat: I think we would all agree that if we were making Riesling, Gewurztraminer or Sauvignon Blanc, we would be looking at sulfur additions to the juice coming out of the press. When you are going into a large tank, you want to be adding small amounts as you go. The alternative is to add to the tank and then fill on top of it, but the last thing you want to do is have the tank full and add it to the top.
Zoran: There's another SO2 use that I need to mention. If we get to the lower range of reducing sugar and we have sluggish fermentation, we look under the microscope; and if we find bacteria present, we might use 20 ppm of SO2 and two pounds per thousand gallons of yeast hulls. Yeast hulls first soak up any toxins and provide sterols for the yeast, and the SO2 immobilizes the bacteria and allows the yeast to finish fermentation.
What would you recommend in the way of SO2 in native yeast fermentations?
John: None. If you are going to add SO2, you kill a lot of the native yeast so why do it?
Pat: It depends on whether they are using true native yeast fermentations or whether they are using leftover inoculated yeast floating in their wineries. You can affect what actually ferments by how much SO2 you add.
John: Ultimately, we know that by the end of a natural fermentation, it's going to be Saccharomyces.
Zoran: In 2003 or 2004 I worked with 100 percent uninoculated whites and reds. I added SO2 according to my normal regimen and had no problem.
John: Then basically you were using your house yeast.
Pat: But a lot of people don't want that. They want the non-Saccharomyces.
Zoran: Well, that's fine, but I would be scared to add no SO2. If I didn't use SO2, then I would be sure to check the juice under the microscope and do a complete analysis. If the pH was high or I had a high level of malic acid, I would acidify to get to a safer pH range because I would be totally afraid of bacillus, especially at high alcohol levels. People are doing crazy things. That's why they are having problems. In the big wineries you just can't afford to take that chance.
Pat: What we are seeing is a lot of home winemakers who turn professional. The first couple of years they are in a relatively clean facility. They do all of this radical, high-risk stuff. They may do fine, but around year four or five they have problems. It's high-risk winemaking, and you need to be trained and know the possible problems before you do it.
Once you have finished wine, what is the purpose of SO2?
John: The main concern in red wine, before malolactic, is total SO2. If it is too high, it seems to inhibit malolactic fermentations. So one of the things people consider, if they are using inoculated fermentations, is how much SO2 does the yeast actually produce. Some yeast can produce up to 200 ppm during fermentation. Most commercial strains finish fermentation at about 10 to 40 ppm. We recommend that you be below 40 ppm total to encourage finished malolactic.
Zoran: I generally don't have problems with finishing malolactic in red wines except for wines with high malic acids. If the malolactic is continuously going and we are not too cold, we won't add SO2.
John: This is a big problem we constantly get calls on. The malolactic started but didn't finish. The temperatures drop when winter comes on and the malolactic stops.
Zoran: Yes, there are two ideas here. We could add like 25 ppm of SO2 to get through the winter safely; and when the spring comes, malolactic will pick up. The other option, if you feel you had sound fruit with little or no spoilage, then you might not add anything; but you have to monitor the wine by taste analysis every two weeks.
John: If the wine cools down, it is hard to get it through malolactic. The cold of winter also retards Brettanomyces, but the concern is that when the wine warms up and malolactic starts to finish, the Brett will take off as well. If someone called me up and said, "What should I do?" I'd tell them that these are their choices. They can add a small amount of SO2, which may increase their risk of finishing, or you can leave it as is which might increase the possibility of undesirable organisms.
Does SO2 usage change with white wines?
John: The danger with white wine is different than with red wine. With white wine your concern is more about oxidation. In red wine it is primarily anti-microbial. With white wine that's going to barrel, I think the concern is oxidation. If it is to over-winter and you can keep the wine cold, I don't think you need to add SO2 if you want to go through malolactic.
Zoran: I totally agree that oxidation is the biggest danger for white wine. It depends on style. If the wine sits on the lees, then the lees will rejuvenate wine and protect it. You just need to keep on stirring, and you don't need to add SO2 until malolactic is finished. You need to monitor the wine closely for off flavors and problems.
John: Remember, bacteria like temperatures between 68° to 72°F. Once you start dropping below that, it makes things much more difficult for the bacteria.
Zoran: It's very hard to convince winemakers to do it; but if you rack the wine from barrel to tanks, where you can better control temperature, it's always successful. In a tank, raise the temperature to 68°F for a week or two, and boom, everything is gone: residual sugar and malolactic acid.
What kind of SO2 additions do you use for white wine that has finished malolactic?
Zoran: If malolactic is finished, before we add SO2, we check for diacetyl by tasting. We might wait a couple of weeks longer because malolactic bacteria will consume the diacetyl. Then we would add 50 ppm.
Pat: You are going to add 50 ppm independent of pH? So you still don't know what your free SO2 is.
Zoran: Correct. This is the first shutdown addition--which we prefer to be larger rather than smaller because experience tells us it is safer, and the total SO2 will be less if you use fewer large additions instead of several smaller ones. Then we revisit free SO2, and we adjust according to the pH.
John: I agree that adding 50 ppm is a good rule of thumb; but then you have to check it to see how much free SO2 you have. From this point on you start looking at molecular SO2 relative to wine pH.
Pat: Red wine is similar. We add 50 ppm and then check the free SO2. From then on we make regular additions to maintain the molecular SO2 relative to pH.
How do you add SO2 to large amounts of wine?
Pat: Ideally you would have some sort of metering system. Tank to tank would be easier; but if you are not racking, then you need to get it moving. You can't just dump it on the top and try to mix it. That just doesn't work.
Zoran: You can have a fitting with a little hose going into it for a venturi effect. That would be ideal. And you should never make an SO2 addition with any other chemical. You have to add separately because it might react immediately with other compounds. Usually we use 6 percent solution for smaller additions. For larger additions to large tanks we use compressed gas.
John: I've been to wineries where you use the liquid coming from the compressed gas. I've also been places where they weigh the metabisulfite out and mix it up to make the addition. Then you have Efferbaktol and other granular effervescent SO2 brands that are very handy for barrels because they are premeasured.
Pat: Basically, you have two options: one is the compressed gas, and the other is potassium metabisulfite in its various forms. From either of those two sources you can make a 4 or 5 or 6 percent liquid solution that can be used with routine ease.
John: Then the issue becomes how soon after that addition you measure the free SO2. It binds quickly; but in general we ask the clients to check the next day unless there is some urgency.
Zoran: If it's just for aging purposes, I would wait for the next day; but if it is for a bottling tank, you'll be checking right away.
Tell me about SO2 at bottling.
Pat: This is getting a lot of attention because of our new closure options. Certain closure manufacturers are recommending that winemakers pay particular attention to SO2 at bottling. They recommend lower levels with screw caps and higher with other closures.
Zoran: You have to be careful with wines going into screw caps. Any sulfide issues need to be dealt with sooner rather than later. With whites we go with free SO2 at 25 to 30 ppm, depending on pH, of course. Red wines need 35 to 40 ppm SO2 because of higher pH.
Pat: It is very hard to determine what the level should be at bottling. We hear about total values and free values, but we are not hearing about molecular. There are recent indications that we should be.
John: But realistically speaking, a winery will not add beyond a certain amount of free because it becomes a sensory issue. For the most part, people try to bottle between 20 and 40 ppm free SO2. It depends on whether you are adding for anti-oxidant or anti-microbial reasons.
Pat: I think it is mostly anti-oxidant by the time you are bottling. What we have learned from most of these closure trials is that total SO2 goes down after bottling, but I don't think we know yet how to quantify the correct amount at bottling.
How do you keep workers safe with SO2?
Pat: Extreme injury can occur. Permanent eye damage or sinus damage. Permanent asthma. This is the reason we have had to reclassify compressed gas when used for sanitation. It now requires a certified applicator's permit.
Zoran: Well, the first thing, before SO2 gets used by anybody in the winery, is to have training. They take a test and have to sign off that they have understanding of the whole thing and the equipment that has to be used: mask, goggles, etc.
Pat: The difference is that California actually has injury and illness prevention plans required for businesses. That is not the case for other states. So in California, for any situation like this, we have to have training, documentation, sign-off and a policy and a plan for anything that is hazardous.
Can you make wine without using SO2?
All: Yes, yeah, of course.
John: I work with a lot of people who make non-SO2 wines, but it's a challenge. Keeping pHs low is always a help. I find that with non-SO2 wines you are much more likely to have mousey wines. Lactic acid bacteria, lactobacillus, in particular, can cause that.
Zoran: It is possible to make wine without SO2; but first of all you have to have good sanitation practices. You would like to have fresh, sound fruit with lower level pHs: below 3.6 for reds and 3.4 for whites.
Pat: You can do it. It's going to take a huge amount of effort. The quality of wine is not likely to be as commercially acceptable as wine made with a judicious use of SO2.
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Scientists are just not normal people, even winemaking scientists. Theirs is a world of the scientific method, of trials, theories and proofs. Their search for an absolute truth makes them powerfully aware that tiny inaccuracies can throw off an entire research project. So when you ask a scientist a question, no matter how innocent, the answer is always loaded with caveats, qualifications and prerequisites.
When you realize that wine is very complex with dozens of compounds continuously interacting with one another and that all those interactions are volatile and affected by alcohol, then it is hard to make solid, irrefutable predictions. Still, we know that sulfur dioxide (SO2) works. It is an effective anti-microbial and anti-oxidant agent. It is relatively safe, and we have a lot of experience with it.
Exactly how SO2 does what it does is another matter. The basics are that if you use SO2, then you better monitor its levels in your wine. Proper levels of SO2 (measured as total) during fermentation control unwanted yeast and bacteria but can retard finishing some primary and malolactic fermentations when they get too high. Proper levels (measured as molecular free) during aging can protect your wine from unwanted bacteria. Correct (but debatable) amounts of SO2 at bottling can be wonderful anti-oxidants.
At this roundtable, I learned that even after decades of use, SO2 remains somewhat of a mystery. Winemakers need to be aware of whether they are using SO2 for its anti-microbial or its anti-oxidant properties because it makes a big difference in how much you need to add. Eschew SO2 and you can still make good wine, but the risk factor goes way up. It behooves winemakers to stay current on research, tests and literature, and it helps to make your decisions about SO2 carefully. wbm
