Fermentation
The biological engine of all alcohol
Fermentation is an anaerobic (oxygen-free) metabolic process — a type of biological process in the domain of biochemistry and food science — in which microorganisms, principally Saccharomyces cerevisiae, enzymatically convert fermentable sugars into ethanol (ethyl alcohol, C₂H₅OH) and carbon dioxide (CO₂) through a sequence of reactions known as glycolysis and pyruvate decarboxylation. Fermentation is the foundational production process shared by all alcoholic beverages — beer, wine, spirits, and traditional fermented drinks — regardless of their base ingredient. Fermentation alone produces beverages up to approximately 15–16% ABV, above which ethanol becomes toxic to the yeast; higher-strength beverages require the subsequent process of distillation.
What fermentation actually is
Every alcoholic drink on earth — from a bottle of Bordeaux to a glass of Nigerian palm wine — begins with the same invisible event: yeast eating sugar and producing alcohol as a waste product.
Yeast is a single-celled fungus. It is alive. When yeast encounters sugar and has no oxygen to breathe, it switches into survival mode and begins fermenting — breaking sugar apart to release the energy it needs, and releasing ethanol and carbon dioxide as byproducts. The ethanol is what makes a drink alcoholic. The carbon dioxide is what makes it bubble.
This is not a human invention. Fermentation happens spontaneously wherever sugar, water, and wild yeast exist together — in overripe fruit, in grain soaked by rain, in tree sap pooling in a hollow. Human beings did not discover fermentation. They noticed it happening naturally and learned to control it.
The earliest documented evidence of intentional fermentation comes from Jiahu in China, dated to approximately 7000 BCE — a mixed fermented drink made from rice, honey, fruit, and hawthorn, identified from residue analysis of pottery shards by Dr. Patrick McGovern of the University of Pennsylvania Museum in a paper published in the Proceedings of the National Academy of Sciences (2004).
Why it matters for what you drink
Fermentation does not only produce alcohol. It produces hundreds of flavour compounds — esters, aldehydes, organic acids, higher alcohols — that give each drink its specific taste, aroma, and character. The tropical fruit notes in a Jamaican rum, the bready complexity in a Belgian ale, the sharp acidity in a natural wine — all of these are produced during fermentation, not during distillation or ageing.
This means that a distiller or winemaker who wants a specific flavour profile must control fermentation precisely — the yeast strain they choose, the temperature, the speed, the nutrients available, the size and shape of the vessel. Two identical batches of grape juice fermented with different yeast strains at different temperatures will produce wines that taste completely different.
The natural limit of fermentation
Yeast cannot survive in its own waste. As ethanol concentration rises during fermentation, it becomes progressively toxic to the yeast. Most yeast strains die at around 12–15% ABV. Certain engineered strains can tolerate up to approximately 20% ABV before dying. This is why wine and beer are naturally limited in alcohol content — not by recipe, but by biology.
To produce anything stronger, you need distillation — a separate process that concentrates the ethanol by exploiting the difference in boiling points between alcohol and water.
The fermentation equation
The overall chemical reaction of alcoholic fermentation is summarised as:
This equation, described by French chemist Joseph Louis Gay-Lussac in 1810, is a summary of a complex enzymatic sequence rather than a single reaction. The actual pathway involves 10 enzymatic steps in glycolysis plus two further steps of pyruvate decarboxylation — all of which occur inside the yeast cell.
The stages of fermentation in practice
Lag phase — yeast acclimatisation (0–12 hours)
Yeast is introduced to the must (grape juice), wort (malted grain liquid), or molasses solution. Yeast cells absorb nutrients, synthesise enzymes, and adapt to the temperature and pH of the liquid. Little visible activity. This phase is critical — yeast under-pitched or introduced to extreme temperatures here will produce off-flavours throughout the ferment.
Exponential growth phase — vigorous fermentation (12–72 hours)
Yeast population doubles rapidly. Visible bubbling as CO₂ is released. Temperature rises due to exothermic reaction — temperature control is critical here. The majority of ethanol is produced during this phase. Most flavour compounds including esters and higher alcohols are also produced now.
Stationary phase — slowing fermentation (days 3–7+)
Sugar supply diminishes. Ethanol concentration increases. Yeast growth slows as the environment becomes less hospitable. CO₂ production decreases visibly. The winemaker or brewer monitors residual sugar to determine when to intervene or allow fermentation to continue.
Decline phase — fermentation completion
Yeast cells die or become dormant. Fermentation ceases naturally when either all fermentable sugar has been consumed (dry fermentation) or ethanol concentration has reached yeast tolerance (typically 12–16% ABV). Dead yeast cells — lees — settle to the bottom of the vessel.
Yeast strains — how they differ and why it matters
Saccharomyces cerevisiae is the dominant yeast species in commercial fermentation globally, but thousands of distinct strains exist — each producing a different flavour profile. Selecting the correct strain is one of the most consequential decisions in any fermented drink production.
| Category | Yeast type / strain example | Key flavour contribution | Fermentation temperature |
|---|---|---|---|
| Champagne / Sparkling wine | S. cerevisiae EC-1118 | Neutral, high alcohol tolerance, minimal esters | 10–18°C |
| Burgundy / Pinot Noir wine | RC 212 (Bourgovin) | Red fruit, spice, low volatile acidity | 20–30°C |
| Belgian Trappist ale | House S. cerevisiae strains (e.g., WY3787) | Phenolics, esters, dried fruit, high attenuation | 18–24°C |
| German Lager | S. pastorianus (bottom-fermenting) | Clean, sulphur compounds reduced on cold conditioning | 7–13°C |
| Jamaican rum (high ester) | Wild / Schizosaccharomyces pombe + bacteria | Extreme ester production — funk, tropical fruit, banana | 28–35°C |
| Scotch whisky | S. cerevisiae distillers strains (e.g., MX, Quest) | Esters, fatty acids — long fermentation develops character | 18–22°C |
| Sake | S. cerevisiae sake strains (e.g., K7, K9, K14) | Floral, fruit esters, amino acids from parallel fermentation | 5–15°C |
The role of fermentation vessel
The material, size, and shape of the fermentation vessel affects temperature distribution, oxygen exposure, CO₂ escape rate, and lees contact — all of which influence flavour development.
| Vessel type | Used in | Documented effect |
|---|---|---|
| Stainless steel tank (closed) | Most commercial wine, beer, spirits | Temperature-controllable. Neutral flavour contribution. Preserves primary fruit character. |
| Open wooden washback (larch or Oregon pine) | Scotch malt whisky | Harbours resident bacteria producing additional fatty acids and esters. SWA regulations do not mandate wood but traditional distilleries cite flavour benefit. Documented by SWA technical papers. |
| Open stone or clay vat | Traditional mezcal (ancestral category) | Wild yeast and bacterial contribution. Required for Ancestral category under COMERCAM regulations. |
| Concrete tank | Natural wine producers, some Burgundy | Slight thermal mass moderates temperature swings. Micro-oxygenation through concrete pores. |
| Large wooden oval (Fuder) | German Mosel wine | Micro-oxygenation, historical yeast populations in wood. |
| Amphora / qvevri (clay) | Georgian wine (UNESCO heritage method) | Skin-contact fermentation. Clay temperature regulation. Documented by Georgian National Wine Agency. |
Glycolysis — the enzymatic pathway in full
Alcoholic fermentation proceeds via the Embden-Meyerhof-Parnas (EMP) pathway of glycolysis, followed by pyruvate decarboxylation. The complete sequence involves 12 enzymatic steps, each catalysed by a specific enzyme within the yeast cytoplasm.
Glycolysis — 10 steps, summary
Glucose (C₆H₁₂O₆) is phosphorylated by hexokinase using ATP, producing glucose-6-phosphate. After isomerisation to fructose-6-phosphate, a second phosphorylation by phosphofructokinase-1 (PFK-1) — the primary regulatory enzyme in glycolysis — produces fructose-1,6-bisphosphate. PFK-1 activity is allosterically inhibited by high ATP concentrations (abundant energy) and activated by AMP and ADP (energy demand), making it the key control point for fermentation rate.
Cleavage by aldolase produces two triose phosphate molecules. These are oxidised by glyceraldehyde-3-phosphate dehydrogenase (GAPDH), producing NADH and 1,3-bisphosphoglycerate. The remaining steps generate ATP via substrate-level phosphorylation, ultimately producing two molecules of pyruvate (C₃H₄O₃) per glucose.
Net yield: 2 ATP, 2 NADH, 2 pyruvate per glucose molecule.
Pyruvate decarboxylation — the alcohol-producing steps
Under anaerobic conditions, pyruvate is converted to acetaldehyde (C₂H₄O) by pyruvate decarboxylase, releasing CO₂. Acetaldehyde is then reduced to ethanol (C₂H₅OH) by alcohol dehydrogenase (ADH), simultaneously re-oxidising NADH to NAD⁺ — regenerating the cofactor required for glycolysis to continue.
This NAD⁺ regeneration is the biochemical reason fermentation exists: it allows glycolysis to continue producing ATP in the absence of oxygen, by providing an alternative electron acceptor. The ethanol is, in this sense, a metabolic waste product.
Secondary metabolites and flavour compound formation
The flavour complexity of fermented beverages derives primarily from secondary metabolites produced during fermentation. The principal classes, their biochemical origins, and their sensory thresholds are documented in the peer-reviewed literature as follows:
Esters
Esters are formed by the reaction of alcohols (primarily ethanol and higher alcohols) with acyl-CoA derivatives, catalysed by alcohol acyltransferases (AATases). The most significant are ethyl acetate (solvent/fruity, threshold ≈ 33 mg/L) and isoamyl acetate (banana/pear, threshold ≈ 1.2 mg/L). Ester production is enhanced by: higher fermentation temperatures, specific yeast strains with high AATase activity, nitrogen limitation, and reduced agitation. The banana character of Hefeweizen beers and the estery profile of Jamaican high-ester rum are direct consequences of ester accumulation documented in Journal of the Institute of Brewing literature.
Higher alcohols (fusel alcohols)
Higher alcohols — principally isoamyl alcohol (3-methylbutan-1-ol), isobutanol, and n-propanol — are produced via the Ehrlich pathway from amino acid catabolism. At concentrations below ≈ 300 mg/L they contribute body and complexity; above ≈ 400 mg/L they produce harsh, solvent-like off-flavours. Their production is increased by high fermentation temperature, nitrogen deficiency, and elevated pitching rates. The practice of removing foreshots during distillation (the first fraction of distillate) is partly designed to concentrate and then discard the fraction with the highest fusel alcohol content.
Organic acids
Succinic acid, malic acid, citric acid, and acetic acid are produced during fermentation. Acetic acid above ≈ 700 mg/L (volatile acidity) is considered a defect in wine, produced primarily by Acetobacter spoilage under aerobic conditions or by certain yeast strains. In natural wine and some traditional fermented drinks, elevated acetic acid is an intentional stylistic element.
Sulphur compounds
Hydrogen sulphide (H₂S), dimethyl sulphide (DMS), and dimethyl disulphide are produced during fermentation, primarily under nitrogen deficiency. In lager production, extended cold conditioning (lagering) is specifically designed to allow H₂S and DMS to off-gas, producing the clean flavour profile characteristic of the style. In some sake styles, low sulphur compound production by specific yeast strains (K9, K14) is a documented breeding objective of the Brewing Society of Japan.
Non-Saccharomyces fermentation and wild fermentation
Commercial fermentation is dominated by Saccharomyces cerevisiae, but significant categories of alcoholic beverages involve other organisms or mixed fermentations:
| Organism | Drink category | Documented role | Source |
|---|---|---|---|
| Schizosaccharomyces pombe | High-ester Jamaican rum; some African traditional drinks | Malic acid deacidification; high ester production under certain conditions | Léauté (1990), J. Inst. Brew. |
| Brettanomyces bruxellensis | Belgian lambic, Gueuze, some red wines | 4-ethylphenol and 4-ethylguaiacol — barnyard, leather, spice. A defect in wine; a defining characteristic in lambic. | Chatonnet et al. (1992), J. Sci. Food Agric. |
| Lactobacillus and Pediococcus spp. | Belgian sour ales, lambic, some natural wines | Lactic acid fermentation producing sourness. Diacetyl (butterscotch) intermediate reduced by extended conditioning. | Gevers et al. (2000), J. Inst. Brew. |
| Aspergillus oryzae (koji mould) | Sake, shochu, mirin, some baijiu | Saccharification — converts rice starch to fermentable sugars via amylase enzymes, occurring simultaneously with S. cerevisiae fermentation (multiple parallel fermentation) | National Research Institute of Brewing, Japan |
| Wild yeast consortia | Natural wine, ancestral mezcal, traditional African and South American drinks | Highly variable. Species including Torulaspora delbrueckii, Lachancea thermotolerans, Metschnikowia pulcherrima contribute to aromatic complexity before being outcompeted by S. cerevisiae. | Jolly et al. (2014), FEMS Yeast Research |
Fermentation in parallel — sake and baijiu
Standard Western fermentation is sequential: saccharification (starch to sugar) precedes fermentation (sugar to ethanol). Japanese sake and Chinese baijiu use multiple parallel fermentation — saccharification and fermentation occur simultaneously in the same vessel, enzymatically coupled.
In sake production, Aspergillus oryzae (koji) cultivated on a portion of the rice produces amylase and glucoamylase enzymes that continuously release glucose from starch. Saccharomyces cerevisiae simultaneously consumes that glucose as it is released. This continuous sugar feed prevents the glucose concentration from ever reaching inhibitory levels, allowing sake to achieve ethanol concentrations of 18–20% ABV before dilution — the highest of any naturally fermented beverage — documented by the National Research Institute of Brewing (NRIB), Japan.