Trouble-shooting help

Hello,

I'm looking for some help/ideas trouble shooting. I've been successfully baking my own sourdough bread for years now, but over the past two months or so, every single loaf I've tried to bake has ended up collapsed in the middle, and I can't figure out why!

For a bit of background, my sourdough starter was originally a dry "San Francisco Sourdough starter" which I bought from bakerybits, but I've had it now for 5+ years I think (lives in the back of my fridge in a glass jar), so who knows what the culture has morphed into. I generally bake a rye/wheat or rye/spelt mix loaf in a tin. After I couple of years of doing this by hand (i.e. in the oven), I found that I could get very reliable and consistent results using a bread machine, using the following method:

Mix 1/2 cup of starter from the fridge with 200g wholemeal rye and 122g wholemeal wheat or spelt and 322g warm water, put into bread machine loaf tin. Cover with cling film and leave in oven (off) with door ajar and incandescent light bulb in (to give about 30C temperature) over night. This gives a soft, strongly bubbling dough next day.

Next day, dissolve 10g salt in 156g warm water and add to tin, then add another 200g wholemeal wheat/spelt and 122g white bread flour on top. The tin then goes into my bread machine (Lidl brand "Silvercrest") on the "wholemeal" setting, which is a 3.5hr program, pre-heating for 30min, then 2hr kneading/rising, followed by 1hr baking. (NB: Slightly odd amounts to get to a total weight of 1125g, which is the max my bread machine will take).

This method has proven super reliable, delivering a good loaf every single time (once a week) for about 3 years now, despite me not being super accurate about overnight rising time, types of flour, exact amount of starter etc. Very robust. Until a couple of months ago, that is.

Now, every single loaf I try to bake has a tall standing "rim" with the middle collapsed (about an inch lower, if not more). The bread is still tasty and consistency is fine in the centre (i.e. not underbaked, not super dense and wet). Today, I've finally observed exactly when the collapse happens - it's during the first 5-10 minutes of baking. The dough is not collapsed before the baking stage starts, nor is it obviously poofed up/overproofed (slight upward bend, still plenty of room to top of tin).

It's been a warm summer, and it crossed my mind that the bread was overproofed and that is why it collapses, but I've tried a shorter bread machine programme without pre-heating, and the same thing still happens. In fact, since the temperature both in the bread machien and the oven during overnight proofing is regulated, I can't quite believe that ambient temperature would have that much of an impact. Also, I don't really think the bread is overproofing. I need to fish out the kneading hooks from the machine some 40min or so before the baking stage begins, and the consistency of the dough feels normal/as usual, nor does it significantly collapse at that stage after being disturbed. I also wondered if it could be flour, as I recently started using a particular type of "ancient" grain flour (Lammas Fayre brand), and it occured to me that the gluten/starch content in that might be different from usual. But swapping back to standard bread flour still gave me a collapsed loaf the next time around.

I'm now starting to wonder if something could be wrong with the bread machine, but I don't quite see how e.g. temperature during the baking stage would lead to collapse? Any ideas where I might be going wrong? I do realise that my method does not provide a particularly tight control over all relevant variables, but I can't wrap my head around why my formerly so reliable method is no longer working, without having made any significant changes.

Thanks for reading to the end!