Fermented red onions are one of the simplest and most vibrant ways to introduce beneficial microbes into your meals. Made using a traditional 2% salt water brine, these lacto-fermented red onions develop a tangy flavour while naturally supporting digestion and gut health.
Whether you’re following the GAPS diet or simply looking to nourish your digestive system, this easy ferment is a delicious addition to your kitchen.
What Are Lacto-Fermented Red Onions?
Lacto-fermented red onions are preserved using a 2% salt water brine (20g salt per 1 litre of filtered water), which allows naturally occurring lactic acid bacteria to thrive.
Despite the term “lacto,” no dairy is involved — the name refers to lactic acid, the naturally produced acid during fermentation.
Fermentation has been a traditional practice for thousands of years, helping cultures worldwide preserve food while enhancing flavour and nutritional value (PMID: 40646907).
Unlike pickled onions, no vinegar is used — the preservation is completely natural.
If you’re new to fermented foods, check out my detailed guide on lacto-fermented foods for gut health.
Why Fermented Onions Are Different From Pickled Onions
Many people confuse fermented onions with pickled onions. Here’s how they differ:
Fermented Onions
- Made with salt water brine
- Naturally probiotic
- Develop beneficial bacteria
- Support gut health
Pickled Onions
- Made with vinegar
- Not probiotic
- Bacteria destroyed by acidity
- Primarily flavour-based preservation
Fermentation encourages the growth of beneficial microbes, while pickling preserves but does not provide live cultures. If your goal is gut support, lacto-fermented red onions are the better choice.
Why It Works – The Microbiology of Fermentation
Fermentation works because salt creates an environment where harmful bacteria struggle to survive, while beneficial lactic acid bacteria thrive.
As these microbes multiply, they produce lactic acid. This acid naturally preserves the onions, stabilizes the brine, and transforms their nutritional profile.
Dr Natasha Campbell-McBride explains: “Fermented foods introduce beneficial microbes into the body, which will play their part in a very complex interaction of various micro-creatures, who already live there. We do not know what exactly they do, but in a clinical setting, regular consumption of fermented foods helps to eliminate digestive symptoms.” (source)
Lacto-fermented foods may also provide anti-oxidant, anti-microbial, anti-fungal, anti-inflammatory, anti-diabetic, and anti-atherosclerotic activity (PMID: 28945458).
Fermented Red Onion Benefits
When properly prepared, fermented red onions provide more than just flavour:
- Supports Digestive Balance – Naturally occurring microbes may support a healthy gut environment, particularly within the GAPS framework.
- Enhances Antioxidant Activity – Fermentation increases flavonoid availability and short-chain fatty acids, boosting the onion’s bioactive properties.
- Improves Nutrient Bioavailability – Fermentation can break down compounds that may otherwise limit absorption, making nutrients easier to digest.
- Encourages Microbial Diversity – Including a variety of fermented vegetables in your diet introduces different strains of beneficial microbes. Explore more ferment options here: 7 Easy Vegetable Ferments
Are Fermented Red Onions GAPS-Friendly?
Yes! Dr Natasha Campbell-McBride recommends including a wide variety of fermented vegetables as part of the GAPS protocol.
Learn more about GAPS here: What is the GAPS Diet?
How to introduce on the GAPS Introduction Diet
- Introduction Stages 1–2: Consume only the fermented juice.
- From Stage 3 onward: Introduce onion pieces gradually, observing tolerance.
Fermented foods can have a powerful effect on the body. If you haven’t eaten home fermented foods before you will need to introduce slowly. Starting with 1 teaspoon of the juice and working your way to eating the onion piece up to cup a day if that’s your jam. Make sure to listen to your body to see how you react during the introduction process.
This approach helps avoid digestive discomfort while building gut resilience.
Why Use a 2% Salt Water Brine?
The 2% brine (20g salt per 1 litre of filtered water) creates ideal conditions for fermentation.
Filtered water is recommended because chlorine and other chemicals in unfiltered water can interfere with microbial activity. Enough salt ensures a stable environment for beneficial bacteria to thrive.
Do You Need a Fermentation Weight?
No, a fermentation weight is optional, but highly helpful to keep onions submerged. When vegetables float above the brine, they are exposed to oxygen, increasing the risk of mold.
Alternatives:
- Place a clean cabbage leaf on top
- Insert a small glass jar inside the larger jar
Airlock lids are recommended as best practice. They allow gases to escape without introducing oxygen, reducing the need to “burp” jars manually.
How Long to Ferment Red Onions
Fermentation time depends on room temperature:
- 3–7 days at room temperature
- Warmer rooms accelerate fermentation
- Cooler rooms slow it down
Taste after a few days and refrigerate once the flavour suits you. For additional home fermentation safety see here.
Troubleshooting Fermented Onions
- Cloudy brine: Normal, indicates active fermentation.
- White film (kahm yeast): Skim off and ensure onions remain submerged.
- Mold: Discard the batch.
- Strong smell: Fermented onions are pungent, but should not smell rotten.
Proper salt ratios and submersion prevent most issues.
My Favorite Ways to Eat These on GAPS
In our home, fermented red onions are a staple. We enjoy them:
- On eggs
- In salads
- On burgers
- Mixed with sauerkraut (Sauerkraut with Dill and Red Onion)
They are an easy way to include fermented vegetables daily without complexity.
Tools you may need
- Knife and chopping board (or mandoline slicer)
- 2 x Wide mouth mason jar with lid (1quart or 1 litre)
- Fermenting weights (optional)
- Fermenting lids (optional)
- Measuring spoons
- Bowl
Ingredients
- 4 Red onions
- 20g Mineral salt
- 1 liter filtered water
How To Ferment Red Onions
Step 1: Prepare the onions – Peel and thinly slicing the onions.
Step 2: Pack your jars – Add the sliced onions into wide mouth mason jars up to the shoulder of the jar, and press onions firmly down.
Step 3: Prepare your salt water brine – Make a 2% brine by add 20g salt to a jar topping with a splash of hot water to dissolve the salt. Add remaining room temperature. (Note: brine should be room temperature) Then pour the brine over your onions making sure that the onions are under the brine.
Step 4: Ferment – Secure fermenting lid and sort at room temperature out of direct sunlight for 5-7 days. Ferment time will depend on room temperature and how you prefer them to taste.
Step 5: Transfer to the fridge – When your fermentation time is complete; remove fermenting lid, replace with a regular lid and store in the fridge.
Tips for making lacto-ferments at home
- Choose your salt. You want to avoid using table salt and instead pick a high-quality mineral salt such as Himalayan pink salt or sea salt.
- Make sure there is enough liquid to cover the ingredients.
- Leave enough space between the shoulder of the jar and rim as the ingredients will expand as the lactic acid is released. This gap will help prevent leaks and you should not have to open the jar to burp it. If you have to do this it means that you filled the jar to high.
- Don’t open the jar, fermentation is an anaerobic process and too much oxygen will ruin the ferment.
- Ferment at room temperature and avoid direct sunlight. The cooler the room, the longer it takes. The warmer the room the faster it is.
- Time depends on individual preference and requirements. A ferment can done in as little as 5-7 days; or you may choose to ferment for weeks or even months. The longer you ferment the more broken down the fiber, it easier to digest and lower in histamines.
- Time and Temperature are linked
Frequently Asked Questions About Fermented Red Onions
Several months in the fridge if kept submerged in brine.
Yes, though the flavour will vary.
Use a fermentation weight or cabbage leaf to keep them submerged.
Yes, but begin with the juice/ brine only during Stages 1–2.
They contain naturally occurring beneficial microbes produced during fermentation.
You can add a small amount of the brine from a previous batch to your new brine to kick-start the fermentation process.
Cloudiness is normal. Skim off any kahm yeast and ensure onions remain submerged.
Fermented red onions are simple, traditional, and nourishing. With just onions, filtered water, salt, and time, you can create a vibrant, probiotic-rich ferment.
Whether added to eggs, salads, burgers, or sauerkraut, they are a practical way to include gut-friendly foods, aligned with GAPS principles and supported by both traditional wisdom and modern research.
Start small, observe your body’s response, and let this easy ferment become a regular part of your kitchen and gut-healing journey.
More Gut-Friendly Ferments to Try
Expand your fermentation repertoire with:
Fermented Red Onions
Learn how to make lacto-fermented red onions with 3 simple ingredients: red onions, salt and water. A delicious way to introduce probiotic goodness into your diet and boost gut health.
Ingredients
- 4 Red onions
- 20g Mineral salt
- 1 liter filtered water
Instructions
- Peel and thinly slicing the onions.
- Add the sliced onions into wide mouth mason jars up to the shoulder of the jar, and press onions firmly down.
- Make a 2% brine by add 20g salt to a jar topping with a splash of hot water to dissolve the salt. Add remaining room temperature. (note: brine should be room temperature)
- Secure fermenting lid and sort at room temperature out of direct sunlight for 5-7 days. Ferment time will depend on room temperature and how you prefer them to taste.
- When your fermentation time is complete; remove fermenting lid, replace with a regular lid and store in the fridge.
Notes
Note: If you do not use fermenting lids leave a larger gap for expansion.
How to Introduce fermented onions
Fermented foods can have a powerful effect on the body. If you haven’t eaten home fermented foods before you will need to introduce slowly. Starting with 1 teaspoon of the juice and working your way to eating the onion piece up to cup a day if that’s your jam. Make sure to listen to your body to see how you react during the introduction process.
GAPS and Gut and Psychology Syndrome™ are the trademark and copyright of Dr. Natasha Campbell-McBride.
Last Updated on 01/03/2026 by Rebecca MacTavish
I’m Rebecca, a Certified GAPS & Nutrition Coach, and I help busy mums restore their family’s gut health—one kitchen, one meal at a time. I share practical tips, simple recipes, and ways to create lasting change.
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Hello!
Do you do 20g of salt for each jar? or is it 10g and 10g?
Hi Jennifer, Dissolve 20g of salt in 1L of water then pour over the sliced onions. 1L of brine is usually enough to cover the onions in both jars.
These are delicious! Not weird-tasting at all, and so good. Also easy to make!