What size of jar, proportionally, should I keep my poolish in to prevent overflow? This morning my ongoing poolish lived up to its troublemaking name. Over half of it (by weight) ran out over the counter overnight, through the screwed-down lid. This is after I saw it hitting the top last night and stirred it down before bed. In the photo below, the "after" photo is stirred down, but still pretty bubbly, so that's not even a completely fallen volume. I never fill the jar more than 1/3 full (at flat volume).




So, overnight, it at least sextupled in size.

Details: Commercial yeast 100% hydration bread flour poolish, maintained for about a month. Kitchen was probably 68F last night. I've let it rise and fall on the counter before, but the last two rest periods have been mostly in the fridge, if that matters.

So, primary question, what size jar, proportional to flat volume, should I use to be safe from overruns like this? I'd been going with 1/3 max, but then... this.

Secondary "why" questions to understand the answer:


Is this level of expansion normal, or at least somewhat frequent, or was this a bizarre fluke?
Did a counter rest after a couple fridge rests contribute to this explosion?
If the "Blob that ate Cincinnati" effect is expected now and then, would a biga be more predictable due to its stiffer consistency?
This is after I saw it hitting the top last night and stirred it down before bed.


You have been warned. But it's certainly not a bizzare fluke.

What could have helped the dough out:


at 100% hydration and low volumes you have a good chance that you get less-than-90% hydration dough by only a slight mistake in measurements; 90% dough can actually be pretty stable to support itself all the way up (especially if you have some better bread flour)
there might be a giant heap of nutrients left for the yeast/bacteria after 2 less-active fridge rests
your jar is quite narrow, which in fact provides lots of support to rising dough


In my opinion, to your original question, filling 1/3 for counter rests is on the edge, 1/4 should be okay. You can add some extra safety by having a wider jar, wetter dough, and feeding the starter much less for fridge rests (I actually always let the culture feed on the new flour for a day or two on the counter, before putting it to fridge).

About your "why"s:


Just 1 gram of consumed sugar produces ~0.248 litres of carbon oxide. Not bizzare at all, just a bit of bad luck that your dough was strong enough to be able to trap it. :]
IMO yes.
Biga is a short-lived starter culture, people usually don't keep it for longer than 1 day. Given there's much less water (40%) the dough usually starts to tear instead of rising very much. Btw. poolish is usually also taken to be short-lived; isn't your poolish actually sourdough-ish?