Lacto-Fermentation vs Other Fermentation Types: The Microbiology Explained

The word “fermentation” covers four different kinds of microbial chemistry. Lumping them together is like saying “car” and “motorcycle” are the same because both have engines. The differences explain why sourdough bread is sour, why kombucha is tart and slightly fizzy, and why a badly managed kimchi batch can go wrong while a properly salted one is essentially self-protecting.

What Lactic Acid Bacteria Actually Do

Lactic acid bacteria (LAB) are gram-positive, non-spore-forming bacteria that get their energy by breaking down sugars — primarily glucose, fructose, and sucrose — and releasing lactic acid as the main byproduct. That acid is the entire point. As it accumulates, pH drops. Most pathogens can’t survive below pH 4.5. By the time a healthy lacto-ferment reaches its stable phase, the environment is chemically hostile to nearly anything dangerous.

The two big genera are Lactobacillus and Leuconostoc. They don’t behave identically.

Leuconostoc mesenteroides typically dominates early in vegetable ferments like kimchi and sauerkraut. It’s heterofermentative, meaning it doesn’t just produce lactic acid. It also generates CO2, small amounts of ethanol, and acetic acid. The CO2 creates an anaerobic environment faster than lactic acid alone could, which protects the ferment while pH is still relatively high. The acetic acid adds that faint vinegar note that makes early-stage sauerkraut taste different from aged sauerkraut.

As pH drops below about 4.5, Leuconostoc retreats. Lactobacillus plantarum and Lactobacillus brevis — more acid-tolerant species — become dominant. L. plantarum is homofermentative: it converts sugar almost entirely to lactic acid with minimal CO2 or other byproducts. The flavor becomes cleaner and more acidic. This is the stable, shelf-stable phase you’re aiming for.

Think of it like a relay race. Leuconostoc runs the first leg, builds the protective CO2 atmosphere and begins acidification. Lactobacillus takes the baton and drives the ferment to completion.

Why Salt Is a Selection Mechanism

Salt doesn’t create LAB. They’re already on the cabbage leaf, the cucumber skin, the surface of every vegetable. What salt does is create conditions where LAB can outcompete everything else.

Most harmful microorganisms — Listeria monocytogenes, E. coli O157:H7, Staphylococcus aureus — are poorly adapted to high-osmolarity environments. At 2-3% NaCl, they’re suppressed before they can establish a foothold. LAB, which evolved in naturally salt-containing environments like mammalian digestive tracts and sea grass, tolerate this concentration well.

Below 1.5% salt, the playing field is too level. Unwanted organisms have time to multiply, produce off-flavors, and potentially reach unsafe levels before LAB can drop the pH far enough to suppress them. Above 5%, even LAB are slowed enough that fermentation becomes unreliable.

The window is narrow, which is why weight-based salt measurements matter more than volume measurements. A kitchen scale is not optional for serious fermentation.

Four Fermented Foods, Four Distinct Microbial Systems

Kimchi is a vegetable lacto-ferment dominated by LAB, primarily Leuconostoc early on and Lactobacillus through the mature phase. The brine from salting the cabbage creates the selection environment. Traditional kimchi can contain dozens of LAB species depending on regional vegetables, ambient temperature, and fermentation vessel. The fish paste or shrimp used in many versions adds amino acids and further complexity to the substrate.

Sourdough is a more complex ecosystem: LAB and wild yeast coexist in a stable relationship. The yeast (often Saccharomyces cerevisiae and wild species like Kazachstania humilis, formerly Candida humilis) converts sugars to CO2 and ethanol, providing leavening. The LAB produce lactic and acetic acid. The ratio of these two acids determines whether the bread tastes milkier (more lactic, warmer fermentation) or more sharply sour (more acetic, cooler and slower). San Francisco sourdough’s distinctive tang comes partly from Lactobacillus sanfranciscensis, now reclassified as Fructilactobacillus sanfranciscensis, which thrives in that region’s flour and temperature conditions.

Kefir uses a grain (a gelatinous matrix of polysaccharide and protein) that houses a stable consortium of LAB, yeasts, and acetic acid bacteria. The LAB produce lactic acid; the yeast produces a small amount of ethanol (usually under 1%); the acetic acid bacteria contribute small amounts of acetic acid. The result is a slightly fizzy, tangy dairy product with a distinct carbonation that ordinary yogurt doesn’t have. Kefir grains are living starter cultures — they’re not reinoculated from scratch like a commercial yogurt culture.

Kombucha is primarily acetic acid fermentation with a yeast component, not lacto-fermentation. The SCOBY contains Acetobacter and Gluconobacter species (acetic acid bacteria) alongside yeast. The yeast first converts sucrose to ethanol. The acetic acid bacteria then oxidize that ethanol to acetic acid. The result: a tart, slightly acidic beverage with pH typically between 2.5 and 3.5, significantly more acidic than most lacto-ferments.

Wild Cultures vs Starter Cultures

A wild ferment relies on whatever microorganisms colonize the ingredients naturally. Traditional kimchi and natural sourdough are wild ferments. The composition varies by region, season, flour variety, and environment. This is why a sourdough starter maintained in San Francisco for a decade has a different microbial profile than one started from scratch in Berlin.

Starter cultures — either saved from a previous batch or purchased commercially — introduce a known microbial population. Yogurt made with a commercial starter containing specific Lactobacillus bulgaricus and Streptococcus thermophilus strains will taste similar batch to batch. It won’t have the complexity of a wild-culture yogurt, but it will be predictable.

Neither approach is superior. Wild ferments reward patience and tolerance for variation. Starter cultures reward control and repeatability. Most home fermenters use a combination: a wild sourdough starter for bread, a purchased kefir grain for kefir.

The pH Drop and Why It Matters for Safety

The safety of lacto-fermented food rests entirely on acidification speed. This is why proper salt concentration and temperature aren’t preferences — they’re functional requirements.

At room temperature (68-72°F), a 2% salt brine with healthy LAB populations should show measurable acidification within 24-48 hours and reach pH below 4.5 within 4-7 days for most vegetables. Below pH 4.5, Listeria, E. coli, and Salmonella cannot multiply. Clostridium botulinum, which produces the botulism toxin, is suppressed at pH below 4.6.

Temperature affects the speed but also the flavor profile. Warmer ferments (75-80°F) acidify faster but can produce softer textures and simpler flavor. Cooler ferments (60-65°F) are slower but develop more acetic acid (tangier, more complex) and maintain better texture. Traditional kimchi is often fermented briefly at room temperature then transferred to cool storage (traditionally underground ceramic pots) for extended aging.

The numbers matter because this is a genuinely predictive system. If you hit the salt concentration and temperature targets, the chemistry takes care of the safety. If you miss them, you’re relying on luck.