The primary gas produced during fermentation is carbon dioxide (CO₂). When yeast breaks down sugar, each molecule of glucose yields two molecules of ethanol and two molecules of carbon dioxide. This is the gas responsible for the bubbles in beer, the rise in bread dough, and the fizz in champagne. But carbon dioxide isn’t the only gas fermentation can produce. Depending on the type of organism and the conditions involved, fermentation also generates hydrogen, methane, and trace amounts of hydrogen sulfide.
Carbon Dioxide From Yeast Fermentation
The most familiar type of fermentation is alcoholic fermentation, carried out by yeast. The chemical equation is straightforward: one molecule of glucose (C₆H₁₂O₆) is broken down into two molecules of ethanol and two molecules of CO₂. This reaction powers the production of beer, wine, spirits, and fuel ethanol from grains like corn, sorghum, barley, and wheat, as well as sugar crops like cane and beets.
The volume of CO₂ produced is substantial. Major breweries install multimillion-dollar recovery systems to capture fermentation CO₂ and reuse it for carbonating beer and purging tanks. Smaller craft breweries typically can’t justify that expense, so they vent the gas and buy CO₂ from outside vendors instead. One recovery system designed for mid-size breweries produces about five tons of carbon dioxide per month, enough for a brewery generating up to 60,000 barrels per year. The CO₂ captured directly from fermenters is actually purer than what breweries typically buy, which often comes as a byproduct of ammonia and urea plants and may carry trace industrial contaminants.
Not All Lactic Acid Fermentation Produces Gas
Lactic acid bacteria come in two varieties, and only one of them generates CO₂. Homofermentative bacteria, like Streptococcus, Lactococcus, and Pediococcus, convert glucose entirely into lactic acid with no gas production at all. This is the type of fermentation happening in yogurt, for example.
Heterofermentative bacteria, like Leuconostoc and certain Lactobacillus species, take a different path. They convert one molecule of glucose into one molecule of lactic acid, one molecule of ethanol, and one molecule of CO₂. This is why some fermented foods, like sourdough bread and kimchi, develop a gentle fizziness. Lab technicians can actually distinguish heterofermentative from homofermentative bacteria by watching for CO₂ bubbles during a simple bench test.
Hydrogen Gas From Bacterial Fermentation
Certain bacteria produce hydrogen gas (H₂) as a major fermentation product. In oxygen-free environments, these organisms use protons as electron acceptors, stripping electrons from organic material and combining them into H₂. The key players are strict anaerobes like Clostridium species and facultative anaerobes like E. coli, which can switch between oxygen-dependent and oxygen-free metabolism.
The specific mix of gases depends on the fermentation pathway. In butyric acid fermentation, Clostridium bacteria break down glucose into butyric acid, acetic acid, hydrogen, and CO₂. In ethanol-type fermentation driven by bacteria like Bacteroides and Fusobacterium, the end products include ethanol, acetic acid, H₂, and CO₂. Mixed acid fermentation, carried out by organisms like E. coli and Salmonella, produces lactic acid, acetic acid, ethanol, formic acid, plus both H₂ and CO₂. In every case, hydrogen is released alongside carbon dioxide rather than on its own.
Methane From Anaerobic Digestion
When organic waste breaks down in the absence of oxygen, a community of microorganisms produces biogas through a multi-step fermentation process. The resulting gas mixture is 50 to 70 percent methane (CH₄) by volume and 30 to 50 percent carbon dioxide, with traces of hydrogen sulfide and water vapor. This is the same process that occurs in landfills, swamps, and the digestive systems of cows. Industrially, anaerobic digesters harness this reaction to convert food waste, manure, and sewage into a renewable fuel source.
Hydrogen Sulfide and Other Trace Gases
Hydrogen sulfide (H₂S), the gas that smells like rotten eggs, is produced in small amounts during many types of fermentation. It’s particularly relevant in winemaking, where yeast strains of Saccharomyces cerevisiae generate H₂S as they metabolize sulfur-containing compounds. The amount varies significantly by yeast strain, the nitrogen content of the juice, and how vigorously fermentation proceeds. Winemakers once assumed that adding nitrogen to grape juice would reduce H₂S, but research using Chardonnay juice and five commercial yeast strains found a more complicated picture: moderate nitrogen additions actually increased H₂S formation, while only high levels reduced it. Even small amounts of H₂S powerfully affect a wine’s aroma.
Fermentation Gas in Your Gut
Your own digestive system is a fermentation chamber. Gut bacteria ferment undigested carbohydrates, especially fiber, resistant starch, and certain sugars your small intestine can’t absorb. The gases produced include hydrogen, carbon dioxide, and in about a third of people, methane. Hydrogen concentrations in intestinal gas range from undetectable to over 40 percent by volume, depending on what you’ve eaten and which bacterial species dominate your gut. Some of this hydrogen gets consumed by other gut microbes, including methane-producing archaea and bacteria that generate short-chain fatty acids like butyrate. The balance between hydrogen producers and hydrogen consumers shapes both the volume of gas you experience and the composition of your gut microbiome.
Why CO₂ Buildup Is a Safety Concern
Because carbon dioxide is colorless and odorless, it can accumulate to dangerous levels in enclosed fermentation spaces without anyone noticing. OSHA lists breweries and fermentation plants as potential sources of hazardous CO₂ exposure. The workplace exposure limit is 5,000 ppm as a time-weighted average, with a short-term ceiling of 30,000 ppm. For context, normal outdoor air contains about 420 ppm.
The earliest symptoms of excessive CO₂ exposure are sweating and headache, caused by blood vessel dilation in the brain and skin. As concentrations rise, people experience shortness of breath, drowsiness, dizziness, and increased heart rate. Concentrations above 10 percent pose an immediate physiological threat. At roughly 30 percent, the gas triggers convulsions and coma. Fermentation cellars, tank rooms, and grain silos are the highest-risk areas, which is why commercial operations use continuous CO₂ monitors and forced ventilation systems in any enclosed space where fermentation occurs.