Fermenting with care: botulism and how to avoid it
Botulism is rare, serious and invisible. You don't avoid it by looking at the jar but by following a process: weighed salt, everything under the brine, measured pH and the fridge.
7 min read First steps Reviewed October 27, 2026 42 facts, each with its source
What botulism is
Botulism is an illness caused by a toxin that attacks the nerves and weakens the muscles1. It's uncommon, but sometimes fatal2. It's made by Clostridium botulinum, a bacterium that lives in soil and in water sediments as spores3, a hardy form that can reach your kitchen with the soil on any vegetable.
What sets it apart is that it gives no warning. The toxin doesn't change the color, smell, taste or texture of food4, and a small taste can be deadly5. That's why, at Fervida, safety never depends on looking at, smelling or tasting the jar.
To keep it in proportion: Spain reported 28 cases in all of 20236, and the greatest risk is still home canning7. The most common sources are homemade foods that were improperly canned, preserved or fermented8. It's rare, and a process prevents it.
Why a well-made ferment holds it back
C. botulinum only grows without oxygen9. A ferment under its brine is exactly that: fermenting creates an oxygen-free environment where spores carried by the vegetables could grow and produce toxin10.
What stops it is acid. Below pH 4.6, the bacterium doesn't develop and the toxin doesn't form11. Lactic acid bacteria make that acid: without oxygen, they turn the vegetables' sugars into lactic acid, which holds back harmful bacteria12. Salt gives them the edge from day one, because it favors the good bacteria and holds back the rest13.
So a ferment isn't protected just by being a ferment. Reaching acidity is what protects it. Until the pH has dropped, the jar is at its most delicate14. And if it spoils later and the pH climbs back above 4.6, spores can germinate15.
Canning works differently. In water-bath canning, heat does the protecting; low-acid vegetables, such as asparagus or green beans, can only be canned at home in a pressure canner16. In a ferment, acid does the protecting. That's why fermented asparagus or green beans are checked with pH.
What Fervida requires in every batch
- Weighed salt, never below 2%. Reducing salt in a vegetable ferment is unsafe17, and the NCHFP warns against cutting back on salt in sauerkraut or fermented pickles18. The 2% is a house rule: the engine won't work out anything below it.
- Everything under the brine, and no mold. If mold shows up anywhere in the ferment, it's discarded19. A spoiled ferment is the one whose pH can climb back up15.
- pH, checked. The acid has to reach the whole product evenly20. The bacterium doesn't grow at pH 4.6 or below21; Fervida recommends waiting for 4.2 before moving vegetables to the fridge, as a house rule, to leave a margin. You check it with pH test strips22 or a calibrated pH meter. Where pH is the safety barrier, as in kimchi, hot sauce, kombucha or fruit in honey, measuring it is required and, without a pH meter, the app asks for strips before you start. For sauerkraut and brined pickles, it recommends it.
- A tested recipe. Only recipes with tested proportions23. At Fervida, the method sets the salt, temperature, times and pH, each figure with its source; the variation only adds flavor.
- The fridge, once it's done. Cold, salt and pH together are what keep the bacterium in check24.
What Fervida never does
Some preparations give the bacterium exactly what it needs. Fervida's engine blocks them, and no suggestion, including from AI, gets around that.
| Never | Why |
|---|---|
| Garlic fermented in honey | There's no data to support the process25 |
| Vegetables, garlic or herbs stored in oil | Oil leaves the jar without oxygen26; chopped garlic in oil has already caused botulism27 |
| Fermented fish or meat | They're among the foods most linked to botulism28 |
| Vacuum-packing a ferment | Botulism is linked to foods packed with little oxygen29 |
| Salt below the minimum | Reducing it is unsafe17 |
Spain's food safety agency gives homemade chili peppers in oil and fermented, salted or smoked fish as examples28. In Alaska, most cases come from fermented fish30.
If you make a garlic or herb oil for cooking, keep it in the fridge and throw it out after 4 days31. And one warning that isn't about ferments, but that Fervida gives whenever a recipe uses honey: no honey for children under one year old32.
What the jar won't tell you
- Bubbles prove nothing. Gas in a ferment is a good sign only if the pH stays below 4.633.
- A domed lid doesn't mean the same thing. In a canned food, bulging, rust, leaks or an odd smell are reasons to throw it out34. In a ferment that is still fermenting, gas is normal and pushes on the lid; what tells you whether it's going well is the pH, along with the smell and the absence of mold.
- Don't taste it to settle a doubt5. If in doubt, throw it out35.
- Don't eat food when you don't know how it was made36, even a jar someone gave you.
How to recognize mold19, a rotten smell or a slimy texture is covered in the guide to reading your jar.
If someone feels ill
The first symptoms are usually intense fatigue, weakness and dizziness, followed by blurred vision, dry mouth and trouble swallowing and speaking37. They usually appear 12 to 36 hours after eating, but can take anywhere from 4 hours to 8 days38.
It's an emergency: see a doctor or go to the emergency room right away, or call 911 or your local emergency number39. Most people recover if it's diagnosed and treated early40, and the only effective treatment is antitoxin41.
Don't throw the jar away. Keep it sealed, untasted and where nobody can eat from it, and tell the doctor: samples of the suspect food help identify the cause and prevent other cases42.
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
The toxin attacks the nerves and weakens the muscles
“Botulism is caused by a toxin that attacks the body's nerves and causes muscle weakness.”
CDC · Botulism: symptoms · read on October 26, 2026
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2
It's uncommon, but sometimes fatal
“Esta enfermedad es poco frecuente, pero en ocasiones es mortal”
AESAN · Conservas caseras · read on October 26, 2026
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3
The bacterium lives in soil as spores
“Este organismo se encuentra en el suelo y los sedimentos acuáticos y puede sobrevivir en estos ambientes en forma de espora.”
Instituto de Salud Carlos III, Centro Nacional de Epidemiología (RENAVE) · Informe de botulismo 2023 · read on October 25, 2026
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4
The toxin doesn't change color, smell, taste or texture
“La presencia de toxinas botulínicas en las conservas no produce cambios organolépticos, por lo que no podemos detectar si un alimento está contaminado por cambios en su color, olor, sabor o textura.”
AESAN · Conservas caseras · read on October 26, 2026
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5
You can't see, smell or taste it, and a small taste can be deadly
“You cannot see, smell, or taste the toxin that causes botulism. But taking even a small taste of food containing the toxin can be deadly.”
Centers for Disease Control and Prevention (CDC) · Botulism Prevention · read on October 25, 2026
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6
28 cases reported in Spain in 2023
“En 2023 se notificaron 28 casos de botulismo (14 confirmados, 6 probables y 8 sospechosos), de los que dos casos fueron importados (uno confirmado de Argentina y uno probable de Francia).”
Instituto de Salud Carlos III, Centro Nacional de Epidemiología (RENAVE) · Informe de botulismo 2023 · read on October 25, 2026
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7
The greatest risk is still home canning
“el mayor riesgo sigue estando asociado a las conservas caseras”
Instituto de Salud Carlos III, Centro Nacional de Epidemiología (RENAVE) · Informe de botulismo 2023 · read on October 25, 2026
-
8
Common sources: homemade foods improperly canned, preserved or fermented
“Common sources of botulism are homemade foods that have been improperly canned, preserved, or fermented.”
Centers for Disease Control and Prevention (CDC) · Botulism Prevention · read on October 25, 2026
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9
It only grows without oxygen
“El Clostridium botulinum es una bacteria anaerobia, lo que significa que sólo se desarrolla en ausencia de oxígeno.”
Agencia Española de Seguridad Alimentaria y Nutrición (AESAN) · Botulismo (ficha informativa, 30/08/2019) · read on October 26, 2026
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10
Fermenting creates an oxygen-free environment where spores could grow
“Whenever we are doing a fermentation, we are creating an anaerobic environment (no oxygen) and this can create an opportunity for Clostridium botulinum spores that are present in the food to grow and produce botulism toxin.”
Ask Extension (Joy Waite-Cusic, Oregon State University; Clackamas County, Oregon) · Fermented honey #822983 (2023) · read on October 25, 2026
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11
It doesn't develop below pH 4.6
“Clostridium botulinum no se desarrolla en condiciones de acidez (pH inferior a 4,6), y por lo tanto la toxina no se generará en alimentos ácidos.”
Agencia Española de Seguridad Alimentaria y Nutrición (AESAN) · Botulismo (ficha informativa, 30/08/2019) · read on October 26, 2026
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12
Lactic acid bacteria turn sugar into lactic acid, which holds back harmful bacteria
“In an anaerobic (oxygen-free) environment, these bacteria convert sugars into lactic acid, which inhibits harmful bacteria and acts as a preservative.”
University of Illinois Extension · Fermentation handout (2020) · read on October 20, 2026
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13
Salt favors the good bacteria and holds back the rest
“In fermented foods, salt favors the growth of desirable bacteria while inhibiting the growth of others.”
National Center for Home Food Preservation (USDA, University of Georgia) · General Information on Fermenting · read on October 26, 2026
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14
Early on, the pH is not yet low enough to stop certain pathogens
“During the early stages of fermentation, the pH may not yet be low (acidic) enough to prevent the proliferation of certain pathogens.”
University of Georgia Cooperative Extension · Homemade Kimchi: A Science-Based Guide to Safe Fermentation (Circular 1362, 2025) · read on October 20, 2026
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15
If a ferment spoils and the pH rises above 4.6, spores can germinate
“C. botulinum spores may germinate in fermented foods when pH rises above 4.6 during spoilage.”
BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.1: Fermented Vegetables (August 2024, updated December 2024) · read on October 25, 2026
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16
Low-acid vegetables are only canned at home in a pressure canner
“Pressure canning is the only recommended method for canning low-acid foods.”
Centers for Disease Control and Prevention (CDC) · Botulism Prevention · read on October 25, 2026
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17
Reducing salt in a vegetable ferment is unsafe
“It is unsafe to reduce salt in lactofermented vegetables.”
UC Master Food Preservers, Inyo-Mono · Brine Math Made Easy · read on October 20, 2026
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18
Don't make sauerkraut or fermented pickles with less salt
“Caution: Do not attempt to make sauerkraut or fermented pickles by cutting back on the salt required.”
National Center for Home Food Preservation (USDA, University of Georgia) · General Information on Fermenting · read on October 26, 2026
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19
Mold anywhere means it's discarded
“If you confirm mold growth on any part of a ferment, it should be immediately discarded.”
UC Davis, Department of Food Science and Technology · Common issues with fermented fruits and vegetables (v1.0, 15-mar-2022) · read on October 20, 2026
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20
The acid must reach the whole product evenly
“There must be a minimum, uniform level of acid throughout the mixed product to prevent the growth of botulinum bacteria.”
National Center for Home Food Preservation (USDA, University of Georgia) · General Information on Fermenting · read on October 26, 2026
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21
It doesn't grow at pH 4.6 or below
“C. botulinum will not grow at pH of 4.6 or below”
BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.1: Fermented Vegetables (August 2024, updated December 2024) · read on October 25, 2026
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22
pH can be checked with test strips
“The pH can be verified using a pH test strip.”
Colorado State University Extension · Understanding and Making Kimchi (2025) · read on October 20, 2026
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23
Only recipes with tested proportions
“Use only recipes with tested proportions of ingredients.”
National Center for Home Food Preservation (USDA, University of Georgia) · General Information on Fermenting · read on October 26, 2026
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24
Cold, salt and pH together keep the bacterium in check
“Las combinaciones de baja temperatura de almacenamiento y contenidos de sal, y/o el pH, se utilizan también para prevenir el crecimiento de la bacteria o la formación de la toxina.”
Agencia Española de Seguridad Alimentaria y Nutrición (AESAN) · Botulismo (ficha informativa, 30/08/2019) · read on October 26, 2026
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25
There's no data to support garlic fermented in honey
“Garlic fermented in honey is one of those foods where we don't have data to support the safety of this process.”
Ask Extension (Joy Waite-Cusic, Oregon State University; Clackamas County, Oregon) · Fermented honey #822983 (2023) · read on October 25, 2026
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26
With oil, the jar is left without oxygen and the bacterium can grow
“Durch die Zugabe von Öl ist in dem fertigen Produkt kein Sauerstoff mehr vorhanden, und die Bedingungen sind für eine Vermehrung von C. botulinum geeignet.”
Bundesinstitut für Risikobewertung (BfR) · Selbst hergestellte Kräuteröle und in Öl eingelegtes Gemüse bergen gesundheitliche Risiken (Mitteilung Nr. 001/2016) · read on October 25, 2026
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27
Chopped garlic in oil has caused botulism
“New sources of botulism have been found. They include chopped garlic in oil”
Centers for Disease Control and Prevention (CDC) · Botulism Prevention · read on October 25, 2026
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28
Most linked to botulism: vegetables in oil, fermented or salted fish, cured meats
“Entre los alimentos más frecuentemente asociados con el botulismo están conservas caseras, principalmente conservas de vegetales como guindillas en aceite, espárragos, judías verdes; también pescados, incluido el atún en lata y los pescados fermentados, salados y ahumados; y productos cárnicos, por ejemplo, jamón y salchichas.”
Agencia Española de Seguridad Alimentaria y Nutrición (AESAN) · Botulismo (ficha informativa, 30/08/2019) · read on October 26, 2026
-
29
Botulism is linked to foods packed with little oxygen
“Por consiguiente, los casos de botulismo frecuentemente guardan relación con alimentos listos para el consumo envasados con poco oxígeno.”
Agencia Española de Seguridad Alimentaria y Nutrición (AESAN) · Botulismo (ficha informativa, 30/08/2019) · read on October 26, 2026
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30
In Alaska, fermented fish causes most cases
“In Alaska, most cases of foodborne botulism have been caused by fermented fish and other aquatic animals.”
Centers for Disease Control and Prevention (CDC) · Botulism Prevention · read on October 25, 2026
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31
Homemade garlic or herb oils: refrigerate, and discard after 4 days
“Refrigerate homemade oils made with garlic or herbs. Throw the oils away after 4 days.”
Centers for Disease Control and Prevention (CDC) · Botulism Prevention · read on October 25, 2026
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32
No honey before age one
“Do not feed honey to a child who is younger than 1 year old.”
Centers for Disease Control and Prevention (CDC) · Botulism Prevention · read on October 25, 2026
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33
Gas is a good sign only if the pH stays below 4.6
“Some fermentation microbes make carbon dioxide gas during the fermentation process. This can be a good sign, but only if the pH stays below pH 4.6”
UC Davis, Department of Food Science and Technology · Common issues with fermented fruits and vegetables (v1.0, 15-mar-2022) · read on October 20, 2026
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34
A can that bulges, rusts, leaks or smells off gets thrown out
“En general se recomienda a los consumidores desechar conservas (caseras e industriales) que presenten abombamiento, óxido, pérdidas de líquido u olores anómalos.”
Agencia Española de Seguridad Alimentaria y Nutrición (AESAN) · Botulismo (ficha informativa, 30/08/2019) · read on October 26, 2026
-
35
If in doubt, throw it out
“If in doubt, throw it out!”
Centers for Disease Control and Prevention (CDC) · Botulism Prevention · read on October 25, 2026
-
36
Don't eat food when you don't know how it was made
“Do not eat food if you do not know whether it was safely canned, preserved, or fermented.”
Centers for Disease Control and Prevention (CDC) · Botulism Prevention · read on October 25, 2026
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37
Fatigue, weakness and dizziness; then blurred vision, dry mouth, trouble swallowing and speaking
“Los síntomas iniciales incluyen fatiga intensa, debilidad y vértigo, seguidos generalmente por visión borrosa, sequedad de boca y dificultad para tragar y hablar.”
Agencia Española de Seguridad Alimentaria y Nutrición (AESAN) · Botulismo (ficha informativa, 30/08/2019) · read on October 26, 2026
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38
Usually starts 12 to 36 hours after; it can range from 4 hours to 8 days
“Por lo general se manifiestan entre 12 y 36 horas después de la ingesta (con un plazo mínimo de cuatro horas y un máximo de ocho días).”
Agencia Española de Seguridad Alimentaria y Nutrición (AESAN) · Botulismo (ficha informativa, 30/08/2019) · read on October 26, 2026
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39
It's an emergency: see a doctor or go to the ER right away
“Botulism is an emergency. People with symptoms should see a doctor or go to the emergency room immediately.”
CDC · Botulism: symptoms · read on October 26, 2026
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40
Most people recover if treated early
“La mayoría de los casos se recuperan si son diagnosticados y tratados precozmente.”
Instituto de Salud Carlos III, Centro Nacional de Epidemiología (RENAVE) · Informe de botulismo 2023 · read on October 25, 2026
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41
The only effective treatment is antitoxin
“El único tratamiento efectivo es la administración de la antitoxina botulínica.”
Instituto de Salud Carlos III, Centro Nacional de Epidemiología (RENAVE) · Informe de botulismo 2023 · read on October 25, 2026
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42
The suspect food is kept sealed for testing
“Las muestras de alimentos vinculados a casos sospechosos se deben obtener inmediatamente, guardar en envases herméticos y enviar al laboratorio para identificar la causa y prevenir otros casos.”
Agencia Española de Seguridad Alimentaria y Nutrición (AESAN) · Botulismo (ficha informativa, 30/08/2019) · read on October 26, 2026