The Science of Sourdough Starter Activity: What's Actually Happening in That Jar

There is a moment, usually sometime around day three or four of building a new starter, where you come downstairs in the morning and the jar has doubled, there are bubbles pressed against the glass, and it smells sharp and alive. It feels like magic. And honestly, even once you understand the science, it still kind of does.

But understanding what is actually happening inside that jar changes everything. It helps you stop guessing and start reading your starter properly. You stop panicking when it seems flat on a cold morning, and you stop overfeeding it out of anxiety. The biology here is genuinely fascinating, and once it clicks, your whole relationship with sourdough shifts.

The two communities living in your starter

A sourdough starter is not one organism. It is a small, self-regulating ecosystem with two main groups of microbes doing very different jobs: wild yeasts and lactic acid bacteria (LAB). They are not in competition. They have, over thousands of years of co-evolution, developed something close to a working partnership.

The wild yeasts, most commonly strains of Saccharomyces cerevisiae and Kazachstania humilis (also called Candida humilis), are responsible for leavening. They consume simple sugars and produce carbon dioxide gas, which inflates the gluten network and makes your dough rise. They also produce small amounts of ethanol as a byproduct, which contributes to flavour.

The lactic acid bacteria, things like Lactobacillus sanfranciscensis (now reclassified as Fructilactobacillus sanfranciscensis, though most bakers still use the old name), ferment sugars into lactic acid and acetic acid. These acids are what give sourdough its characteristic flavour. They also lower the pH of the starter, which creates an environment that harmful bacteria find inhospitable. The starter essentially makes itself safe to eat.

The rise and fall cycle, explained properly

When you feed your starter, you are introducing fresh flour and water. The flour contains complex carbohydrates (starches) that need to be broken down into simpler sugars before the yeast and bacteria can use them. Enzymes called amylases, present naturally in flour, do this work. At cooler temperatures this process is slower; warmer conditions speed it up.

Once simple sugars are available, the yeast and bacteria get to work. Carbon dioxide production ramps up, bubbles form throughout the starter, and the whole thing expands. This is peak activity, what most bakers call the starter being at its peak. After this point, the food supply runs low, acid levels build up, and the microbes slow down. The starter collapses back on itself. That liquid layer you sometimes see on top (called hooch) is a sign the starter has been hungry for a while, not that it is dying.

The timing of that peak is what you are chasing when you bake. Using your starter just as it reaches its highest point, or just slightly past, gives you the most active, gassy culture to drive fermentation in your dough.

Why the float test is a rough guide, not gospel

Drop a spoonful of starter into water and see if it floats , you have probably heard this one. It works because a gassy, active starter traps enough CO2 to be buoyant. But it is not foolproof. A starter can be highly active and still sink if the gluten structure is weak and cannot hold the gas. It can also float briefly even when slightly past its peak.

More reliable signals: the starter has visibly doubled, smells yeasty and pleasantly sour (not nail-varnish sharp, not unpleasantly cheesy), has a domed or just-starting-to-fall top, and feels airy when you stir it. These combined signals tell you much more than the float test alone.

Acids, flavour, and why they are not the same thing

The two acids your LAB produce have different characters. Lactic acid is smooth, yoghurt-like, mild. Acetic acid is sharper, more vinegary. The balance between them is influenced by several factors: hydration, temperature, and how long fermentation runs.

Higher hydration starters (runnier) tend to produce more lactic acid, giving a rounder, milder sourness. Stiffer starters and cooler, slower ferments push towards more acetic acid production, resulting in a more pronounced tang. This is why a cold overnight retard in the fridge tends to produce more complex, sharper flavour compared to a fast room-temperature bake.

It is also why spring baking can feel unpredictable. Your kitchen might be 18°C one morning and 22°C the next as the weather shifts about. Those four degrees make a real difference to which acids your culture is producing and how quickly everything moves.

What stress does to your starter

Starters are more resilient than people give them credit for. They can handle being neglected in the fridge for weeks, being fed inconsistently, or being moved between different flours. But chronic stress (irregular feeding, extreme temperatures, wildly fluctuating ratios) does shift the microbial balance over time.

Think of it a bit like debugging code. If the output is wrong, check the inputs first. Sluggish starter with poor rise? Check the flour (low-protein flour gives the microbes less to work with), the water temperature, and the feeding ratio. Overly sour with weak lift? The yeast population may have been outcompeted by bacteria, often from feeding too infrequently. Most starter problems have a root cause, and adjusting one variable at a time usually finds it.

If you find yourself going in circles troubleshooting your starter or your bakes, the DoughRise Coach is worth a look. It gives you personalised bake plans, technique guidance, and unlimited AI coach messages, so instead of posting a photo of your starter in a forum at 11pm and waiting for answers, you can just ask directly and get something specific to your situation.

Keeping your starter in good shape through the warmer months

Spring and into summer is when starters start behaving differently for a lot of home bakers. Warmer ambient temperatures mean faster fermentation, which can catch you off guard if you are used to winter timings. A few practical adjustments help:

• Use cooler water when feeding (around 18 to 20°C rather than 25°C) to slow the cycle down slightly and give yourself more flexibility.
• Feed with a higher ratio if you cannot bake every day. A 1:5:5 ratio (starter:flour:water) will extend the peak timing considerably.
• Keep your starter jar away from direct sunlight and anywhere near a warm oven. Counter position matters more than you think in a sunny kitchen.
• If your starter is consistently peaking too fast to catch, consider moving it to the fridge between bakes and refreshing it the evening before you plan to bake.

None of this has to be complicated. The more you observe your starter across different conditions, the more you develop a feel for it that no amount of reading fully replaces. It becomes intuitive, eventually. But the science helps you get there faster.

Happy baking! Find everything you need at doughrise.store

Photo by Thomas Zimball on Unsplash