This article should have been posted for Halloween. That’s why you see “trick or treat?” on the title. I’m running a little late on my schedule… but I hope you enjoy reading it as much as I did working on it.
Sulfur dioxide (SO2) is a natural by-product of winemaking as a small quantity is produced during the alcoholic fermentation by yeasts. Its use goes back to the end of the 18th century and, due to its antiseptic and antioxidant properties, it makes it almost impossible for the winemaker to avoid adding it during the several processes involved in making and storing wine. It is one of the most efficient additives used in winemaking; however, due to health concerns and a recent growing consumer interest in wines with less additives, its use has been a fertile field of discussion in many different segments of the wine industry. Also, the Organization of Vine and Wine has been reducing the maximum legal amount of SO2 present in wines, so there’s a very important legal issue related with its use. This paper will cover both the advantages and drawbacks of the addition of SO2 in winemaking, as well as some possibilities of eliminating or reducing its use.
The use of SO2 in winemaking is a complex subject and there’s a need to understand the factors involved in its efficacy (or the opposite) before anything else is discussed. Its antimicrobial and antioxidant efficiency in wine depends on a wide range of factors, but the most important ones are: pH, residual sugar (RS), and ethanol content. What it means is that these factors will determine the amount of SO2 that needs to added to determine a final safe level of free SO2 in the wine (10-20 mg/L) in accordance to the following basic rules: the lower the pH, the less SO2 needs to be added; the higher the alcohol content, the less SO2; and the lower the RS, the less SO2 needed. Acid and alcohol make the environment more hostile to bacteria, while the presence of sugar has the opposite effect. Temperature is also a very important factor: the lower the temperature, the better the efficacy of SO2 and the slower the oxidation of the wine. Worth of note is the fact that now it is known that at high pHs and low concentrations SO2 just acts as a fungistatic, and at low pHs and high concentrations it is a fungicide. A better understanding of the chemistry of SO2 in recent years, combined with an increased hygiene in the wineries, has enabled winemakers to substantially reduce the SO2 concentrations used during the vinification process. Salvo Foti is making wine in the slopes of Mount Etna in Sicily with no addition of SO2 and he has become a “rock’n’roll star” for people who are adepts of the so called “natural wine movement”, but, in fact, even if some wines are interesting, some of them are clearly faulty (premature oxidation has occurred) and there’s huge bottle variation.
There are multiple advantages for the use SO2 in winemaking. From the moment the grapes are harvested until the moment the bottle of wine is opened and it is finally consumed, SO2 has very important properties. It is antiseptic and antimicrobial (molecular SO2) as it inhibits the growth of all types of microorganisms, such as yeasts and bacteria. In practice what it does is to chemically help the winemaker to inhibit bacterial spoilage (more importantly on lactic acid bacteria, but also, to a lesser extent, on acetic acid bacteria), Brettanomyces, Zygosaccharomyces, and mycodermic yeasts (flor yeasts) growth, prevention of yeast haze formation, unwanted malolactic fermentation, and secondary fermentation of wines that contain residual sugar (dessert white wines mostly). Before fermentation, SO2 (and its antioxidasic properties) protects must from oxidation; at low concentrations it even stimulates fermentations, making them complete to dryness more rapidly than otherwise in most cases; it helps with yeast selection (Apiculated yeasts, which tend to produce wines of lesser quality and lower alcohol levels, are more sensitive to SO2); it favors the dissolution of organic acids, minerals and the so important (in red winemaking) amalgamation of tannins and anthocyanins to the wine; SO2 also binds with ethanal and other similar chemical compounds (such as ketonic acids) helping to avoid a possible perception of a wine being “flat”; and some specific thiols (or mercaptans) can be beneficial in determining the characteristic aromas of grape varieties such as Sauvignon Blanc and Riesling.
As an antioxidant, it works because it binds with dissolved oxygen and it is much more important during wine storage than during winemaking. What it means is that it can protect the wine from chemical oxidation, but it doesn’t avoid enzymatic oxidation. It is also a helping tool to the establishment of a low oxidation-reduction potential and wine aroma and taste development during the aging process is heavily dependent on it. It also reduces the rate phenolic polymerization that causes color loss observed during the aging process. Not to mention that when a wine is aged in a barrel, it may not be sufficient to add SO2 to the wine, but rather a disinfection of the barrel with SO2 gas (or burning sulfur) is necessary to prevent the growth of Brettanomyces. Transportation is a key issue as well: if the wine is going to travel long distances and be exposed to drastic temperature variations, SO2 is essential to save the wine from premature oxidation.
So what are the drawbacks or issues with the use of SO2 in winemaking? Of course, first and foremost, excessive doses must be avoided, mostly because of health reasons, but also because it can have a negative impact on aromas and flavors (pungent and sulphorous). It can also be illegal in most countries to be over a certain limit of total SO2 in a specific wine (red, white, sweet, etc). However, the issue with SO2 is not about only excessive additions of it, even low doses of SO2 added during a pre-fermentation skin contact promote the extraction of protein. This is not good, especially in white winemaking, as a larger quantity of bentonite will be needed to stabilize the wine. This may cause a negative impact in the organoleptic properties of the wine (mainly aroma loss).
There are many methods available today to minimize the use of SO2 in winemaking today. Some of these possibilities would complement the use of SO2, as completely replacing it seems to be a lot harder. The adequate treatments can be chemical or physical: On the chemical side, phenolic compounds also have the antiseptic and antioxidant properties that SO2 has. It even provides a better sensorial perception than wines made with SO2 additions. However, a very high concentration of these compounds are needed in order to achieve its antiseptic properties. Another one is Dimethyl dicarbonate (DMDC). It’s been proposed to replace SO2 in winemaking. It is very effective against yeasts (killing them), but not so much against bacteria. Also, lysozyme is a very effective to inhibit bacterial development (especially Oenococcus spp). On the physical side ultrasound has been considered as a promising alternative to conventional thermal treatments. It has a very high antiseptic effect, as well as a good ability to preserve wines. Ultraviolet radiation can inactivate both yeasts and bacteria. However, it seems to be much more effective in white wines than in reds, as the phenolic compounds present in reds seem to absorb the radiation. Pulsed electric fields increase the permeability of cell membranes. It makes it both effective to destroy yeasts and bacteria, as well as a good tool to help with the extraction of the phenolic compounds present it wine.
The main issue about the use of SO2 today, however, doesn’t seem to be about the health and/or quality problems mentioned above. Most of the discussions are about the consumer desire to acquire “natural products” and the addition of SO2 in wine is perceived as something “unnatural” (an addition is viewed by some segments of the market as an interventionist technique to make wine). In other words, a winery may attract an audience (and consumption) for a “natural product” by avoiding the use of SO2. It seems that the use (or not) of SO2, falls under this category: A philosophical decision. But the truth is: unless you suffer from some serious allergy from SO2 (such as the one asthmatics may have), the use of it cannot be considered a “trick” as the disadvantages of using it are clearly related (and limited) to its excessive use. If all the advantages are considered, it’s clear to see that the proper additions of SO2 are a real “treat” to people who love wine. Without it, we wouldn’t have such a great diversity of high quality wines that we enjoy today.
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I believe some of you will agree with my arguments… but some others won’t… so I would love to hear your thoughts on it! 🙂
Cheers,
Luiz Alberto, #winelover
Luiz – congratulations on a comprehensive look at the issues surrounding SO2, the most important additive in wine.
First I want to pick you up on one word that I don’t believe is technically correct and may cause confusion, though I admit my formal studies are long behind me, although in wine as we all know you learn something knew every day. The paragraph starting “As an oxidant, it works because it binds with dissolved oxygen…” surely you mean “As an anti-oxidant”?
Secondly, I just want to add to a couple of things you mentioned briefly. Not only has the quantity of SO2 used in wine reduced due to legal limits being lowered, it has been reduced willingly by most good winemakers all over the world. In a few cases the levels have been reduced perilously low and there is discussion as to whether, for example, this may have been one of the causes for the Premox issues in white Burgundies. But, the absolutely no. 1 reason that SO2 levels have been able to be reduced over the past 20 years has been improved hygiene in wineries, something you touched upon, but that is not discussed enough. When Australian and other New World winemakers started consulting and/or working harvest in France and the rest of Europe, and some younger French and other European winemakers went over there to work a vintage, hygiene – and notably the use of masses of water in wineries – started to improve dramatically in Europe, which 20+ years ago had a lot of filthy wineries! It is primarily this that has allowed producers to use lower SO2 levels than before, as better hygiene in brief translates as lower risk of bacterial spoilage.
Last harvest I spent a day with a small Jura winery that uses no SO2 unless it absolutely has to pre-bottling and shipment (see http://jurawine.co.uk/2012/11/15/sorting-the-ladys-trousseau/) and I was struck by how absolutely immaculate everything was in the grape-receiving area. These are some of the few no-SO2 wines I really enjoy. A mutual friend of ours spent 2011 harvest with a natural wine producer and was struck by how filthy everything was – needless to say she discovered that she didn’t like her employers wines!
As someone who doctors technically refer to as asthmatic (i.e. I was a bad asthmatic as a child and although I grew out of the worst it never goes completely), I can say that only a tiny number of asthmatics are affected by SO2 in correctly used low doses, and that it exists in far more food packaging than most wine drinkers could ever possibly imagine. In general I believe judiciously used it’s a good thing. But, I have become more open to no-SO2 wines with the experience of tasting some excellent examples, but remain convinced that this is a route that only suits certain grapes and certain styles of wines, and only then when used with extreme caution and with the use of lots of water in the winery!
Thank you so much for taking the time Wink! Yes, you’re correct. Not doubt I meant “antioxidant”… Funny that when I wrote it previously on a word file it’s correct… Anyway, it’s fixed!
And yes, I agree that hygiene in the winery is a critical issue in this discussion. Thanks again!
Cheers, Luiz
Thank you for this very interesting article about the use of SO2.
Hey Luiz, this is a subject dear to my heart.
I think there are some important slants that you don’t cover:
1. the different stages during winemaking when So2 could be used – or not used (Wink does of course hint at this, but I am ramming it home) . In conventional modern winemaking there are of course 3 main points when So2 is added: first, just after the grapes are picked, to stop any premature oxidation, and to kill off any natural yeasts, so that the chosen cultured yeast can be used to inoculate the grapes. 2nd, after fermentation, and before malolactic. And third, at bottling time, to ensure that there is no possibility of refermentation or any unwelcome bacteria entering the final wine.
The important point is that many so called “natural” winemakers dispense with the 1st and 2nd uses, but the 3rd is much harder to do without. And it is generally accepted that if you want your wine to stand any chance of being durable and stable, you will need to use a small amount of So2 at bottling time.
2. It’s worth mentioning that it’s far harder to dispense with SO2 use in white wines, than in red, because the tannins and anthocyanins (from the skins) in red wine act as a natural preservative. Hence also why Georgian Qvevri wines are such an interesting example – because even the white wines, made with skin contact, have some of this natural preservative, meaning that So2 usage can be minimised – although note that even Alaverdi Monastery use some So2 in their white wines. Not a problem, as only the red wines are offered up to god (and thus cannot contain *any* additive of any sort).
Simon
Hi Simon, thanks for taking the time.
We have been to 2 trips (Etna and Georgia) together last year when “natural wine” was a big part of our program, so I know exactly the way you feel about it… I can’t say I’m passionate about natural wines but, as you could see in Georgia, I can really love those amazing qvevri wines! However, I have a huge problem when the wines are faulty… then I could care less if the wine is natural or not. The first obligation of a wine is too be good! And I’m sure you remember when I asked you to read your tasting note after we had the white wine from Salvo… and you said “a little oxidized”, didn’t you? And that’s a young wine that hadn’t even left the winery! Imagine if that wine had been transported here to the US and stayed in a shelf for a few months… how do you think it was going to show? It would be another faulty wine… and I can’t stand those… I’m a #winelover for heaven’s sake! 🙂
Anyway, thanks for adding these 2 very important points to the discussion. Much appreciated!
Cheers,
Luiz
“As an antioxidant, it works because it binds with dissolved oxygen” – do some research please…”The direct reaction of SO2 with oxygen appears to be insignificant in wine. Oxygen reacts with polyphenols before it can be removed by SO2. Instead, the main antioxidant role of SO2 appears to be its reaction with hydrogen peroxide (H2O2) [Danilewicz, 2003]”
Hi Aaron. Thanks for taking the time. I did my research before writing this paper… “Some researchers are of the opinion that SO2 reacts with oxygen directly and so protects polyphenols and other wine constituents from oxidation (Ribéreau-Gayon etal. 2000, Clarke and Bakker 2004).
Does that meet your demand?
Cheers,
LA