Over-kneading sourdough dough results in a tough, dense bread with a less open crumb structure due to gluten breakdown. The dough will also be slack and difficult to shape.
Kneading develops the gluten in the dough, which provides structure and elasticity. However, over-kneading breaks down the gluten strands. In sourdough, this is particularly problematic because the dough already has a longer fermentation time, which naturally weakens gluten. When you over-knead, you're essentially working the gluten past its breaking point. This leads to a dough that feels slack and sticky, making it hard to shape. The resulting bread will be dense and chewy, lacking the airy, open crumb that is characteristic of well-made sourdough. The overdeveloped gluten also loses its ability to trap gas produced during fermentation, further contributing to a dense loaf.
Instead of relying solely on kneading time, pay attention to the dough's texture. Aim for a smooth, elastic dough that springs back slowly when poked. Windowpane test is a good indicator. If the dough tears easily when stretched thin, it needs more kneading. If it becomes overly stretchy and loses its shape quickly, it's likely over-kneaded.