We want to cool wort after boiling as fast as possible in order to pitch our yeast, get fermentation started, and thereby avoid potential bacteria infections. The quicker we can chill wort, the better! Chill haze can be reduced by the addition of hydrated Irish moss in the final 15 minutes of the boil, but quick cooling of the wort also permanently precipitates cold-break proteins out of solution. Slow cooling over a period of hours will not do the trick!
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If you're a typical home brewer using extracts and ending up with about 2 gallons of hot wort, super cold water is your best solution. Do the math. If you pour 2 gallons of boiling wort into 3 gallons of 70 degree (summer) tap water, you’ll be waiting many hours for the wort to cool below 80 degrees, even with your fermenter in a sink full of ice water. Take three empty milk/juice gallon jugs, fill each with 1/3 cup baking soda and hot water, let stand for a few hours, then rinse and sanitize. Fill with cold water and recap, place in a freezer for 2 hours before you’ll finish your boil. The handles will likely be solid and a bit of ice crust will be forming inside the jugs. Pour this super cold water into your fermenter, then pour in the hot wort, and your wait time to get under 80 degrees will be dramatically reduced!
Avoid using commercial ice to chill your beer as it often contains bacteria that could spoil your beer. Even household freezers may contain bacteria that can end up in your ice. Yes, the packaging for that steak or salmon you brought home from the grocery store was likely not sanitized!
If you're instead boiling 4-6 gallons (for a 5 gallon batch), you won’t have the luxury of adding cold water to cool your wort. Many people have tried the “snow bank” wort cooling method in the winter, it doesn’t work (Don’t Eskimos live in igloos?) You will need some sort of heat exchanger to cool your wort. The following are some of the varieties of wort chillers available:
How to Cool Wort with an Immersion Chiller
Immersion wort chillers are the most popular and inexpensive variety of homebrewing wort chillers. They are a coil of copper or stainless steel tubing with hoses attached to both ends. These chillers are placed into the boil pot for the last 15 minutes(to kill any possible bacteria). After the heat is turned off and cold water is run through the chiller to remove heat from the wort. Recirculation or gentle stirring is needed to maximize heat exchange from the tubing. Depending on the time of year, anywhere from 15-30 minutes is required.
How to Cool Wort with a Counterflow Chiller
Counterflow wort chillers are more complex in design(and cost) than immersion wort chillers. Their inner copper tube is surrounded by another larger diameter copper tube. Hot wort is pumped through the inner tube, while cold water is pumped in the opposite direction through the outer tube. Wort flow is restricted so you have a higher ratio of cold water to wort, allowing you to output the chilled wort directly into the fermenter. The inner tubing needs to be flushed/sanitized between uses.
How to Cool Wort with a Plate Chiller
Plate wort chillers are also more complex in design (and cost) than immersion wort chillers. Similar to counterflow chillers, plate chillers have an inner path for wort to flow through with a water jacket around it. Wort flow again is restricted through the plate chiller(usually with an attached ball valve) so the ratio of cold water to wort is high, resulting in fully cooled wort being outputted to the fermenter. Plate chillers are also susceptible to clogging if free hops, orange peel, hot/cold break proteins or other ingredients are not filtered prior to entering the chiller, and like counterflow chillers, need to be cleaned and sanitized between uses.