Fermenting tames onions. Sliced and salted, then left under their own juice for 3–7 days, they lose much of their raw bite and turn tangy, soft-crisp and gently sweet – good on burgers, tacos, grain bowls and sandwiches. The recipe card below is the everyday version: sliced onions with 2% salt by weight, kept under the liquid with a weight. Pearl onions work too, whole in a brine (below).
What Happens in the Jar
As with other vegetables, salt lets the lactic acid bacteria already on the onions outcompete spoilage microbes (NCHFP). In a study of onion fermentation, the main microbes were lactic acid bacteria (including Lactobacillus plantarum), acetic acid bacteria and yeasts; lactic and acetic acid rose, and esters, alcohols and aldehydes formed that give fermented onion its flavor. Salt changed the result: salted onions built up less acid but kept more of their onion character, because salt limited the breakdown of the sulfur compounds (Cheng et al., 2014). So expect a fermented onion that’s milder and tangier than raw, but still clearly oniony – not a caramelized onion.
Pearl Onions in Brine
Whole pearl onions don’t release enough liquid, so they need a brine. To peel them, cut a small cross in the root end, blanch 30 seconds, then plunge into cold water and slip off the skins. Pack about 450 g into a jar with ½ tsp peppercorns, ½ tsp mustard seeds and a bay leaf, and cover with a 3% brine (15 g salt in 500 ml water). Weight them down and ferment 5–10 days at 18–22°C, until tangy right through, then refrigerate. They make good cocktail onions for a Gibson.
Why We Don’t Ferment Onions in Honey
Recipes for “honey-fermented onions” pack raw onions in raw honey at room temperature. We’ve dropped that method. Onions are low-acid – pH about 5.3–5.9 (K-State pH chart) – and botulism bacteria can’t be ruled out in low-acid foods kept moist, airless and warm. You can’t see, smell or taste botulinum toxin (CDC). Our fermented honey guide explains the same problem with garlic in honey. For a sweet-sour onion condiment, drizzle a little honey over salt-fermented onions when serving.
Good to Know
- Red onions turn bright pink as the brine acidifies; that’s normal.
- Smell: a strong, sulfury onion smell is normal while fermenting. A putrid smell, slime, or fuzzy colored mold means discard.
- FODMAPs: onions are a common trigger for people with IBS, and there’s no testing showing fermented onions are low-FODMAP. If you avoid onions, treat these the same.
- Nutrition: raw onions have about 40 kcal, 1.7 g fiber and 7.4 mg vitamin C per 100 g (USDA). Eaten uncooked, fermented onions also contain live lactic acid bacteria.
Onions also go into other ferments: Salvadoran curtido, fermented salsa and kimchi. For a milder allium, try fermented leeks. If your jar misbehaves, see our troubleshooting guide.
