From Grape Juice to Wine: The Chemistry of Fermentation
Fermentation is one of humanity's oldest biotechnologies, predating written history. The Sumerians brewed beer 6,000 years ago; wine vessels have been found dating to 7,000 BCE. Yet it was not until Louis Pasteur's work in the 1850s that fermentation was understood as a biological process — and Eduard Buchner's 1897 demonstration that yeast extracts could ferment sugar without living cells confirmed that enzymes, not life force, drove the chemistry. Buchner won the 1907 Nobel Prize in Chemistry for this insight.
What Is Fermentation?
In the strict biochemical sense, fermentation is an anaerobic metabolic process that regenerates NAD⁺ from NADH, allowing glycolysis to continue in the absence of oxygen. Cells cannot keep running glycolysis unless they can oxidize the NADH produced — fermentation accomplishes this by transferring electrons to an organic molecule instead of to oxygen.
In common usage, fermentation refers more broadly to any microbial transformation of organic substrates, including aerobic processes used in industrial fermenters.
Alcoholic Fermentation
Alcoholic fermentation is carried out primarily by Saccharomyces cerevisiae (baker's and brewer's yeast), though other yeasts and some bacteria also perform it.
The pathway begins with glycolysis, which converts one molecule of glucose (C₆H₁₂O₆) to two molecules of pyruvate (CH₃COCOO⁻), yielding 2 ATP and 2 NADH:
C₆H₁₂O₆ → 2 CH₃COCOO⁻ + 2 ATP + 2 NADH
Under anaerobic conditions, pyruvate is processed in two steps:
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Decarboxylation: pyruvate → acetaldehyde + CO₂ (enzyme: pyruvate decarboxylase, requires thiamine pyrophosphate)
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Reduction: acetaldehyde + NADH + H⁺ → ethanol + NAD⁺ (enzyme: alcohol dehydrogenase)
Net reaction: C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂ + 2 ATP
The CO₂ produced is responsible for the bubbles in beer, the carbonation in Champagne, and the rise in bread dough. The ethanol (C₂H₅OH) is the intoxicating component of alcoholic beverages.
Wine Chemistry
Grape must (freshly crushed juice) contains 150–250 g/L of sugars (glucose and fructose in roughly equal proportions, collectively called "invert sugars"). Yeast ferments these to ethanol and CO₂ over 1–2 weeks.
The alcohol content (ABV) of the final wine is approximately: ABV ≈ 0.59 × (sugar concentration in g/L) / 10. A must with 200 g/L sugar yields approximately 12% ABV wine.
Beyond ethanol, wine contains hundreds of secondary metabolites that create its complex flavor: - Glycerol (5–15 g/L): produced when yeast reduces glyceraldehyde-3-phosphate; adds body and sweetness - Esters (ethyl acetate, isoamyl acetate): fruity aromas produced by esterification of alcohols and acids - Higher alcohols (propanol, isoamyl alcohol, 2-phenylethanol): formed from amino acid catabolism via the Ehrlich pathway; contribute to complex aromas - Organic acids: tartaric, malic, citric (from grapes), plus succinic and lactic acids produced by yeast - Sulfur dioxide (SO₂): naturally produced in small amounts by yeast; also added as a preservative and antioxidant
Malolactic fermentation (MLF) is a secondary fermentation carried out by lactic acid bacteria: malic acid (tart, apple-like) → lactic acid (softer, creamy). Common in red wines and some white wines (e.g., Chardonnay).
Beer Chemistry
Beer is produced from malted barley (or other grains). The malting process germinates the grain, activating amylase enzymes that break starch into fermentable sugars:
Starch (polymer) + H₂O → maltose + glucose (amylase reaction during mashing)
Hops (Humulus lupulus) flowers contain: - α-acids (humulones): isomerized during boiling to iso-α-acids, which provide bitterness (measured in IBUs) - Linalool, myrcene, and terpene oils: floral and citrus aromas - Antibacterial compounds that help preserve beer
Different yeast strains (lager vs. ale yeasts) and fermentation temperatures produce dramatically different flavor profiles. Lager yeasts (S. pastorianus) ferment at 4–10°C and produce clean, crisp flavors. Ale yeasts (S. cerevisiae) ferment at 15–24°C and produce fruitier, more complex profiles due to greater ester formation.
Lactic Acid Fermentation
Lactic acid fermentation reduces pyruvate to lactic acid (lactate):
Pyruvate + NADH + H⁺ → Lactate + NAD⁺ (enzyme: lactate dehydrogenase)
In Human Muscle
During intense exercise, when oxygen delivery to muscles is insufficient for aerobic respiration, muscle cells switch to lactic acid fermentation to regenerate NAD⁺ and maintain ATP production. Lactate diffuses into the bloodstream and is transported to the liver, where it can be converted back to glucose (Cori cycle).
The burning sensation of strenuous exercise was long attributed to lactate accumulation, but recent research suggests it is the accompanying H⁺ ions (acidification) and inorganic phosphate that cause fatigue, not lactate itself.
In Food Production
Lactic acid bacteria (LAB) — including Lactobacillus, Streptococcus, and Leuconostoc species — perform lactic acid fermentation and are essential in food preservation and production:
- Yogurt: Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus lower pH by producing lactic acid, denaturing milk proteins (casein) into a gel
- Cheese: different LAB strains, molds, and aging times create the diversity of cheeses
- Sauerkraut and kimchi: Leuconostoc and Lactobacillus species ferment cabbage, lowering pH below 4, inhibiting pathogenic bacteria (natural preservation)
- Sourdough bread: a mixed culture of LAB and wild yeasts; LAB produce lactic and acetic acids for flavor, yeast provides CO₂ for leavening
Industrial Fermentation
Beyond beverages and food, fermentation is a cornerstone of industrial biotechnology:
- Bioethanol: yeast fermentation of corn, sugarcane, or cellulosic biomass produces fuel ethanol; approximately 100 billion liters produced annually worldwide
- Citric acid (E330): produced by Aspergillus niger fermenting glucose or molasses; 2 million tonnes/year, used in food, pharmaceuticals, and detergents
- Amino acids: glutamate (Corynebacterium glutamicum) for MSG; lysine for animal feed
- Penicillin and antibiotics: Penicillium chrysogenum in stirred bioreactors; fermentation yields have increased 10,000-fold since Fleming's discovery through strain improvement and optimization
- Recombinant proteins: human insulin is produced by recombinant E. coli fermenting in large bioreactors; similarly, erythropoietin, growth hormone, and monoclonal antibodies
Secondary Metabolites and Flavors
Many of fermentation's most valued products are secondary metabolites — compounds not essential to the organism's primary metabolism but with important roles in ecology (and enormous value to humans):
The diacetyl (buttery flavor), acetaldehyde (green apple), and hydrogen sulfide (rotten egg) found in off-flavor beers result from fermentation intermediates not fully processed by yeast. Control of fermentation conditions — temperature, pitching rate, oxygen during yeast growth — minimizes these off-flavors.
Fermentation exemplifies how simple biochemistry — just a few enzymatic steps downstream of glycolysis — can be harnessed to transform raw materials into products of extraordinary cultural, economic, and industrial importance.