Why exactly does cold crashing work?

I think once you guys read any number of bio papers like Mechanisms of Yeast Flocculation: Comparison of Top- and Bottom-Fermenting Strains, by PASCALE B. DENGIS, L. R. NE´LISSEN, AND PAUL G. ROUXHET, you will realize that the mechanisms underlying cold crashing are probably actively regulated by yeast, and not passive processes.

I mean you are dealing with an organism that was found with 1.6 million SNPs (gene variants) in a sample of a thousand yeast cultures collected globally (Genome evolution across 1,011 Saccharomyces cerevisiae isolates, Nature 2018). Next to nothing passively 'just happens' to them.

Cold Crashing clarifies cider by causing the yeast to clump together, or "flocculate." These clumps of yeast then fall out of suspension much faster than individual cells (due to some fluid dynamics that I won't pretend to understand). Flocculation depends on many factors, chief among them, yeast strain, but temperature plays a major factor. For the majority of yeasts, a decrease in temperature induces flocculation, although the ideal temperature for flocculation depends on the strain.

As a side note, aging beer at cold temperatures also causes proteins to bond to polyphenols/tannins and form a "chill haze," which precipitates readily.

If you're curious, other factors that can affect flocculation include pH, calcium concentration, ethanol concentration, as well as presence of certain "inhibiting" sugars (glucose, mannose, maltose, and others).

Temperature at its core is a measurement of particle velocity. The colder something gets the slower the particles and atoms are moving.

When you cold crash you are slowing down the nano-scopic movement of all these particles (macro and otherwise). The less they collide with each other and the slower the move, the more effective gravity is at pulling them towards the bottom of the vessel.

I think there are several reasons why cold crashing works, but they all come down to affecting various parts of the equation for drag (in other words, gravity is doing all of the work).

First, cold promotes early flocculation of yeast. Yeast clump together and form flocs as a survival reaction to adverse environmental conditions, with cold being one of those conditions. These flocs are larger than single yeast cells, therefore experiencing more drag in the beer, and sinking to the bottom. Of course, this does not work as well with non- or low-flocculant strains of yeast.

A second reason has to do with the slightly greater density of the beer at cold crashing temps. This promotes drag on suspended particles, and allows gravity to act on them, and cause them to settle. I am guessing that most of the particles that settle in this manner (besides floccing yeast) are larger, such as break material and hop debris.

A third reason is that beer is a colloidal substance (a mixture of substances that do not dissolve well with each other). Chill haze is a visible demonstration of this. Cold crashing helps foster reactions between proteins in the beer and tannins/polyphenols that cause chill haze. While chill haze is usually undesirable, having it early in the process allows it to settle out to the bottom of the fermenter, I have read. Because the particles that cause chill haze are on the 0.10 micron scale, I doubt that this is a significant effect.

Edit: typo

At the simplest level, cold crashing is about reducing any exothermic heat from yeast metabolism, since this causes convection and interferes with the sedimentation.

Cold crashing temperatures can (and should) be a good deal cooler than serving temps. Despite what is common practice, most, if not all beer, be served above ice-cold temperatures to properly convey the flavor and nuances the brewer has worked hard to produce. So, assuming the correct serving temperature, Ales served at cellar temps of 10°C/50°F may still have some yeast activity which prevents effective sedimentation, so temperatures close to freezing are used to cold crash to be sure there is no fermentation activity.