Slowly the BRF+spent substrate jars started smelling a bit, not too bad at first, but now it's unmistakingly the smell of bacteria. The jars with cardboard and a bit of oat bran mixed with the spent substrate don't smell.
Anyway, the experiment of making cakes with this substrate is off now.. I never had any bacterial issues with BRF, also the jars are on the dry side, so it must be the mix that doesn't work. I guess there is too much nutrition left in the spent substrate to mix it up with fresh flour, evenif it's all sterilized for 1,5 hours. But any other ideas about why it doesn't work would be appreciated.
I'll probably try using the rest of my bag simply as casing as I had originally intended, though I guess the casing might well still be so nutritious it's actually more of a bulk substrate.