The Science of Acetic vs Lactic Acid in Sourdough: Why Your Loaf Tastes the Way It Does

There is a moment, usually somewhere around mid-October, when the sourdough baking urge shifts gear. The kitchen feels a bit warmer when the oven is on, the loaves feel more justified, and you start caring a bit more about flavour depth rather than just getting a good rise. That is exactly when it is worth thinking about something most home bakers never quite get to: what is actually creating the taste in your sourdough, and can you influence it?

The short answer is yes, you absolutely can. And the mechanism behind it comes down to two organic acids produced during fermentation: acetic acid and lactic acid. Getting your head around these two changes how you think about every decision you make in the process, from feeding ratios to dough temperature to how long you let things sit.

Two Acids, Very Different Personalities

Your sourdough starter is home to a community of lactic acid bacteria (LAB) alongside wild yeast. The yeast produces CO2 and alcohol, which give you the rise. The bacteria produce acids, which give you the flavour and the preserving power. But not all LAB work the same way, and not all conditions favour the same acid output.

Lactic acid is the same acid you find in yoghurt and kefir. It is smooth, round and mild. When your bread has a creamy, almost buttery tang to it that lingers without biting, that is lactic acid doing the work. It dissolves easily in water and is produced more readily in warm, wet, relatively oxygen-poor environments.

Acetic acid is vinegar. Literally. It is sharper, more pungent, and when you have a loaf that hits you with a pronounced sour punch, the kind that makes you sit up a bit, that is acetic acid. It is less water-soluble, takes longer to produce, and is favoured by cooler, drier, more aerobic conditions.

What Controls the Balance?

This is where it gets genuinely interesting, because the same starter, fed the same way, will produce different acid profiles depending on a handful of variables you are already adjusting without realising it.

Temperature is the biggest lever. Lactic acid bacteria are more productive in the 24 to 28 degree Celsius range. Drop below 18 degrees and the acetic-acid-producing strains (particularly Lactobacillus sanfranciscensis, now reclassified as Fructilactobacillus sanfranciscensis if you want to get nerdy about it) start to dominate. Your autumn kitchen, sitting around 17 or 18 degrees without the heating fully kicked in, is naturally nudging you towards a more acidic loaf. That is not a problem, it is just something to be aware of.

Hydration plays a role too. A stiffer dough or starter creates pockets of lower water activity, which suits acetic acid bacteria. A wetter, looser ferment tends to favour lactic. This is why high-hydration loaves often taste milder and more yoghurty, while a stiff rye loaf can be bracingly sharp.

Fermentation time matters in a less obvious way. Both acids accumulate over time, but lactic acid builds relatively quickly in the early stages of fermentation. Acetic acid production ramps up later, particularly during extended cold ferments. A 48-hour cold proof will have more acetic character than an 18-hour one, even at the same temperature.

Starter feeding ratio is the variable people overlook most. A 1:1:1 starter (starter to flour to water by weight) fed at its peak and used relatively fresh will tend to produce a more lactic, milder profile. A starter that has been left longer, or fed at a higher ratio like 1:5:5 and used young, will have a different acid balance entirely. If you want to get precise about this without doing it all by feel, the sourdough starter calculator is genuinely useful for working out ratios before you commit to a bake.

How These Acids Affect More Than Just Taste

Flavour is the obvious one, but the acid balance also affects the texture and structure of your loaf in ways that matter.

Lactic acid is gentler on the gluten network. It provides some strengthening without over-softening the dough, which is part of why a moderately fermented, lactic-forward loaf holds its shape well and has a soft, open crumb. Acetic acid, being harsher, can weaken the gluten structure over time if fermentation goes too far, which contributes to the slightly denser, chewier crumb you sometimes get in heavily acidic loaves.

Crust behaviour is also affected. A higher acetic acid content creates a slightly more complex Maillard environment (the browning reactions during baking), contributing to those deeper, more complex crust flavours in a well-fermented loaf. The crust on a longer, cooler ferment often has more going on than one baked quickly and warm.

There is also the question of shelf life. Acetic acid is a more effective preservative than lactic acid at equivalent concentrations. That is why an old-fashioned country sourdough, baked for longevity and made to last a working week, would traditionally have had a more pronounced sour character. It was functional, not just flavour.

Practical Ways to Shift Your Acid Profile This Autumn

You do not need a lab to make this work in your favour. Here is what I actually do when I want to adjust where a loaf lands on the flavour spectrum.

For a milder, more yoghurty loaf: keep the dough around 24 to 26 degrees during bulk, use a starter that is at or just before peak, and keep the total fermentation time tighter. A single overnight cold proof (12 to 14 hours) rather than two days will keep the acetic acid lower.

For a more pronounced, complex sour: let the dough sit cooler during bulk, extend the cold proof to 36 or even 48 hours, and use a slightly stiffer starter. October kitchen temperatures are actually working in your favour here without you having to do anything special.

If you want someone to actually look at your specific situation and tell you what to adjust, that is exactly the kind of thing DoughRise Coach is built for. You can describe your current process and get personalised guidance on shifting your acid balance, adjusting your timing, or troubleshooting why the flavour is not where you want it.

Happy baking! Find everything you need at doughrise.store

Photo by Vicky Ng on Unsplash