One large whole in middle of sourdough loaf? Breadit

Good evening. I've been making the Basic Country Loaf from the Tartine book for a few months now. I've made the recipe 6-8 times. The last three times or so I'm getting one very large cavity in the middle of the loaf, and poor oven spring (I think as a result). The bread is also a bit dense and moist. Any diagnoses? I've been following the recipe and have a kitchen scale. I do not have a proofing box, so the bulk fermentation is a bit variable. I'm in NC in air conditioning, probably 73-74 room temp. I often let one loaf do a final rise at room temp and one rise over night in the refrigerator. I used to get better results from the refrigerator dough, but the last two bakes have been very similar (large cavity). I bake in a cast iron combo cooker. I've tried varying oven temps and time, amount of starter used to mix in the leaven (200-250 g), and bulk fermentation time. I feel like this must be over fermented / proofed, but the last bake I used 3 hours only on the bulk fermenation and final rise and it still came out this way.

Submitted October 18, 2016 at 05:30AM by phasedweasel http://ift.tt/2efXAd6 Breadit