The Science of Sourdough (What Is Actually Happening in That Jar)

Ever wondered what is actually going on inside your sourdough starter? This guide explains the science behind fermentation, gluten, and everything in between, in plain English

8/30/20263 min read

Sourdough often gets treated like a bit of kitchen magic, but there is real, fascinating science behind every stage of the process. Understanding it will not just satisfy your curiosity, it will genuinely make you a better baker, because you will understand why the advice you follow actually works.

What Is a Starter, Really?

Your starter is a living culture of wild yeast and lactic acid bacteria, living together in a mutually beneficial relationship. Both are naturally present in flour and in the air around us. When you combine flour and water, leave it out and feed regularly, you are essentially creating the ideal environment for these organisms to establish themselves and multiply.

The yeast consumes sugars in the flour and produces carbon dioxide (the gas responsible for rise) and small amounts of alcohol. The bacteria produce organic acids, mainly lactic and acetic acid, which is where sourdough’s characteristic tang comes from.

Why Flour Choice Matters

For feeding your starter, plain (all-purpose) flour is genuinely all you need, since you are simply providing food for an already established colony rather than worrying about gluten structure. Occasionally you can give your wild yeasts a treat by feeding with wholemeal or rye flour, as this gives the yeast extra nutrients. These flours are more nutritionally dense than plain white flour, which can noticeably speed up how quickly your starter rises and peaks.

For your actual dough, protein content becomes far more important. Flour with a higher protein content (ideally a minimum of 10 percent, but the higher the better) allows for stronger gluten development. Gluten is the elastic protein network that forms when flour is hydrated and worked, and it is what allows your dough to trap and hold onto the gas bubbles being produced by fermentation. Lower protein flour simply cannot hold structure as well, resulting in a denser and smaller final loaf.

Why Chlorinated Water Can Be a Problem

Tap water in many areas contains chlorine, added specifically because it inhibits microbial growth and keeps our drinking water safe. Unfortunately, that same property makes it less than ideal for a starter, since you are actively trying to encourage microbial growth, not prevent it. This is why many bakers use filtered, bottled, or rested tap water (left out uncovered for at least an hour to allow chlorine to evaporate) instead. Note that if your water supplier uses chloramine rather than standard chlorine, it will not evaporate off as easily with resting or boiling alone - you should be able to check online which one your supplier uses.

Why Temperature Has Such a Big Effect

Wild yeast, like most living organisms, has an ideal temperature range in which its metabolic processes run efficiently. Warmer temperatures speed up yeast activity (and therefore fermentation), while cooler temperatures slow it down considerably. This single biological fact underpins almost every piece of temperature-related sourdough advice you will ever read.

Why Hydration Affects Texture

The percentage of water relative to flour in your dough (hydration) affects how easily gluten develops, how much the gas bubbles can expand, and how much steam is available during baking to help create oven spring. Higher hydration doughs generally produce a more open, irregular crumb with larger air holes, while lower hydration doughs are easier to handle and typically produce a more even, tighter crumb, better suited to sandwich slices. I personally prefer a lower hydration loaf so always modify my recipes to match with my own personal tastes.

Why Salt Matters (And Why Some Bakers Delay Adding It)

Salt strengthens gluten structure and controls fermentation speed by slightly inhibiting yeast activity. Some mixing techniques deliberately delay adding salt for the first 20 to 30 minutes, allowing fermentation to get a head start before the salt’s inhibiting effect kicks in.

Why Cold Slows Everything Down (But Does Not Stop It)

Refrigeration significantly slows yeast and bacterial activity but does not stop it entirely, which is exactly why your dough continues to rise slowly, and continues developing flavour, during a cold proof, just at a much gentler pace than at room temperature.