Fermentation · Science

How Fermentation Works: The Science Behind Every Ferment

Understand the biology of fermentation, from glycolysis to lactic acid and alcoholic pathways. Learn how enzymes, pH, temperature, and CO2 shape every fermented food and drink you make.

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An airlocked alcoholic ferment beside a submerged vegetable ferment

Updated August 2026 · 12 min read

Fermentation is one of the oldest biological processes humans have harnessed, predating written history by thousands of years. At the molecular level, it is a way for microorganisms to extract energy from sugar without oxygen. The byproducts of that energy extraction (acids, alcohol, gases) are what transform cabbage into sauerkraut, grape juice into wine, and flour paste into sourdough.

Understanding the science behind fermentation gives you control over your ferments. When you know why salt concentration matters, why temperature changes flavor, and why some ferments need an airlock while others need open air, you stop following recipes blindly and start making informed decisions.

What fermentation actually is

Fermentation is an anaerobic metabolic process. Microorganisms (bacteria, yeast, or molds) break down sugars to produce energy for themselves. The byproducts they generate, including acids, alcohols, and gases, are what we value as fermenters.

In strict biochemistry, fermentation refers specifically to ATP production without an electron transport chain or oxygen. In everyday food science, the term covers a broader range of microbial transformations, including some that involve oxygen (like vinegar production).

Anaerobic fermentation Occurs without oxygen. This includes lactic acid fermentation (sauerkraut, yogurt, kimchi) and alcoholic fermentation (beer, wine, bread). Most food and beverage ferments fall into this category. The absence of oxygen is maintained by submerging in brine, sealing vessels, or using airlocks.
Aerobic fermentation Acetic acid fermentation is the primary aerobic example. Acetobacter bacteria require oxygen to convert ethanol into acetic acid (vinegar). This is why vinegar crocks are covered with cloth rather than sealed. Kombucha also involves aerobic processes at the surface where the SCOBY contacts air.
Mixed fermentation Many real-world ferments involve multiple organisms and both aerobic and anaerobic stages. Kombucha starts with aerobic yeast and bacteria activity at the surface, while anaerobic conditions develop deeper in the liquid. Traditional sourdough hosts both LAB (anaerobic) and wild yeast (facultative anaerobic).
10,000+ years humans have practiced fermentation
2 ATP energy molecules produced per glucose in fermentation
36 ATP produced per glucose in aerobic respiration (for comparison)

Glycolysis: the universal first step

Every type of fermentation begins with glycolysis. This is a ten-step metabolic pathway that breaks one molecule of glucose (a six-carbon sugar) into two molecules of pyruvate (a three-carbon compound). Glycolysis happens in the cytoplasm of the cell and does not require oxygen.

During glycolysis, two molecules of ATP (the cell’s energy currency) are produced, along with two molecules of NADH (an electron carrier). The net energy gain is small compared to aerobic respiration, but it works without oxygen and is fast.

  1. 1
    Energy investment phase (steps 1-5) The cell spends 2 ATP molecules to phosphorylate glucose and split it into two three-carbon molecules (glyceraldehyde-3-phosphate). This initial investment is required to destabilize the glucose molecule so it can be broken apart.
  2. 2
    Energy payoff phase (steps 6-10) Each three-carbon molecule is oxidized and rearranged, producing 2 ATP and 1 NADH per molecule. Since there are two molecules, the total yield is 4 ATP and 2 NADH. Subtract the 2 ATP invested, and the net yield is 2 ATP per glucose.
  3. 3
    Pyruvate is the decision point What happens to pyruvate after glycolysis determines the type of fermentation. In lactic acid fermentation, pyruvate is converted directly into lactate. In alcoholic fermentation, pyruvate is first decarboxylated into acetaldehyde, then reduced to ethanol. This step also regenerates NAD+ so glycolysis can continue.

The regeneration of NAD+ is the entire purpose of fermentation from the cell’s perspective. Glycolysis needs NAD+ to keep running. Without oxygen (which normally regenerates NAD+ through the electron transport chain), fermentation provides an alternative way to recycle NADH back into NAD+.

Lactic acid fermentation

Lactic acid bacteria (LAB), primarily species of Lactobacillus, Leuconostoc, and Pediococcus, convert pyruvate into lactic acid. This is the pathway behind sauerkraut, kimchi, yogurt, kefir, dosa, and many pickled vegetables.

There are two sub-types of lactic acid fermentation. The distinction matters because it affects flavor, texture, and gas production in your ferments.

Homofermentative LAB Convert glucose almost entirely into lactic acid. They produce a clean, sharp sourness with minimal CO2. Lactobacillus acidophilus and Lactobacillus delbrueckii (used in yogurt) are homofermentative. These bacteria are efficient acid producers and dominate in later stages of vegetable ferments when pH drops below 4.0.
Heterofermentative LAB Produce lactic acid along with ethanol, acetic acid, and CO2. The flavor profile is more complex, with tanginess and slight fizziness. Leuconostoc mesenteroides is a heterofermentative species that dominates the early stages of sauerkraut fermentation, producing the initial bubbling and tangy notes before acid-tolerant homofermenters take over.
Sauerkraut and kimchi Wild LAB present on cabbage leaves drive the fermentation. Leuconostoc species start the process at higher pH, producing CO2 and mild acidity. As pH drops below 4.0, Lactobacillus plantarum and related species dominate, driving pH down to 3.2-3.5 over 3-6 weeks.
Yogurt A defined culture of Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus works symbiotically. S. thermophilus starts first, lowering pH and producing formic acid that stimulates L. bulgaricus growth. L. bulgaricus then produces amino acids that feed S. thermophilus. This cooperation produces the characteristic tang and thick texture.
Sourdough LAB (primarily Lactobacillus sanfranciscensis and related species) outnumber yeast by roughly 100:1 in a mature sourdough starter. The LAB produce lactic and acetic acid, giving sourdough its sour flavor. The wild yeast provides leavening through CO2 production. The two organisms coexist because LAB cannot consume maltose efficiently, leaving it for the yeast.

Alcoholic fermentation

Yeast, primarily Saccharomyces cerevisiae, converts pyruvate into ethanol and CO2 through a two-step process. First, the enzyme pyruvate decarboxylase removes a carbon dioxide molecule from pyruvate, producing acetaldehyde. Then alcohol dehydrogenase reduces acetaldehyde to ethanol, regenerating NAD+ in the process.

This pathway is responsible for beer, wine, mead, cider, sake, and the leavening of bread. The CO2 produced creates bubbles in beer and lifts bread dough. The ethanol is the source of alcohol in beverages (and evaporates during baking).

Beer and wine fermentation Brewers and winemakers select specific yeast strains for their flavor profiles. Ale yeast (S. cerevisiae) works at 15-24°C (59-75°F) and produces fruity esters. Lager yeast (S. pastorianus) works at 7-13°C (45-55°F) and produces cleaner flavors. Wine yeast strains tolerate higher alcohol levels, up to 14-18% ABV, compared to beer yeast which typically stalls around 8-12%.
Bread fermentation Baker's yeast produces the same ethanol and CO2 as brewing yeast. The CO2 gets trapped in the gluten network, causing the dough to rise. The ethanol evaporates during baking. Commercial baker's yeast is selected for fast, vigorous CO2 production rather than flavor, while sourdough yeast works more slowly and contributes more complex aromas.
Ethanol tolerance Most yeast strains die or go dormant when alcohol concentration reaches their tolerance limit. Standard ale yeast stops around 8-12% ABV. Wine and champagne yeast can reach 14-18%. Specialized strains like turbo yeast can push to 20%+, but produce harsher flavors. This natural limit is why distillation is needed to produce spirits above 20% ABV.
C₆H₁₂O₆ glucose: the starting sugar molecule
2 C₂H₅OH two ethanol molecules produced per glucose
2 CO₂ two carbon dioxide molecules released per glucose

Acetic acid fermentation

Acetic acid fermentation stands apart from the other pathways because it requires oxygen. Acetobacter and Gluconobacter bacteria oxidize ethanol into acetic acid on the liquid’s surface, where air contact is available. This is how vinegar is made.

The process happens in two stages. First, yeast or another organism produces ethanol through alcoholic fermentation. Then acetic acid bacteria convert that ethanol into acetic acid. Apple cider vinegar, for example, starts as apple juice, ferments into hard cider (alcoholic), and then ferments further into vinegar (acetic).

The vinegar mother A vinegar mother is a cellulose biofilm produced by Acetobacter bacteria. It floats on the surface of the liquid, providing a structure for the bacteria to live on while accessing both the ethanol below and the oxygen above. A healthy mother looks like a translucent, rubbery disc. You can use a piece of an existing mother to start a new batch.
Why vinegar needs air Unlike lactic acid and alcoholic fermentation, acetic acid production is an oxidative process. The bacteria use oxygen as the final electron acceptor when converting ethanol to acetic acid. This is why vinegar crocks are covered with cloth (to allow airflow while keeping insects out) rather than sealed with an airlock.
Unwanted acetic acid In beer, wine, and other alcoholic ferments, acetic acid is a flaw. It produces a sharp, vinegary off-flavor. This is why brewers and winemakers minimize oxygen exposure during and after fermentation. If your beer smells like vinegar, oxygen got in and Acetobacter found it.

The role of enzymes

Every chemical reaction in fermentation is catalyzed by a specific enzyme. Without enzymes, these reactions would happen too slowly to be useful. Understanding which enzymes are at work helps explain why conditions like temperature and pH have such dramatic effects on fermentation.

Glycolysis enzymes Hexokinase phosphorylates glucose in the first step. Phosphofructokinase is the rate-limiting enzyme that controls how fast glycolysis runs. Pyruvate kinase catalyzes the final step, producing pyruvate and ATP. These enzymes are present in nearly all living organisms.
Pathway-specific enzymes Lactate dehydrogenase (LDH) converts pyruvate to lactate in lactic acid fermentation. Pyruvate decarboxylase removes CO2 from pyruvate in the alcoholic pathway. Alcohol dehydrogenase (ADH) reduces acetaldehyde to ethanol. Each pathway has its own characteristic enzyme that determines the final product.
Why temperature matters to enzymes Enzymes are proteins with a three-dimensional shape that determines their function. Heat increases reaction speed up to a point, but excessive heat denatures (unfolds) the protein, destroying its activity permanently. This is why fermenting too hot kills the process. Most fermentation enzymes work optimally between 20-40°C (68-104°F), with activity dropping sharply above 45°C (113°F).

pH changes during fermentation

pH drops during most fermentations as acids accumulate. This falling pH is one of fermentation’s built-in safety mechanisms. It creates an environment hostile to pathogenic bacteria while favoring the acid-tolerant fermentation organisms.

Tracking pH gives you a window into what is happening inside your ferment without opening it.

  1. 1
    Starting pH: 5.5-7.0 Most raw ingredients start near neutral. Fresh cabbage for sauerkraut has a pH around 6.0-6.5. Wort for beer starts around 5.2-5.6. Milk for yogurt starts at about 6.5-6.8. At this pH, many organisms can grow, including undesirable ones.
  2. 2
    Early fermentation: pH 4.5-5.5 As LAB or yeast begin producing acids, the pH drops below 5.0. This inhibits most pathogenic bacteria, including Clostridium botulinum, E. coli, and Salmonella. The ferment becomes microbiologically safer as acidity builds.
  3. 3
    Mid fermentation: pH 3.5-4.5 At this stage, only acid-tolerant organisms survive. In vegetable ferments, the transition from Leuconostoc (which dies off below pH 4.0) to Lactobacillus (which tolerates pH 3.0+) happens here. Flavor complexity increases.
  4. 4
    Finished ferment: pH 3.0-3.8 Sauerkraut finishes around pH 3.2-3.5. Yogurt reaches about pH 4.0-4.6. Kombucha lands between 2.5-3.5. Wine finishes around 3.0-3.6. At these low pH values, the ferment is shelf-stable (when kept in appropriate conditions) and resistant to spoilage.

Watch for

  • A pH above 4.6 does not reliably inhibit Clostridium botulinum. Fermented foods that stay above this threshold for extended periods pose a safety risk. Proper salt concentration (2-3% for vegetables) ensures pH drops quickly enough.
  • Do not use pH strips for precision measurement. They are only accurate to about 0.5 pH units. A digital pH meter gives readings accurate to 0.01 units and costs as little as $15-20.
  • Rinsing vegetables before fermenting removes surface LAB that drive wild fermentation. If your recipe calls for wild fermentation, skip the rinse or add a small amount of brine from a previous ferment as a starter.

CO2 production and pressure

Carbon dioxide is a major byproduct of both alcoholic and heterofermentative lactic acid fermentation. Managing CO2 determines whether your ferment stays safely sealed, carbonates properly, or blows up a jar on your counter.

Alcoholic ferments produce the most CO2 Every molecule of glucose fermented by yeast releases two molecules of CO2. A typical 5-gallon beer batch produces enough CO2 to fill a large room. This is why airlocks and blow-off tubes are necessary during active alcoholic fermentation. The pressure can crack sealed containers if there is no way for gas to escape.
Vegetable ferments produce less CO2 Heterofermentative LAB produce some CO2 during early fermentation, causing bubbling in sauerkraut and kimchi jars. Homofermentative LAB produce negligible CO2. The gas production in vegetable ferments is much lower than in alcoholic ferments, which is why simple weight-on-top setups work without airlocks.
Carbonation happens under pressure CO2 dissolves into liquid under pressure, creating carbonation. This is the basis of bottle conditioning in beer (adding priming sugar before sealing) and the natural fizz in kombucha and water kefir. The amount of CO2 that dissolves depends on pressure and temperature. Colder liquid holds more CO2, which is why carbonated drinks are served cold.

Wild vs. cultured fermentation

The choice between wild and cultured fermentation affects predictability, flavor complexity, and the level of control you have over the process.

Wild fermentation Relies on microbes already present on ingredients or in the environment. Sauerkraut, kimchi, and traditional sourdough are wild ferments. The microbial community shifts over time as conditions change (pH drops, sugars are consumed). Results vary between batches because the starting microbial population is never identical. This variation is part of the appeal for many fermenters.
Cultured fermentation Uses a defined starter culture added to the ingredients. Yogurt (S. thermophilus + L. bulgaricus), beer (selected yeast strains), and kombucha (SCOBY) are cultured ferments. The results are more consistent because the dominant organism is chosen and added in quantity. Commercial food production almost exclusively uses cultured fermentation for batch-to-batch consistency.
Backslopping A middle ground where a portion of a finished ferment is added to a new batch as a starter. This carries over a complex microbial community (not a single strain) while giving the beneficial organisms a head start over spoilage microbes. Sourdough starters, kefir grains, and ginger bugs all work on this principle.

Why temperature changes everything

Temperature affects fermentation at every level: enzyme activity, microbial growth rate, flavor compound production, and which organisms dominate the ferment.

Warmer = faster but rougher Higher temperatures increase metabolic activity, speeding up fermentation. But they also increase the production of off-flavor compounds like fusel alcohols (in beer), excess acetic acid (in vinegar), and sulfur compounds. A sauerkraut that ferments at 30°C (86°F) finishes faster than one at 18°C (64°F) but tastes sharper, less complex, and can develop mushy texture.
Cooler = slower but cleaner Lower temperatures slow microbial activity and enzyme reactions, extending fermentation time. The trade-off is cleaner, more nuanced flavors. Lager beer fermented at 10°C (50°F) takes weeks longer than ale fermented at 20°C (68°F) but has a notably smoother, crisper character. Sauerkraut fermented slowly at 18°C (64°F) develops deeper, more complex acidity.
Too hot kills Most fermentation organisms die above 45-50°C (113-122°F). Yeast is typically killed above 40°C (104°F). LAB are somewhat more heat-tolerant, with some strains surviving up to 50°C (122°F). If your ferment is exposed to extreme heat, the microbes die and fermentation stops permanently. You cannot restart a cooked ferment by cooling it down.

Every ferment has a temperature sweet spot. Staying within that range gives you the best balance of fermentation speed, flavor development, and food safety. The temperature guide for specific ferments covers the ideal ranges for every common type.

FAQ

Frequently asked questions

What is the difference between aerobic and anaerobic fermentation?
Anaerobic fermentation happens without oxygen. Lactic acid fermentation and alcoholic fermentation are both anaerobic processes. Acetic acid fermentation (vinegar making) is the notable exception. It requires oxygen because Acetobacter bacteria need it to convert ethanol into acetic acid. Most food fermentation takes place in anaerobic or low-oxygen environments.
Why does fermentation produce CO2?
Alcoholic fermentation produces CO2 as a byproduct when yeast converts pyruvate into ethanol. For every molecule of glucose fermented by yeast, two molecules of CO2 are released. This is what causes bread to rise, beer to carbonate, and airlocks to bubble. Lactic acid fermentation, by contrast, produces little to no CO2.
How does salt affect fermentation?
Salt creates a selective environment that favors lactic acid bacteria over spoilage organisms. At 2-3% salt concentration (by weight of vegetables), LAB thrive while most harmful bacteria and molds cannot grow. Salt also draws water out of vegetables through osmosis, creating the brine that submerges them and maintains the anaerobic environment.
Can fermentation go wrong?
Yes. Common problems include mold growth from oxygen exposure, off-flavors from fermenting too warm, mushy textures from too little salt, and stalled ferments from temperatures that are too cold. Most failures trace back to contamination, incorrect salt ratios, or temperature extremes. Following tested ratios and keeping ferments submerged and at the right temperature prevents the majority of issues.
What is the role of enzymes in fermentation?
Enzymes are proteins that catalyze every step of the fermentation pathway. Hexokinase and pyruvate kinase drive glycolysis. Pyruvate decarboxylase and alcohol dehydrogenase handle the alcoholic pathway. Lactate dehydrogenase converts pyruvate to lactic acid. Without these enzymes, the chemical reactions would be too slow to produce meaningful amounts of fermentation products.
What is wild fermentation vs. cultured fermentation?
Wild fermentation uses microbes already present on the raw ingredients or in the environment. Sauerkraut, traditional kimchi, and sourdough rely on wild fermentation. Cultured fermentation adds a specific starter culture to control the process. Yogurt, commercial beer, and kombucha (using a SCOBY) are cultured ferments. Wild fermentation is less predictable but can produce more complex flavors.
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