Fermenting lager at 20PSI/12C in fridge.
The source of spontaneous fermentation is clearly visible.
You mean the puddle of now beer syrup on the bottom of the old fridge? That's just years of it being a keg serving fridge. It's not intented to be sterile or even clean. Doesn't need to be. As long as what is inside the vessels is sane and anything that touches the inside of them.
It's unlike to ferment though
Not even wild yeast are that desperate.
I reused the yeast "trub" after using the pressure within the fermentor itself + the 19L keg tank at 20PSI to push the beer into the 12L keg. Then I brewed a new 22L batch of lager last evening. Opened the fermentor used my brand new BBQ tonqs w/sanitizer wash to lift the floating wifi hydrometer out to charge and clean and ... pumped the new lager straight in.
Thing went off like a rocket. No lag, no messing around, the cell count per litre was already so high it had hit 25PSI and completed a quarter of the ferment by morning.
The pressure fermentor has a 35PSI PRV!