Fermentation: Alcoholic and Lactic Acid Types Explained

Fermentation is an anaerobic process in which cells regenerate NAD+ from NADH by reducing pyruvate, allowing glycolysis to continue producing ATP without oxygen. The two best-known types are alcoholic fermentation, carried out by yeast, which produces ethanol and carbon dioxide, and lactic acid fermentation, carried out by muscle cells and Lactobacillus bacteria, which produces lactate. No oxygen and no electron transport chain are involved.

It matters because fermentation keeps ATP flowing when oxygen is absent or insufficient — during a sprint, in waterlogged roots, or inside a brewing vat. Entire industries of bread, beer, wine, curd, and cheese rest on it. It also explains the burning sensation of hard exercise and how our bodies clear lactate afterwards.

Key Concepts

Both types start from glycolysis products: 2 pyruvate and 2 NADH per glucose. Alcoholic fermentation: pyruvate → acetaldehyde + CO2 (pyruvate decarboxylase), then acetaldehyde + NADH → ethanol + NAD+ (alcohol dehydrogenase). Lactic acid fermentation: pyruvate + NADH → lactate + NAD+ in one step (lactate dehydrogenase), with no CO2 released. The essential product in both is regenerated NAD+, not the ethanol or lactate themselves. Net energy remains glycolysis's 2 ATP per glucose; fermentation steps add zero ATP. Ethanol above about 12-14 percent concentration kills yeast, limiting natural wine strength.

Formulas

Alcoholic: C6H12O6 → 2C2H5OH + 2CO2 + energy (2 ATP net from glycolysis). Glucose (180 g per mol) yields ethanol (2 × 46 = 92 g) and CO2 (2 × 44 = 88 g) theoretically. Lactic acid: C6H12O6 → 2C3H6O3 + energy (2 ATP net); glucose converts to two lactic acid molecules with no gas released. Assumptions: anaerobic conditions, complete conversion; real industrial yields are lower due to side products and cell growth. Both formulas summarise glycolysis plus the NAD+-regenerating steps combined.

Worked Example

A brewery ferments 180 g (1 mol) of glucose completely by alcoholic fermentation. Step 1: From the equation, 1 mol glucose → 2 mol ethanol + 2 mol CO2. Step 2: Ethanol mass = 2 × 46 g = 92 g. Step 3: CO2 released = 2 × 44 g = 88 g, which as gas at STP occupies 2 × 22.4 = 44.8 litres — this is the gas that raises bread dough and foams beer. Step 4: ATP harvested by the yeast = 2 mol ATP net. In practice yield is about 90 percent of theoretical because yeast uses some carbon for growth. The same 180 g under lactic fermentation would give 180 g lactic acid (2 × 90 g) and no gas.

Common Misconceptions

• Fermentation produces lots of ATP. Correction: It produces no ATP itself; the only ATP is the net 2 from glycolysis, far below aerobic respiration's roughly 30.
• Lactic acid fermentation releases CO2. Correction: Only alcoholic fermentation releases CO2; lactic acid fermentation converts pyruvate directly to lactate with no gas.
• Lactate causes next-day muscle soreness. Correction: Lactate clears within an hour; delayed soreness comes from micro-damage to muscle fibres, not retained acid.

Real-World Applications

Bakers use yeast CO2 to leaven bread; brewers and winemakers harvest ethanol; distillers concentrate it further. Curd, yoghurt, cheese, and pickles depend on bacterial lactic acid, which also preserves food by lowering pH. Bioethanol fuel is produced by fermenting sugarcane and corn at massive scale. Athletes train to raise lactate threshold, and clinicians monitor blood lactate in critical care. Industrial lactic acid also feeds biodegradable PLA plastic production.

Fermentation recycles NAD+ by converting pyruvate to ethanol plus CO2 in yeast, or to lactate in muscle and bacteria, keeping glycolysis's 2 ATP flowing without oxygen. Small in energy but huge in industry and survival, it is life's anaerobic backup plan.