Ethanol fermentation takes place in the cytoplasm of cells, specifically in the fluid portion called the cytosol. It does not require mitochondria or any membrane-bound organelle. This is true whether the cell belongs to yeast, a bacterium, a plant root, or even certain animal tissues. Beyond the cellular level, ethanol fermentation happens across a surprisingly wide range of living systems and industrial settings.
Inside the Cell: The Cytosol
All the chemical steps of ethanol fermentation occur in the cytosol, the liquid interior of the cell outside the nucleus and organelles. The process begins with glycolysis, which splits a molecule of glucose into two molecules of pyruvate. Under low-oxygen or no-oxygen conditions, the cell then converts pyruvate into ethanol through two additional reactions rather than sending it to the mitochondria for aerobic respiration.
First, an enzyme called pyruvate decarboxylase strips a carbon atom from pyruvate, releasing carbon dioxide and producing acetaldehyde. Then a second enzyme, alcohol dehydrogenase, converts that acetaldehyde into ethanol. The entire sequence, from glucose to ethanol and CO₂, generates a net yield of 2 ATP molecules per glucose molecule. That’s far less energy than the 30-plus ATP produced by aerobic respiration, which is why cells only resort to fermentation when oxygen is scarce or absent.
Yeast: The Primary Fermenter
The organism most associated with ethanol fermentation is the yeast Saccharomyces cerevisiae, the same species used in bread baking, brewing, and winemaking. In industrial sugarcane ethanol production, selected strains of this yeast convert sugars into ethanol at concentrations of 7 to 11 percent by volume, reaching up to 92 percent of the maximum theoretical yield. A smaller number of bacteria also perform ethanol fermentation, most notably Zymomonas mobilis, which uses a slightly different metabolic route and produces only 1 ATP per glucose instead of 2.
Yeast cells ferment whenever oxygen is limited or when sugar concentrations are very high. When oxygen becomes available again, they shift back toward aerobic respiration, a phenomenon known as the Pasteur effect. Under oxygen-free conditions, yeast keeps consuming large amounts of glucose to compensate for the low energy return, and ethanol accumulates until it eventually reaches concentrations toxic enough to halt fermentation entirely.
Ethanol Fermentation in Plants
Plants are not typically thought of as fermenters, but their root cells regularly produce ethanol when waterlogged soil cuts off their oxygen supply. Under these hypoxic conditions, root cells switch from aerobic respiration to glycolysis paired with ethanol fermentation, using the same pyruvate decarboxylase and alcohol dehydrogenase enzymes found in yeast.
This is a survival strategy, not a permanent solution. The fermentation provides just enough energy to keep root cells alive during flooding. Waterlogging-tolerant species like rice, cucumber, cotton, and soybean ramp up the expression of genes coding for these fermentation enzymes, buying time until oxygen returns. Prolonged waterlogging, however, still causes root damage, reduced growth, and eventually yield loss, because the energy output of fermentation simply cannot sustain normal plant function for long.
Animals That Produce Ethanol
A handful of vertebrates can produce ethanol in their own tissues. Goldfish and crucian carp survive prolonged oxygen deprivation by converting stored glycogen into ethanol. In these fish, the brain and heart initially produce lactate under anoxic conditions, but that lactate is then transported to skeletal muscle, where alcohol dehydrogenase activity is concentrated. The muscle tissue is where the final conversion to ethanol and CO₂ takes place exclusively. The ethanol then diffuses out through the gills into the surrounding water, preventing toxic buildup.
This adaptation is extraordinarily rare. Only three carp relatives and two species of turtles are known to survive without any oxygen for days using this mechanism. All other vertebrates die within minutes under the same conditions. When researchers first documented this in goldfish, they described it as the first vertebrate known to match the biochemical capacity of yeast.
Fermentation in the Human Body
Human cells do not normally produce ethanol. However, in a rare condition called auto-brewery syndrome, fungi or bacteria in the gastrointestinal tract, oral cavity, or urinary system ferment dietary sugars into ethanol. People with this condition can become measurably intoxicated without drinking alcohol. The primary trigger is an overgrowth of fermenting microorganisms, often yeast, in the gut. Reducing simple and complex sugars in the diet decreases the amount of ethanol produced.
Industrial Fermentation Vessels
At the commercial scale, ethanol fermentation takes place in large bioreactors, often called fermenters. In Brazil, the world’s second-largest ethanol producer, most distilleries run fed-batch fermentation, where sugar is added gradually to avoid overwhelming the yeast with high substrate concentrations. About 20 to 30 percent of Brazilian distilleries instead use continuous fermentation, where fresh sugar solution flows in and fermented broth flows out in an ongoing cycle.
Fermentation temperatures in industrial and optimized laboratory settings typically fall in the range of 30 to 38°C, with 35°C being a common target for S. cerevisiae. The process generally runs for 24 to 72 hours depending on the setup. One of the main challenges is that byproducts like organic acids and the ethanol itself progressively inhibit yeast metabolism, so conditions need careful management to keep yields high.