Updated: Jan 28
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 (thank you yeast!), 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
Early in the wine's life, very little SO2 is needed to prevent spoilage. A small dose of 30-50 parts per million (ppm), can be used prior to fermentation to suppress any wild bacteria or undesirable yeast strains.
Wine yeast (Saccharomyces Cerevisiae) is very effective at metabolizing any oxygen that may get into the wine. A young red wine is also loaded with tannin, which can bind with oxygen. During active fermentation, the wine is mostly saturated with CO2, and will have a nice inert gas blanket over the surface to act as a barrier against air. And finally, yeast cells are extremely competitive and will generally dominate over other bacteria as long as a healthy population is achieved.
If you are planning to allow the wine to undergo malolactic fermentation (MLF), then you will generally not add much, if any SO2 until after MLF has completed. In certain wines, where MLF is not wanted, like crisp white wines, then make sure to sulfite immediately after primary fermentation completes.
After malolactic fermentation is complete (or after alcoholic fermentation if no MLF desired), you will generally want to maintain a molecular SO2 of 0.8 parts per million. Molecular SO2 is dependent on the pH of the wine. The higher the pH, the more SO2 is required to achieve 0.8ppm molecular SO2. Below is a chart showing the required free SO2 necessary to achieve 0.8ppm molecular SO2.
Over time, the free SO2 will react with compounds in the wine and oxygen entering the wine which will gradually reduce the amount of free SO2 present. Oxidative processes like racking and bottling will usually consume about 10-12ppm of free SO2 during each activity. I generally like to add 30-40ppm extra during the first aging-period addition, and allow it to naturally drop down to the range that I need for bottling.
How do I measure how much SO2 is currently in the wine?
The total SO2 is the total of all the sulfites that have been added to the wine including bound and free SO2. This number is much less useful than the free SO2 when determining how safe the wine is from spoilage. Free SO2 is the portion of the sulfites that are un-reacted and can readily grab up oxygen molecules or suppress microbial growth. There are two methods of measuring free SO2 that relatively easily to perform as a home winemaker or a small winery. The quickest method is the ripper method. The aeration-oxidation method is a lot slower but can be very accurate.
In the ripper method, a small sample of wine is primed with an acid and starch indicator solution. Iodine is then added to the wine slowly. Initially the iodine will react with the free SO2 in the wine until the free SO2 is gone. With no remaining SO2 to react with, the iodine will react with the starch indicator, creating a blue tint to the wine. By measuring how much iodine was needed to trigger the blue reaction, the free SO2 can be calculated. Unfortunately, it is difficult to see the color shift in a red wine which greatly impacts the accuracy of the test. To accurately measure the endpoint of the iodine/SO2 reaction, several companies offer SO2 analyzers with a specialized probe. The Vinmetrica SC-100A and SC-300 come equipped with the SO2 probe and greatly simplify this test.
To run the free SO2 test with a Vinmetrica SC-100A or SC-300, the following procedure is used.
1. Measure 25mL of wine using a burette or syringe
2. Fill a 5mL syringe to the 5mL line with SO2 Titrant solution
3. Turn on the SO2 Analyzer, and make sure that it is on the SO2 Setting
4. Using separate pipettes, add about 2mL of Acid Solution and 2mL of reactant
5. Place the SO2 probe in the wine sample
6. While swirling the sample, add SO2 Titrant one drop at a time.
7. When the meter starts beeping, wait to add another drop until the beeping stops
8. When the meter beeps 20 times in a row, the test is done. Stop adding titrant.
9. Measure the amount of Titrant used and multiply by 20. This is the amount of free SO2 in the wine.
-If 1.5mL of titrant was used, then the free SO2 in the wine is 30ppm (1.5 x 20).
This method can be performed using a simple aquarium pump based kit like this one. The test involves two small flasks and several chemicals.
The second flask (furthest from pump) will contain a solution of distilled water, hydrogen peroxide, and SO2 indicator solution. This solution is then titrated with hydrochloric acid until a light pink color is achieved. In this kit, the color is then adjusted back to grey/green with several drops of sodium hydroxide before starting the test. This flask is then sealed with the stopper and tube apparatus.
The first flask, closest to the pump is where the wine sample will go. A 20mL sample of wine is added to the flask. Next 10mL of phosphoric acid is added to the wine sample for further acidification. The stopper is placed on the flask, and now it is time to turn on the aquarium pump.
Let the pump run for 10 to 15 minutes to assure that all of the free SO2 has left the wine sample. The flask with hydrogen peroxide will turn magenta as the SO2 transfers. Once complete, Sodium Hydroxide is then added to the peroxide sample until the color changes back to the original color. From this point, the free SO2 can be calculated by multiplying the mL of Sodium Hydroxide used by 16. Make sure to read the instructions of your kit, as the volumes can be different depending on the source of the kit.
When measuring free SO2, always wear gloves and safety glasses, since you will be working with strong acids. It is also a good idea to keep some baking soda around to neutralize any acid that may spill.
Calculating how much Potassium Metabisulfite to Add
If the measured free SO2 in the wine is below the amount necessary in in the chart above, make an adjustment with potassium metabisulfite. Because the process of checking the free SO2 may introduce a bit of air, I generally will over shoot the target by about 5 or 10 ppm (or sometimes more, if it is early in the aging period).
Potassium metabisulfite contributes about 57% by weight to Free SO2. When calculating how many mg necessary, multiply by 0.57 to account for the 57% free SO2. The calculation is as follows.
(ppm free SO2 needed) x (Liters of Wine)*0.57=(mg of potassium metabisulfite to add)
When adding Potassium Metabisulfite, make sure to dissolve fully in a small sample of water or wine before adding. Stir the SO2 solution into the wine thoroughly to assure that it mixes well. If you don't have one yet, get a small gram scale to make precise wine additions. This Scale works well for home winemaking applications.
In general, the processes for testing SO2 at home are relatively simple once you get the hang of it. I personally prefer the ripper method, with the Vinmetrica tester but the equipment is a little more expensive. Until you are able to test SO2 levels at home, you can make estimates as to how much you need to add, and add a little extra when in doubt. The sensory threshold for SO2 is very high, but if the levels fall below the safe level, the wine can spoil relatively quickly. Don't forget to keep your carboys and barrels topped up to minimize unnecessary air contact, and good luck out there! If you have any home winemaking tips, mention in the comments section below, and be sure to checkout the new Smart Winemaking Store!
For more information, checkout my youtube video on testing SO2 levels at home!