Let me cut to the chase and provide dessert before the entrée: unless you are asthmatic or one of the 1% club of people who have a bone fide sulfite allergy, it is highly unlikely that sulfites are the cause of your hangover or any other ill effects from drinking wine.
I will make this hypothesis: you like high alcohol wines, you perhaps drink too much wine in one sitting, and/or you tend to mix types of alcohol (including types of wine) in the course of an evening or event. Gotcha! Those choices are the real culprits. Oh yes, and if you think that red wines cause extra problems because of sulfites, you should know that they tend to have lower levels than white or sweet wines. And your theory about European wines having fewer sulfites because you can drink them without ill effects? As a general notion, it’s just not true. Wine made in Tuscany and sold in Italy is the exact same wine that is exported to the U.S. There’s only one difference in the U.S.: the bottle has the required warning “Contains Sulfites” on the back label.
Intrigued? Read on.
This is a complex topic, and I am keen to help simplify it. I cannot be the only reasonably well-educated wine-lover to get confused by the differences among sulfur, sulfur dioxide and sulfites, or about their health effects. Do they matter in the winemaking process? Sure, but why and how? Why is it that sulfites get blamed for many maladies and keep otherwise enthusiastic sippers away from vino?
For wine enthusiasts not terribly intrigued by the chemistry of wine, this analysis may be a bit of a rat-hole. But let’s level-set the chemistry backdrop.
Sulfur (or sulphur) is the tenth most abundant chemical element in the universe. Usually found in salt deposits, it is pale yellow, odorless and brittle. Because it existed naturally near volcanoes such as Pompeii, sulfur was known and used as a preservative in the winemaking process in ancient Roman times, but it was not officially named a chemical element until the 1770s.
Sulfur dioxide (SO2) forms when fossil fuels (e.g., coal, oil) are burned or sulfur-rich mineral ores (e.g., copper, zinc, lead, iron) are smelted. It can be deadly in high concentrations. Commercial SO2 is typically used as a bleaching agent, solvent, disinfectant and refrigerant. SO2 is also used to preserve fresh foods and as a preservative ingredient in processed foods such as jam, soda, canned soups and dried fruit.
Sulfite is a class of sulfur compounds. Because SO2 is one of the best known sulfur compounds, the terms “sulfur dioxide” and “sulfite” tend to be used interchangeably (and thus constitute the main source of consumer confusion).
To simplify things for the balance of this article, I am going to use primarily the compound name SO2 rather than the class name of sulfites. Please hang on to the truth that they are often used interchangeably (and at times incorrectly) in many references as one and the same.
Where does SO2 come from? Two sources: yeast and the winemaker. Some yeast is wild. It may be found in the vineyard on grapes and in the winemaking facility. Most commercial strains of yeast produce 10-30 ppm of SO2 during the normal process of alcoholic fermentation.
Why is SO2 added during the winemaking process? SO2 is considered to be the most important additive used in making wine. As author Jamie Goode describes it in the second edition of his book Wine Science, sulfur dioxide is the “chemical custodian” of wine. (I highly recommend this book as an excellent resource for both consumers and professionals, as it is written in a very approachable style.) SO2 acts a preservative in wine just as it does in solid food. It is both antimicrobial (kills bacteria and fungi) and antioxidant (inhibits the damaging effects of oxidation). The degree to which SO2 exists in finished wine is expressed as “parts per million” or ppm for short. (This is the exactly same liquid measure as milligrams per liter, or mg/l.)
Wine is inherently volatile due to many changes that occur from grape to glass. It can easily spoil from bacteria, turn to vinegar, or develop other faults during fermentation and aging. In Roman times, sulfur-based candles were burned in amphorae (aging vessels). This unnamed discovery stabilized and preserved the wines which were so popular two millennia ago. In modern times, winemakers employ a similar burning process, using SO2 to clean barrels. Sulfur dioxide is increasingly used to augment good cellar practices as part of a winery’s “housekeeping” regime to clean hoses, fermentation tanks, valves and other processing hardware.
When is SO2 added? SO2 is often added to protect and preserve the wine’s character, flavor and color. Among other things, it inhibits enzymes that cause browning, controls the balance of bacteria, controls wild yeasts from growing, helps to extract pigment making red wines “redder,” and prevents secondary fermentation in the bottle. The winemaker’s challenge is to use SO2 judiciously.
Sulfur dioxide can be added at four main stages in the winemaking process depending on the condition of the grapes and desired style of the finished wine:
- Arrival of fresh grapes to control wild yeasts and/or to protect the berries from disease as the surface is broken in the process of destemming and crushing the grapes.
- Beginning of alcoholic fermentation to prevent browning from oxidation and premature malolactic fermentation (MLF) for fruit-forward wine styles that are not intended to be barrel-aged.
- After MLF to achieve certain wine styles, and to preserve and stabilize the wine for barrel aging.
- Prior to bottling to support bottle aging.
How much SO2 is added, and why? What factors influence this decision? Two main factors influence how much SO2 should be added at any stage in the winemaking process to achieve antimicrobial and antioxidant goals. The proper conventional dose of SO2 is determined by a technical formula that maximizes the effects of SO2 in the context of alcohol by volume (abv) and pH level.
Alcohol by volume (abv) is the total percentage of ethanol (the type of alcohol in wine) expected in the finished wine. This measure matters because the higher the abv, the less SO2 protection is required. There is no longer a threat of secondary fermentation in the barrel or bottle, and ethanol enhances the bacteria-killing effects of SO2.
The pH scale, which ranges from 0-14, measures acidity (0-6) vs. alkalinity (8-14). Seven is considered neutral. The higher the pH, the more SO2 needs to be added to prevent MLF, Brettanomyces or other bacteria. Acidity (low pH) helps to keep those things under control.
While it may seem counterintuitive, for a quick mnemonic, remember these ratios. It is these ratios that matter.
- higher pH = lower acidity (which needs more added SO2)
- lower pH = higher acidity (needs less added SO2)
- higher alcohol (and drier wine) = less need for SO2
- lower alcohol (and sweeter wine) = more need for SO2
White wines tend to be higher in acidity than red wines, so applying this ratio, they should require less SO2. But that’s not usually the case. They actually require more SO2 because they are prone to oxidation. The more color in the grape pigment, and the longer the period of maceration (contact with skins and stems), the less added SO2 is needed. In most cases, red wines do not even need to have SO2 added because they contain these natural antioxidants, but conventional winemakers may still add it as a precaution. Additionally, higher pH wines (alkaline) made from very ripe red fruit to enhance youthful drinkability are more susceptible to bacteria, thus may require excessively high levels of added SO2. Wines with higher levels of residual sugar need more SO2 to prevent secondary fermentation. The typical order from most-to-least use of SO2 is as follows: sweet white dessert wines, blush and semi-sweet white wines, dry white wines, and dry red wines.
What happens to SO2 during the winemaking process?
The basic formula for sulfur dioxide in wine is this: total SO2 = bound SO2+ free SO2.
When SO2 is added to wine, it is initially “free” to serve intended antimicrobial and antioxidant purposes. Over time, a portion of the SO2 binds (or dissolves) with other components of the wine and is becomes inactive, undetectable to most people except the small percentage who are genuinely sensitive. Some proportion of the SO2 evaporates as gas, but the unbound portion remains “free,” continuing to work as an antimicrobial and antioxidant agent. Of the total SO2 in finished wine, 50-90% tends to be bound. The remaining 10-50% is active, or free. It is the only portion of the original dose of SO2 still actively working in the wine, and is thus the component most likely to have a negative impact on sulfite-sensitive wine drinkers.
Labeling Laws and Consumer Protection
It’s a conundrum. Labels are frankly not helpful. But how would a consumer know any of this?
All bottles of wine sold in the U.S. with more than 10 ppm of SO2, regardless of country of origin, must be labeled “Contains Sulfites.” By now you should be quite clear that this information borders on irrelevance. All wines will have at least this level of SO2, whether naturally from yeasts or added by winemakers. It simply isn’t a helpful warning.
In the United States, the Food & Drug Administration discovered nearly 30 years ago that about 1% of the population is severely allergic to sulfites, and that about 5-10% of people who have asthma are prone to having adverse effects from sulfites. This prompted the health warning label “Contains Sulfites” starting in 1987.
Why you may ask? Other countries do not do this. For example, SO2 limits vary in the European Union according to type of wine, from 160 ppm for red wine to 400 ppm for sweet wine, but the EU does not require this information to be disclosed on bottles sold/purchased in the EU.
My recommendation is to create a worldwide standard for sulfite content and labeling requirements that actually achieve the intended goal of health protection and education for the consumer. The warning “Contain Sulfites” is neutered by so many misunderstandings of its meaning.
The Bottom Line
If you are one of the rare people with a known sulfite allergy, or if you suffer from asthma plus sulfite sensitivities, please be careful. If you are not certain, do check with a physician who understands the nuances of this type of allergy. Sulfite reactions induce symptoms such as hives, itching, swelling, nausea, diarrhea and low blood pressure. Headaches and hangovers are another matter.
Choosing your wines carefully is another option. If you are particularly sensitive, look for “no sulfites added” on the label. Know your winemaker and his/her practices. (Ask if you aren’t sure – many winemakers can tell you the amount of sulfur dioxide added or the total ppm of SO2 in the finished wine.) Organic wines typically limit SO2 to 110 ppm or less, a good option for people with sulfite sensitivities. Biodynamic wines go a step further, typically building on organic farming and winemaking practices. This class of “natural” wines is an interesting subject unto itself.
And get to know your wine merchant. There’s no substitute for good advice in buying wine.
For everybody else? Don’t drink too much, choose wines that have a low-to-moderate alcohol level, and don’t mix too many different types of alcohol. Don’t buy cheap wine that may have been heavily processed unless you know how the wine was made. If SO2 is an aromatic annoyance, decant and chill the wine before serving. Wines served at warmer temperatures tend to release their free SO2 compounds, which can also be mitigated by chilling and decanting.
In Pursuit of More Knowledge
For another angle on this topic, please read the Vino Ventures blog post from July 2013 “I Can’t Drink Wine. It Gives Me a Headache.” In coming weeks, there will be more posts about natural wine trends, an exploration of various types of wood and toasts used in barrels for aging, and other influences on the overall wine experience.
The references used for this post are primarily secondary based on quality original source material.
- Butzke, Christian. “Use of SO2 in High-pH Wines.” Purdue University Extension. March 2010.
- Good, Jamie. Wine Science: The Application of Science in Winemaking, Second Edition. Octopus Publishing Group Ltd (Mitchell Beazley), 2014.
- Henderson, Pat. “Sulfur Dioxide: Science Behind this Anti-microbial, Anti-oxidant, Wine Additive.” Practical Winery & Vineyard Journal. Jan/Feb 2009.
- Jancou, Pierre. “Sulphites in wine.” www.morethanorganic.com. Undated.
- Miller, Mike. “How SO2 and pH are Linked.” Acuvin.com. Undated.
- Robinson, Jancis et al. Oxford Companion to Wine.
- www.winefolly.com. “The Bottom Line on Sulfites in Wine.” January 15, 2014.
Photo credits: www.practicalwinery.com; www.savagechickens.com; www.nzqa.govt.nz