Fermented Red Onions

I LOVE fresh and pickled red onions... a lot. We always have red onions in our house and we never, ever run out. We put them in quinoa bowls, in burrito bowls, on avocado toast, and pretty much any savory meal. Want to know what we love more than pickled red onions though? Fermented Red Onions! It's the same great taste of pickled red onions, but with an awesome purpose: gut health! 

We fermented some for two weeks and some for four weeks, and they are delicious! I prefer the four week fermented batch because they are just a bit more tart. Four weeks allows for the Lactobacillus in stage three of the fermentation to make more lactic acid, dropping the pH a bit more, and allowing the flavor to develop. 

If you are curious what the elusive probiotic Lactobacillus look like, here is a picture from my microscope:


  This is a diluted sample of the two week fermented red onion brine. It's rich in probiotic Lactobacillus and there are still a few Leuconostoc bacteria remaining. The chain of rod shaped bacteria in the middle is Lactobacillus. The two little round bacteria to the right are Leuconostoc bacteria. The big blob to the left is a piece of onion.  I love using microscopy, because it validates the safety and probiotic quantity of everything we ferment.  When developing recipes we always keep the focus on the microbes, whats best for selecting just friendly species of microbes, and what allows for a high population of those friendly microbes. Then we share recipes like this based on science and facts! 


The brine method: It is explained in detail in the instructional steps below. Basically, we chop and then weigh the ingredients, then add 2.5% of that weight in salt. Then we add a 2.5% salt water solution to the mix. Read the science behind why salt concentration matters. 
If you're going to master fermentation you'll need to use weight measurements for your fermentation ingredients. That means you need a kitchen scale. In order to select for the best probiotic bacteria (the ones that are actually beneficial and not pathogens) in your ferments, you must weigh salt to create a specific salt concentration. Weighing salt is the only way to create a salt concentration that will select for only probiotic microbes to thrive.  This is the scale we use in our home kitchen to weigh salt.  


Supply List:


If you're new to fermentation, we offer fermentation supply starter kits on our shop page that includes everything you need to start fermenting some veggies at home, you just need to have a scale and fresh veggies. 

To shop for a Wide Mouth Jar Starter Kit click here and to shop for a Regular Mouth Jar Starter Kit click here

Fermented Red Onions




  1. First, make a 2.5% saltwater brine. To do this, put an empty large jar on your kitchen scale, and tare/zero the scale. (Taring/Zeroing the scale with a container on it subtracts the weight of the container, allowing you to weigh only what is added to the container). Fill the jar with warm filtered water and record the weight of the water (in grams).
  2. Next multiply the weight of the water by 0.025, the number you get is the amount in grams of salt you need to add to the water.
  3. Place a small separate bowl on your scale and tare the scale. Weigh out your Fermentation Salt in grams. 
  4. Add your salt to the water and stir it in until it dissolves. Then let the mixture cool. 
  5. While the salt water cools, chop your onions however you desire. I perfer to chop the onions in slivers.  
  6. Place an empty bowl on your kitchen scale and tare it. Place the chopped onion in the bowl and record the weight in grams. Using the same math method, multiply the weight of your chopped onion by 0.025. The number you get is the grams of salt you need to add to the chopped onion.
  7. Place a small separate bowl on your scale and tare the scale. Weigh out your Fermentation Salt in grams. 
  8. Add your salt to the onions and mix until evenly distributed. If mixing with your hands, you may want to wear gloves so your hands dont stain purple or smell like onion.
  9. Once you have mixed the onion and the salt thoroughly, it is time to add the mixture into an empty, clean jar. We used a pint sized mason jar for this recipe. Before you add in the onion pieces, you need to dump any liquid from the bowl into the jar. Then add in all the onion pieces.
  10. Place a fermentation weight in the jar to hold everything down. Then once the saltwater brine you made is cool, pour it over the onions and fermentation weight until everything, including the fermentation weight is submerged. 
  11. Store your fermentation jar in a cool place at room temperature out of direct sunlight. It should be kept in an area that stays around 70-75 degrees F
  12. During the first few days of fermentation carbon dioxide and  bubbles will be produced. During this time if you are using a regular mason jar lid, you will need to burp the jar to let the gas escape. To burp the jar, just remove your regular mason jar lid and let the gas out. Both those using and those not using an airlock lid must observe the jar and make sure that the fermentation weight and vegetable pieces are staying submerged BELOW the brine. You may have to remove the lid, press everything down with a clean spoon/tamper, and then replace the lid. 
  13. The onions should ferment for 2-4 weeks, so that all the stages of bacterial succession can occur. The time line is a little different from a cabbage fermentation, due to a different micorbial population on onions. 2-4 weeks also allows enough time for the final stage Lactobacillus bacteria to produce enough lactic acid to drop the pH to preservation levels. Once the fermenation process is finished, store the onions in the refrigerator for up to a year. Visit our Science of Fermentation blog to read more about the stages of fermentation!

We tracked our onions throughout the fermentation process. By checking the progress of microbial stages under the microscope we have provided you with this handy timeline! If you follow our recipe and directions, your timeline of red onion fermentation should approximately match ours! 

24 - 72 hours: all contents in the jar should be submerged beneath the brine. At this time there are still Gram negative bacteria and possible pathogens present. 

72 hours - 5 days: After 72 hours you should start to see lots of bubbles being produced. This is the stage in which you will burp the jar. This is when the ferment enters stage two of vegetable fermentation. Leuconostoc bacteria begin to thrive and produce a lot of carbon dioxide. Gram negative organisms die off. 

5 - 14 days: The bubbles in the brine will decrease, as the ferment leaves stage two and enters stage three. The red onions will become cloudy and start to develop a pleasant sour smell. The onions will also start to turn fully pink. Lactobacillus species are most abundant during this time period.

14 - 28 days: Lactobacillus make up majority or all of the microbial population. They produce copious amounts of lactic acid, and make the ferment smell even more pleasantly sour. This is the time in which the vegetable mixture becomes preserved. This is when you want to smell and taste test.
 30 days: Wait for the onions to smell and taste as you like, and refrigerate when you find the smell and taste most pleasant! We like ours best when we refrigerate at about 30 days, but they are also good at around 15-20 days. The longer the red onions ferment, the more the flavors develop.


Peace, Love and Probiotics,