Lactic Fermentation

Lacto-fermentation is the engine behind sauerkraut, kimchi, and brined cucumbers. Lactic-acid bacteria (LAB) — naturally present on the surface of every vegetable — metabolize the plant’s own sugars and excrete acid, mostly lactic acid. As acid accumulates the pH falls, and below roughly pH 4.0 the brine becomes hostile to spoilage organisms and pathogens. You add nothing but salt and time; the vegetable supplies the sugar and the microbes.

The salt (default 2.0% by weight for dry-salt kraut, 2.5% equilibrium for submerged) does two jobs: it pulls water and sugar out of the cells to make a brine, and it suppresses competing organisms just enough to give LAB the head start they need.

The mechanism: acid drops the pH

LAB ferment hexose sugars (glucose, fructose) into organic acids. There are two metabolic styles, and a single ferment uses both in sequence:

Phase	Dominant organism	Metabolism	Products	Typical pH
Early (heterofermentative)	Leuconostoc mesenteroides	Heterofermentative	Lactic acid + acetic acid + CO₂ + ethanol	6.0 → 4.5
Late (homofermentative)	Lactiplantibacillus plantarum	Homofermentative	Lactic acid (almost exclusively)	4.5 → 3.4

Leuconostoc starts the ferment: it grows fast at cellar temperature, and its CO₂ purges oxygen from the brine to establish anaerobic conditions. Its acetic acid and CO₂ build the characteristic tang and “lift.” But Leuconostoc is acid-intolerant — once the brine reaches roughly pH 4.5 it stops, and the more acid-tolerant Lactiplantibacillus plantarum takes over, driving the pH down to its finished range and converting residual sugar to clean lactic acid.

Why it is self-preserving

A finished lacto-vegetable ferment lands at pH 3.2–3.8 (sauerkraut tightens to 3.4–3.6). That matters for two reasons:

Info

At pH 3.2–3.8 the brine sits far below the pH 4.6 botulism line — the threshold under which Clostridium botulinum cannot grow or produce toxin. Lactic acid at 1.0–2.0% total acidity also lowers water activity and out-competes spoilage molds and putrefactive bacteria. The product preserves itself, no canning required, as long as it stays submerged and cold.

Worked example

Start with 1,000 g shredded cabbage at ~4% fermentable sugar (≈40 g sugar). Salt at 2.0% = 20 g. Over 2–4 weeks at 18–22 °C / 64–72 °F, LAB convert most of that sugar to acid, yielding roughly 1.5% lactic acid in the brine and a finished pH near 3.5. Confirm with a calibrated pH meter — taste is never a safety test.

Caution

pH is the only safety verdict. A ferment can taste sour and still sit above 4.0 if salt was low or temperature was high. Measure with a calibrated meter and confirm the brine is at or below pH 4.0 before you call it shelf-stable. Source: NCHFP — Fermenting.

Tip

Keep everything under the brine. The acid only protects what it touches; vegetable exposed to air is where kahm yeast and mold start.

See also

• Microbial succession — the full population curve and who shows up when.
• Acidity and pH — measuring pH/TA and the safety thresholds.
• Brine salinity calculator and pH safety checker.