Fermented onions – supercharged for health

Raw onions may leave a pungent taste in your mouth. Cooked onions provide a milder option. Pickled (canned) onions are less pungent but more acidic. However, fermented onions provide the best flavour option of all: least pungent, subtle sweetness, and mild acidity from natural chemical changes taking place under a brine, no added sugar or vinegar.

Fermentation transforms raw onions into an enjoyable and palatable vegetable, while retaining its crispiness, enhancing its digestibility and nutrition, and adding good microbes into the mix.

Onions and quercetin

Additionally, fermentation enhances quercetin. This phytochemical (plant compound) is one of the most ubiquitous flavonoids in herbs, vegetables, fruit, and red wine. It is a well-known antioxidant with anticancer, anti-inflammatory, antihistamine, antiviral, cardio- and neuroprotective actions (1).

Fermentation enhancement

Most quercetin in onions is in the papery onion peel and outer 1-2 layers. While the peel is unappealing to eat, fermentation draws quercetin (and other nutrients and phytochemicals) from the peel into the brine. Brine that changes from clear to purplish reveals pigment (anthocyanin and quercetin) transfer from within red onions into the brine. This recipe includes the onion peel – the part most people toss away – placed at the bottom of the fermentation vessels to harness the maximum amount of quercetin. Once fermentation is complete and the onions are gone, toss of compost onion peels at the bottom of your fermentation jar. 

Bioavailability booster

Fermentation also enhances the bioavailability of quercetin as evidence by higher antioxidant effects than from non-fermented onions (2). Translation: Fermented red onions deliver quercetin in a form that is easier to absorb and use than non-fermented onions, meaning you receive more disease-preventing, health-promoting benefits for dealing with life in a modern world (or in this global pandemic).

High concentrations of good microbes are in the brine. Consume the brine raw as a gut shot, a tablespoon at a time before meals, preferably. Start with one teaspoon, and work your way up to a tablespoon.

How to use fermented onions

Use the fermented onion slices as you would raw onions. The milder flavour lends to consuming raw fermented onions, but cooking them is also an option since most good microbes are in the brine. Consider the following options for using fermented onion:

  • soups, stews, stir-fries, casseroles, or any skillet meal 
  • burgers, tacos, quesadillas, wraps, or sandwiches
  • cocktails
  • on charcuterie boards
  • salads 
  • homemade salad dressings

Always ensure fermented onion slices are submerged under the brine. That is, consume the onions quicker than the brine. Viability of good bacteria require the fibres and sugar from the onions. 

Read “Onion peels: 7 benefits and 6 uses” for more tips to increase your daily intake of quercetin and why onions are an excellent source.

Print Recipe
5 from 2 votes

Fermented Onions Recipe

Subdue the pungent taste of raw onion and add a subtle sweet note by lacto-fermenting onions. Fermentation makes raw onions more palatable, while reserving and enhancing the health benefits. Onions, red onions in particular, are a high source of quercetin, a flavonol plant compound, with anti-inflammatory, antioxidant, antihistamine, antiviral, and cardio- and neuroprotective properties. Fermentation boosts the power of quercertin, so it can work better in your body.
Prep Time10 minutes
Cook Time0 minutes
Fermentation time28 days
Course: Side Dish


  • 1.5 L Airlock Fermenter, knife, cutting board, (mandoline, optional)


  • 5-6 organic red onions, peeled and sliced (reserve the peel)
  • 600 mL non-chlorinated, filtered water
  • 12 grams non-iodized sea salt (about 3/4 tablespoon fine-grain salt)


  • Make a 2% brine: dissolve the salt in hot water; set aside to cool.
  • Wash onions to remove any dirt from the papery onion peel; remove and reserve. Cut any other papery area of the next outer layer underneath the peel; reserve. Half the remaining onion from tip (scape) to root. Slice onions crosswise, about 3-5 mm thick. Determine the thickness of your slices depending on how you intend to use these onions after fermentation (i.e. on hamburgers or tacos, in soups or stews). Alternatively, chop the onions in whatever shape lends well to recipes that call for onions.
  • Place the reserved onion peels into the bottom of a clean 1.5 L Airlock Fermenter. Layer the sliced onions over top, gently packing them with a clean fist or fermentation tamper. Pack jar with onions half an inch below the shoulder. Place a flat glass weight on top. Pour brine overtop to the neck of the jar or until it covers the glass weight by half an inch. (You may have brine left over.)
  • Fill the large airlock with water to the fill line, and insert through the lid. Latch lid and clamp shut. Store at room temperature, between 18-21ºC, out of direct sunlight for 7 days days, then transfer to a fridge or cold storage (<12ºC) for 3 weeks. Will keep for 1 year in an unopened Airlock Fermenter.
  • When ready to eat, replace airlock with the plug. Refridgerate after opening. Ensure onions stay submerged under brine. Discard the onion peels once the onions are consumed. Use within 3-6 months.

3 replies on “Fermented onions – supercharged for health”

Adding the fermented onion, or any ferment, to dishes and foods that are heated will destroy the good bacteria and the benefits gained by eating them. I would advise against heating.

5 stars
Can I ferment onion juice made from pureed red onions and then drink the juice. I plan on keeping it in the refrigerator.

Great article.

5 stars
Great tips on the quercetin. I have a batch bubbling away now. I Added a little starter from some fermented tomatoes. Next time I will try without. I was wondering why you transfer to the fridge for 3 weeks? Is this to continue the ferment slower to complete. If left out at room temperature to complete would something bad happen?

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