A “tight crumb” aka small holes in the interior of your bread can be the result of different factors: under-fermenting, over-fermenting, and a lack of gluten development.
The most confounding part of sourdough bread baking is that the rise times of recipes are just a suggestion or range. You as the baker have to watch the dough to make sure it is fermenting nicely, and you might have to stop the process earlier if your kitchen is very warm or let the process go longer if it’s cold or if your sourdough starter is a bit sleepy or you used less than the recipe called for.
This article, Demystifying Sourdough Bread Baking, is a great read to help you understand sourdough fermentation, and here are some fermentation diagnostics to help you understand what may have happened with your dense bread:
- If the interior of your bread is a mix of very dense areas and large holes, and the loaf is relatively tall; then the bulk fermentation was likely too short and the gluten development limited, with some compensation during the final proof
- If the interior of your bread is extremely dense and the loaf is flat, then the bulk fermentation and final proof were likely too short.
- If the interior of your bread is moderately aerated but the loaf has a deflated look to it with no opening to the score, then either or both the bulk fermentation and final proof were too long.
For a more open crumb, fermentation must be accompanied by a strong gluten structure. Gluten is developed through time (no knead) and actions on the dough, such as stretching and folding or kneading. Here is a useful blog post about Gluten Development with videos showing various techniques to employ on the dough to strengthen it and allow it to hold large pockets of air.
