Potassium metabisulfite is a necessary preservative in wine making. It provides sulfur dioxide which helps prevent microbial spoilage and fight off 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 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 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 30 ppm 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 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.
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.
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 dissipate. Chemically there isn’t much you can do.
Also, potassium metabisulfite can be very agitating to your eyes and respiratory system. Handle the powder carefully and do not let the dust into your eyes or breathe it in.
In Conclusion
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.
Photograph by: Amy
Excellent and thorough about potassium metabisulfite. I really appreciate how you explained it in simple terms.
Thanks Jason! There’s a lot of science in wine making but I try to make it approachable and simple. Cheers!
This is excellent information. One small problem the aspiring winemaker is left with, however, is the use of “gallons”. Oh dear … with mg and L and grams surely the capacity measure needs to be in Litres. I believe there are only two countries now still using the ancient imperial measures.
Hi Neil,
Units are a bit of an issue. I do try to cover my bases with metric and imperial, however, those imperial units are so ingrained in how I think that I sometimes struggle to wrap my mind around the metric measurements. I’ll do my best though to present both.
Thanks,
Matt
Hello Matt im new at wine making ive got my first batch of feijoa wine down and i think im close to adding my potassium sorbate but im not sure how long i should leave it before i add the tannins and potassium ive had the wine in now for 6 weeks and it been 3 weeks since the primary fermeting process can you help
Hi Josh! Great question.
Sorbate is usually added just prior to bottling. As long as you get it mixed in well there’s no need to wait. The same goes for potassium metabisulfite as long as the dose is correct.
Tannins are usually added in the beginning before fermentation has begun. This way the yeast can help mix them in and make the wine more homogeneous.
Has your wine finished fermenting? Also, do you plan to back sweeten it?
Cheers! -Matt
Hi Josh,
Yes you can add sugar or unfermented juice until it tastes good to you. At the same time you’re going to want to add potassium sorbate to keep whatever yeast is left in there from fermenting the sugar you’re adding. Here is an article on back sweetening: http://winemakersacademy.org/back-sweeten-wine/.
The best way to determine if fermentation is over is to take two specific gravity readings a few days a part. If the specific gravity has not changed at all in that time then the wine is no longer fermenting.
Cheers! -Matt
Hello Matt
I am new to the winemaking, I have bought the grape must in a wine shop and added the yeast, mixed it air tighten it and fermented it for about 2 months without racking it. I was wondering if I have already left it for to long? Also I have been given a sachet with a mixture of potassium sorbate and potassium metabisulphate to add it after rackin. In additional to that I have been given kieselsol and chitosan witch I prefere not to use as it looks nasty ( is jelly). I would appreciate it if you could get back to me. Am I still in with a chance? I have about 25 litres in total.
Kind regards,
Eryon
Hi Ery,
Your wine is probably fine as long as the must did not contain any fruit. Must that contains chunks of fruit or grape skins can start to decay after a week or two in the must. It doesn’t always happen but it’s a risk.
I recommend taking a specific gravity reading at this point and see where you are in the fermentation process. My guess is that it is over.
If it is over I would rack into a clean and sanitized carboy and put it back under the airlock. Add the potassium metabisulfite as instructed and I recommend adding the sorbate only if you plan to back sweeten this wine.
When you rack take a small sample, smell it, and taste it as well to see how things are progressing. Pay special attention to the flavors and aromas. We don’t want to run into anything sulfur like.
Once the wine is clear and provided that it smells and tastes good you can proceed with bottling whenever you’re ready.
I hope this helps! Let me know if you have any other questions.
Cheers!
-Matt
Hi Matt
Thank you very much for getting back to me. I will do as you say. I am going to siphon the wine into another sanitized container and then back into the airlock container once sanitized as well.
The only difficulty I will have will be how to tell if it is sulfur like or gone off (will the smell be strong)? How long do you recommend I leave it for the next fermentation? Will a few days do? Or does it matter if I leave it for weeks?
You really doing a great job! Thank you very much!
Best regards,
Ery
Hi Ery,
While I haven’t run into sulfur issues myself what I have gathered from those that have is that it is a pretty distinct flavor and aroma. It does start off subtle but gets bad in a hurry.
If you pick up a burnt toast or rubber like flavor or aroma you could be in the beginning stages of a sulfur issue.
As for the timing of the rest of fermentation, if it is still going, you can leave it in there as long as you like. Rack if the yeast lees gets to be 1/2 inch thick or more to prevent off flavors.
Once fermentation is over and you’ve stabilized the wine you can let it sit for weeks, months, or even years if you like. Bulk aging will yield more consistent results from one bottle to the next once you do bottle it. If you’re thirsty though go ahead and bottle it and enjoy 🙂
With the proper protection from oxygen (the airlock) and spoilage organisms (the airlock and potassium metabisulfute) time is on your side. White wines and fruit wines are generally better younger. Reds need more time to mature. Your best gage though is tasting the wine as time goes on.
Let me know how it turns out Ery!
-Matt
Sir, I am a beginner to wine making and my pineapple wine has lost its sweetness and some of my wine is too sweet ! How much Potassium Metabisulfite must I put for five ltr’s or one ltr of wine ?
In India we dont get chemicals mentioned in the wine making as mentioned on site but I managed to get Potassium Metabisulfite 50 grams. from a friends lab. what could be the substitute for the remaining, (Fining , To stop fomentation, Nutrients ?)
Hi Jude, great questions! When a wine ferments it is the sugars that are converted into alcohol. A finished with that is completely dry has no fermentable sugars left and therefore not sweet. For the most part there the only way to stop a fermentation is through filtration or by ensuring that the alcohol level gets too high for the yeast to keep fermenting and they die before the sugar is gone.
The wines that have lost their sweetness have probably fermented to dryness. The wines that are still sweet either have not finished fermenting or have stuck fermentations where, for some reason, the yeast has stopped consuming sugar. This can be because the temperature isn’t right, not enough nutrients, or the alcohol level got too high.
As for how much potassium metabisulfite to add to a liter of wine, that depends largely on your pH level. If you check out the chart on this page it will tell you what the ideal sulfite levels are for the pH of your wine. pH alters how effective sulfites are.
If you haven’t yet added any sulfites to your wine you could add 0.22 grams of potassium metabisulfite to reach 25ppm of free sulfur dioxide. This would be in the goal range for wines with a pH of 3.4 to 3.6 which is a fairly common range to be in.
Please post your pH if you happen to know it.
Cheers! -Matt
Another great question Jude! There are several things you can do to replace commercial fining agents and yeast nutrients. Stopping a fermentation, however, requires the use of a sterile filtration system. A sterile filter has openings of 0.45 microns or less and will catch micro-organisms as the wine passes through.
For many years egg whites and egg shells have been used as fining agents. When using egg whites you want to be really careful that everything you use is very clean and has been sanitized. Robert Mondavi used egg whites to clear some of his wines. I can find a recipe for this if you like.
The most common thing I’ve heard of wine makers using to provide yeast nutrients without using the additive is raisins. It is recommended you cut them in half to pierce the skin. I was once advised to use 3-5 raisins cut in half per gallon of wine. I don’t have any scientific information to back this practice up but it is something I’ve seen quite a few wine makers use.
Cheers Jude! -Matt
I think your formula underestimates the needed amount of K-bisulfite, since it does not take into account the amount of SO2 that will become bound to other compounds already present in wine. So only between 40 and 70% of total SO2 available in K-bisulfite will be available to become free SO2.
See Table IV in the following link
http://www.practicalwinery.com/janfeb09/page5.htm
This consideration is not only shown in this article, many others do so also.
Hey there young autumn, thanks for sharing the link. The fourth step addresses the difference between the amount of sulfites added and the amount that will remain free to protect the wine. According to Alison Crowe of the Winemaker’s Answer Book about 57% of the sulfites added will remain free. This sits in just about the middle of the 40-70% range discussed in the article you linked to.
If you take a look at the equation I divide by 0.57 which effectively increases the dosage by 1/0.57 = 75% to account for this.
Please let me know if I’ve misunderstood anything or missed something entirely. I strive to keep everything on the Academy website as accurate as possible and I’ll gladly make a correction if this seems off.
Cheers! -Matt
I have a question merely because I am curious. This all sounds a bit hi-tech to me,
which leads me to my question: What did they do in ancient times e.g. BC, we
have I believe empirical evidence that grapes or maybe juice was fermented in
animal skins and sloshed about as nomadic people moved from place to place. Did
they just not worry about it?
Hi Howard, this is a great question! I’ve been looking into this but haven’t found anything specific. So many resources claim that “winemakers have used sulfites for centuries”, however, there’s no explanation as to how sulfites were added or used. I’ll keep digging and let you know what I find out.
Just recently I came across some public domain books on wine making from the early to mid 1800s. Surely there is something in there that might give us a clue.
-Matt
Thanks for the reply Matt, I just have this image in my mind of the first person transporting what they believe to be plain juice and the look on the face after the first gulp which leads to the question of what the winery might have looked and functioned like in a desert? Maybe a heard of goats with skins attached to back packs wandering around in an endless circle… boggles the mind!
Matt, I believe that, according to the paper in the link to practicalwinery.com from “young autumn”, the 0.57 factor accounts for the fraction of SO2 in the potassium metabisulfite. Then the 40-70% accounts for the free SO2.