We do not knead high-hydration dough. Kneading is a technique proper for low hydration, no gluten doughs. Whole wheat sourdough bread, on the other hand, is a high-hydration, glutenous affair. Gluten does not exist in plain wheat flour but establishes itself when gliadin and glutenin, two smaller proteins native to wheat flour, combine − as made possible in high-hydration dough. Gliadin and glutenin form a new, larger protein structure in the dough called gluten.
Gluten can be hard to digest for some people. However, high-protein whole wheat sourdough bread is very glutenous, almost by design. The gluten structure in the dough is what contributes to the dough’s rise, all without the addition of rising agents like baking soda or baking powder.
Gluten forms naturally in wet wheat dough, but it can be helped if the bread is supposed to rise strongly, come out of the baking process with an open crumb, and even with a decorative lip on top. Stretch and folds are the techniques or actions performed to develop a strong gluten structure.
While the yeast in the dough produces CO2 and alcohol, the lactic bacteria produce the lactic acids that give sourdough its tang or sourness. The CO2 then collects in the airtight pockets of the gluten structure, and those pockets expand during the baking process and fill with air as the CO2 eventually dissipates, giving the loaf its crumb − open or tight.
This is to say that unless the gluten structure is strongly developed to contain CO2 and air, the pockets are not there to expand, and the dough will not rise much and only develop a tight or dense crumb.
A baker will notice that an incorporated mass of water and wheat flour turns out to be slack to begin with, but that after being rested, mixed, stretched, and folded a bit, becomes elastic and extensible. Elasticity refers to the dough’s behavior of pulling back when stretched, much like a rubber band. Extensibility refers to the dough’s behavior of being stretchable to begin with − again, much like a rubber band. Both extensibility and elasticity are the effects of the gluten (protein) structure in the dough.
Ever looked at a rare flank steak? Protein! You’ll likely see the tough muscle strands, and you know that after cooking the meat, you need to cut it across the strands for tender fajitas.
Stretch and folds, as well as the related technique of slap and folds, are the performative actions to stretch and align the dough’s gluten or protein strands and aerate the dough a bit. If the gluten structure stays weak and unaligned, the dough will have little extensibility and elasticity. The dough will have no muscle to hold its eventual shape. Mixing and stretch and folds develop the dough’s strength.
A baker, when handling the dough with his or her hands, will feel the dough’s extensibility and elasticity. First, we grab and stretch a handful of dough away from the edge of the batch and then fold it over into its middle. A rested and relaxed dough will let the baker stretch the dough quite a bit before the dough pulls back due to its elasticity.
After a few stretch and folds, the baker will notice that the dough seems to be resisting the stretches more and more. The dough begins to lose its extensibility and firms up. If the baker were to keep stretching, the dough’s elasticity would break. That is, the dough’s gluten structure would be damaged, with the dough becoming slack again. A broken rubber band is of no use to almost anyone. What a baker wants is a firm and not a slack dough. What a baker wants is a dough that can hold its shape.
Resting the dough in between stretch and folds allows the gluten structure to relax again to be extensible enough for the next stretch and fold − until the dough is strong enough to stay in shape.
Working and getting a feel for the dough takes practice.