Does Oxidation Effect Fermentation?



This picture above is by Melanie of Pickle Me Too.  I asked for her permission to use it because it shows that the top inch or so of the ferment has oxidized- proof that the environment is not made anaerobic simply by being under the brine or having a lid on the container. Melanie reports that it was self-brining and the top inch or so oxidized the entire time they were eating it.  As they would pull more out, the unoxidized would be exposed to oxygen despite the container having a lid and would turn brown before they would open it again.


Oxidation is a bad thing in food, no matter where it occurs.  It doesn’t matter whether it’s a ferment, a fat, or another food, you want to avoid oxidation. Oxidation might not be a familiar word to you, but if you’ve read Nourishing Traditions, you’d recognize oxidation by another name- rancid. Rancid refers to oxidized fat.  A quick perusal of any natural health book will tell you to avoid oxidized or rancid foods at all cost.


What causes Oxidation?

In a word?  Oxygen.  Oxidation is the interaction between oxygen and a material. Oxygen is almost always bad news for food (outside of kombucha and vinegar) because it ends in unfavorable changes in flavor, texture, nutritional value and it shortens storage length. Oxygen (among other things) causes foods to decompose and oxygen kills off some of the beneficial properties of the food. You’ve seen oxidation if you’ve ever cut some fruit and left it out without dipping it in an antioxidant like vitamin C- it turns brown and after a while it gets yucky.

Lacto-ferments are no different- when exposed to oxygen, they can create all sorts of nasties you don’t want to eat.  Oxidation begins long before you can taste the flavor changes- foods are far along in the oxidation process before you can see or taste the changes. Oxidation can not be reversed.


Oxidation Creates Free Radicals

The cells of your body can also be oxidized from eating oxidized food or being exposed to toxins.  Oxidation always creates free radicals.  Free radicals are unstable molecules, atoms or ions that damage cells in your body because they scavenge their needs to make themselves stable.

Free radicals are greedy little thieves.  They rip electrons off from your cells (or from the uneaten food) to try to become stable, creating a chain reaction because the molecule that just had its election ripped off is now unstable and it then goes in search of stability, grabbing an electron from another molecule, and the chain reaction continues until two free radicals join together to form a stable molecule- that can take a while and wreak a lot of havoc in the process. The molecules made up of two free radicals are called ‘stable oxidation products.’  If this reaction happens in your food, the smell you associate with rancidity is caused by a lot of stable oxidation products in a food. So, as you see, many of them have to be present before you can smell it.

When it happens in your body, this chain reaction causes cell death- if you’re lucky.  If they don’t kill the cell, the injury can cause lots of trouble, like mutations and DNA problems- that would be unlucky. When that damaged cell replicates, both of the new cells it creates has the same problem.  On top of that, free radicals are known for causing cells to speed up their replication rates, a known issue in the development of tumors and cancer.

Not only are free radicals implicated in cancer, they’re also implicated in cardiovascular disease, Alzheimer’s, Parkinson’s, multiple other diseases and some things associated with aging in your body, like wrinkles. Oxidized cholesterol (cholesterol is a fat) is condemned as a source of blood vessel damage in Nourishing Traditions (pages 436 and 444). Oxidation is bad news for your body.

You avoid free radicals by avoiding cigarettes, second hand smoke, pollution and other toxins.  You should also avoid oxidation and the resulting free radicals in your food.


Why Eliminating Oxygen From A Ferment Is Important

Anti-oxidants are molecules that protect other molecules from damage by free radicals and oxygen- it’s a category of many different molecules. Many different things function as antioxidants and many antioxidants also function as natural food preservers. It’s critical to get them in your food as they are protective for your health and they are one of the main reasons fruits and veggies are so highly recommended as part of the diet. Antioxidant intake is known to be protective from many illnesses and diseases, including neurological disorders, radiation, cancer, stroke, arthritis and more.

Antioxidants are produced by lactic acid bacteria during fermentation and even in your gut after they are consumed. There are many studies showing this effect, I’ll just list a few here, here, here, here and here. That’s one reason why it’s so important to keep oxygen-loving bacteria out of your ferments- you want to consume as many lactic acid bacteria as possible and if they’re having to compete with other bacteria &/or give them an unfavorable condition to grow in, you won’t have as many to consume. Oxygen kills off many of the products of fermentation, from some of the lactic acid bacteria to all of the metabolites, like antioxidants, they produce and the enzymes that the antioxidants use to stop oxidation.

In fact, the food industry uses anaerobic fermentation to create antioxidants, nutraceuticals and more used for food and supplement production, which they then extract from the resulting ferment.

The presence of  light in a ferment destroys anti-oxidants. It’s critical if you want to consume antioxidants in your ferments that you keep them in the dark. That’s why things like oils that are prone to go rancid, like olive oil, come in tinted bottles.

Anti-oxidants protect your body from oxidation by preventing, breaking off, absorbing or stopping the free radical chain reaction.They lessen the effects of free radicals by stopping the process before it otherwise would end.

In a food, anti-oxidants can absorb the free radical and stop the reaction even before the food is eaten. As free radicals develop in a ferment because you have oxygen or light around, antioxidants have died off so they can not stop it. However, in an anaerobic system, the anti-oxidants are there to stop the reaction while it’s still in the jar, before you take your first bite.

One antioxidant most people are familiar with is Vitamin C.  If your ferment is getting oxygen, it will have reduced levels or no Vitamin C in it.  The same is true for light- light kills Vitamin C.  That is why it is important to ferment in anaerobic conditions with no light.  I wrap my Pickl-Its in a towel when fermenting and the Harsch is opaque so no worries there.

If you don’t allow your kraut to go through all four stages of fermentation, you might not be getting any vitamin C from it at all! The lactic acid bacteria use up all of the C in the ferment and then around week 7 the anaerobic bacteria begin regenerating Vitamin C back into the brine. If you have oxygen in the mix or you don’t have anaerobic LABs in your ferment, you won’t be getting the Vitamin C you expect.


How Do I Know if My Lacto-Ferment Has Oxidized?

Look for the tell-tale color changes of oxidation.  If a food changes colors, it’s oxidizing.  Throw it out! Only eat unoxidized food.

This means you’re going to be throwing away a lot of oxidized food if you aren’t fermenting in an anaerobic environment.


Avoiding Oxidation

Fermentation gases can protect the foods because carbon dioxide is heavier than oxygen (we discussed this in the post about brine) in the right environment if you don’t fiddle with your ferments and your container is airtight.  Go ahead and go read that post.  I’ll wait.


You back?  OK, good.

If you have an airleak letting oxygen in, even a tiny amount, it changes the pressure in the vessel and disrupts that protective layer of fermentation gasses in ways that aren’t beneficial.

How do you know your vessel is airtight? If you have a truly airtight vessel, excess brine will be pushed up and out the airlock and not out the sides of the jar where the lid and body join. If brine can get out, oxygen can get in. Containers that don’t seal tightly enough to have brine go out the airlock won’t have enough pressure to keep the carbon dioxide layer in place and protect the ferment from oxidation.


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I'm KerryAnn Foster. I live in the mountains of Western North Carolina with my husband, Jeff, and our two kids, a teen and a tween. I blog here at Intentionally Domestic (formerly Cooking Traditional Foods). I blog about Paleo, beauty, health, family, homeschool and lifestyle for women in their 30s and beyond. I have over sixteen years of real food and natural lifestyle and health experience.

I am also an It Works! Global Triple Diamond Independent Distributor. I love that crazy wrap thing! I have been extremely happy with how the It Works Products have tightened up my loose skin and healed my stretch marks after losing 179 pounds and having a 10-pound baby.

Read about my journey to health through celiac disease, PCOS, food allergies, obesity, adrenal fatigue and heavy metals.


  1. lydia says:

    good post – important to realize even when not fermenting. I have had ferments that the top layer changes color – but not often thankfully, I always pitch the discolored part. I recently just composted a bunch of carrots that were cut up (not feremented) because they were losing their color and just looked vapid and not edible at all – I think our instincts can even tell us discolored food is not desirable!
    lydia recently posted..The Role of Minerals in the Body

  2. Nerdymom says:

    I’m really enjoying your series, thanks for all the research you are sharing with us! I appreciate that I can comprehend what you are saying even with my mommy/lactation brain!

    I think it’s interesting that you said “How do you know your vessel is airtight? If you have a truly airtight vessel, excess brine will be pushed up and out the airlock and not out the sides of the jar where the lid and body join.” I bought an airlock lid for my mason jars (before you started this series) and I’ve only had them leak brine from the airlock, not from around the rings. I’m now more encouraged about the products that I’m making, although I have to make them in small batches to keep them from oxidizing before we consume it all.
    Nerdymom recently posted..Chaos! Mayhem! Panic! (and thrift store show and tell)

    • I’m glad you’re enjoying the series. I totally understand mommy brain- I’ve got two myself, and that’s a big reason why I’m writing this series in small bites instead of trying to write one big tome.

      Do be aware that once the active fermentation stops, the homemade airlocks are no longer airtight- you can’t get a true, airtight seal that is continuous with anything that is threaded on instead of wire bailed or held via weight and water like the Harsch. Threading doesn’t exert enough pressure on the seal to prevent air leakage in differing pressures that you face once the active off-gassing stops. So you will potentially face problems after active fermentation is over.

      • Nerdymom says:

        Yes, I’ve found that I have to make it in small batches and use it quickly or the quality deteriorates, from what I’ve gathered on your blog, oxidization during storage is my big problem.

  3. Maretta says:

    Do you have an article where you discuss the four stages of fermentation?

    Also any suggestions for keeping veggies under the brine in the pickle it? The small weight often isn’t enough.

    Thank you very much for sharing your knowledge and experience!

  4. Katie P says:

    If you ferment your food correctly, how do you keep it from oxidizing in the refrigerator or root cellar as you are consuming it?

    • Katie, in a correctly shaped vessel with an airtight seal, that contains more pressure inside than out and that has an airlock, the oxygen is lighter than the carbon dioxide being off-gassed, so it gets pushed out the airlock first, preventing it from coming into contact with your food. It’s no problem to open up a Harsch or a Pickl-It and get some out because any oxygen that is introduced will quickly be eliminated.

  5. Patricia Prenosil says:

    So I was wondering the same thing about using up the oxygen once you are eating it. If you are opening the jar once a day or maybe more can the oxygen truly be used up that quickly? I know you mentioned switching it to a smaller jar once there is less ferment in the jar, but it just seems unlikely that the oxygen is used up quicly enough even in an almost full jar if you are opening it all the time. And if that’s the case then what would the pickl-it really be doing once it’s finished fermenting? Just trying to wrap my head around it all. Thanks for all your info.

    • ‘Finished fermenting’ really don’t mean the process has stopped, it means that it’s gone through to the point it can be eaten. The bacteria continue to eat the nutrients in the veggies for months. It’s considered fully cured or ‘finished’ once it’s in the fourth stage, but that fourth stage can last a year or much more in some ferments. Cabbage can go two or more years.

      The oxygen isn’t being used up by the bacteria as much as it’s getting pushed out the top of the airlock because oxygen is a lighter molecule than carbon dioxide- if you’ve ever watched dry ice, it falls. The CO2 sinks to the lowest point and sits there. Some of the LABs can use some oxygen- they can store a small amount. But the different pressure between the inside and the outside of the jar pushes the oxygen out. That’s why the jar shape is important. Straight-sided vessels with little to no neck don’t hold in the CO2 as well.

  6. Jaime says:

    My question is in regard to soaking grains. I’ve noticed, when I soak whole wheat flour overnight for making biscuits or pancakes, the top layer turns brown and yucky looking. I am assuming this is also from oxidation, though I seal the bowl very well with plastic wrap. If oxidation is so dangerous, how do we soak grains without the top layer becoming oxidized?

    • KerryAnn says:

      Jamie, I’ve thought about that as well and I’m changing some of my practices and doing a little more research. I’ll post when I’ve come to a final conclusion.


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