Potassium Metabisulfite: The Overview

If you read some of my other articles on sulfites and sorbates, you will know that I am not an advocate of using Campden, potassium metabisulfite, sulphite, SO2, or whatever other term you use for this preservative and sanitizing compound. The biggest reason I am not an advocate is because I think it’s overused, especially in the treatment of juice prior to fermentation. However, my main goal at PricklyCider.com is to educate people on making and enjoying hard cider. Part of that is giving you the knowledge to make the appropriate decision for your situation. I fully understand why a commercial producer would advocate for using sulfites in their process. However, I also believe we should be trying to eat more locally produced, fresh, and raw products. My cider making methods are always evolving. But, I am focused of finding ways to improve the taste while making it as healthy as possible.

You may not realize that the goal of adding Campden or potassium metabisulfite is to create sulfites and ideally, SO2. Metabisulfite is a common dosage method because it comes in an easily measurable powered form. Potassium and sodium are often bound to the sulfites to create the metabisulfite compound. The potassium metabisulfite has a 57.6% theoretical yield of SO2 while the sodium metabisulfite has a 67.39% theoretical yield(1). It sounds like you should use the sodium version because you could use less. The concern is with the binding agent. Do you want sodium or potassium in your cider? Sodium is generally considered less healthy and not common in apples versus potassium that is quite common in apples. For this reason, it is recommended that potassium metabisulfite be used for treatments.

When potassium metabisulfite (K2S2O5) is added to your cider, it reacts and forms various new compounds called sulfites(4). There are 3 main compounds: molecular SO2 (SO2), bisulfite (HSO3-), and sulfite (SO3=). The bisulfite and sulfite are negatively charged. The pH of your cider determines how much of the various compounds are present. In the presence of water (H2O), these compounds are shifting from one compound to another in order to maintain equilibrium. However, given the acidic nature of cider, it generally contains more molecular SO2 and bisulfite. Initially, these sulfites are in equilibrium and able to move freely between the three forms. When they encounter others compounds like sugar, acetaldehyde, and polyphenols, they react with them and become bound to them. In this way, they are removed from the equilibrium process. Therefore, you now have free and bound sulfites. Binding sulfites to other compounds is why you added the metabisulfite to your cider.

The amount of free sulfites in your juice or cider defines the number of reactions that can take place. If you added sulfites, you want reactions so you want some free sulfites to be present. You might therefore be thinking you should add more sulfites to ensure you have lots of free sulfites and will never run out. Sulfites are recognized as a GRAS (Generally Recognized As Safe) food additive(1). However, there are limits set on the amount of free sulfites you can have in beverages like cider and wine and they are prohibited for use in meats and fresh fruits and vegetables by the FDA. In other words, they are generally recognized as safe. Sulfites are a known allergens to some individual, especially infants and those with respiratory problems. There are documented fatalities linked to exposure to SO2 exposure from food(1). This is where my motto for life kicks into action: all things in moderation. The decade of 1960 to 1970 saw a 30-70% increase in the amount of sulfites used for food preservation in the US(2). Is a little SO2 added to our food going to kill us? No. However, as we consume more and more food with sulfites and other preservatives, is it good for us? You will have to decide whether using sulfites is the right thing for your hard cider or not.

Let’s explore the two main times you would apply potassium metabisulfite in your cider making process. The first is pre-fermentation, which could occur as part of the processing of your fruit or applied directly into your juice after processing. The second is post-fermentation where you apply it directly into your cider. This could occur immediately after fermentation is complete or later once the cider has matured. The timing is important because it is all about controlling the maturation process. Let’s explore the two times in more detail.

  • Pre-fermentation Treatment
  • Post-Fermentation Treatment

Pre-fermentation Treatment

Sulfites, Potassium metabisulfite or SO2, can be added to fruit as soon as it’s picked or during processing. However, most will add it directly to the juice once it’s processed. Adding it with the fruit may reduce enzymatic browning but adding it to the juice will often reverse any browning as well so the differences are generally associated with the condition on your fruit and whether you are trying to stabilize it prior to processing. In his book on fruit wines, V.K. Joshi highlights the effects that 50-200 ppm of free SO2 will have on juice and hard cider. Note that most countries limit free SO2 to below 200 PPM in wine and cider. The following table is an adaptation of that information(4).

Area of EffectLevel of Impact
Human PathogensNone
Lactic and Acetic AcidStrong
Weak Fermenting YeastsMedium
Strong Fermenting YeastsWeak
Glycerol ProductionWeak
Impact of 50-200 PPM SO2 on Apple Juice Pre-Fermentation
  • Human Pathogens: As assessed by G. Menz and associates, sulfites will not kill or suppress human pathogens like listeria or E. Coli(3). What ultimately protects you from these pathogens is the alcohol. It is also helped by the acidity and phenolic compounds found in the cider.
  • Lactic Acid and Acetic Acid Bacteria: If you want kill or suppress Latin acid bacteria or acetic acid bacteria, sulfites do an excellent job. However, so does yeast. The fermentation process will quickly consume the oxygen and nutrients in your juice, which deprives these bacteria and prevents them from growing. There are always exceptions as some LAB are resistant to sulfites.
  • Weak Fermenting Yeast: Even at 50-200PPM, sulfites will suppress many of the weak fermenting yeasts naturally found in your apple juice. This includes many of the Hanseniaspora spp., Candida spp., Pichia spp., and similar yeast genera. Note that many of these yeasts are known for creating complex aromas and increased levels of glycerol. Also note that inoculating with a stronger fermenting yeast, will effectively disable these yeast because it again consumes the oxygen and nutrients that these would need.
  • Strong Fermenting Yeast: Sulfites at 50-200PPM will not have much if any impact to strong fermenting yeasts like Saccharomyces spp. and even Torulaspora Spp. Even some medium yeasts like Lachancea spp. may not be effected by sulfites at low levels. If you over sulfite, you can inhibit about anything but, that includes your health. Note that some Brettanomyces yeast are also resistant to sulfites.
  • Browning: Sulfites do a very good job of limiting enzymatic and non-enzymatic browning. If you want to retain phenolic compounds, sulfite additions will bind with many weak phenolic compounds and prevent or even reserve their oxidation. This will prevent that amber color forming. If you want a silver cider, aerating your juice and encouraging oxidation will ensure many of these weak phenolic compounds are processed out during fermentation, which will leave your cider more silvery or pale yellow. Adding sulfites will retain these phenolic compounds. However, many of those compounds are subject to browning and precipitating out during maturation. You will still need to deal with them post-fermentation.
  • Glycerol Production: Sulphite can increase the production of glycerol, which as I have noted in other articles has a lot of positive contributions to cider quality. Glycerol can increase the residual sweetness and mouthfeel of cider. However, many of the weaker yeasts that the sulfite suppresses also increase glycerol production. These yeast are known to produce more complex esters and the amount of glycerol produced by non-Saccharomyces yeast is generally higher than those produced by Saccharomyces, even with sulfite present.

Post-Fermentation Treatment

Once fermentation is complete, there should not be any free sulfites in your new cider regardless of whether you added any to your juice or not. Remember that yeast naturally produce SO2. Some yeasts will create more than others. But, all of the free SO2 is bound to other compounds during the fermentation process. So from a free sulfite perspective you are basically at ground zero. However, your cider may lack the natural compounds desired for maturation if you added sulfites pre-fermentation. Understanding the state of your cider and where you want it to go next is critical to understanding what you might need to add to your cider to achieved your desired results. Let’s explore how adding sulfites post-fermentation can impact your cider. Again, I created a table to give you a rough guide as to how impactful it can be to key post-fermentation maturation.

Area of EffectLevel of Impact
Lactic and Acetic AcidStrong
Spoilage YeastsMedium
Color Strong
Residual SweetnessWeak – Medium
Impact of 50-200 PPM SO2 on Apple Juice Post-Fermentation
  • Lactic Acid and Acetic Acid Bacteria: If you added sulfites to your juice pre-fermentation, you will have probably killed or suppressed the lactic and acetic acid bacteria. That means much of the natural maturation aspects can’t occur. A prime example,is malolactic fermentation (MLF). Many lesser bacteria are killed naturally during fermentation but the natural lactic acidic bacteria used for MFL will endure. It is post-fermentation when MLF normally occurs and while it needs nutrients, it doesn’t need much sugar. Yeast autolysis often provide the nutrients that are needed. If you don’t want MLF because you are using low acid apples already, adding sulfites pre-/post-fermentation will help ensure that doesn’t happen. It will also help prevent acetic acid bacteria from becoming active. Note, acetic acidic bacteria requires large amounts of oxygen so if you are adding sulfites to prevent your cider from turning to vinegar, just keep it away from large amounts of oxygen. The main impact from adding sulfites is to prevents further maturation of your cider from occurring due to LAB and other bacteria.
  • Spoilage Yeast: Bacteria isn’t the only potential spoilage organism in your cider. Brettanomyces spp. can live through fermentation and they normally become active during maturation. Brettanomyces normally comes from your equipment and was more prevalent when cider was aged in wooden barrels. While some people enjoy the funky nature Brettanomyces can create, it’s generally considered a fault. The best way to limit Brettanomyces is good sanitation processes with your equipment but if you find you are still having issues, treatment with sulfites can be an effective way to suppress it. However, it’s important to note that some Brettanomyces are not sensitive to SO2 so sulfites might not solve this problem and you will need to dig deeper on your sanitation practices.
  • Color: Hard cider that doesn’t include sulfites will continue to mature and evolve as it ages. Part of that evolution will be the continued precipitation of particles, including colors. It will also be oxidation if you age in barrels, in plastic, or under cork, which can all allow oxygen to enter and enable oxidative reactions. Adding sulfites post-fermentation is one of the best ways to prevent color changes to your cider. It is also known for making colors more vivid.
  • Residual Sweetness: The last area of post-fermentation where sulfites can have a large impact is the residual sweetness. If you want a sweet cider and are planning to back-sweeten it, sulfites can help. Sulfites are capable of suppressing yeast and fermentation but that usually would require an excessive amount, beyond the 50-200 PPM normally allowed. Sulfites are much better at impacting bacteria so it would have weak or limited application for preventing added sugar from fermenting. That is where you should add potassium sorbate, which creates sorbic acid. The sorbic acid prevents yeast from fermenting sugars. However, there are bacteria that can break down sorbic acid and it creates an aroma that smells like geraniums. Sulfites in combination with sorbate combine to suppress any bacteria that might break down the sorbic acid. This will allow the sorbic acid to keep suppressing the yeast and prevent it from fermenting the added sugar. Recognize that there are alternatives to these preservatives. One method is pasteurization, another is filtering, and a third is keeving or nutrient deprivation.

I hope I’ve help you understand how and when you might use sulfites to address specific challenges you are facing with your cider. I can see the value of using sulfites and how they could provide benefits. However, I also hope I’ve help you understand why you might not want to use sulfites or if you do to use them for specific purposes. Hopefully, you also recognize that there are alternatives for most scenarios. Either way, I hope you are better prepared in making your decisions and wish you luck in your cider making endeavors.

(1) Z. Berk, Chapter 25 – Chemical Preservation, Food Science and Technology, Food Process Engineering and Technology, Pages 591-606, 2013

(2) S.L. Taylor and associates, Sulfite in Foods: Uses, Analytical Methods, Residues, Fate, Exposure Assessment, Metabolism, Toxicity, and Hypersensitivity, Advances in Food Research, Vol. 30, 1986

(3) Henderson, P., 2009. Sulfur Dioxide: Science behind this anti-microbial, anti-oxidant, wine additive. Prac. Winery & Vineyard Journal Jan/Feb 2009.

(4) V.K. Joshi and associates, Science and Technology of Fruit Wines: An Overview, 2017

(5) G. Menz, P. Alfred, and F. Vriesekoop, 2011. Growth and Survival of Foodborne Pathogens in Beer. J. Food Prot. 74:1670-1675.

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