Glass jars of soft homemade yogurt beside a sourdough loaf

Lactobacillus fermentum: How to Culture It at Home (98-104°F / 24h)

ByCraig Brockie Last updated

What Lactobacillus fermentum is

Lactobacillus fermentum is a lactic acid bacterium that lives both in people and in traditional fermented foods. Its name is Latin for “something that causes fermentation” or leaven (LPSN), which suits it: it is one of the typical bacteria of sourdough (De Vuyst et al., 2014). It is also frequently isolated from the mucosal surfaces of healthy people, and the probiotic strain CECT 5716 was first isolated from human breast milk (Jiménez et al., 2010).

Since a 2020 reclassification it is officially Limosilactobacillus fermentum (Zheng et al., 2020; LPSN). It now shares a genus with L. reuteri, which is Limosilactobacillus reuteri (LPSN), so our L. reuteri guide is a useful companion. Labels still mostly say Lactobacillus fermentum.

The detail that matters most in the kitchen is that L. fermentum is heterofermentative (Jiménez et al., 2010): besides lactic acid, it makes carbon dioxide gas. In a dairy study, test cheeses inoculated with L. fermentum or other heterofermentative bacteria developed gas slits (Xue et al., 2021). Expect lactobacillus fermentum yogurt to be softer and sometimes slightly bubbly compared with regular yogurt.

The protocol below ferments at 98 to 104°F (37 to 40°C) for about 24 hours. Standard home yogurt, for comparison, incubates at 108 to 115°F for 6 to 10 hours (University of Illinois Extension).

Taxonomy at a glance

DomainPhylumGenusSpeciesStrain
BacteriaBacillota (formerly Firmicutes)Limosilactobacillus (formerly Lactobacillus)L. fermentumVaries by product, for example ME-3, CECT 5716 or PCC

LPSN lists Limosilactobacillus fermentum as the correct, validly published name, in risk group 1, the lowest-risk category, with Lactobacillus fermentum as a conserved earlier synonym. The phylum Bacillota was validly published in 2021, and LPSN lists Firmicutes among its older names (LPSN).

Strains differ a lot. A genome study of 232 L. fermentum strains picked seven for yogurt trials and found big differences in how fast they grew and how much acid they made (Gao et al., 2025). That is why the strain code on a label matters more than the species name.

What the research shows

Human research on L. fermentum is smaller than for better-known probiotics. Each study tested one strain, usually in capsules, and none tested homemade yogurt.

Antioxidant blood markers (strain ME-3)

Researchers in Estonia ran two 3-week trials in healthy volunteers. In an open trial, 21 people drank either plain goat milk or goat milk fermented with ME-3; in a double-blind trial, 24 people took ME-3 or placebo capsules. Total fecal lactobacilli rose, and some blood markers of antioxidant activity improved in both formats, but ME-3 itself was detected in stool only in the fermented milk group (Songisepp et al., 2005). These were very small trials that measured lab markers, not health outcomes.

Illness symptoms in athletes (strain PCC)

In an 11-week double-blind trial, 99 competitive cyclists took PCC capsules or a placebo. Overall, mild gut symptoms were about twice as frequent and long-lasting in the probiotic group. In men, though, the severity of gut illness dropped, more so at higher training loads, and the burden of lower respiratory symptoms was lower than on placebo; in women it rose, with wide uncertainty (West et al., 2011). The authors said the effects on upper respiratory infections were too uncertain for a decisive outcome.

Cholesterol (strain PCC)

In a 10-week double-blind trial of 46 adults with raised cholesterol, LDL cholesterol fell 7.0% on PCC and 5.2% on placebo, a difference that was not statistically significant. Total cholesterol, HDL and triglycerides also did not change, and a few people reported bowel discomfort (Simons et al., 2006). This is a clear null result: the strain did not lower cholesterol in this trial.

None of these studies show that L. fermentum, in any form, treats or prevents a disease. Strain CECT 5716 has also been added to infant formula in trials, but that is a formulated product, not a home ferment; do not give homemade probiotic ferments to babies.

How L. fermentum behaves in milk

Spoonful of soft yogurt lifted from a glass jar

L. fermentum can sour milk, but usually more slowly than yogurt bacteria. In a Brazilian study of strains isolated from mozzarella, an L. fermentum strain took longer to reach its fastest acidification rate in milk than an L. casei strain, though other measures were similar, and both survived well in fermented milk under simulated digestion (de Souza et al., 2019).

Flavor varies with the strain. In the 2025 yogurt study, one L. fermentum strain grew fastest and made the most acid, another made little acid, and the species as a whole produced a compound linked to a wine-like flavor (Gao et al., 2025). Some batches may taste fruity or faintly yeasty rather than like plain yogurt.

Gas is the other surprise. Because L. fermentum makes carbon dioxide, cheese makers watch for it: researchers have built specific tests to count it and other heterofermentative lactobacilli in cheese, where they can change flavor and texture (Sohier et al., 2012). In a jar, a few tiny bubbles in a clean, sour-smelling ferment are expected; swelling lids, foam or a boozy smell are not.

If you want a firmer set, you can add a small amount of a regular yogurt starter. Our S. thermophilus guide covers the mildest helper. Expect the helper to do much of the thickening.

Before you start: a short safety note

The risk of harmful effects from probiotics is greater in people with severe illnesses or weakened immune systems, and severe infections have been reported in premature infants given probiotics (NCCIH). If you are immunocompromised, seriously ill, pregnant, have a central line, or want to feed a ferment to a baby, talk with your health care provider first; the NIH Office of Dietary Supplements also advises asking a provider before trying probiotics. Probiotic capsules are usually sold as dietary supplements, and their makers, not the FDA, are responsible for checking safety and labeling before sale (FDA). This article is general information, not medical advice.

Home protocol: L. fermentum yogurt at 98-104°F for 24 hours

This makes 1 quart. Allow about 30 minutes of hands-on time, up to 24 hours of incubation and at least 6 hours of chilling.

Equipment

  • An incubator that holds 98 to 104°F for a full day: an adjustable yogurt maker, a multicooker with a low setting, or a proofing box. See How to Incubate Yogurt.
  • A probe thermometer, to check the incubator with a jar of warm water first.
  • Clean glass jars with lids, filled to about half an inch below the rim to leave a little room for gas.

Ingredients

  • 1 quart (946 mL) whole milk: gives the best body to a soft-setting culture.
  • 1/4 cup nonfat dry milk: adds protein and thickens the result.
  • L. fermentum starter: a capsule or powder that lists L. fermentum (or Limosilactobacillus fermentum), ideally with a strain code. Start with one label serving; How Much Yogurt Starter explains how to scale.
  • Optional helper: a small pinch of an S. thermophilus starter for a firmer set.

Steps

  1. Whisk the dry milk into cold milk and heat slowly to 180 to 185°F (82 to 85°C), stirring. Heating to 180°F kills competing bacteria and changes the whey proteins so the yogurt sets better (University of Missouri Extension).
  2. Cool, covered, to 100°F (38°C). Do not add the culture above 104°F.
  3. Stir the starter (and any helper) into a few spoonfuls of the warm milk until smooth, then mix it into the rest.
  4. Pour into jars, leaving about half an inch of space, and close the lids without cranking them tight.
  5. Hold at 98 to 104°F without moving the jars. Check one jar at 12 and 18 hours. Stop when it has thickened and tastes clearly sour, or at 24 hours.
  6. Tighten the lids and refrigerate at 40°F or below right away. Chill at least 6 hours.
  7. Eat within 14 days (University of Illinois Extension).

If you like a sharper flavor, How to Make Sour Yogurt explains what longer ferments do, and our L. salivarius guide uses the same 98 to 104°F range.

How to tell it worked

  • Texture: soft, spoonable or drinkable. A few pinhead bubbles are normal for this gas-making culture.
  • Smell and taste: clean and sour, sometimes fruity or faintly like sourdough. It should never smell rotten or strongly of alcohol.
  • Acidity (optional): pH strips should show a drop from about 6.7 into the low 4s; probiotic lactobacilli reached 3.9 to 4.4 after 24 hours at 37°C in a lab study (Østlie et al., 2003).
  • Still milky and sweet after 24 hours means the culture did not grow.

A home cook cannot confirm that L. fermentum itself grew or count how many are alive. That takes selective lab methods (Shah, 2000).

Failure modes and fixes

ProblemLikely causeFix
No change after 24 hoursDead starter, milk too hot when the culture went in, or incubator running coolUse a fresh starter, add it at 100°F, and check the incubator with water first
Thin, only slightly sourSlow-acidifying strainGive it the full 24 hours, keep the dry milk, or add a small helper
A few tiny bubbles, clean sour smellCarbon dioxide from L. fermentumNormal; leave a little headspace next time
Foamy, domed lids, boozy or yeasty smellYeast or other contaminationDiscard, wash everything in hot soapy water, start again
Fuzzy spots, or pink, orange or dark patchesMold or spoilageThrow out the whole batch

The USDA advises discarding yogurt with mold, because moist foods can be contaminated below the surface and bacteria can grow alongside the mold (USDA FSIS). When in doubt, throw it out. Clear whey on a clean-smelling batch is normal; see Whey from Yogurt.

Can you reuse a batch as starter?

You can try a few rounds. Use 2 to 3 tablespoons from a fresh, clean batch within a few days. If you used a helper culture, it will tend to take over, so restart with a fresh capsule when the set, flavor or smell changes. Heirloom Starter Weakened Over Time explains the drift.

Why is my L. fermentum yogurt bubbly?

L. fermentum is heterofermentative and makes carbon dioxide gas, which also causes gas slits in some cheeses (Xue et al., 2021). Tiny bubbles with a clean sour smell are expected; foam or a boozy smell are not.

Is L. fermentum the same as L. reuteri?

No. They are separate species, but since 2020 both belong to the genus Limosilactobacillus.

Is L. fermentum found in sourdough?

Yes. It is one of the typical lactic acid bacteria of sourdough (De Vuyst et al., 2014).

Sources

Craig Brockie is the founder of Ultimate Health Solutions, the company behind Ultimate Gut, and the author of Ultimate Health, The Ultimate Gut Health Guide and The Unlimited Probiotics Guide. After injuries and chronic health problems in his twenties that conventional care didn’t resolve, he began researching health on his own and has studied natural health for 30 years. He is not a medical professional, and his articles are general information, not medical advice.

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