Fermentation Biochemistry: Alcoholic and Lactic

Bread rises, grape juice becomes wine, milk turns into yoghurt, cabbage becomes sauerkraut, and your muscles keep working during a sprint — all thanks to fermentation. It’s one of the oldest technologies humans use, and one of the oldest metabolic pathways in living things. Chemically, fermentation is a clever solution to a specific problem: how to keep making ATP when there’s no oxygen to accept electrons. This article explains the biochemistry of the two most important types, alcoholic and lactic.

What fermentation is — and isn’t

In everyday language, “fermentation” means any process where microbes change food. In biochemistry it has a more precise meaning:

Fermentation is a way of making ATP from organic molecules (such as glucose) in which the electrons removed during oxidation are passed to an organic molecule made within the pathway, rather than to oxygen or another external acceptor.

In other words, the cell oxidises part of the glucose and reduces another part, balancing the books internally.

The problem fermentation solves: recycling NAD⁺

Both fermentation types begin with glycolysis, which takes place in the cytoplasm and doesn’t need oxygen (see glycolysis). Overall:

glucose + 2 ADP + 2 Pᵢ + 2 NAD⁺ → 2 pyruvate + 2 ATP + 2 NADH + 2 H⁺ + 2 H₂O

Glycolysis produces a net 2 ATP per glucose. But notice that it also converts NAD⁺ into NADH. NAD⁺ is a coenzyme that accepts electrons in one step of glycolysis — the oxidation of glyceraldehyde-3-phosphate (see cofactors and coenzymes). A cell has only a small amount of NAD⁺. If it’s all converted to NADH, glycolysis stops, and so does ATP production.

With oxygen available, NADH passes its electrons to the electron transport chain, regenerating NAD⁺ and producing much more ATP. Without oxygen (or in cells that lack mitochondria), that route is closed. Fermentation’s whole purpose is to regenerate NAD⁺ so glycolysis can keep running. It makes no extra ATP of its own.

Lactic acid fermentation

The reaction

In lactic fermentation, pyruvate itself is the electron acceptor. The enzyme lactate dehydrogenase transfers electrons (as a hydride ion) from NADH to pyruvate:

pyruvate + NADH + H⁺ → lactate + NAD⁺

CH₃COCOO⁻ + NADH + H⁺ → CH₃CH(OH)COO⁻ + NAD⁺

The ketone group (C=O) of pyruvate is reduced to a secondary alcohol (CH–OH), and NADH is oxidised back to NAD⁺. Overall:

glucose → 2 lactate + 2 H⁺ (net 2 ATP)

No carbon dioxide is produced — all six carbons of glucose end up in the two three-carbon lactate ions.

Where it happens

• Human muscle during intense exercise, when glycolysis runs faster than mitochondria can take up pyruvate (see what happens chemically when you exercise). The lactate is later used as fuel or converted back to glucose in the liver (see lactic acid myths).
• Red blood cells, which have no mitochondria and rely entirely on glycolysis and lactate production.
• Lactic acid bacteria (such as Lactobacillus and Streptococcus thermophilus), used to make yoghurt, cheese, sourdough, kimchi, sauerkraut and pickles.

Why it preserves food

Lactic acid bacteria convert sugars in milk or vegetables into lactic acid, which lowers the pH to around 4–4.5 or below. Most spoilage and disease-causing bacteria can’t grow at this pH, so the food keeps much longer. In yoghurt, the falling pH also makes the milk protein casein lose its negative charge and clump into a gel — the same chemistry as curdling milk with lemon juice (see denaturation).

Alcoholic fermentation

The reactions

Yeasts (such as Saccharomyces cerevisiae) and some bacteria use a two-step route from pyruvate:

1. Decarboxylation: the enzyme pyruvate decarboxylase removes a carbon dioxide molecule from pyruvate, giving ethanal (acetaldehyde). This enzyme needs thiamine pyrophosphate (from vitamin B₁) and magnesium ions.

   CH₃COCOO⁻ + H⁺ → CH₃CHO + CO₂

2. Reduction: alcohol dehydrogenase transfers electrons from NADH to ethanal, producing ethanol and regenerating NAD⁺.

   CH₃CHO + NADH + H⁺ → CH₃CH₂OH + NAD⁺

Overall:

C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂ (net 2 ATP)

Interestingly, alcohol dehydrogenase is the same type of enzyme your liver uses to oxidise ethanol — running in the opposite direction (see how the body metabolises alcohol).

Uses

• Bread: the CO₂ forms bubbles trapped by the gluten network, making dough rise. The small amount of ethanol evaporates during baking.
• Beer and wine: the ethanol is the product. In brewing, grain starch must first be broken into sugars (malting and mashing), because yeast can’t digest starch directly. In winemaking, grape sugars are fermented directly.
• Bioethanol fuel: fermentation of sugars from sugarcane or maize.

A worked example

What mass of ethanol could, in theory, be produced from 1.00 kg of glucose?

• M(C₆H₁₂O₆) = 180.16 g mol⁻¹; moles = 1,000 ÷ 180.16 = 5.55 mol.
• 1 mol glucose → 2 mol ethanol, so ethanol = 11.10 mol.
• M(C₂H₅OH) = 46.07 g mol⁻¹; mass = 11.10 × 46.07 = 511 g.

The remaining mass leaves as CO₂ (11.10 × 44.01 = 489 g). In practice yields are lower (often around 90 % of theoretical), because yeast uses some sugar to grow and makes by-products such as glycerol. You can practise these calculations with the molar mass calculator.

Why wine stops at about 15 % alcohol

As ethanol accumulates, it disrupts yeast cell membranes and denatures proteins. Most wine yeasts stop working at around 13–16 % alcohol by volume. Stronger drinks such as spirits are made by distillation — separating ethanol (boiling point 78 °C) from water (100 °C) by heating the mixture and condensing the vapour, which is richer in ethanol.

Comparing the two pathways

Feature	Lactic fermentation	Alcoholic fermentation
Organisms	Animal muscle, red blood cells, lactic acid bacteria	Yeast, some bacteria, some plants when flooded
Electron acceptor	Pyruvate	Ethanal (from pyruvate)
Enzymes after glycolysis	Lactate dehydrogenase	Pyruvate decarboxylase, alcohol dehydrogenase
Products	2 lactate	2 ethanol + 2 CO₂
CO₂ produced?	No	Yes
Net ATP per glucose	2	2
Reversible in the organism?	Yes (lactate → pyruvate)	Not in yeast (the decarboxylation is irreversible)
Food uses	Yoghurt, cheese, sauerkraut, sourdough	Bread, beer, wine

How efficient is fermentation?

Fermentation yields only 2 ATP per glucose, compared with about 30–32 from aerobic respiration (see aerobic vs anaerobic respiration). Most of the chemical energy stays locked in the products — which is exactly why ethanol can be burned as a fuel and lactate can be used by the heart.

So why do yeasts ferment even when oxygen is present, in sugary conditions? Fermentation is fast: yeast can process sugar quickly, and the ethanol it produces suppresses competing microbes. Brewers exploit this by keeping sugar high.

Common misconceptions

• “Fermentation makes ATP in the final step.” The ATP comes from glycolysis; the final step only regenerates NAD⁺.
• “Humans make alcohol in their muscles.” Human muscle makes lactate, not ethanol.
• “Fermentation needs no oxygen at all, so oxygen stops it.” Many fermenting organisms tolerate oxygen; yeast ferments in sugary conditions even with oxygen present.
• “Anaerobic respiration and fermentation are the same thing.” Strictly, anaerobic respiration uses an electron transport chain with a non-oxygen inorganic acceptor (such as nitrate or sulfate); fermentation uses no electron transport chain.

Key takeaways

• Fermentation regenerates NAD⁺ so that glycolysis can keep making ATP without oxygen.
• Lactic: pyruvate is reduced to lactate; no CO₂.
• Alcoholic: pyruvate is decarboxylated to ethanal, then reduced to ethanol; releases CO₂.
• Both give a net 2 ATP per glucose.
• Fermentation gives us bread, wine, beer, yoghurt, cheese and preserved vegetables.