How the grain you feed your starter shapes its bacterial community, its flavor and your bread.
By Hendrik Kleinwächter, The Bread Code/The Sourdough Framework
Every baker who has swapped flour knows it: the bread changes — not just its structure or its color, but its smell, its tang, its personality. For years, we chalked this up to protein content or ash levels. But a growing body of research tells a different story. The flour you feed your starter does not just nourish microbes; it selects them. Your flour is running an audition, and only certain bacteria get the part.
I have spent the last decade studying sourdough fermentation, writing The Sourdough Framework, and baking thousands of loaves across dozens of countries. I have seen bakers obsess over feeding schedules, fuss over hydration percentages and debate the ideal room temperature. These things matter, but the single most powerful lever you have over your sourdough’s character is something far simpler: the bag of flour on your counter.
The NC State Discovery
A team at North Carolina State University recently published a study in Microbiology Spectrum that made the flour question impossible to ignore. Taheri and colleagues in Rob Dunn’s lab took three types of flour (whole wheat, bread flour and all-purpose), started fresh sourdough cultures with each, and tracked what grew over the course of a month.
The results were striking. Whole wheat flour consistently selected bacteria in the genus Companilactobacillus. Bread flour favored a completely different group: Levilactobacillus. These are not minor taxonomic footnotes. These genera are known to differ in their fermentation pathways, producing different ratios of lactic and acetic acid, and contributing to distinct flavor profiles. The flour had set the microbial trajectory from day one.
Here is what makes this finding even more interesting: the yeast did not care. Kazachstania, a yeast genus, dominated every single starter regardless of flour type. The team also tested two different feeding schedules (once daily vs. twice daily) and found no meaningful effect on community composition.
Your feeding schedule is not the main driver; your flour is. It is the selecting agent, the casting director, the one calling the shots.
Six Acts in Every Starter’s Life
The NC State study built on earlier work from the same lab. In 2023, McKenney and colleagues tracked 40 sourdough starters made from 10 different flour types over 14 days. They identified six distinct phases of microbial succession, and these phases followed a predictable arc regardless of flour. Early on, the starters smelled toasty. By mid-fermentation, fruity aromas appeared. Eventually, the familiar sour tang took over.
But the destination, the climax community that settled in after the drama of succession, was flour-specific. Each flour type produced its own stable microbial fingerprint, with its own aromatic profile. Think of it this way: every starter goes through adolescence, but where it grows up depends on what you feed it.
This pattern shows up across continents. Boreczek and colleagues in Poland found that rye flour starters began with Weissella bacteria before shifting to Lactobacillus during early fermentation, while wheat and spelt starters were dominated by Lactobacillus from the start. Different grain, different opening act, different microbial theater. Yet one ratio held constant across all of them: roughly 1,000 bacteria for every single yeast cell. The balance between bacteria and yeast appears to be a universal feature of sourdough, even when the specific species change.
What This Means for Flavor
If different flours select different bacteria and different bacteria produce different metabolites, then flour choice is a flavor tool. This is not speculation; it is biochemistry.
Lactic acid bacteria (the “LAB” in sourdough science) can be broadly grouped by their fermentation pathways. Some species are homofermentative, producing mainly lactic acid, which gives a smooth, yogurt-like sourness. Others are heterofermentative, producing both lactic and acetic acid along with CO2, which adds the sharp, vinegary bite and contributes to rise. When your flour selects Companilactobacillus over Levilactobacillus (or vice versa), you are shifting the ratio of these acids. You are tuning the flavor.
Liu and colleagues recently demonstrated this principle with an unconventional substrate: jujube flour. Adding jujube to a sourdough system shifted the bacterial community toward Lactiplantibacillus plantarum, and the resulting bread showed improved aroma profiles compared to a standard wheat control. The substrate changed, the microbes changed and the flavor followed.
Photo by Hendrik
The Whole-Grain Advantage
If flour selects microbes, then it follows that more complex flour selects more diverse microbes. And that is exactly what the research shows.
Whole-grain flour carries more of the original grain: bran, germ, and the microbes that live on them. In The Sourdough Framework, I wrote that “your flour is contaminated with millions of microbes” and that “a whole flour works better because you have more natural contamination.” The Taheri study confirmed this quantitatively. Whole wheat starters developed richer bacterial communities than their white flour counterparts.
Baev and colleagues found the same pattern in Bulgarian sourdoughs: Weissella bacteria were more abundant in whole grain cultures. Working with faba bean flour, Coda and colleagues showed that including more of the hull increased microbial diversity. Munch-Andersen and colleagues went further, testing legume flours and finding that these alternative grains produced entirely different microbiomes compared to wheat.
The mechanism makes sense. Whole grains provide a wider range of substrates: more types of starch, more proteins, more minerals, more fiber. A more varied buffet attracts a more varied crowd. More bacterial diversity means more metabolic pathways active during fermentation, which translates into more complex flavors.
A more varied buffet attracts a more varied crowd. Whole grains give your starter more to work with, and it shows in every bite.
Your Kitchen Has a Signature
Before you rush to change your flour, here’s a word of nuance. Flour is the primary driver of microbial selection in new starters. But in established bakeries, the environment fights back.
Comasio and colleagues studied 17 sourdoughs from four countries and found that in bakeries with a long history of sourdough production, the “house microbiota” could override flour effects. Fructilactobacillus sanfranciscensis (once called Lactobacillus sanfranciscensis, and yes, it was first identified in San Francisco) dominated eight out of 17 of these bakeries, regardless of what flour they used.
Scheirlinck and colleagues found the same interplay in Belgian bakeries back in 2007: both flour type and geography shaped the bacterial communities. Minervini’s survey of 19 Italian sourdoughs found at least 20 LAB species and 4 yeast species, with ingredient choices and bakery environment both playing a role.
This makes intuitive sense. A bakery that has made sourdough for 50 years has seeded its walls, its bowls, its air with microbes. Those resident populations have a home advantage. They are already adapted to that environment. Calvert, Madden and Dunn described this beautifully in their 2021 review, calling starter management “analogous to land management.” You are not just feeding a culture; you are stewarding an ecosystem.
For home bakers, the implication is freeing. Your kitchen is not a sterile lab, and it does not need to be. Your environment is part of your bread’s identity.
Your Starter Fights Back
Here is something most bakers never consider: your sourdough is not just passively fermenting; it is actively defending itself.
The lactic acid bacteria in sourdough produce an arsenal of antimicrobial substances. These include bacteriocins, small protein-based antibiotics that kill competing bacteria. Researchers have identified specific ones in sourdough cultures, including bavaricin A and plantaricin ST. Think of them as targeted weapons: they destroy closely related species that might compete for the same resources, while leaving the rest of the community intact.
But the defense goes further. Sourdough LAB also produce antifungal peptides that inhibit mold growth. This is one reason a healthy, mature starter resists contamination so well. The bacteria are not just surviving; they are clearing the field.
Perhaps the most remarkable discovery is reutericyclin, a tetramic acid antibiotic first discovered in sourdough cultures. It is produced by Limosilactobacillus reuteri and is active against a range of gram-positive bacteria.
Your starter does not just grow; it fights. Every feeding makes its resident bacteria stronger, better armed and harder to displace.
On top of these targeted weapons, the organic acids produced during fermentation (lactic and acetic acid) drop the pH low enough to make the environment hostile for most invaders. It is chemical warfare on multiple fronts: specific protein antibiotics, antifungal compounds and a broadly acidic environment that few newcomers can tolerate.
This explains why established starters are so remarkably stable. The resident bacteria have built up their defenses over hundreds of feedings. They produce bacteriocins that target competitors, antifungal peptides that prevent mold, and acids that keep the pH inhospitable. With each feeding, your starter becomes more adapted, more resilient, more itself. A mature culture is not fragile. It is a fortress.
A Starter from the Coral Reef
A few years ago, I found myself on Rarotonga in the Cook Islands with no flour, no starter and a craving for sourdough. So I tried something unconventional. I collected saltwater from the coral reef lagoon, mixed it with sterilized sugar and waited.
It worked. Within days, I had a bubbling culture with a character entirely its own. The marine environment carries its own lactic acid bacteria and wild yeasts, living in biofilms on coral and seaweed. The resulting starter tasted nothing like any flour-based culture I had ever grown. It was proof of a principle I keep coming back to: sourdough microbes are everywhere. On grain, on fruit, in the sea. The substrate you offer determines who shows up.
Bakers around the world have started cultures from raisins, apples, grapes and pineapple juice. Each fruit surface carries its own wild yeasts and LAB, creating a different starting community. Your flour is the most important ongoing influence. But the microbial world is larger than any single bag of grain.
Practical Takeaways
From 16 studies on four continents, we get one clear message: your flour is your most powerful tool for shaping your sourdough. Here is how to use that knowledge.
Want more complexity? Set up your starter with whole-grain flour. The bran and germ carry more microbes and more diverse substrates. You will get a more complex bacterial community from day one, and with it, more layered flavors. The flour you begin with sets the trajectory.
Reactivating an old or weak starter? Consider using white flour. White flour has less natural contamination, meaning fewer competing microbes. This gives your existing culture less competition during recovery, allowing the established bacteria to rebuild their population without fighting off newcomers.
Want different flavors? Try making a starter from fruits or other sources, such as raisins, apples, grapes, honey or even saltwater. Every environment hosts unique microbes ready to ferment. Each source brings its own bacterial and yeast community, creating starters with completely different character.
Want to start with a proven culture? Purchase a starter from an established bakery. An established starter comes with a mature, battle-tested community that has already produced its arsenal of bacteriocins and antifungal compounds. With each feeding, this culture becomes even stronger and more adapted to your kitchen. You skip the unpredictable early succession phase and start with a culture that knows how to defend itself.
Loafy: Your Sourdough Companion
Loafy is a tool I built to help bakers connect the science to their practice. Tell Loafy what flour you are using, and it explains how that choice influences your starter’s microbiome, your dough’s fermentation and your bread’s final character. It draws on the same research behind this article.
Whether you are a professional baker experimenting with heritage grains or a home baker wondering why your rye starter behaves differently from your white flour one, Loafy gives you science-backed answers in plain language. Find it at loafy.ai.
A software engineer by design, Hendrik Kleinwächter is a sourdough nerd at heart. His book The Sourdough Framework democratizes the art of breadmaking by showing that you don’t need expensive equipment to make great bread. Check out his Youtube channel: www.youtube.com/@the_bread_code
(This article appeared in the Spring 31 issue of Pastry Arts Magazine)
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