Why does cold crashing work?

[stadelman]

I've been fermenting in a temp controlled deep freeze for quite some time. I cold crash at the end of fermentation. This seems to drop yeast and hops out of solution in short order. I just cold crashed my last batch, a stout that had two pounds of shredded coconut in it. The coconut floats. I swirled it around a few different times, some of the mass drops for a bit, but it pops back up because it floats. I just cold crashed it and whammo two pounds of otherwise floating shredded coconut drops to the bottom.

Why?

I know the liquid is going to become more dense with the decrease in temp, but the size of the coconut doesn't change. It displaces the same amount of liquid, which would be even more weight because of the increase in density. Even if the coconut or yeast or whatever does condense as well, it would be at a similar rate to the change in water density.

[DannyW]

Wild guess: at warmer temps CO2 is bubbling out and sticking to the coconut, making it rise. At colder temps the CO2 stays in solution, does not form bubbles, therefore there is nothing to lift the coconut. You might get the same effect if you let it sit there long enough at warm temps to completely degas.

[stadelman]

The coconut went in after active fermentation was over. Fermentation picked back up after adding, presumably there were some fermentable sugars in the coconut, but within a day all was calm again. Over the next week I swirled it a few times. What I'm saying is that, although CO2 was undoubtedly in solution, it's not like it was actively bubbling to buoy up the coconut. Also, I suspect gas in solution decreases the buoyancy of solids. I know this is true for bubbling liquid. This is one of the possible explanations for the Bermuda triangle. Bubbling gases would sink ships.

Long story short, I don't believe the CO2 in solution is the primary factor.

[Brandon]

Maybe I am misunderstanding the question. But I am under the impression that the colder something is the heavier it becomes and will sink. Like in scuba diving you refer to the thermocline where the warm water sits on the cold.

[stadelman]

You do understand my question. That's probably it.

So, let's take a single coconut flake as an example. The temp decreases, the flake decreases in size and it is displacing less liquid. But... the liquid that is displacing has also contracted, so it weighs more. The flake is displacing less liquid, but the same amount of weight (maybe) because of the increased density of the water.

If this is the answer then the increased weight of the liquid must not be enough to offset the decreased displacement caused by the shrunken solids. Maybe... solids are less dense than liquids therefore they have more room to contract as temperature decreases.

[Bellmer]

I'm going to throw something else that's a wild possibility out there. Coconut has a significant amount of oil in it to begin with, and if you used pre-grated coconut, then it has even more oil on it than freshly grated. Oil floats on water because it is less dense (has a lower specific gravity I think), and your beer I'm guessing didn't finish at a lower density (specific gravity) than water. So, it might be due to the oil content making them float. Just an idea to add to the pile, not saying it's right.

[stadelman]

That could have something to do with cold crashing as it relates to coconut, but coconut is just an example solid. Cold crashing also works with yeast, trub and hops.

[Mylo]

I suppose it's also possible that the yeast found the coconut flakes as neucleation points for their flocculation. As they attached, then brought the coconut down with them.


Mylo