Does Sourdough Contain L. reuteri? The Surprising Truth About Baked Bread

November 28, 2025 •

4 min read

Sourdough starter is a complex, living ecosystem of wild yeasts and lactic acid bacteria that can harbor species like Lactobacillus reuteri. While this beneficial bacterium can thrive in the pre-baked dough, the high heat of baking largely eliminates these live microbes, meaning baked bread does not contain active probiotics.

Quick Summary

Sourdough starters often contain L. reuteri, but the baking process kills the live bacteria. The finished bread, however, still offers health benefits through prebiotics and postbiotics.

Key Points

Starter Contains L. reuteri: Sourdough starter is a living culture of wild yeast and lactic acid bacteria, including Lactobacillus reuteri.
Baking Kills Live Bacteria: High temperatures during baking destroy most live bacteria in sourdough, meaning the finished bread is not a source of active probiotics.
Prebiotics Remain in Bread: The baked bread contains prebiotics, which are fermentable fibers that feed your gut's resident beneficial bacteria.
Postbiotics Provide Health Benefits: Beneficial compounds created during fermentation (postbiotics) survive baking and offer health advantages.
Sourdough Improves Digestibility: Fermentation partially breaks down gluten and phytic acid, making sourdough bread easier to digest and more nutritious.
Factors Influence Microbiome: The specific strains present in a sourdough starter, including L. reuteri, depend on the flour, temperature, and maintenance practices.

The Sourdough Starter: A Living Ecosystem

A sourdough starter, a fermented mixture of flour and water, serves as a breeding ground for a symbiotic culture of wild yeast and lactic acid bacteria (LAB). This intricate microbial community gives sourdough bread its distinctive flavor, texture, and leavening. Among the many species of LAB that can colonize a starter, Lactobacillus reuteri has been identified as a stable member in some sourdoughs. Researchers have even found that certain L. reuteri strains isolated from sourdoughs are of intestinal origin, demonstrating the potential for cross-environmental adaptation.

The Role of L. reuteri in Fermentation

During the fermentation of the starter, L. reuteri and other LAB metabolize sugars in the flour, producing organic acids like lactic acid and acetic acid. These acids create the signature tangy flavor of sourdough and lower the pH of the dough, which inhibits the growth of unwanted microorganisms. The presence and activity of L. reuteri can also contribute to the unique sensory qualities and preservative properties of the resulting bread. The specific strains and their activity, however, are highly dependent on the flour used, fermentation conditions, and geographical location.

From Starter to Bread: What Happens During Baking?

The journey from a live, active starter to a finished loaf involves a critical step: baking. While the starter itself is teeming with live microorganisms, the high temperatures inside an oven are designed to cook and sterilize the dough. A baking temperature of around 220°C (428°F) effectively kills most, if not all, of the live yeasts and bacteria, including any L. reuteri that were present.

Prebiotics and Postbiotics Survive Baking

This heat-induced elimination of live cultures is why baked sourdough bread cannot be considered a probiotic food. However, the story of sourdough's gut health benefits doesn't end there. The beneficial compounds produced during fermentation—known as postbiotics—can survive the baking process. Furthermore, sourdough acts as a prebiotic food. Prebiotics are fermentable fibers and starches that serve as food for the beneficial bacteria already living in your gut microbiome. Sourdough fermentation also increases the bioavailability of certain nutrients and can lower the glycemic index of the bread, offering additional health advantages.

Factors Influencing the Presence of L. reuteri

The specific microbial composition of a sourdough starter is not static and can be influenced by several factors:

Flour Type: Whole-grain flours, particularly rye, provide more nutrients and microbes, which can promote a stronger and more diverse fermentation. Different flours can also favor different strains of LAB.
Temperature: The fermentation temperature can influence the ratio of lactic acid to acetic acid and favor certain microbial species over others.
Hydration: The water content of the starter affects bacterial activity and yeast growth. A wetter starter can favor lactic acid production, while a drier one might favor acetic acid.
Backslopping: The practice of consistently using a portion of mature sourdough to inoculate a fresh mixture ensures the survival and dominance of competitive, adapted strains over time.
Environmental Contamination: The ambient environment, and even the baker's hands, can introduce different microorganisms to the starter.

Comparison Table: Sourdough Starter vs. Baked Sourdough Bread


Feature	Sourdough Starter (pre-baking)	Baked Sourdough Bread
Live Cultures (Probiotics)	Yes, contains live yeast and lactic acid bacteria, including L. reuteri.	No, live cultures are killed by high baking temperatures.
Fermentation Products (Postbiotics)	Yes, rich in organic acids and other beneficial metabolites.	Yes, postbiotics survive the baking process and offer health benefits.
Fiber Content (Prebiotics)	Yes, contains fermentable fibers that feed the starter's microbes.	Yes, contains fermentable fibers that act as prebiotics for your gut bacteria.
Digestibility	Easier to digest than bread dough with commercial yeast due to fermentation.	Easier to digest due to the partial breakdown of gluten and phytic acid during fermentation.
Lower Glycemic Index	Not applicable to dough.	Yes, the fermentation process contributes to a lower glycemic index compared to conventional bread.

Conclusion: The Final Verdict on L. reuteri in Sourdough

To conclude, L. reuteri can be a component of a sourdough starter, but it does not survive the heat of the baking process. Therefore, while the sourdough starter contains live L. reuteri, the finished sourdough bread does not provide a probiotic benefit from these live bacteria. Nonetheless, the baking process does not eliminate all the health advantages. The bread is rich in prebiotics, which nourish your existing gut flora, and beneficial postbiotics, which are the byproduct of the bacteria's fermentation. For gut health, consuming baked sourdough bread offers valuable support, even without the live probiotic cultures.

For more information on the beneficial microbes found in sourdough, see research from the American Society for Microbiology.

Frequently Asked Questions

No, while the sourdough starter contains live bacteria, including L. reuteri, the high temperatures of the baking process kill most, if not all, of these live probiotic cultures.

Yes, it is still beneficial. The baked bread contains prebiotics, which are fermentable fibers that nourish the good bacteria already in your gut. It also contains beneficial postbiotic compounds created during fermentation.

Probiotics are the live, beneficial bacteria found in the starter. Prebiotics are the food source (fiber/carbs) for gut bacteria, found in both the starter and baked bread. Postbiotics are the healthful compounds produced by the bacteria during fermentation, which survive baking.

Yes, it is possible to introduce specific strains like L. reuteri to a starter, either by inoculation or by optimizing conditions. However, commercial sourdough starters and spontaneous ones can also contain it naturally.

Use whole-grain flours, as they contain more nutrients and fiber (prebiotics). Consider extended fermentation periods (cold-proofing) to maximize the breakdown of starches and gluten.

Yes. The long fermentation process breaks down gluten and phytic acid, which makes sourdough bread easier on the stomach and helps with mineral absorption.

To consume live bacteria from sourdough, you would need to consume the unbaked starter or a food product that has been fermented with L. reuteri but not subsequently baked at high heat. The finished bread is not a reliable source of live probiotics.