Cold Crashing Beer: Temperature, How Long, and How to Do It Without Oxygen

Cold crashing is dropping finished beer to just above freezing for a day or three before you package it. Yeast, hop particles and haze-forming proteins all settle faster in the cold, so the beer that goes into the keg is bright and the sediment stays in the fermenter. It is the simplest clarification method there is, and the one every brewery uses.

Here is the temperature, the timing, the one mistake that oxidizes a lot of homebrew, and what to do if you cannot crash.

What cold crashing does

Three things settle when beer gets cold:

• Yeast. Cold triggers flocculation, the clumping of yeast cells into flocs that sink far faster than single cells. Most strains drop out almost completely within 48 hours at 34°F.
• Chill haze. Proteins and polyphenols that are dissolved at room temperature bind together in the cold and fall out. If you crash before packaging, they fall out in the fermenter instead of in the glass.
• Hop and trub particles. Anything from a dry hop that is still floating.

It does not change the flavour directly. It removes the things that would have made the beer cloudy and yeasty, and it gets the beer to serving temperature ready to carbonate.

Cold crash temperature

33 to 38°F (1 to 3°C). Colder is faster. 34°F is the usual target; a fridge that only reaches 38°F still works, it just takes a day longer. Do not go below 30°F: beer freezes around 28°F, and a tank against a chiller wall can freeze locally before the middle does.

How long to cold crash

24 to 72 hours once the beer is actually at temperature. Yeast drop in the first day, chill haze in the second. Two days is the usual answer for ales. Lagers go straight from the crash into lagering, which is the same thing held for weeks (lagering explained).

Count from when the beer reaches temperature, not from when you turned the fridge down. Five gallons in a fridge takes 12 to 24 hours to get cold. A jacketed tank on glycol does it in a few hours.

When to crash

1. After terminal gravity, confirmed by the same reading two days apart. Crashing an unfinished beer leaves it sweet and, worse, buttery: the yeast never get to do their diacetyl cleanup.
2. After the diacetyl rest: two or three days warm after terminal gravity.
3. After the dry hop. Dry hop warm for 2 to 4 days, then crash. Crashing drops the hop material out with the yeast, so you transfer clear beer.

The suck-back problem, and how to avoid it

This is the part most guides skip, and it matters more than the temperature. When you cool a sealed fermenter the gas inside contracts. Five gallons of headspace CO2 cooling from 68°F to 34°F shrinks by about 7%, and the beer itself shrinks a little too. Something has to fill that space, and in a fermenter with an airlock, it is room air pulled back through the airlock, sometimes with the sanitizer in it.

Oxygen at this stage is what turns a hoppy beer dull and a pale beer to wet cardboard within weeks. Ways to stop it:

• Crash under pressure. In a pressure-rated fermenter, set a spunding valve to 5 to 10 psi before you crash. The tank stays positive as it cools; nothing comes in. This is the whole answer if you have a Unitank.
• Feed it CO2. Connect a CO2 line at 1 to 2 psi to a gas post during the crash. The regulator supplies gas as the headspace shrinks.
• A CO2 bag. Attach a mylar balloon or a sanitized bag filled with CO2 to the airlock port. It collapses instead of drawing air. Crude, cheap, works.
• At minimum, replace the airlock with a solid stopper and accept a small vacuum. Better than an airlock full of sanitizer emptying into your beer.

Step by step in a jacketed fermenter

1. Confirm terminal gravity and finish the diacetyl rest.
2. Set the spunding valve to 5 to 10 psi (or put CO2 on the gas post).
3. Set the controller to 34°F. With a glycol chiller on a 13-gallon tank that takes 3 to 6 hours.
4. Hold 48 hours.
5. Open the cone valve for a second to dump the yeast and hop sludge.
6. Rotate the racking arm above the remaining sediment and push the beer to a purged keg with the tank's own pressure.

Does cold crashing kill yeast?

No. It puts them to sleep and makes them clump. Enough yeast remain in suspension to bottle-condition, though it will be slower and you may want to add a small pinch of fresh yeast at bottling for beers over 7% or crashed longer than a week. If you harvest yeast from the cone after a crash, they are dormant but viable; warm them slowly and make a starter.

Is cold crashing necessary?

No. Time does the same job, more slowly: a week or two cold in the keg clears most beers. Crash if you want clear beer at packaging, want to leave the yeast and hops behind in the fermenter, or are transferring to a keg you will serve from soon. Skip it for hazy IPAs on purpose, for bottle-conditioned beers where you want yeast in suspension, or if you simply cannot get the fermenter cold.

Other clarification methods

Method	When	What it removes	Notes
Kettle finings (Irish moss, Whirlfloc)	Last 10 min of the boil	Proteins, in the kettle	Cheap, do it every batch
Cold crash	After fermentation	Yeast, chill haze, hop particles	This post
Gelatin	During the crash	Yeast, haze	1 tsp in ½ cup water, heat to 150°F, add to cold beer, 24 to 48 h. Not vegan
Biofine / silica finings	During the crash	Yeast, proteins	Vegan alternative to gelatin; commercial standard
Time (lagering)	Weeks cold	Everything, eventually	Slow but free
Filtration	At transfer	Everything down to the filter size	Strips some flavour and body; rarely worth it at home

A note on cider

The same rules apply, with one difference: cider yeast are less flocculent than beer yeast, so cider often needs the full 72 hours and benefits from gelatin or pectic enzyme (for pectin haze, added before fermentation). Crash at 33 to 35°F, under CO2 if you can.

If you cannot crash

The fermenter is too big for the fridge, the garage is 60°F, and you would like clear beer anyway. This is exactly what a jacketed fermenter solves. A Unitank with a temperature control kit crashes on tap water in a cool basement and on glycol anywhere else, at any size from 8 gallons up. Whether that is worth it for you is a fair question, and we answer it honestly.