Frederick Breidt on Understanding the Science Behind Vegetable Fermentation

Frederick Breidt on Understanding the Science Behind Vegetable Fermentation

The buffer powder and a pH meter I use at home to test acidity levels in fermented vegetables

The buffer powder and a pH meter I use at home to test acidity levels in fermented vegetables

At a glance

Food scientist Frederick Breidt shares how to use science to ferment vegetables the right way.

What you'll learn

  • Why measuring the pH of the fermented vegetables is important
  • Whether nitrates and nitrites generated in the vegetable fermentation process is a concern
  • Difference between probiotics and fermented vegetables
  • Why adding starter cultures may not be a good idea
  • The benefit of adding vinegar

Let's begin.

I reached out to Frederick Breidt after Sandor Katz of <The Art of Fermentation> told me that Fred is the perfect person to learn about the science of vegetable fermentation.

After an hour-long phone conversation with him and multiple exchanges of emails, I developed great respect for this food scientist who works for the USDA because of his dedication to his research, his genuine heart to serve people, and his patience in explaining details to me. 

Our talks gave me a lot of reassurance, confidence, and clarity in terms of making fermented vegetables. I believe every consumer can benefit from his sharing. Below are the highlights of what I have learned (you can find more details in my book).

Note that the answer given by Fred might not be the exact words that he used. But they convey the same meaning.

How to make sure fermented vegetables I make are ready to serve?

The pH of the fermented vegetables should drop to at least 4.6 before you eat them.

Understanding the pH of the batch of fermented vegetables is important. pH 4.6 is the key cutoff because that is the point when Clostridium botulinum gets killed. Generally, the longer you wait, the lower pH you get, and the safer they are to eat.

Tracy’s note: this bacterium can cause botulism, which is a rare but serious paralytic illness which leads to weakness in the body, poor vision, trouble speaking, and even potentially death. To measure the pH, you can consider using either pH testing papers or a pH test meter. Check out this page to learn what I use at home and how to use them.

Some people are concerned with the nitrates and nitrites in fermented vegetables. What is your take on that?

We don’t know enough about this yet. In the US, it may not be a big problem; and more investigations are looking into this right now, too.

Tracy’s note: through my research, I also found out that nitrates and nitrites should not be a concern, as long as you make and consume fermented vegetables properly. The pH should reach 4.6 (as mentioned above) or below.  Also, always remember that fermented vegetables should only be consumed in moderation only. If interested, you may stay up to date with my latest findings by filling out the form on Vegetable Fermentation Resources page.  

How to deal with molds and yeasts during fermentation?

In general, if you see a small amount of surface yeasts, you don’t have to always worry about that. Usually, skimming the brine surface will be okay. But if you see there is a thick layer of yeasts and molds on the top of the jar, you should throw away the whole batch. The most important point is to watch the pH. If the pH starts to rise, then you have a problem!

Can you tell us more about the difference between probiotics and fermented vegetables?

Within a species of bacteria, there are different strains. Probiotics mean specific strains which have been characterized to show specific human health effects, while other strains of the same species may not have them and in fact probably don’t.  To solve a particular health problem, we might need very strain-specific probiotics to solve it.

There might be general characteristics of lactic acid bacteria in fermented vegetables that could be helpful to you. But some Lactobacillus strains might not be as helpful to you in fighting disease, which requires a specific strain in the species to trigger the immune system to reduce incidences of disease, whereas other strains may not.

Tracy’s note: a lot of people wonder if taking probiotic supplements and eating fermented vegetables will give the same health benefits. Fred’s answer sheds some light here. If you simply aim to improve overall health, eating fermented foods may give you some benefits. But if you hope to treat a specific health problem, certain probiotics may give you a better chance of healing the body. Make sure you consult your doctor for more knowledge in this regard.

What is your opinion on using starter cultures to ferment vegetables?

Starter cultures can cause the pH to drop so quickly that the acidity levels become unstable.

Starter cultures can speed up the fermentation process by adding bacteria and sugar (a lot of the freeze-dried starter cultures contain sugar as well). One concern for the sudden drop of the pH is that it can influence the stability of the acidity levels. In other words, the pH can rise again even after it is well below 4.6.

While it is totally fine to use starter cultures for vegetable fermentation, you always have to make sure the pH of the vegetables is stable. That’s why checking the pH from time to time is recommended.

Is it okay to add vinegar?

A Large scale of commercial processors add a little bit of vinegar, which lowers the pH and helps create a favorable environment for fermentation; at the same time, a small amount of vinegar serves as a “buffer” to prevent the pH from dropping too quickly to allow a slow and steady fermentation.

I found that the fermentation happens very slowly in fermented leafy vegetables. And the pH wouldn’t drop to below 5 even after two weeks. How to solve this problem?

Slow fermentation in leafy vegetables can be due to there being not enough sugar content. Adding vegetables with more sugar content – such as cabbage – can help speed up the fermentation process.

Tracy’s note: because of what I learned from my own kitchen experiments and Fred’s comments, I recommend you add vegetables with more sugar content if you plan to ferment leafy greens.

My thoughts

Testing the pH of fermented vegetables is a great way to ensure food safety. I highly recommend you do this if you are new to vegetable fermentation and look for a benchmark to rely on to decide when to start enjoying your first batch.

At the same time, it can be very powerful if we revive traditional practice while perfecting it with science. That way we not only pass traditional food wisdom down to next generations but also leverage it with the best of our ability to receive clearer guidance in following our ancestors’ path to wellness.

In closing, I’d like to share this quote from my all-time favorite book Nourishing Traditions:

The wise and loving marriage of modern invention with the sustaining, nurturing food folkways of our ancestors is the partnership that will transform the Twenty-First Century into the Golden Age.
— Sally Fallon and Mary Enig

About Fred

Frederick Breidt is a food scientist and microbiologist who works for Agricultural Research Service (ARS) at United States Department of Agriculture (USDA) since 1990. He is also a professor in the Department of Food, Bioprocessing, and Nutrition Sciences in North Carolina State University.

Fred and his team have a website called Pickle Bibliography that hosts existing and upcoming research papers regarding vegetable fermentation.


  • The pH of the fermented vegetables should reach at least 4.6 or below before you consume them.
  • You don’t have to worry about the nitrates and nitrites in fermented vegetables as long as you make them properly and eat them only in moderation.
  • If you only see a thin layer of surface yeasts, you just need to scrape them off. But if you see a thick layer of molds, you should throw away the whole batch.
  • Both probiotics and fermented vegetables have their health benefits.
  • Using starter cultures can cause pH instability in the fermented vegetables. You’ll have to make sure the pH is steady and below 4.6 before you eat them.
  • Adding a little bit of vinegar is beneficial for vegetable fermentation.
  • If you want to ferment leafy greens, consider adding veggies with more sugar content (e.g.  cabbage).

My question to you

What is the one thing you will do differently as you ferment vegetables next time?


Have fun and always keep exploring!

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Want to learn more details about the topics discussed above and my other research findings? Check out my new book.

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