Vegan Fermentation Basics: Yeast, Lacto, and Mycelium
- May 4
- 9 min read
Fermentation is one of the oldest forms of food transformation. At its core, it is a controlled process where microorganisms — yeast, bacteria, or mold — break down sugars and starches in food and leave behind acids, gases, or alcohol. The food does not spoil. It changes.
For plant-based cooks, fermentation is a quiet superpower. It turns soybeans into tempeh, cabbage into kimchi, oats into yogurt, flour into sourdough — and once the underlying science clicks, the same principles apply across every ferment in a plant-based kitchen. This guide covers the four main fermentation types, the microbes responsible for each, and what they produce.
How Fermentation Works
Fermentation is metabolism. Microorganisms eat sugars and excrete byproducts. Those byproducts are what define the finished food — the sour tang of sauerkraut, the carbonation in kombucha, the alcohol in wine, the rise in sourdough.
Three things govern every ferment:
The microbe — yeast, lactic acid bacteria, mold, or acetic acid bacteria. Each produces different byproducts.
The substrate — the food being fermented. Sugars, starches, and proteins all behave differently.
The environment — temperature, salt concentration, oxygen availability, and time. Shift any of these and the dominant microbe shifts with them.
Most home fermentation does not require buying a starter culture. Wild microbes already live on vegetables, fruit, and grains. The technique is about creating conditions where the wanted microbes thrive and the unwanted ones cannot.
Yeast Fermentation: Sourdough, Tepache, Ginger Bug
Yeasts are single-celled fungi. The most studied species in food, Saccharomyces cerevisiae, converts sugars into ethanol and carbon dioxide through alcoholic fermentation. The CO₂ leavens bread. The ethanol flavors beverages. Both are happening simultaneously in any yeasted ferment — the question is which one matters more for the final product.
In a sourdough starter, wild yeast captured from flour and air feeds on the starches in the dough, releasing CO₂ as it does. That gas gets trapped in the gluten network of the dough, expanding it from the inside out — this is what gives sourdough its rise. Alongside the yeast, lactic acid bacteria produce lactic and acetic acid for the characteristic tang. The two organisms work together: yeast lifts the loaf, bacteria flavor it. This is why sourdough has a more complex flavour and a longer shelf life than commercial yeasted bread, and why the same starter, given more time and a little less hydration, becomes the base for naturally leavened pizza dough.
In tepache, the yeast already living on pineapple skin starts feeding on the added sugar and the fruit's natural sugars as soon as the mixture is left at room temperature. The bubbling visible on the surface within 24 hours is CO₂ escaping from the liquid. Within 48 to 72 hours, the brew is mildly tangy, lightly fizzy, and gently alcoholic — though in most home versions the alcohol stays well under 2 per cent.
A ginger bug is a small jar of grated ginger, sugar, and water that is fed daily until it becomes thick with wild yeast. Once active, it can be used as a starter for naturally carbonated sodas. A few tablespoons of an active ginger bug stirred into fruit juice, herbal tea, or sweetened tamarind water will, given a couple of days at room temperature, produce a fizzy, lightly sweet beverage with no commercial CO₂ involved.
A few common yeast-driven ferments in plant-based cooking:
Sourdough bread and pizza dough — wild yeast and LAB working together to leaven and flavor
Tepache — fermented pineapple peel drink, driven by yeast from the fruit's skin
Ginger bug — wild yeast starter used for naturally carbonated sodas
Beer, wine, cider — alcoholic fermentation as the primary goal
The parameter to watch is temperature. Yeast prefers 24–28°C for active fermentation. Below 18°C it slows dramatically. Above 35°C, it stresses and produces off-flavours. The common mistake is over-proofing — leaving a yeasted ferment too long, which exhausts the sugar supply and collapses the structure.
Lacto-Fermentation: Kimchi, Sauerkraut, Vegan Yoghurt
Lactic acid fermentation, often called lacto-fermentation, is the workhorse of plant-based fermentation. It is run by lactic acid bacteria (LAB) — primarily species of Lactobacillus, Leuconostoc, and Lactococcus. These bacteria are anaerobic, meaning they thrive without oxygen, and they convert sugars into lactic acid for energy.
Lactic acid lowers the pH of the food, making it sour and inhospitable to spoilage organisms. This is why properly fermented vegetables can sit for months without rotting — the environment they create is hostile to almost everything except the LAB themselves.
The two key levers in lacto-fermentation are salt and oxygen. Salt at 2–3 percent by weight slows down spoilage organisms long enough for LAB to take over. An anaerobic environment (vegetables submerged under brine) keeps mould and yeast from competing for the sugars on the surface.
In kimchi, the cabbage is salted to draw out water, then mixed with chilli, garlic, ginger, and other vegetables. Within hours, LAB native to the cabbage begin multiplying. By day three, the brine is cloudy with their activity. By day seven, the kimchi has dropped to a pH around 4.0 — sour, complex, and stable enough to keep refrigerated for months.
Lacto-fermented fruits and vegetables work on the same principle, but the pace varies with the substrate. Vegetables low in sugar — radishes, cucumbers, carrots — ferment slowly and stay crisp longer because LAB have less fuel to work with. Fruits with high natural sugar — mango, plum, pineapple — ferment faster and softer because the bacteria multiply rapidly on the abundant sugar supply. Lacto-mango produces a tangy, almost cheesy condiment within four to five days. Preserved lemons, a Moroccan staple, are lacto-fermented in their own salted juice for weeks until the rind softens completely and the flavor turns deep and savory. Lacto-fermented berries become a slightly fizzy, savory-sweet sauce. Even chili pastes — the base of sriracha and most modern hot sauces — begin life as a lacto-fermented mash, where the chili sugars feed the bacteria and the resulting acidity rounds out the heat.
Vegan yogurt is lacto-fermentation in liquid form. Plant milks (soy, cashew, coconut, oat) are warmed, inoculated with the same bacterial cultures used in dairy yogurt — Lactobacillus bulgaricus and Streptococcus thermophilus — and held at 40–45°C for 6 to 12 hours. The LAB convert plant-milk sugars into lactic acid, which thickens the milk by coagulating its proteins. The result is a tangy, spoonable yogurt that tracks closely with dairy yogurt in flavor and behavior, though the firmness depends heavily on which plant milk is used.
Common mistakes in lacto-fermentation include too little salt or vegetables floating above the brine. Both let mould colonise before LAB can acidify the environment.
Mycelium Fermentation: Tempeh and Vegan Cheese
Mycelium fermentation uses molds — filamentous fungi that grow as a network of root-like threads called hyphae. This network is the mycelium. In food, two molds matter most for plant-based cooks: Rhizopus oligosporus for tempeh and Penicillium species for some aged vegan cheeses.
Tempeh is the clearest example. Soaked, partially cooked soybeans (or other legumes) are inoculated with Rhizopus oligosporus spores and incubated at 30–32°C for 24 to 48 hours. The mold grows as fine white threads, weaving between and through each bean. As it grows, it digests the bean's surface starches and proteins, breaking them into simpler compounds — amino acids, peptides, and free sugars — that are easier for the human digestive system to absorb. By the end of incubation, the mycelium has knit the entire batch into a firm, sliceable cake bound by a dense white mat. The flavor at this stage is nutty and mushroom-like, and far less beany than the soybeans started out as.
Vegan cheese-making borrows the same principle. A cultured nut base — typically cashew or almond, often inoculated with kefir grains or yogurt cultures first — can then be aged with surface molds. Penicillium candidum gives a soft white rind. Penicillium roqueforti produces blue veining. Over weeks of aging in a controlled humid environment, the mold breaks down fats and proteins inside the wheel, deepening flavor and softening the texture from the rind inward. The end result behaves much like a traditional aged cheese, even though no dairy is involved.
The parameter to watch with mycelium fermentation is humidity and temperature stability. Rhizopus needs warmth and airflow but not excess moisture. Too dry and the spores never germinate. Too wet and competing molds — usually black or green — take over.
The common mistake is rushing tempeh by raising the temperature. Above 35°C, the mycelium stresses and can produce undesirable off-flavors or grey patches.
Acetic Acid Fermentation: Kombucha and Vinegars
Acetic acid fermentation is a two-stage process. First, yeast converts sugars to alcohol. Then Acetobacter bacteria — which require oxygen — convert that alcohol to acetic acid. The result is vinegar, or in the case of kombucha, a lightly acidic, lightly carbonated tea.
Kombucha is fermented using a SCOBY: a symbiotic culture of bacteria and yeast. The cellulose mat that forms on top of the brew is the visible structure of this community. Inside the brew, the yeast component ferments the sweetened tea into alcohol. The Acetobacter then converts most of that alcohol into acetic and gluconic acid. What remains is a tart, lightly sweet liquid with the leftover sugar feeding a slow, ongoing fermentation.
After the primary fermentation finishes (usually 7–14 days), kombucha is often bottled with fruit, ginger, or herbs for a second fermentation. Sealed in the bottle, the remaining yeast feeds on the new sugar and produces CO₂ — which now has nowhere to escape. This is what gives finished kombucha its sharp, beer-like fizz. The same principle, scaled up and given more time, is what produces apple cider vinegar from cider, or red wine vinegar from wine.
Kombucha sits best at 22–28°C and finishes in 7–14 days depending on conditions. The common mistake is fermenting too long, which produces a sharp, vinegar-forward drink rather than a balanced one.
How Plant-Based Fermentation Differs from Dairy and Meat Fermentation
The microbes are largely the same. The substrates are not.
Dairy fermentation relies on lactose — a sugar specific to milk — as the primary fuel for LAB. Plant milks contain different sugars (sucrose, glucose, fructose, and oligosaccharides), so cultures may behave differently. Soy milk ferments quickly because of its protein and sugar content. Oat milk needs added starches or thickeners for body. Coconut milk ferments well but separates without an emulsifier.
For mycelium ferments, legumes replace dairy as the protein matrix. Soybeans, chickpeas, black beans, and lupin all support Rhizopus growth, though each yields a slightly different texture and flavour profile.
The takeaway: the fermentation principles transfer cleanly across the plant-animal divide. The recipes need calibration, not reinvention.
Is Fermented Food Better for You?
Fermentation does three useful things to plant foods:
Predigests anti-nutrients — phytic acid in legumes, oxalates in some greens, and lectins in raw beans are reduced through fermentation, which improves mineral absorption
Adds B vitamins and bioactive compounds — particularly some B vitamins in fermented foods, though levels vary, and tempeh is not a reliable B12 source on its own
Introduces live cultures — fermented foods that have not been heat-treated carry beneficial bacteria into the gut
Research published by the Stanford School of Medicine suggests that regular consumption of fermented foods may support gut microbial diversity, though the effect's strength depends on the specific food and the individual.
FAQ
What are the four main types of fermentation?
The four most relevant fermentation types in cooking are yeast (alcoholic) fermentation, lactic acid fermentation, acetic acid fermentation, and mold or mycelium fermentation. Each is driven by a different group of microorganisms and produces different byproducts: alcohol and CO₂, lactic acid, acetic acid, and protein-modified solids respectively.
Is fermentation safe at home?
Properly executed fermentation is one of the safest food preservation methods. The acidic or alcoholic environment created by the dominant microbes inhibits pathogens. The risk lies in improper conditions — too little salt, too much oxygen, contaminated equipment — which allow spoilage organisms to take over before the desired culture establishes.
What is the difference between lacto-fermentation and yeast fermentation?
Lacto-fermentation is anaerobic and produces lactic acid through bacteria. Yeast fermentation produces alcohol and carbon dioxide through fungi. Lacto-fermentation is used for vegetables, plant yoghurts, and most pickles. Yeast fermentation is used for bread, beer, wine, and tepache. Many traditional ferments — sourdough, kimchi, kombucha — use both microbes simultaneously.
Can plant milks be fermented like dairy?
Yes. Plant milks ferment readily with the same lactic acid bacteria used for dairy yoghurt. The texture varies because plant proteins coagulate differently than casein. Soy milk produces the firmest set; oat and almond milks usually need added thickeners or starches for a yogurt-like texture.
Is tempeh fermented?
Tempeh is a fermented soybean cake produced by inoculating cooked beans with Rhizopus oligosporus, a mold whose mycelium binds the beans into a firm block. Fermentation also breaks down some soy proteins and reduces phytic acid, thereby improving digestibility.
How long does fermentation take?
It depends on the ferment and the temperature. Tempeh takes 24–48 hours. Sauerkraut and kimchi take 5–14 days. Sourdough rises in about 4 hours, depending on the starter's strength. Kombucha takes 10–14 days. Aged vegan cheeses can take weeks or months.
Where to Take Fermentation Next
Reading about fermentation is one thing. Watching a tempeh block knit itself together overnight, or tasting the difference between a 5-day kimchi and a 14-day kimchi, is another. The principles in this guide come together in practice, where every variable can be felt and adjusted in real time.
The Vegan School's fermentation module covers all of the above as hands-on practice: sourdough bread and pizzas, tepache, kombucha, plant-based yogurt, kimchi, lacto-fermenting fruits and vegetables, vegan cheese, ginger bug, and tempeh. Each ferment is taught alongside the science behind it, so the technique transfers to any future ferment a cook decides to try.
Fermentation rewards patience and observation. The microbes do most of the work. The cook sets the stage.
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