The Importance of Rapid Cooling After The Boil

After you’re finished boiling, the hot wort (wort is the name of beer before it’s fermented) must cool down before it is safe to add yeast.  Yeast can’t be added while the wort is hotter than 90 or 100 degrees, but even that is still too hot for comfort.  Ideally you should cool the wort to a temperature in the 70’s.

The extreme “low tech” method of cooling wort is to let it sit until it cools off on its own.  This takes a long time, you’re dealing with 2 to 5 gallons of very hot liquid.  It’s not good to let hot wort sit for such a long time.  Here are some of the reasons why:

Although yeast can’t live in wort until it’s cooled to near room temperature, there are bacteria that can survive in wort as hot as 140 degrees. These bacteria can get a head start on the yeast during a long cool-down.  This can cause contamination which may ruin the flavor of your beer.  The sooner you can pitch yeast  (pitch yeast means adding yeast to the wort) after the boil, the less is the risk of contamination.

A long, slow cool-down will give you beer off-flavors.  Off-flavors are tastes and aromas that don’t belong!  

The most common off-flavor in home brewed beer is DMS (Dimethyl Sulfide).  DMS is created naturally any time wort is hot.  During the boil it escapes in vapor form.  This is why it’s important to always boil vigorously & with the lid off! 

After you stop boiling, DMS is still being produced, but it can’t escape as vapor.  If your wort sits hot for a long time, it builds up DMS.  DMS is often described as a “butterscotch” or “buttery” flavor, but we could describe it as “homebrew” flavor.  If you hand someone a glass of beer without telling them what it is, they taste it and say, “you made this yourself, didn’t you?” then you probably have a DMS problem.  This is a pity, because it’s an easy off-flavor to avoid.

If you cool your wort rapidly, there will be little time for DMS to build up.  Below a few parts per million, it is not detectable by human taste buds.

Rapid cooling also has the benefit of causing more of the naturally occurring haze-proteins in the wort to settle in the kettle, so finished beer will be clearer. These proteins, along with hop particles that settle after the boil, are called trub. After cooling, the clear wort is poured off of the trub into the fermentor. It is OK if some of the trub makes it into the fermentor, it will settle to the bottom anyway.

Fast cooling also preserves delicate hop aroma. Aroma hops are added to the end of the boil because high temperature rapidly destroys hop aroma as it transforms it into bitterness. 

As you have learned, there are many advantages to rapid cooling of wort.  Here are three different ways to do it:

The Ice-Water Bath
The low-tech option is to use an ice water bath.  An ice water bath only works well if you are boiling partial volume of 2 or 3 gallons.  A larger volume of wort is difficult to chill in this manner.

After boiling, cover the pot and place it in a sink or tub that is filled with ice and water.  Put in more ice and cool water as it melts.  This should cool your wort in about a half hour.  Don’t use just ice with no water, there won’t be as much contact area, which will greatly slow the process.

Placing your pot outside in the cold or snow is very ineffective.  Snow doesn’t have enough mass to pull away much heat.  BE VERY CAREFUL anytime you’re moving a pot of hot wort, and make sure that you have enough ice. A 16 lb. bag of ice will usually work.

Immersion Wort Chillers
Most wort chillers, called immersion chillers consist of a coil of copper tubing.  The coil is placed in the boiling wort for the last 15 minutes of the boil.  The high temperature of the boil ensures that the chiller is sterile. 

After the boil is over, cold water from your garden hose or faucet is run through the copper tube. The cold water absorbs heat from the wort and carries it away. The cooling water does not mix with the wort.

When the wort is cool, it’s poured or siphoned into the fermentor.  Immersion chillers work very well, and are recommended for beginners.  Immersion chillers are cheaper and much easier to keep sterile since they are placed in the boil.

Counterflow Wort Chillers
Another type of wort chiller, called counterflow chillers, cool the beer as it’s drained from the kettle into the fermentor.  Counterflow chillers are theoretically more efficient, but in reality they’re a lot of extra work for no additional benefit.  You only want one if you have a distinct love of gadgets.

Counterflow chillers are only truly practical if your brewpot has a built in drain, otherwise you’ll need a stainless steel racking cane to withstand the heat of the kettle. 

Counterflow chillers are the fastest chillers, but they are not as easy to clean and sterilize as immersion chillers. A counterflow chiller works by running the hot wort through a length of copper tube. The copper tube is surrounded by a larger tube that carries cold water. The wort cools as it flows through the chiller.

It is important to keep a counterflow chiller clean and sterile because the inside of the copper tube can harbor bacteria which can contaminate wort as it runs through it.

Free Advice
Don’t get caught up in the wort-chiller hype.  Lots of companies market “mega efficient wort chillers,” but on the small scale, when you’re chilling 5 or 10 gallons, they make little or no difference.  Remember that there are absolutely no rules governing the claims you can make about how great your chillers work.  Your best bet is a reliable immersion chiller or one of the simpler counterflow chillers, one that doesn’t use any exotic parts or fittings.

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