Crumb Query

This happens with most loaves, regardless of shaping. And while it's certainly possible to over-compress the crumb while shaping, thus causing an even tighter center, even a gently shaped and well-formed loaf will exhibit this phenomenon to one degree or another.

As I see it, it's mainly a matter of compressive and tensional forces. The gas cells towards the center of the loaf bear a greater burden of compression than do the gas cells towards the surface of the loaf, because they support the greater portion of the load (the weight of the loaf). Because they're undergoing greater compression, they require a greater counter force to expand than do the cells towards the surface of the loaf. 

The gas cells towards the edge of the loaf are under much less compression, therefore they require less expansive force to grow. So they have a greater inclination to grow larger. But this also makes them less stable as the cell walls are stretched further and further. They are more prone to collapse, and some will. The more surface tension the loaf contains, the more likely those cells will collapse and coalesce (into larger cells). However, the looser the dough, the more likely those cells will collapse and expel their gasses (deflation). Both can happen at the same time in a loaf.

The gas cells towards the center of the loaf are much more stable than the cells towards the surface. They experience a better balance of compressive and expansive forces. And because they don't grow as much, or collapse and coalesce as much, they are more numerous as well.

So at the center of the loaf you have a stable structure formed from many small and sturdy cells. And towards the edge of the loaf you have a structure containing fewer, larger and more unstable cells.

Once the loaf is placed in the oven, the resulting expansion will magnify all these effects. The already less-stable cells towards the surface of the loaf undergo greater turbulence than the sturdier cells towards the center of the loaf. Additionally, as the crust hardens it begins to trap some of the steam and gasses that are being expelled from the still-pliable center of the loaf. Since the gas can't escape the loaf, it will accumulate under the surface forming ever-larger alveoli. Essentially, ovenspring exaggerates the cell structure. 

But all that long explanation is somewhat pointless anyway. Those are just the general forces that affect the crumb structure of all loaves (there are other forces as well). Some loaves will be affected to a greater degree than others. And there are always outliers that seem to defy all physics.

If your crumb seems excessively tight in the center then it's most likely a fermentation or handling issue. Open crumb is 80% proper fermentation and handling. Both are skills. The more practice you get, the better your skills get. And the better your skills get, the better your crumb gets. 

Cheers!

Trevor

As trevor explained above, the middle of the dough is usually more dense due to [physics] (lol)

You can reduce this effect by shaping your dough into a flatter, wider loaf to lessen the tension on the middle of the dough. Shaping it into a longer loaf might also help. Anything to reduce the tension in the middle, since it's what's causing it.

If you do not want to change the shape of your loaf, either just accept the fact that it's normal or try higher hydration recipes for an even more open crumb, hopefully making the effect less noticeable.

someone on this forum cant remember who made a good point - its not so much the holes but the space between the holes you should be looking at...to me a good crumb is a variance in ho a proliferation of holes throughout the loaf....sort of like a lesson i learnt in life drawing in college - look at the negative space around the figure to find the true shape not the line of the figure...if you get what i mean and as Trevor says fermenting and handling...and that only comes with experience...i had a problem with a dense centre and so decided to push my loaves (in time) to as long as i thought feasible in bulk and proofing and worked backwards from there....as a result my loaves are now better....i think theres a tendency for people to underproof....to me its always a game of chicken with dough....    

Personally I don't think the explanations regarding tension have nearly as much impact on crumb development as do the effects of heat. A good open crumb will develop if you can cause gases to expand faster and earlier. The bigger the loaf the more time it takes for the interior to heat up and its as simple as that at least in my experience. If you want holes inject as much possible heat as fast as possible. A big boule will take more and more and more time to transfer that heat into the middle so the trick is really to get the middle hotter and faster - that's just my 2c worth :)