Rehydrating yeast. How much head space is needed?

[kiwipen]

How much head space is needed in the container when rehydrating dry yeast?

I have a 500 ml lab bottle like the one in the picture, but I'm not sure if it's big enough.

Hydrating 4 packets of dry yeast requires 400 ml of water, and the 44 grams yeast itself will fill up a little more of the volume when dry.

But how much volume will it take up when it's finished rehydrating after 15-30 minutes?

 

[McKnuckle]

Perhaps I am missing something, but if the water is taken up into the yeast, then there will be no overall volume expansion within the container. Volume will simply shift from being in airspace, so to speak, to being within the yeast cells. Water in the airspace will reduce and be absorbed into the yeast. Overall volume would be the same both pre- and post-absorption.

Again, perhaps I am missing something.

 

[kiwipen]

I haven't rehydrated in a long time, but I think it foamed and expanded a bit.

 

[VikeMan]

It does foam up. I've never measured it though.

 

[RM-MN]

Some of the yeast suppliers are now advocating that you don't bother with rehydrating. Just dump the dry yeast into the fermenter.

I've rehydrated, I've dumped the yeast into the fermenter and added the wort, I've dumped the wort into the fermenter and sprinkled the yeast on top. I can't see any difference in the beer.