Hi all,

So, I'm beginning to bake with natural levain and I've the problem given in the subject (which I never had with dried yeast).

I'm following the receipe given in Hamelman for the Vermount Sourdough, using the levain with 125% of hydratation.
What I do, is that I make the final mix at ~7PM, I fold twice, and give the final shape at ~10PM (oval loaves). The temperature during the process is 22-23 C.

I let the loaves raise ~1hr at 22C seams up, and then put them in the fridge at 11PM (temp 10C) seams up. They are lean on baking paper and covered with plastic film.

At 8.30AM the next morning I remove them from the fridge, let them "warm" at room temperature for 30-45min, I turn them in such a way that what was the bottom becomes the top (that is, they are baked with seams down), score them and bake (250C, steam).

What happens it that the loaf is not fully developed at the top, but it craks on the bottom instead, during the oven spring. I can attach picture, if you want (and if I discovered how to do that!).

When that happened the first time, I though it might be due to a gradient of preassure (sorry, I'm a physicist :-) ) in the loaf, with the part that stayed on the bottom for the whole night more dense (and/or humid) than the top. So today I made again two loafs, but this time I left one of the two with the seams down while I turned the other. The outcome is that both of them cracked on the bottom :-)
I don't think the problem are the seams not well sealed, as the cracks do not happen in correspondence to the seams lines. 

Hence, it seems I've no idea of why this is happening, and suggestions are welcome!!

Thanks,
S.