Measurement Of Sulfur Dioxide In Wine
This page summarises the procedures and equipment needed for two commonly used techniques for measuring; sulfur dioxide in wines . Automated systems are available for SO2 analysis and offer considerable benefits to laboratories that routinely analyse relative large numbers of samples.
Description: Sulfur dioxide is sparged from an acidified wine sample in an air stream and trapped in a solution of hydrogen peroxide which oxidises the sulfur dioxide to sulfuric acid. The sulfuric acid formed is then titrated with standardised sodium hydroxide, and the amount used is proportional to the amount of sulfur dioxide in the wine. Total SO2 is determined by heating an acidified sample during the aspiration step .
Equipment: 100 mL round bottom flasks, 2-necked pear-shaped flasks, condensers, retort stands, bunsen burners, flasks, buretteReagents: Phosphoric acid solution, standardised sodium hydroxide solutionServices: Water supply, sink, natural gas supply, compressed airSpace required: Bench space
Equipment: Flasks, buretteReagents: Sulfuric acid solution, Standardised iodine solutionServices: Wash-up area
References and further reading
Understanding Sulfur Levels In Wine
Back in Ancient Rome, winemakers would burn candles made of sulfur in empty wine containers to keep the wines from turning to vinegar. Winemakers began using sulfur in the winemaking process during the early 1900s to stop bacteria and other yeasts from growing .
Sulfur dioxide or SO2 is also used everywhere in the food industry, as it is a proven way to protect perishable items for oxidation. The chart below, from Wine Folly, shows the levels present in many of the types of food we consume every day.
Wine has fewer ingredients than a pot of Beef Bourguignon, cioppino, paella or gumbo. An ingredients label for wine might simply read: grape juice, yeast and sulfur dioxide. This last ingredient however is very complex and tricky to use.
Sulfur dioxide is by far the most important additive used in wine. Many times it is the only additive. Its value derives from its ability to perform several crucial functions. It preserves wines freshness and fruit characters by virtue of antioxidant, antimicrobial and anti-enzymatic properties. There is no other additive available to winemakers that can single-handedly play all of these roles. SO2 is required to make high-quality wines that can stand the test of time and more importantly your cellar.
When SO2 has already reacted and becomes unavailable for any other functions, it is considered bound.
Free Sulfur or FSO2
Bottling SO2 Addition
Some of our bottling analysis:
The Essential Guide To Measuring So2 In Wine
The;Hanna Instruments team doesn’t just want to sell you great instrumentation; we want to work with you to make your product the best it can be. That’s why we’re constantly creating resources for our customers in all industries.
Our Measuring;SO2;in Wine;eBook is designed as a resource for winemakers who need to put an effective SO2 testing program in place. It;covers how SO2 affects wine quality, the necessary tools needed to measure SO2 levels, and how to use these tools to get accurate results.
Here is a brief recap of the three sections included in the eBook.
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Control Dissolved Oxygen For Distinct Quality
Get consistently accurate dissolved oxygen measurements to achieve the right balance in your wine.
Whether it’s reduced or alternatively oxidized, the taste of wine can be dramatically affected by oxygen. Monitor DO for timely and effective micro-ox implementations and to ensure consistent process control and operation of equipment.
How To Use A Titret Kit
If you have too little sulfur dioxide in your wine, the wine will age prematurely. If you have too much, you will notice a salt like flavor in your wine. Using a titret kit will help you determine your levels of SO2 in your wine.
How to Measure Sulfite in Wine Titrets®
Once you have your parts per million, youll know where you stand. Ideally, most wines will have a free SO2 reading between 50 70 ppm.
Campden tablets: 1 tablet / gallon of wine yields 50 ppm SO2
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Use And Measurement Of Sulfur Dioxide In Wine
Sulfur dioxide is a gas under normal conditions. It is very soluble in water .
It has been used in wine making since early Egyptian and Roman times.
Reasons for its use include:
- It is an antioxidant .
- Keeps wine fresher.
- Can inhibit some yeast .
- Inhibits acetic bacteria .
- Inhibits lactic bacteria
- It helps stabilize color in red wines.
- It allows longer storage of wine.
- Not only used directly in wine but also in disinfecting equipment
- Nitrogen levels may be better preserved for wine yeast that otherwise would be competed for with wild yeast strains.
Some General Points to Consider:
- Because of the above information, it is our belief that by using sulfur dioxide, better wine will be made.
- Levels of SO2 are measured in milligrams per liter which is the same as saying parts per million . Example: 45 mg/l = 45 ppm.
- Use is somewhat more important in white wines as they have less of other antioxidants than do red wines.
- The Food and Drug Administration cautions that there may be potential health problems for people with asthma. TTB regulations say any wine with a level greater than 10 mg/l must say contains sulfites on the label. The maximum level allowed is 350 mg/l. That is way above a level needed. Keep in mind that many yeast strains produce levels that exceed 10 ppm.
- There are considerable differences amongst people as to their threshold for detection of sulfur dioxide.
Two forms of SO2 will develop when added to wine. The two forms add up to Total SO2
Why Adding Potassium Metabisulfite Is Complicated
With most wine additives you just measure out a specific amount to treat whatever volume of wine you have on hand. Not so with sulfites additives like potassium metabisulfite and campden tablets.
The first thing to understand about sulfites is that they bind with other things in your wine. They bind with micro-organisms, oxygen, solids, yeast, acids, bacteria, and sugars.
When this chemical bond happens the sulfite goes from being free to bound. Bound sulfite has already done its job and while it is still in your wine it is not free to bind with anything else. Thus we have to different sulfite levels to worry about, free and total.
Free sulfites are unbound sulfur dioxide molecules that are available to bind with the bad guys to keep your wine safe. Total sulfites is a measurement of free sulfites and sulfites that have already chemically bonded with something in your wine.
As winemakers we want to know that our wine is protected against the many things that can spoil it. Protection comes only from free sulfites. Thus we need to know how much we have in our wine already that is free and how much free sulfites we would like to have.
When adding sulfites to wine, usually in the form of potassium metabisulfite, some of it will become bound while the rest will remain free. You cant predict how much will become bound so youve got to add potassium metabisulfite, test it, then adjust as necessary.
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Sulfur Dioxide Measurement And Overestimation In Red Wine
Jenny Savits, Enology Field Specialist
Sulfur dioxide is used in winemaking as a preservative to prevent oxidation and microbial spoilage. It exists in three forms; bisulfite , molecular SO2, and sulfite . The equilibrium is pH dependent with the predominate form at wine pH being bisulfite. Most of the rest is molecular and very little, if any, remains in sulfite form. These forms make up what is termed as free SO2. Free SO2 can be lost through volatilization or binding, thus management is important. A target of 0.6-0.8 mg/L molecular SO2 has been deemed sufficient to keep wine protected. Charts are commonly available to relate the desired free SO2 level to a given pH of the wine to hit the target molecular SO2.
When bisulfite forms binds with other molecules in wine , this is termed bound SO2. ;Once bound, the SO2 is no longer available to protect the wine. The total SO2 is achieved by adding the free and bound, and has a legal limit set by the TTB of 350 mg/L .
Coelho JM, Howe, PA, and Sacks GL. 2015. A Headspace Gas Detection Tube Method to Measure SO2 in Wine without Disruption SO2 Equilibria. Am. J. Enol. Vitic. 66:257- 265. DOI: 10.5344/ajev.2015.14125
Howe PA, Worobo R, and Sacks GL. 2018. Conventional Measurements of Sulfur Dioxide in Red Wine Overestimate SO2 Antimicrobial Activity. Am. J. Enol. Vitic. 69:210-220. DOI:10.5344/ajev.2018.17037
S For Testing Free So2 In Wine
When you add sulfite to wine, sulfur dioxide ionizes to the sulfite ion, SO3, and bisulfite ion, HSO3. A small fraction remains in the molecular form, SO2. It is this molecular form that protects the wine from spoilage organisms and oxidation. As sulfite reacts with other wine components, it becomes bound to them and is no longer available to participate in producing molecular sulfite.
We cannot measure molecular sulfite directly. Rather, we measure free sulfite, and use a table of wine pH values to predict the amount of molecular sulfite we will achieve. This is why it is so important to frequently measure your free sulfite. No matter how high your total sulfite , it is only the free sulfite number that really counts. Dont just guess and toss some sulfite in. Analyze it first, then add it. To this end, we offer some advice on ways to keep up with testing your SO2.
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Use And Labeling Regulations
In 1986, the U.S. Food and Drug Administration banned the use of sulfites as preservatives on foods intended to be eaten fresh . This has contributed to the increased use of erythorbic acid and its salts as preservatives.
Generally, U.S. labeling regulations do not require products to indicate the presence of sulfites in foods unless it is added specifically as a preservative; still, many companies voluntarily label sulfite-containing foods. Sulfites used in food processing are required to be listed if they are not incidental additives ), and if there are more than 10 ppm in the finished product )
Products likely to contain sulfites at less than 10 ppm do not require ingredients labels, and the presence of sulfites usually is undisclosed.
In Australia and New Zealand, sulfites must be declared in the statement of ingredients when present in packaged foods in concentrations of 10;mg/kg or more as an ingredient; or as an ingredient of a compound ingredient; or as a food additive or component of a food additive; or as a processing aid or component of a processing aid.
Sulfites that can be added to foods in Canada are potassium bisulfite, potassium metabisulfite, sodium bisulfite, sodium dithionite, sodium metabisulfite, sodium sulfite, sulfur dioxide and sulfurous acid. These can also be declared using the common names sulfites, sulfates, sulfiting agents.
In the European Union, “EU law requires food labels to indicate contains sulfites without specifying the amount”.
Managing Sulfites In Wine
Sulfur Dioxide, also known as “sulfite” or SO2 is a commonly misrepresented, yet critical component of wine. In the United States, wineries are required to use the statement “Contains Sulfites” on the label which has caused somewhat of a hysteria over the substance. SO2 occurs as a natural bi-product of alcoholic fermentation by yeast, whether in nature or in the winery. The human body can produce sulfite at a rate of about 1g per day. You are probably familiar with the smell of sulfite if you have ever lit a match, as SO2 is responsible for the characteristic smell. The amount of SO2 found in a glass of wine is extremely low at around 0.005 to 0.010 grams, while the concentration in dried fruits can be 10 to 20 times higher.
In winemaking, SO2 is the first line of defense against oxidation and microbial spoilage and a key contributor to a wines aging potential. As a winemaker, you are intentionally allowing the fruit juice to decompose into wine , but stopping it before it continues the decomposition cycle to volatile acids like vinegar and then later into water, as would happen in nature.
Many new winemakers may choose to go sulfite free but in doing so will likely end up with high concentrations of arguably worse substances like acetaldehyde, ethyl acetate, and acetic acid. Eventually, without some form of protection against oxidative spoilage, the wine won’t be wine at all anymore.
How Much SO2 is Necessary to Prevent Spoilage
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Sulfur Dioxide Measurements In The Winery
Overestimating antimicrobial activity in red wine
This article is based on the publication Conventional Measurements of Sulfur Dioxide in Red Wine Overestimate SO2 Antimicrobial Activity by Patricia A. Howe, Randy W. Worobo and Gavin L. Sacks. American Journal of Enology & Viticulture, 69 2018. ajevonline.org/content/early/2018/02/23/ajev.2018.17037) Raquel Kallas was a viticulture support specialist with Cornell Universitys New York Statewide Viticulture Extension Program. Gavin Sacks is an associate professor of food science at Cornell University. Patricia Howe is director of wine technical services at Constellation Brands in Napa, Calif. Randy Worobo is a professor of food microbiology at Cornell University.
Scheduling So2 Additions In Wine
Initial pre-fermentation sulfite may be added at 50-65 ppm to grapes or juice that is free of rot or mold. The presence of a lot of mold, or grapes in otherwise bad condition, might require twice that amount. Under average conditions the information that follows should keep about 20 to 30 ppm of free SO2 available throughout the wines cycle of production through bottling. If you plan to use ML bacteria, pre-ML sulfite additions should be kept below 50 ppm.
Above pH 3.5, you will notice that the amounts of free sulfur dioxide required become quite high. It is best to lower the pH by adding tartaric acid early in the fermentation cycle.
Continue testing every 6-8 weeks, adding SO2 as required to keep at least 20-30 ppm available in the wine.
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How To Calculate Potassium Metabisulfite Additions
1. Take a pH reading.
The least expensive way to get a pH reading is with test strips. Test strips are quick and easy, however, may be difficult to interpret. For a small investment of about $40 you could get a digital pH meter which will be more accurate.
2. Read the recommended SO2 range from the following chart.
Based on your pH from step one determine what your free sulfur dioxide goal is. This is where we want our free sulfur dioxide levels to be after we add the potassium metabisulfite. Any where in the gray range should be sufficient.
This chart is included with the Accuvin Free SO2 Test Kit. The folks at Accuvin were kind enough to grant me permission to share this table with you.
The sulfur dioxide levels presented in this chart are in ppm which is equivalent to mg/L. So if your goal is 30 ppm that is the same as 30 mg/L.
3. Measure the free SO2;presently;in your wine.
To do this youll need a Free SO2 test kit. Accuvin makes a really easy to use and read kit.
There are machines you can use to measure free sulfur dioxide, however, they are much more expensive and require a bit of maintenance to keep them in working order.
4. Determine how much free SO2 you need to add to your wine.
Subtract the amount of free sulfur dioxide you already have in your wine from your free sulfur dioxide goal from step 2. This tells us how much free sulfur dioxide we need to add.
5. Calculate Your Potassium Metabisulfite Addition.
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Which Corrects For Ph Of The Wine
The level of free sulfur dioxide in wine is measured in terms of parts per million . The portion of the measured concentration that is active is greatly affected by the pH of the wine. The active portion is referred to as molecular SO2. There are two common target values, one at 0.8 ppm molecular which will inhibit Malo-lactic bacteria and the other at 0.5 ppm molecular which allows Malo-lactic fermentation to proceed. The pH of the wine or must has a great effect on the portion of free SO2 which is in molecular form. Since the portion of molecular SO2 decreases as the pH of the must or wine increases, accurate additions will require that you can also measure the pH of your must or wine. Use the following steps and tables to guide you in additions of bisulfite to your wine.
For a 10% SO2 stock solution: