How to Keep Fermented Foods Anaerobic

I’ve seen a lot of confusion about keeping vegetable ferments anaerobic and just wanted to share some facts about vegetable fermentation that will help clear up the confusion. Keeping fermented foods anaerobic is quite simple: you don’t create an anaerobic environment, microbes do. You don’t ferment, microbes do. You just have to set them up for success.

Keeping Fermented Foods Anaerobic

Fermenting vegetables become anaerobic in the first few days of fermentation. This happens because salt-tolerant, oxygen-utilizing microorganisms proliferate in the first stage of vegetable fermentation. These microbes consume most of the oxygen within the liquid. After about two days they use up all of the oxygen and make the vegetable ferment anaerobic. Specifically, everything submerged within the brine is safe from oxygen exposure after this occurs. To keep things submerged all you need is a fermentation weight.

Once most of the oxygen is used up by aerobic microbes, the aerobic microbes can’t survive and die off. This makes room for anaerobic heterolactic fermentative bacteria called Leuconostoc spp. They make CO2 (aka bubbles), acetic acid, and lactic acid. The Leuconostoc bacteria further contribute to anaerobiosis by producing copious amounts of CO2, and you can see this happen by observing bubbles in the fermentation.

The Leuconostoc bacteria end up producing so much CO2 and acids that they can no longer survive in the environment and they die off. This process of microbes thriving, self-limiting, then dying is called bacterial succession.

Then Lactobacillus spp. are able to dominate and thrive. They are homolactic fermenters and produce ample amounts of one substance: lactic acid. Lactobacillus love salt, anaerobic conditions, and acid. They produce enough acid to fully preserve fermented vegetables. They can survive for about 6-9 months in refrigerated conditions once they are the dominant population.

So lack of oxygen does not preserve the vegetables, and salt does not preserve the vegetables. Lack of oxygen in tandem with the presence of salt encourages certain types of probiotic microorganisms to produce acids. The acid preserves the vegetables.

To summarize:

1. You provide a specific salt concentration and vegetable matter.
2. Oxygen using microbes, metabolize most of the oxygen and create an anaerobic environment within the brine.
3. You now have a very selective, salty, oxygen-free environment.
4. This environment causes oxygen-using microbes to die and allows Leuconostoc bacteria from the vegetables to thrive.
5. Leuconostoc bacteria contribute further to anaerobiosis and acidification by producing carbon dioxide, lactic acid, and acetic acid.
6. You now have vegetables that are salty, oxygen-free, and acidic.
7. The only thing that thrives in this three-fold selective environment is Lactobacillus spp.
8. Lactobacillus spp. produce enough lactic acid to drop the pH and fully preserve the fermenting vegetables.

Reference Materials

National Research Council (US) Panel on the Applications of Biotechnology to Traditional Fermented Foods.Washington (DC): National Academies Press (US); 1992.