The Science of Sourdough Sourness: Why Some Loaves Taste Mild and Others Pucker Your Cheeks

⏱ 6 min read

Spring is one of my favourite times to bake sourdough. The kitchen isn't freezing, the starter wakes up properly without you having to beg it, and there's something about longer days that makes a Saturday morning bake feel genuinely relaxed rather than a race against the cold. But this time of year also throws up something I get asked about constantly: flavour. Specifically, why does one loaf taste properly sour and complex, while another (same flour, same starter, same rough method) tastes almost like a mild white sandwich bread?

The answer comes down to microbiology and chemistry, but don't let that put you off. Once you understand what's actually happening inside your dough, you get real control over how your bread tastes. Not guesswork. Not vibes. Actual, repeatable results.

Two Acids, One Loaf

Sourdough flavour is mostly a story about two organic acids: lactic acid and acetic acid. Your starter contains wild yeast and lactic acid bacteria (LAB), and it's the bacteria that produce these acids as they ferment the sugars in your dough. Lactic acid gives you that smooth, yoghurty sourness , think mild and rounded. Acetic acid is the sharper one, the vinegary tang you get in a proper San Francisco-style loaf that hits the back of your palate.

Both acids are always present in sourdough. What changes is the ratio between them, and that ratio is what makes your bread taste the way it does.

Temperature Is the Main Dial

This is the big one. Lactic acid bacteria produce more lactic acid at warmer temperatures (roughly 27 to 35°C) and more acetic acid at cooler temperatures (below 20°C, especially in the fridge). So if you've been fermenting warm and getting a mild loaf, that's not a mystery. You're optimising for lactic acid production.

A long, cold overnight proof , the kind where the dough sits in the fridge for 12 to 18 hours , actively shifts production towards acetic acid. That's why cold-proofed loaves tend to have that sharper, more complex tang. The cold slows everything down, but it doesn't stop acid production. It just changes which acids accumulate.

Warmer bulk ferments (say, 26 to 28°C in a kitchen that's finally comfortable in April) will give you faster fermentation with a softer, more lactic flavour profile. Neither is wrong. It just depends what you're going for.

Hydration Affects Sourness Too (This One Surprises People)

Acetic acid bacteria prefer a lower-moisture environment. A stiffer, lower-hydration dough (think 65 to 70%) actually encourages more acetic acid production, while a wetter, higher-hydration dough pushes you towards lactic. This is part of why old-school German rye breads, made with lower-hydration rye stiff starters, have that intense, sharp sourness. The stiff starter is basically an acetic acid factory.

On the flip side, if you're baking something light and open-crumbed at 78 to 80% hydration and wondering why it's not as punchy as you'd hoped, part of the answer might simply be the hydration itself creating the wrong conditions for acetic production.

The Starter's Role in All This

Your starter isn't just a leavening agent. It's the culture that inoculates your dough with specific strains of bacteria and yeast, and those strains carry flavour-producing tendencies. A well-maintained, regularly fed starter with a healthy population of diverse LAB will produce more complex flavour than a sluggish one that's been left in the fridge for three weeks and half-forgotten.

In spring, when ambient temperatures are climbing back up (even in London, things are finally feeling liveable again), I find my starter responds quicker and the fermentation gets lively. That's great for timing, but it can sometimes mean you catch the dough at peak and bake before enough acidity has developed. A slightly longer bulk ferment, even at the same temperature, gives the bacteria more time to produce acids alongside the yeast activity.

Think of it like debugging: if the flavour isn't right, check the inputs. How long was the bulk? How cold was the proof? What was the starter like when it went in? Change one variable at a time and actually track it.

How to Track What's Actually Working

This is where most home bakers plateau. You bake a great loaf, you want to replicate it, and you can't quite remember whether you proofed for 14 or 16 hours, or what the kitchen was sitting at that afternoon. I've been there plenty of times, and honestly the solution is just keeping better notes.

I use Doughrise Pro for this now. It saves unlimited formulas, keeps a full bake history, and you can export everything to PDF or CSV so your data doesn't just live in an app. When I'm tweaking acid development across batches , adjusting proof times, hydration, starter percentage , having that history makes the pattern obvious. Flavour is reproducible once you understand what produced it.

What About Flour?

Whole grain flours (wholemeal, rye, spelt) have more natural sugars and minerals for the bacteria to work with, which generally means more acid production and more flavour complexity. Even a modest addition , 10 to 20% wholemeal or rye in an otherwise white loaf , can noticeably deepen the flavour without making the bread heavy.

Spring is a nice time to experiment with this. The fermentation is active enough that whole grain additions don't need babying, and the slightly earthier, more complex flavour pairs well with the kinds of things you'd actually be eating in April , sharp cheeses, cured meats, butter and radishes if you're that way inclined.

Happy baking! Find everything you need at doughrise.store

Photo by Olga Petnyunene on Unsplash