Why do we stretch and fold sourdough?

Most bread bakers know that kneading dough is an important part of the process. Let's take a moment to think about why. (I'm going to try not to get too into the weeds and keep it simple, but we all know I struggle on that front).

Flour contains two main proteins: glutenin and gliadin. As the flour and water are combined, these proteins link together, Initially, these protein strands are a tangled mess, like a big bowl of sticky spaghetti. As we knead our dough, we create the gluten structure: a web that is formed as these strands create bonds. This structure is what gives our bread the strength to hold itself together, rising tall rather than squashing out into a flat pancake. 

When you are making a yeast dough, the process is fast. You make the dough, knead, proof the bread for a brief time, then bake. The gluten structure remains intact.

Sourdough fermentation complicates the process. 

We know that your sourdough starter is essentially a mix of two components: wild yeast and Lactic Acid Bacteria (LAB). The yeast produces the rise by releasing gas, while the LAB create acid creating that signature sourdough tang. During your initial mix and knead, you will begin creating a gluten structure, just as with yeast bread. The "first proof" will be much, much longer when baking sourdough, and during this time the gluten will be weakened.

The first culprit is our bacteria friends. As they consume the fresh food you've provided, they create acid waste, lowering the pH of your dough. We want this acidity - it creates the flavor we are seeking through sourdough baking! But it does have a dark side. This acidity activates the proteases naturally found in flour, breaking the gluten bonds and snipping the strands into smaller pieces.

Next, our yeast. Through the fermentation process, our living yeast is partially digesting the gluten we are creating. Again - we want this! It's why sourdough bread is more gut-friendly than commercial yeast breads. But this digestion further weakens the gluten structure of our dough.

Enter: the stretch and fold!

As fermentation begins, we need to rebuild our gluten structure as our sourdough starter weakens it. Traditional kneading is far too aggressive for the gluten in its weakened state. You will quickly notice your dough becoming even stickier as the delicate strands tear. This is why we need to use a gentle method of manipulation, with periods of rest between.

Performing a stretch and fold is simple. Grab one side of your dough, slowly lift it up allowing the dough to stretch, then fold it toward the center of the dough. Rotate your bowl 90 degrees and repeat this process once per side. Pulling tightens the protein bonds. By being as gentle as possible, we prevent the weakened strands for tearing while reforming the gluten network.

Why do we do multiple sets of stretch and folds, and why rest in between?

Generally, I recommend four sets of stretch and folds with a 30 minute rest between each. The rest period allows these tightened, newly-bonded proteins to relax again and continue to hydrate. This makes them more extensible (stretchy). You will notice when you begin your next set of stretch and folds, the dough goes a little further each time. The rest periods allow you to continue to build the gluten network without creating tears.

When should I use a coil fold instead of stretch and folds?

Coil folding is just an alternate form of manual gluten development. Rather than lift the dough from the sides, you will lift from the center and allow the dough to fall from the sides. Coil folds are even more gentle as they rely on gravity rather than pulling, so your dough is less likely to tear. If your dough ever feels tight or difficult to pull, switch to a coil fold. 

How does this benefit the final product?

Creating a tight gluten network is directly responsible for the rise and final shape of your bread. The gluten web traps the gases produced during fermentation. During baking it coagulates, forming the signature bubbly structure and high rise we look for in a perfect loaf of sourdough bread.

Are stretch and folds mandatory? Can I just mix the bread and let it ferment?

It's not mandatory. You can still make a delicious loaf of sourdough bread by skipping this step entirely. But a little extra effort goes a long way toward improving your bread and taking it to the next level. Even if you only have time for a few sets or stretch and folds, you will notice a difference in the final product.