Probiotics: alive or not
A homemade ferment carries live microbes, but that doesn't make it a probiotic. What the word means, what kills microbes, and what is and isn't known.
6 min read First steps Reviewed October 26, 2026 26 facts, each with its source
What a probiotic is
The definition FAO and WHO use is this: live microorganisms that, when taken in adequate amounts, confer a health benefit on the person taking them1. It has three conditions, and all three count. They have to be alive, there have to be enough of them, and a benefit has to have been shown. What's more, that effect seems to depend on the specific strain, not just the species2.
Probiotics are found in yogurt and other fermented foods, in supplements and even in beauty products3. But not everything fermented is a probiotic.
Fermented isn't the same as probiotic
A panel of experts in microbiology and nutrition put it plainly in 2021: the word “probiotic” should only be used when a benefit has been shown with well-defined, characterized live microorganisms4. That's why fermented food and probiotic are not interchangeable terms5.
Their example is sauerkraut. A traditional, spontaneously fermented one most likely carries several strains of Lactiplantibacillus plantarum. But they're unidentified strains, at unknown doses, and they don't qualify as probiotics6. Your homemade sauerkraut is alive. That doesn't make it a probiotic.
For what is alive, the right wording is that it contains “live and active cultures,” as long as they haven't been removed7. For what no longer is, that it's a food “made by fermentation”8. Fervida uses those words and never calls a homemade ferment a probiotic: it's a house rule.
What's alive and what isn't
| Food | Live microbes? |
|---|---|
| Unheated sauerkraut, kimchi and pickles | Yes9 |
| Kombucha | Most, yes10; some, no11 |
| Pasteurized or heat-treated fermented vegetables, vinegar | No11 |
| Sourdough bread | No: it's baked after fermenting12 |
| Wine and most beers | No: they're removed during production13 |
Having no live microbes doesn't stop them being fermented foods14. They still have the acids, the aromas and the changes fermentation brought.
What kills the microbes
- Heat. It's what kills them most. Bread loses its microbes in the oven12, and fermented vegetables lose theirs when pasteurized11. The same goes for what you cook yourself: fried tempeh, or miso boiling in a soup.
- Canning. To can sauerkraut at home, it's brought to a boil with its juice before going into jars15. Canned sauerkraut is no longer alive.
- The freezer. Some bacteria die, but not all16.
- The fridge. It doesn't kill them: it slows them down. In the cold, fermentation slowly continues17, and unheated sauerkraut stays alive.
What is and isn't known
We know microbes in fermented foods can survive the stomach and reach the colon18. We also know they're unlikely to settle in the gut for long19: they're passing through.
What we don't know is how much it matters that they're alive. In a trial with people with irritable bowel syndrome, raw and pasteurized sauerkraut gave the same result: the effect may not depend on live microbes20. In another, with children on antibiotics, live kefir did no better than heat-treated kefir at avoiding diarrhea21. The thinking is that what counts is everything together: the ingredients, what fermentation leaves behind, and the microbes, alive and dead22.
The same goes for probiotics in general: they've been studied a lot, but much remains to be learned about what they help with and how safely23.
When to talk to your doctor
The risk of harmful effects from probiotics is greater in people with severe illnesses or weakened immune systems24. There have been serious infections, some fatal, in premature infants who were given them25. A homemade ferment carries live, unidentified microbes.
If your immune system is weakened, you're pregnant or you want to give them to a baby, talk to your doctor first. And if you're thinking about a probiotic supplement, check with them before you start too26.
Where every number comes from
Every number in the text leads to its figure: the exact sentence from the official guide it comes from, in its own language.
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1
Probiotic: live microorganisms that, in adequate amounts, confer a health benefit
“Live microorganisms which when administered in adequate amounts confer a health benefit on the host”
FAO and WHO · Probiotics in food: health and nutritional properties and guidelines for evaluation (FAO Food and Nutrition Paper 85, 2006) · read on October 26, 2026
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2
The probiotic effect seems to be strain-specific
“the probiotic effect seems to be strain specific.”
FAO and WHO · Probiotics in food: health and nutritional properties and guidelines for evaluation (FAO Food and Nutrition Paper 85, 2006) · read on October 26, 2026
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3
Probiotics are in yogurt and other fermented foods, supplements and beauty products
“Probiotics are live microorganisms that are intended to have health benefits when consumed or applied to the body. They can be found in yogurt and other fermented foods, dietary supplements, and beauty products.”
National Center for Complementary and Integrative Health (NIH) · Probiotics: usefulness and safety · read on October 26, 2026
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4
It's only a probiotic if a benefit is shown with well-defined microbes
“the term ‘probiotic’ should only be used when there is a demonstrated health benefit conferred by well-defined and characterized live microorganisms.”
Nature Reviews Gastroenterology & Hepatology, Marco et al. · The International Scientific Association for Probiotics and Prebiotics (ISAPP) consensus statement on fermented foods (2021) · read on October 26, 2026
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5
Fermented food and probiotic are not interchangeable
“For these reasons, the terms ‘fermented food’ and ‘probiotics’ cannot be used interchangeably”
Nature Reviews Gastroenterology & Hepatology, Marco et al. · The International Scientific Association for Probiotics and Prebiotics (ISAPP) consensus statement on fermented foods (2021) · read on October 26, 2026
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6
The unidentified strains of traditional sauerkraut, at unknown doses, aren't probiotics
“traditional, spontaneously fermented sauerkrauts likely contain multiple strains of Lactiplantibacillus plantarum (previously Lactobacillus plantarum), but these uncharacterized and unidentified strains, at unknown doses, would not qualify as probiotics.”
Nature Reviews Gastroenterology & Hepatology, Marco et al. · The International Scientific Association for Probiotics and Prebiotics (ISAPP) consensus statement on fermented foods (2021) · read on October 26, 2026
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7
The right wording is “live and active cultures” if they haven't been removed
“manufacturers should state only that their product contains “live and active cultures” provided the food is not processed to remove or kill the fermentation microorganisms”
Nature Reviews Gastroenterology & Hepatology, Marco et al. · The International Scientific Association for Probiotics and Prebiotics (ISAPP) consensus statement on fermented foods (2021) · read on October 26, 2026
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8
Without live microbes, “made by fermentation”
“For pasteurized fermented foods without live microorganisms in the final product, it is acceptable to label those foods as “foods made by fermentation””
Nature Reviews Gastroenterology & Hepatology, Marco et al. · The International Scientific Association for Probiotics and Prebiotics (ISAPP) consensus statement on fermented foods (2021) · read on October 26, 2026
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9
Unheated fermented vegetables have live microbes
“Non-heated fermented vegetables”
Nature Reviews Gastroenterology & Hepatology, Marco et al. · The International Scientific Association for Probiotics and Prebiotics (ISAPP) consensus statement on fermented foods (2021) · read on October 26, 2026
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10
So do most kombuchas
“Most kombuchas”
Nature Reviews Gastroenterology & Hepatology, Marco et al. · The International Scientific Association for Probiotics and Prebiotics (ISAPP) consensus statement on fermented foods (2021) · read on October 26, 2026
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11
Pasteurized or heat-treated vegetables, vinegar and some kombuchas don't
“Heat-treated or pasteurized fermented vegetables, sausage, soy sauce, vinegar and some kombuchas”
Nature Reviews Gastroenterology & Hepatology, Marco et al. · The International Scientific Association for Probiotics and Prebiotics (ISAPP) consensus statement on fermented foods (2021) · read on October 26, 2026
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12
Bread is baked after fermenting, and baking kills the microbes
“Fermented foods, such as leavened breads, are baked after fermentation, effectively killing the fermentation microorganisms.”
Nature Reviews Gastroenterology & Hepatology, Marco et al. · The International Scientific Association for Probiotics and Prebiotics (ISAPP) consensus statement on fermented foods (2021) · read on October 26, 2026
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13
Most wines and beers go through steps that remove live microbes
“The manufacture of some fermented foods (for example, most beers and wines) includes steps to remove live microorganisms from finished products.”
Nature Reviews Gastroenterology & Hepatology, Marco et al. · The International Scientific Association for Probiotics and Prebiotics (ISAPP) consensus statement on fermented foods (2021) · read on October 26, 2026
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14
Without live microbes, they're still fermented foods
“Although microbial inactivation or removal is not common to all fermentation processes, these products still qualify as fermented foods.”
Nature Reviews Gastroenterology & Hepatology, Marco et al. · The International Scientific Association for Probiotics and Prebiotics (ISAPP) consensus statement on fermented foods (2021) · read on October 26, 2026
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15
For canning, sauerkraut is brought to a boil
“Hot pack – Bring kraut and liquid slowly to a boil in a large kettle, stirring frequently.”
National Center for Home Food Preservation (USDA, University of Georgia) · Sauerkraut · read on October 20, 2026
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16
Freezing kills some bacteria, but not all
“Some of the beneficial bacteria could die during the freezing process, but freezing will not kill all of the bacteria.”
Virginia Cooperative Extension (Virginia Tech) · Vegetable fermentation (FST-328, reviewed 2025) · read on October 26, 2026
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17
In the fridge, fermentation slowly continues
“Expect subtle changes in flavor even during refrigeration (the process slowly continues).”
University of Illinois Extension · Fermentation handout (2020) · read on October 20, 2026
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18
They can survive the stomach and reach the colon
“Multiple studies in humans have shown that microorganisms in fermented foods can survive gastric transit and reach the colon”
Nature Reviews Gastroenterology & Hepatology, Marco et al. · The International Scientific Association for Probiotics and Prebiotics (ISAPP) consensus statement on fermented foods (2021) · read on October 26, 2026
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19
They're unlikely to settle in the gut for long
“Although these microorganisms are unlikely to maintain long-term residence in the intestine”
Nature Reviews Gastroenterology & Hepatology, Marco et al. · The International Scientific Association for Probiotics and Prebiotics (ISAPP) consensus statement on fermented foods (2021) · read on October 26, 2026
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20
Sauerkraut's effect may not depend on live microbes
“This may suggest that the perceived health benefit of sauerkraut is independent of the live microbes.”
Nutrients (MDPI), Dimidi et al. · Fermented foods: definitions and characteristics, impact on the gut microbiota and effects on gastrointestinal health and disease (2019) · read on October 26, 2026
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21
Live kefir did no better than heat-treated kefir against antibiotic diarrhea
“This study showed that kefir with live microorganisms did not improved the rates of antibiotic-associated diarrhoea compared to control”
Nutrients (MDPI), Dimidi et al. · Fermented foods: definitions and characteristics, impact on the gut microbiota and effects on gastrointestinal health and disease (2019) · read on October 26, 2026
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22
Live microbes, dead ones and what fermentation leaves are all thought to count
“It is expected that the modulation of the human immune system by fermented foods would be the result of the combined effects of compounds present in the starting ingredients and those formed during fermentation as well as of living and dead or inactivated microorganisms.”
Nature Reviews Gastroenterology & Hepatology, Marco et al. · The International Scientific Association for Probiotics and Prebiotics (ISAPP) consensus statement on fermented foods (2021) · read on October 26, 2026
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23
Much remains to be learned about their usefulness and safety
“A great deal of research has been done on probiotics, but much remains to be learned about whether they’re helpful and safe for various health conditions.”
National Center for Complementary and Integrative Health (NIH) · Probiotics: usefulness and safety · read on October 26, 2026
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24
More risk with severe illness or a weakened immune system
“The risk of harmful effects from probiotics is greater in people with severe illnesses or compromised immune systems.”
National Center for Complementary and Integrative Health (NIH) · Probiotics: usefulness and safety · read on October 26, 2026
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25
Serious infections have occurred in premature infants given probiotics
“Cases of severe or fatal infections have been reported in premature infants who were given probiotics”
National Center for Complementary and Integrative Health (NIH) · Probiotics: usefulness and safety · read on October 26, 2026
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26
Before a probiotic supplement, talk to your doctor
“If you’re considering a probiotic dietary supplement, consult your health care provider first.”
National Center for Complementary and Integrative Health (NIH) · Probiotics: usefulness and safety · read on October 26, 2026