Am I over proving my bread, or might there be some other issue? I'm using a fairly standard baker's mix. 750g flour, 450g water, salt, quick blend yeast (Doves).
I bulk prove for 40 minutes, then shape reasonably tightly and into a large, heavy duty loaf tin. 30 minutes in the oven at about 200 degrees c. (Fan oven).
You can see this effect in the image, where I've held it up against a light.
Best Answer
I think everyone who bakes bread has had this issue at some point.
Two key things:
Make sure the bread is fully baked before removing it from the oven. Use a probe style thermometer to check the middle of the bread. For a sandwich style bread like you've shown above, the internal temperature should be about 195-205˚F.
Let the bread cool fully before cutting (30-60 minutes). When you remove bread from the oven the interior is partially in a gel-like state... It will continue to cook and the internal structure will stabilize as it cools and the water vapor redistributes throughout the crumb. As tempting as it is to eat warm bread right out of the oven, cutting into the loaf immediately will guarantee that some portion of the center will be undercooked.