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What is cold crashing?
When is it done?
How long does it take?
Best temperature to make it happen?

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2 Answers 2

up vote 13 down vote accepted

Cold Crashing

Why It's Done

  • To allow yeast and other matter to settle out
  • To improve flavor
  • To precipitate chill haze
  • To help prevent oxidation

When It's Done

  • After fermentation has finished
  • Usually also after diacetyl rest
    If you crash the beer too early, the yeast will become inactive (below 40°F) and won't reabsorb fermentation byproducts like diacetyl. Additionally, if you crash while the yeast are still finishing fermentation, they will excrete more ester compounds.

Cooling Rate and Time

  • Traditional lagering uses a rate of cooling between 1°F and 2°F per day.
  • The beer is then racked into a closed lagering tank for carbonation.
  • If fermentation is finished, and the yeast have had time to reabsorb any off-flavors, you can crash much more quickly.
  • Several weeks is typical, although each batch can vary. Stronger beers tend to need more time.

Temperature Ranges

  • Yeast activity continues above 40°F, so if yeast action is required, a rest period in the 50°F - 55°F range is recommended.
  • If no yeast activity is necessary, temperatures can be brought down to near-freezing.
  • The actual temperature chosen will often depend on equipment limitations. For instance, if your temperature controller has a variance of 5°F, you wouldn't want the temperature setpoint to be 34°F, as the beer may freeze. Likewise, many homebrewers don't have dedicated equipment to achieve these temperatures, but may have a cool basement where cold conditioning can take place.
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Great answer. I've cold crashed for as little as 5 days and seen a big improvement in clarity on a completely finished fermentation. I usually am at 38-39F as well. –  therulebookman Dec 8 '10 at 17:31

Cold crashing is a technique to get the yeast to flocculate (settle to the bottom of the fermenter). This is generally done to get clearer beer (or wine).

It should be done when fermentation is complete, since there will be very little (if any) fermentation activity afterwards. This is because you are effectively removing most of the yeast from the beer. This is not a reliable way to stabilize the beer, though, so I would not recommend thinking of it for that. You should still assume that there is active yeast in the beer, and if you were to add more fermentable sugars to the beer (e.g. to backsweeten or to prime it for bottle conditioning), fermentation may restart.

This process takes at least a couple of weeks, though the longer you do it, the more effective it will be.

Regular refrigerator temperatures (in the 30s F) should work pretty well.

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The amount of time needed for cold crashing will be dependent on the strain and flocculation rate of the yeast you are using. Highly flocculant strains can clear in less than a week. –  Northern Brewer Chris Dec 1 '10 at 16:03

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