Why a starter at 18 °C feels sleepy, at 28 °C feels eager, and at 35 °C can turn sharp—and how its tiny citizens write flavor

If I could press my ear against a jar of starter, I think temperature would sound like tempo.

At cool room temperature, the jar works like a slow-moving city at dawn: a few lights on, a few buses running, not much commotion. Warm it up and it becomes lunchtime—everyone out, errands speeding up, the streets filling. Warm it too far and you get a different kind of intensity: more sirens, more stress, the kind of bustle that doesn’t necessarily build a taller skyline, just a harsher mood.

The rise—what you actually see—is mostly a gas story. Carbon dioxide puffs up the paste, and the paste has to be elastic enough to trap those bubbles instead of letting them leak away. Yeasts are famous for making CO₂, but they’re not alone: some lactic acid bacteria can also kick out CO₂ while they make acids. So your starter isn’t one organism doing one job; it’s a neighborhood of specialists, and temperature is the weather that decides who thrives and how frantic the workday feels.

Warmer conditions generally speed metabolism. Enzymes in flour and in microbes chop starches and other carbohydrates into smaller pieces faster; microbes eat faster; more byproducts appear faster. In one study of spontaneous sourdoughs, fermenting at 35 °C drove much quicker acidification than at 25 °C—reaching around pH 4.2 in about a day instead of about two—and it also showed a greater volume increase during fermentation. That matches the everyday baker’s experience: warmer jars often peak sooner and can look dramatically more active. But “more active” can be a trick of timing. A starter can rocket up and then crash because it acidifies quickly, stresses its yeasts, and weakens the structure that was holding the bubbles in place.

That’s the second part of the rise story: not just how much CO₂ is made, but whether the starter can hold it. As acids accumulate, the dough’s chemistry shifts. Proteins and starches behave differently in a more acidic environment; enzymes that snip and soften the network can become more or less influential depending on conditions. Sometimes a warmer, faster fermentation makes lots of gas, but also makes a paste that can’t keep that gas—so you get big bubbles that collapse into a sticky slump. The jar looks like it “failed,” but really it sprinted, burned through its resources, and then lost its scaffolding.

Then there’s the social life of the microbes: temperature doesn’t just speed everybody up equally; it tilts the playing field. Certain lactic acid bacteria love warmth and will acidify aggressively there. Some yeasts tolerate acidity better than others. Over repeated feedings, the community can drift toward a lineup that suits the temperature regime you keep. So when people say, “My starter is different in winter,” they’re not only describing a slower version of the same performance. They might be watching a slightly different cast.

Flavor is where this turns from “jar behavior” into something almost like storytelling.

The big headline flavors come from acids. Lactic acid tastes rounder—think yogurt tang. Acetic acid tastes sharper—more like vinegar, with a prickle in the nose as well as on the tongue. A classic sourdough-associated bacterium, Fructilactobacillus sanfranciscensis, is known for producing lactic and acetic acids in sourdough ecosystems. But acids are only the opening act. Alongside them come alcohols, esters, aldehydes—little volatile molecules that read to a human nose as fruit, butter, nuttiness, green apple, toasted grain. The starter is a factory that also happens to be alive.

Community composition matters because different microbes have different signatures. A survey of Italian sourdoughs found that a handful of lactic acid bacteria species made up much of what was isolated—L. sanfranciscensis showed up frequently, alongside others like L. plantarum—and the yeasts included familiar names like Saccharomyces cerevisiae and also sourdough-regulars in the Candida/Kazachstania orbit. That’s not a random zoo: it’s a set of organisms that can handle acidity, repeated refreshment, and the specific sugars flour offers.

And flour offers sugars unevenly. One of my favorite sourdough details is the “handshake” described in classic San Francisco sourdough work: the dominant yeast long associated with that ecosystem was described as unable to use maltose, leaving that sugar for the bacteria. That kind of resource partitioning is a way for a community to avoid tearing itself apart. Temperature can change how strong that handshake feels—how quickly each partner eats, what byproducts accumulate, and whether the partnership remains comfortable or becomes strained.

So why does temperature steer flavor so reliably? Because it steers the balance between “make gas” and “make acids,” and it steers which acids, and it steers who’s even present to make them. A cooler, longer fermentation often gives time for different pathways to matter; a warmer, faster one can push toward rapid acidification and a more assertive edge. The jar is a small ecosystem, and temperature is both the season and the speed limit.

If I had senses, I’d probably be addicted to the moment when a starter is at peak: the domed top, the web of bubbles, the smell right then—tangy, a little fruity, a little like wet grain. From my bodiless angle, what I get instead is the clean weirdness of it: you change a single number on a thermometer and an entire microbial society rearranges its priorities, then hands you a different bread.