[Richard]

This question was raised on the BeerSmith forum, but has not been answered. It was asked in the context of BIAB, where some people grind very fine. Does that make any difference to how much water is absorbed by the grain?

[Bob357]

If you're comparing an average crush to a finer crush that might be considered typical for many BIAB brewers, any difference would be negligible. In the context of BIAB brewing the big influence on absorption would be how effectively the bag was drained.
If we're talking a ridiculously coarse crush and/or a very short mash time, you might see a measurable difference.

[Andy Farke]

I haven't noticed much difference across crushes. One area where I _do_ note significant differences is if I have an appreciable amount of flaked grains (e.g., oats, rye, wheat). I typically have to adjust my water volumes up to compensate.

[BitterItDown]

A very fine pile of grain dust will not have the intracellular structure to absorb and retain the water that a whole kernel would.

If you're doing BIABag and you're squeezing the bag or using a press plate on a BIABasket the amount of liquid retained is less.

.1 - .125 gallons per pound for a standard crush with no squeeze/press
.04 - .08 gallons per pound for a BIAB crush (finer) while squeezing/pressing

Maybe try to measure your setup by taking a pound of crushed grain and placing it in a gallon of water... strain the grain and measure the volume of water, now press the grain and measure the volume of water.

[denny]

I haven't noticed much difference across crushes.

Same here

[BitterItDown]

You shouldn't notice much of a difference based on crush, but squeezing/pressing will make a difference.

At a certain point the grain dust will saturate the water but chances are nobody's crushing their grain into a pile of dust and making a barley dough/paste - there's always a higher percentage of coursely cracked kernels (relative to dust) that are absorbing the water.

Flaked grains and different varieties of grain have difference absorption capacities... Wheat, rye, barley, corn, etc...