Lactic acid bacteria, yeasts and molds
Every jar holds bacteria, yeasts and molds. Salt, the lack of air and acid decide who wins, and almost everything you do is meant to help lactic acid bacteria win.
7 min read First steps Reviewed October 26, 2026 33 facts, each with its source
Three groups worth knowing
Bacteria and fungi do the work in a ferment, and the fungi come in two very different kinds: yeasts and molds1. A freshly cut cabbage arrives in the jar with a bit of everything2. Nobody plants anything: the competition starts on its own.
What you do is tip the balance. The amount of salt is what steers fermentation toward lactic acid bacteria3, and the lack of air and the acid do the rest.
| Lactic acid bacteria | Yeasts | Molds | |
|---|---|---|---|
| What they are | Bacteria | Single-celled fungi | Threadlike fungi |
| What they make | Lactic acid and gas | Alcohol and gas | Threads, spores and, in some, toxins |
| You want them in | Sauerkraut, kimchi, pickles, kefir | Bread, wine, cider, kombucha, kefir | Koji, miso, tempeh |
| In a jar of vegetables | Bubbles and a sour taste | A smooth film on the surface | Fuzz or colored spots: throw it out |
Lactic acid bacteria
They are the stars of almost every Fervida recipe. They make acids, above all lactic acid, grow with or without oxygen and tolerate salt4. Those three traits explain the method: with salt and no air, they keep working while nearly all their rivals slow down.
They live in low numbers on fresh produce and grow fast when conditions are right5. That is why you don't buy them for sauerkraut. And that is why you wash vegetables but don't scrub them hard: you would remove the bacteria you need6.
They are not a single species. In vegetables the most important are Leuconostoc, Lactobacillus and Pediococcus7, and they take turns. A ferment starts with one kind, such as Leuconostoc, and moves on to another, such as Lactobacillus8: as acidity rises, some die off and the more acid-loving ones take over9. That is why a one-week sauerkraut doesn't taste like a one-month one.
Besides acid, they make compounds that hold back other microbes, and that matters for the safety of the ferment10. In the end they bring the pH below 4.611, the limit the guides set.
Yeasts
They are single-celled fungi12. They turn sugars into alcohol and carbon dioxide13: that is how bread rises and how wine, cider and mead are made.
In some ferments they work as a team. In kombucha, yeasts make alcohol and bacteria then turn it into acetic acid14. Kefir grains combine lactic acid bacteria, acetic acid bacteria and yeasts15.
In a jar of vegetables, though, yeast shows up as a white, gray or pink film on the surface16. It is usually the yeast known as kahm17. It isn't mold, but it isn't good company either: in sauerkraut it is removed from time to time18, because too much of it causes off flavors and textures and lowers acidity19. Too much salt favors it, too: it slows lactic acid bacteria and makes room for yeast20. The right amount of salt isn't only about safety; it also prevents this.
Molds
They are many-celled fungi that grow in threads21. Some are grown on purpose: miso, tempeh and koji are made with them22. That works because a pure, adequate culture is added, which grows fast and outcompetes unwanted microbes23. And they follow their own rules, because as mold grows it raises the food's pH, the opposite of lactic acid bacteria24.
In a jar of vegetables, mold is always bad news:
- It needs air. It grows on the surface, where the ferment meets the air25, or on a piece sticking out.
- You can tell it by color and texture. It is usually green, blue, brown or black26, fuzzy or spotty.
- It can't be spooned off. Its roots are thin threads, hard to see, that can reach deep into the food27. Some molds also produce mycotoxins, substances that make people sick28.
- The batch goes. If you confirm mold anywhere in the ferment, the whole thing is discarded29.
Salt isn't enough to stop it: mold tolerates salt and sugar better than most other microbes30. What stops it is taking away the air. That is what Oxygen and brine is about.
Spontaneous or cultured fermentation
Sauerkraut or pickles are spontaneous fermentations: they are done by microbes already on the vegetables and the utensils31. They work well, but they depend on the acid arriving in time. If it takes too long, unwanted microbes can grow32. That is why you weigh the salt, keep the vegetables submerged and measure the pH.
Kombucha and kefir start from a culture: a SCOBY, grains or a little of the last batch. That way the active microbes are there in large numbers from day one33, and the acid comes sooner.
What to do with all this
- Wash vegetables without scrubbing them hard.
- Weigh the salt and use what the recipe asks for: not less, for safety, and not much more, so yeasts don't get the upper hand.
- Keep everything under the brine.
- If a smooth white film forms, skim it off.
- If you see fuzz or colored spots, throw the batch out.
- If you can't tell mold from kahm, use the Mold or kahm yeast? tool.
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
Bacteria and fungi, molds and yeasts, do the work in fermentation
“Microbes used in fermentations include bacteria and fungi (moulds and yeasts).”
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
Vegetables arrive with a mixture of bacteria, yeasts and molds
“Cabbage and vegetable ingredients will have a mixture of Gram-positive and Gram-negative bacteria, yeasts and moulds.”
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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3
The amount of salt steers fermentation toward lactic acid bacteria
“The salt concentration is a critical control point (CCP) to drive the primary fermentation towards specific species of LAB groups.”
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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4
Lactic acid bacteria make acid, grow with or without oxygen and tolerate salt
“This means these bacteria produce acids, in particular lactic acid and can grow with and without oxygen and are salt tolerant.”
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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5
They are present in low numbers on fresh produce and grow fast when conditions are right
“LAB are commonly found in low numbers on many types of fresh produce, and grow rapidly with acid production when conditions are right.”
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
Don't scrub produce hard: you would remove the good bacteria
“Do not vigorously scrub the produce; this can remove good bacteria needed for the fermentation process.”
University of Minnesota Extension · Preserving food at home: Fermentation (reviewed 2026) · read on October 25, 2026
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7
In vegetables the most important are Leuconostoc, Lactobacillus and Pediococcus
“Of the many groups of fermenting bacteria, those from the lactic acid bacteria family are the most important in vegetable fermentation – specifically Leuconostoc, Lactobacillus and Pediococcus species.”
University of Illinois Extension · Fermentation handout (2020) · read on October 20, 2026
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8
A ferment starts with one kind of lactic acid bacteria and moves on to another
“Successful ferments using LAB may start with one type of LAB, such as Leuconostoc spp., before progressing to another type, such as Lactobacillus spp. and Streptococcus spp.”
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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9
As acidity rises, some die off and more acid-loving ones take over
“as the conditions during the fermentation become more acidic, populations of one type of LAB will die off, with other more acid loving LAB growing next in the succession.”
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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10
They make compounds that hold back other microbes, which matters for safety
“During growth, LAB can also produce antimicrobial compounds such as bacteriocins that will inhibit other microbial growth and this is important to the safety of the ferment.”
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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11
They 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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12
Yeasts are single-celled fungi
“Yeasts are single-celled microbes in the fungi kingdom.”
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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13
They turn sugars into alcohol and carbon dioxide
“Yeasts ferment sugars and carbohydrates into ethanol and carbon dioxide.”
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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14
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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15
Kefir grains combine lactic acid bacteria, acetic acid bacteria and yeasts
“Milk kefir grains look similar to small clumps of cauliflower florets with white to yellow colouration and contain a mixture of LAB, AAB, and yeasts that are immobilized on a polysaccharide and protein matrix.”
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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16
White, gray or pink films can form on the surface
“White, grey, or pink films may form on the surface of fermenting fruits or vegetables.”
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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17
That film is usually yeast, known as kahm
“This film is typically yeast, often referred to by home fermenters as Kahm yeast.”
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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18
In sauerkraut, yeast is removed from time to time
“For other ferments, such as sauerkraut, yeast should be periodically removed during fermentation.”
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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19
Too much yeast causes off flavors and lowers acidity
“Excessive yeast growth can lead to off flavors or textures and reduce acidity.”
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
Too much salt slows lactic acid bacteria and lets yeasts grow
“High salt can slow the growth of LAB allowing yeast growth and may result in longer fermentation times.”
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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21
Molds are many-celled, threadlike fungi
“Moulds are multi-celled and filamentous fungi.”
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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22
Molds are used to make miso, tempeh and koji
“Moulds are used to ferment soybeans into miso, tempeh, doenjang, amazake and koji.”
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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23
A pure, adequate culture grows fast and outcompetes unwanted microbes
“However, addition of adequate pure mould culture to a properly prepared food substrate will allow the mould culture to quickly grow and out-compete other spoilage and pathogenic microbes.”
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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24
As it grows, mold raises the food's pH
“Mould fermentations, from a food safety perspective, initially appear to be higher risk because as mould multiplies in the food substrate (e.g., soybean) the pH of the food increases.”
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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25
Mold needs oxygen and grows where the ferment meets the air
“Molds require oxygen to grow. Molds can grow on the surface of the ferment at the air interface.”
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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26
Mold is usually green, blue, brown or black
“Mold is typically green, blue, brown, or black in color.”
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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27
Its roots are thin threads that can reach deep into the food
“Molds have branches and roots that are like very thin threads. The roots may be difficult to see when the mold is growing on food and may be very deep in the food.”
USDA Food Safety and Inspection Service · Molds on Food: Are They Dangerous? · read on October 20, 2026
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28
Some molds produce mycotoxins, which make people sick
“And a few molds, in the right conditions, produce "mycotoxins," poisonous substances that can make people sick.”
USDA Food Safety and Inspection Service · Molds on Food: Are They Dangerous? · read on October 20, 2026
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29
Mold anywhere means discarding the batch
“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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30
Mold tolerates salt and sugar better than most other microbes
“Molds also tolerate salt and sugar better than most other food invaders.”
USDA Food Safety and Inspection Service · Molds on Food: Are They Dangerous? · read on October 20, 2026
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31
Spontaneous fermentation is done by microbes already on the food and utensils
“Spontaneous refers to the actions of microbes already present on the food substrate, or on the equipment and utensils that contact the food substrate.”
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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32
If acid takes too long, unwanted microbes can grow
“If the time is too long before acid development pathogens or spoilage present may multiply in the food.”
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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33
With a culture, active microbes are already there in large numbers
“In this guidance we differentiate spontaneous ferments from backslopping because during backslopping, the active microbial agents are already present in high numbers.”
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