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I'm trying to understand the mechanism behind cold crashing. When cold crashing, you cool the fermented brew, yeast and suspended solids settle. With my first cider, a very turbid bottle visible cleared in a day or two. Why do solids settle whan cooling?

Generally, sedimentation happens continously and is faster at low viscosities of the suspending liquid. Cooler water is actually more viscous so you'd expect slower sedimentation. Another purely physical explanation would be that with colder temeperatures there's less diffusive mass transport and the actual sedimentation dominates. But the temperature difference between ordinary storage and cold crashing is not that much, and in other sedimeantion applications we don't talk about temperature at all. So I assume it's no purely physcial process.

So I would assume that by cooling, we immobilize the yeast cells leading to them sedimenting at the bottom, but that's pure conjecture on my part and I'd like to hear a detailed explanation of the hows and whys of cold crashing.

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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

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Your third reason - density of the beer - actually works against sedimentation, the rest seems highly plausible to me. +1! –  mart Feb 3 at 21:57
@mart. I think the idea is that the yeast and other partiles are denser than the wort at any temperature, and that the effect of the increased drag outweighs the buoyancy effect of being suspended in a slightly denser fluid. –  Chino Brews Mar 13 at 18:04
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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.

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interesting. Do you have any source? –  mart Feb 3 at 21:58
No source - it's widely published/known that fermentation is exothermic and causes visible churning in the beer during fermentation. –  mdma Feb 4 at 3:56
While it's true that the heat of yeast metabolism causes mixing during the active part of the fermentation, this is not really a factor after the fermentation is complete. Also, at the time of serving, there should be no yeast activity causing convection. Perhaps I'm misunderstanding. Could you please clarify? –  EmpiricAles Feb 4 at 19:07
Yes, the yeast still do create energy when fermentation is over - they have to or they'll die (or hibernate). For example, diacetyl reduction is also exothermic and will release energy. Unless it is highly flocculative (like WLP002) yeast tend not to floc until they are dormant - the cold temps of cold crashing force the yeast into dormancy. –  mdma Feb 4 at 21:07
In my experience, the heat generated by the yeast after fermentation is negligible. I'd be curious so see a source to support your claim that significant convection continues post fermentation. –  EmpiricAles Feb 8 at 22:51
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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.

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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).

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+1 the increased chill haze is probably the main reason to cold crash. Some brewers cold crash just below freezing to produce a bad chill haze which is then sedimented or filtered out. –  mdma Feb 4 at 20:36
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