What fermentation is
Fermenting means letting microbes transform a food and preserve it. You can't see them, but you decide the conditions they work in.
6 min read First steps Reviewed October 26, 2026 34 facts, each with its source
A short definition
Fermenting means letting microbes turn a food into something we like better1. Bacteria and yeasts eat its sugars and turn them into acids, gas and other compounds, such as alcohol2. That is how cabbage becomes sauerkraut, milk becomes kefir and sweet tea becomes kombucha.
Along the way, the food keeps longer: what would have spoiled in a few days lasts for months3. Before modern canning, fermentation was one of the main ways people preserved their harvest4.
Fermenting or spoiling
Fermenting and spoiling are the same thing: microbes growing in food. What differs is the result. If you get something desirable, it is fermentation; if you get decomposition, it is spoilage5. Sometimes it is even the same microbe, and the conditions decide6.
So fermenting isn't about adding anything magic. It is about setting things up so the microbes you want win and the others lose. Almost everything you will read on Fervida, from salt to time, exists for that.
What happens inside a jar of vegetables
Most Fervida recipes are lactic fermentations. The name is misleading: "lacto" has nothing to do with milk. It refers to lactic acid, made by bacteria that already live on the surface of all fruits and vegetables7. You don't need to buy them.
It happens in four stages:
- Salt. It draws the juice out of the vegetables and favors fermenting bacteria over the ones that would spoil them8.
- Acid. Without oxygen, lactic acid bacteria turn the sugars into lactic acid, which holds back harmful bacteria and preserves the food9. It is also what makes it taste sour10.
- Gas. As they work they release carbon dioxide, which makes the bubbles11 and helps keep the jar low in oxygen12.
- pH. The acid lowers the food's pH13, and lactic acid bacteria take it below 4.614.
That 4.6 is the limit set by food safety guides. Fervida waits for 4.2 before sending a jar of vegetables to the fridge: that is a house rule, with a margin. The pH guide explains why.
Four kinds of fermentation
Not every ferment is lactic. Depending on who does the work and what they make, there are four kinds:
| Kind | Who works | What it makes | Examples |
|---|---|---|---|
| Lactic | Lactic acid bacteria | Lactic acid and gas11 | Sauerkraut, kimchi, pickles, yogurt |
| Alcoholic | Yeasts | Alcohol, with little or no oxygen15 | Wine, cider, mead, beer |
| Acetic | Acetic acid bacteria | Acetic acid, from alcohol16 | Vinegar, kombucha |
| Alkaline | Protein-digesting bacteria | Ammonia: a low-acid result17 | Natto, fish sauce |
Many ferments combine two. In kombucha, yeasts make alcohol and then bacteria turn it into acetic acid18. Fervida doesn't cover alkaline fermentation: it leaves a low-acid food, and fermented fish is on its red list.
Fermenting is not pickling
"Pickle" covers two different things: vegetables with added acid, usually vinegar, and vegetables in which microbes make the acid19. Quick vinegar pickles are not fermented20. In lactic fermentation no vinegar or lemon juice is added: the acid comes from the bacteria21.
The difference matters. A vinegar pickle is acidic from the first minute. A ferment starts out low in acid and takes days or weeks to sour, and during that time everything depends on how you set it up. Every Fervida recipe says which method it uses.
What you decide
You can't see the microbes, but you decide the conditions they work in. There are five, and each has its own guide:
- The recipe. Use a tested recipe22: the salt and the timing have already been measured by someone who checked them.
- Salt. You weigh it and never go below the minimum. Salt: how much and why covers it.
- Air. Lactic acid bacteria work without it; mold needs it23. That is why everything stays under the brine, as Oxygen and brine explains.
- Temperature. The ideal is 68–72 °F (20–22 °C)24. Above 78 °F (26 °C) it can over-ferment and spoil; below 60 °F (16 °C) it barely ferments25.
- Time. Give the bacteria time to make enough acid26. The pH tells you when it is enough.
Where the risks are
The serious risk in a home ferment is botulism. Its common sources include homemade foods that were improperly canned, preserved or fermented27, and its toxin can't be seen, smelled or tasted28. That is why Fervida never asks you to judge a jar by its smell. It asks for enough salt, vegetables kept under the brine and a pH that drops.
Some things Fervida won't calculate or recommend, however often they show up online:
- Going below the salt minimum. Reducing the salt in a vegetable ferment is not safe29.
- Storing anything in oil. Chopped garlic in oil is among the sources of botulism30.
- Garlic in honey. It is a widely shared recipe, but the guides consider it risky31.
- Fermented fish or meat. In Alaska, most foodborne botulism comes from fermented fish and other aquatic animals32.
Fermenting safely covers this in detail.
Where to start
If it is your first time, start with homemade sauerkraut: cabbage, salt and a jar. It brings everything above together in one ferment, and at 70–75 °F (21–24 °C) it is ready in 3 to 4 weeks33. If you prefer whole vegetables, try fermented dill pickles, which ferment in brine.
If you are in a hurry, Napa cabbage kimchi ferments much faster than sauerkraut34. And homemade kombucha shows you the other half of the map: yeasts and acetic acid bacteria working together.
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
Fermentation transforms food through microbial action
“Fermentation transforms food by microbial action into a food with desirable quality and taste characteristics.”
BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 1: Introduction to fermented food safety (2024) · read on October 25, 2026
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2
Microbes turn sugars into acids, gas and other compounds, such as alcohol
“Microbes will consume the available sugars and/or break down the complex carbohydrates of the food’s substrate to form simpler sugars, and use them to form acids, gas and other chemicals, such as ethanol.”
BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 1: Introduction to fermented food safety (2024) · read on October 25, 2026
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3
Fermentation preserves foods that would otherwise spoil and extends their shelf life
“Fermentation preserves foods that would otherwise spoil and increases the shelf life.”
BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 1: Introduction to fermented food safety (2024) · read on October 25, 2026
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4
Before modern canning, fermentation was one of the main ways to preserve a harvest
“Until the introduction of modern canning, fermentation was one of the chief methods people used to preserve their harvest.”
K-State Research and Extension (Kansas State University) · Taking a New Look at Fermented Foods (MF3491, June 2020) · read on October 20, 2026
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5
If the result is desirable it is fermentation; if it decomposes, it is spoilage
“If the process results in desirable food it is termed fermentation; when it results in decomposition, it will be called spoilage.”
BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 1: Introduction to fermented food safety (2024) · read on October 25, 2026
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6
The microbe that ferments can be the same one that spoils
“A microbe used for fermentation may be the same microbe that results in food spoilage, depending on the specific situation.”
BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 1: Introduction to fermented food safety (2024) · read on October 25, 2026
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7
“Lacto” comes from lactic acid, made by bacteria found on all fruits and vegetables
“the “lacto” refers to lactic acid that’s produced during the fermentation process by lactic-acid producing bacteria which are present on the surface of all fruits and vegetables.”
University of Illinois Extension · Fermentation handout (2020) · read on October 20, 2026
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8
Salt favors fermenting bacteria over spoilage microbes
“Aids in the growth of fermenting bacteria over spoilage bacteria, yeasts and molds”
University of Illinois Extension · Fermentation handout (2020) · read on October 20, 2026
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9
Without oxygen, these bacteria turn sugars into lactic acid, which holds back harmful ones
“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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10
Lactic acid gives the sour flavor
“It's also what gives fermented foods their characteristic sour flavor”
University of Illinois Extension · Fermentation handout (2020) · read on October 20, 2026
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11
Lactic acid bacteria turn sugars into lactic acid and release gas
“Bacteria, such as Lactobacillus spp., convert sugars into lactic acid and release carbon dioxide, which creates bubbles in the liquid or brine.”
University of Minnesota Extension · Preserving food at home: Fermentation (reviewed 2026) · read on October 25, 2026
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12
Carbon dioxide helps keep the jar low in oxygen
“the carbon dioxide produced during the fermentation process helps maintain the low-oxygen conditions necessary for the fermenting bacteria to flourish.”
University of Illinois Extension · Fermentation handout (2020) · read on October 20, 2026
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13
The acid lowers the food's pH
“The acid produced lowers the pH of the food.”
University of Minnesota Extension · Preserving food at home: Fermentation (reviewed 2026) · read on October 25, 2026
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14
Lactic acid bacteria bring the pH below 4.6
“LAB produce lactic acid as the major end product during the fermentation of carbohydrates, reducing the pH of the end product to less than 4.6.”
BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 1: Introduction to fermented food safety (2024) · read on October 25, 2026
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15
Yeasts turn sugars into alcohol with little or no oxygen
“Yeast will ferment sugars to produce ethanol under low or no oxygen conditions”
University of Minnesota Extension · Preserving food at home: Fermentation (reviewed 2026) · read on October 25, 2026
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16
Then bacteria turn the alcohol into acetic acid
“After alcohol fermentation, bacteria convert ethanol and sugars into acetic acid.”
University of Minnesota Extension · Preserving food at home: Fermentation (reviewed 2026) · read on October 25, 2026
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17
In alkaline fermentation, bacteria break down proteins and release ammonia: the result is low in acid
“Bacteria break down proteins and produce ammonia, creating a low-acid final product.”
University of Minnesota Extension · Preserving food at home: Fermentation (reviewed 2026) · read on October 25, 2026
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18
In kombucha, yeasts make alcohol and bacteria turn it into acetic acid
“A simplified overview of the chemistry in a kombucha tea ferment is that the sugars in the tea are converted into alcohol by yeasts, followed by AAB which convert the alcohols into acetic acids.”
BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 1: Introduction to fermented food safety (2024) · read on October 25, 2026
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19
“Pickle” can mean vegetables with added acid or fermented vegetables
“Pickled vegetables or pickles can refer to vegetables that are acidified (acid is added) or fermented (acid is created by microbes), so the use of the term ‘pickle’ can be confusing.”
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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20
Quick pickles are not fermented
“Fresh-pack or quick-process pickles are not fermented”
National Center for Home Food Preservation (USDA, University of Georgia) · General Information on Fermenting · read on October 26, 2026
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21
No vinegar or lemon juice is added in lactic fermentation
“An acidifying component, such as vinegar or lemon juice, is not added.”
University of Minnesota Extension · Preserving food at home: Fermentation (reviewed 2026) · read on October 25, 2026
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22
Use a tested recipe
“Use a research-based, tested recipe.”
University of Minnesota Extension · Preserving food at home: Fermentation (reviewed 2026) · read on October 25, 2026
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23
Mold needs oxygen
“Molds require oxygen to grow.”
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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24
Ideally 68–72 °F (20–22 °C)
“A temperature between 68 and 72 degrees Fahrenheit is ideal.”
University of Minnesota Extension · Preserving food at home: Fermentation (reviewed 2026) · read on October 25, 2026
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25
Above 78 °F (26 °C) it can spoil; below 60 °F (16 °C) it barely ferments
“Temperatures over 78 degrees Fahrenheit can cause over fermentation and spoilage. Temperatures below 60 degrees Fahrenheit will inhibit fermentation.”
University of Minnesota Extension · Preserving food at home: Fermentation (reviewed 2026) · read on October 25, 2026
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26
Give the bacteria time to make enough acid
“Allow enough time for the bacteria to produce enough acid to lower the pH of the food.”
University of Minnesota Extension · Preserving food at home: Fermentation (reviewed 2026) · read on October 25, 2026
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27
Homemade foods improperly preserved or fermented are common sources of botulism
“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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28
You can't see, smell or taste botulinum toxin
“You cannot see, smell, or taste the toxin that causes botulism.”
Centers for Disease Control and Prevention (CDC) · Botulism Prevention · read on October 25, 2026
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29
Reducing salt is not safe
“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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30
Chopped garlic in oil is a source of 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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31
Garlic fermented in honey is risky
“Fermenting garlic cloves in raw honey is thought to prevent colds and flu, particularly in children. But this is risky for many factors.”
Kansas State University, Rapid Response Center (Karen Blakeslee) · Honey Fermented Garlic May Be Risky (2020) · read on October 25, 2026
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32
In Alaska, most botulism cases come from fermented fish
“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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33
Sauerkraut ferments in 3 to 4 weeks at 70–75 °F (21–24 °C)
“At temperatures between 70º and 75ºF, kraut will be fully fermented in about 3 to 4 weeks”
National Center for Home Food Preservation (USDA, University of Georgia) · Sauerkraut · read on October 20, 2026
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34
Kimchi ferments much faster than sauerkraut
“Kimchi ferments much more quickly than sauerkraut”
K-State Research and Extension (Kansas State University) · Taking a New Look at Fermented Foods (MF3491, June 2020) · read on October 20, 2026