Sure!Could you talk a little bit about how to achieve each of these?
The holes are exactly what you want!
Contributors to an open crumb like that are: high hydration, limited kneading, low gluten dough, gluten degradation by acid, and baking from a cold dough. These are all tricky to finesse, especially with a sourdough, because these doughs can be quite delicate and easily deflate.
I usually knead dough until it passes the windowpane test. When you say "limited kneading," would that mean the dough does not pass the windowpane test?
Yes, that means not kneading until the dough passes the windowpane test. When you fully knead a dough, you are making sure that the gluten is developed and interlinked as fully and evenly as possible. This equals a regular crumb.
If you knead substantially less than that, and I sometimes knead only until the dough seems fully mixed, there will be some places where the gluten is more fully developed and interlinked than others. In addition, there is some development and interlinkage of gluten that happens purely by chemical means with no kneading needed (this is how "no knead" bread dough works). All these things lead to a more irregular crumb.
Low gluten dough: would you suggest using AP flour, rather than bread flour, to limit the protein and gluten?
Yes. I also prefer the flavor of AP flour over bread flour.
What do you mean by "gluten degradation by acid"?
Acid actually breaks down the gluten. This is why a sourdough that has been fermented too long will simply break apart: because the gluten has been degraded to the point where it is not able to hold the dough together. This is also why, the longer a sourdough is fermented, the more delicate the dough is. It is the central challenge of sourdough baking, because longer fermentation equals more flavor but it also equals a weaker dough -- so you're always playing a game, trying to push the fermentation as long as you can but still having a dough strong enough to produce an open crumb instead of a doorstop.
Some information on this phenomenon may be found in this paper: Effects of Acid-Soluble and Acid-Insoluble Gluten Proteins on the Rheological and Baking Properties of Wheat Flours. Preston et al. CChem 57:314 (1980)
More or less what this is saying is that the part of gluten that is responsible for loaf volume is also the part that can be degraded by acid.
Gluten, isolated from a hard red spring wheat flour, was fractionated into acid-soluble and acid-insoluble protein fractions. The effects of adding increasing levels of these fractions and of unfractionated and reconstituted gluten upon the rheological and baking properties of two base flours varying in baking quality were investigated. Results with the mixograph and farinograph suggested that the dough-strengthening effects obtained when gluten proteins were added to the base flours were mainly due to proteins present in the acid-soluble gluten fraction, whereas the acid-insoluble gluten proteins at higher levels had a slight dough-weakening effect. Addition of increasing levels of gluten to the base flours significantly increased loaf volume with both the Grain Research Laboratory's Chorleywood and remix baking procedures. Similar increases in loaf volume were also obtained by addition of the acid-soluble gluten proteins. Addition of acid-insoluble gluten proteins significantly reduced loaf volumes.
I haven't had much success baking a cold dough - this is the only step in jackal10's lesson that I don't follow. When I bake directly out of the fridge, the final loaf tends to bulge out on one side. So I usually leave the dough out for about 2 hours prior to baking.
Try larger slashes in the dough.