Potassium metabisulfite is a necessary preservative in wine making. It provides sulfur dioxide which helps prevent microbial spoilage and fight of oxygenation. However, figuring out how much to add can be complicated.
By the end of this article you’ll understand how this stuff works, how to calculate what you need to add, and what equipment you need to do it. Let’s get to it.
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 sulfer 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 can’t predict how much will become bound so you’ve got to add potassium metabisulfite, test it, then adjust as necessary.
If that wasn’t enough variables here’s another twist. The effectiveness of sulfites change with the pH of the wine. The higher the pH the more sulfites you’ll need to do the same job as you would in a wine with a lower pH. Don’t worry though there’s any easy way to figure out what your ideal range is.
The Effects of Time
Over time the free sulfur dioxide will bind with things in your wine or it can also leave as a gas. Thus you’ll need to monitor your sulfite levels throughout the wine making and aging process.
With each racking and addition of additives you’ll be using up free sulfites. These become bound and are not affected at protecting your wine. So don’t think that if you’ve added it once in the beginning that you’re good to go, you’ll likely have to add more.
As you can see there are many variables we have to deal with when figuring out how much potassium metabisulfite we need to add to our wines. Now that we understand how sulfites work and where they go let’s take a look at how to calculate the right amount of potassium metabisulfite to add to your wine.
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 30ppm that is the same as 30 mg/L.
3. Measure the free SO2 presently in your wine.
To do this you’ll 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 sulfer 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.
According to The Wine Maker’s Answer Book only about 57% of potassium metabisulfite powder is free. This will be accounted for in our calculations below.
5. Calculate Your Potassium Metabisulfite Addition.
The following equation can be used to determine how much potassium metabisulfite you need to add in grams. This formula is based on US gallons of wine and comes from The Wine Maker’s Answer Book by Alison Crowe (affiliate link).
If you don’t have a scale to measure your potassium metabisulfite a generally accepted equivalent is 1/4 tsp = 1.4 grams.
Mix in your potassium metabisulfite by gently stirring your wine. Be careful because the more oxygen and solids introduced to your wine the more SO2 that your adding will become bound and not free to protect your wine.
Caution: Potassium Metabisulfite is Serious Business
Please use caution when adding SO2. Once it’s been added it is not easy to remove. The only remedy is to give it lots of time to dissapate. Chemically there isn’t much you can do.
Also, potassium metabisulfite can be very agitating to your eyes and respritory system. Handle the powder carefully and do not let the dust into your eyes or breath it in.
The addition of sulfites is not a simple process. I’ve done my best to break it down as simply as I can, however, there are limits to how much simpler this can be made. If you add too much you’ll be able to taste sulphur in your wine and that’s nasty.
Sulfites as a food additive is controlled by law for a reason. Use caution and add only what you need. If you have to add more later that is better than having to figure out how to remove it if you’ve added to much.
I invite you to check out the resources listed in this post so that you can have a more in depth understanding of this.
Equipment & Resources
4. Scale (optional)
The product links in this post are affiliate links. You’ll pay the same price regardless of whether or not you use them. However, if you do use them a portion of the sale price will be paid to the Academy. This helps ensure that there will be more winemaking experiments and tutorials in the future.
Photograph by: Amy