Once malt sugars (either from extracts or malts) have been collected, it is brought to a boil. Boiling will sterilize the wort, coagulate proteins, drive off off-flavors, and allow hopping. There are several factors when it comes to boiling, including: watching out for boil-over, accounting for boiloff, time of the boil, boil kettle size, and cooling after the boil.
As the wort heats towards boiling, it will begin to foam. The foaming is a coagulation of malt proteins, and is called “hot break”. Be careful here, and try to prevent the foam from boiling over. This can be done by either reducing the heat as foam builds, or keeping a spray bottle of cold water on hand to break about the foam. If the foam is allowed to boil-over, a big, sticky mess will be on your hands. Once the foam settles back into the wort, boiling has begun.
Boil vigor and boiloff
Keep the boil at a constant, steady rate. If you boil too hard, you may end up with less volume than you expected, due to excessive boil-off. Depending on your heat source, ambient temperature, and strength of the boil, boil-off can range between ~1/2 gallon per hour up to 1 ½ gallons per hour. Take good notes, with regards to volume before and after the boil, and you should get a good idea how much you boil off, and account for that loss on your next batch.
Always boil with the lid off your kettle. A chemical called DMS (dimethyl sulfide) is driven off during the boil. If the DMS is allowed to condense on the inside lid of your kettle, and dropped back into the wort, a strong, cooked-corn flavor can be formed in your final beer. Thus, you should always boil with the lid off your kettle.
Timing the boil
Most beers will be boiled for 1 hour, enough for all hops additions needed. Longer boils are sometimes performed to enhance malt complexity. The longer the wort is boiled for, the more maillard reactions (browning reactions, similar to cooking) will occur, darkening the wort and adding increased malt perception. This is commonly done for barley wines, which can be boiled for as long as 120 minutes.
Make sure to keep a timer during the boil, counting down from the beginning of the boil. Hops (and other) additions will refer to this boil timer. Typical recipes will call for an additive and a time. The time listed is the amount of time the ingredient is boiled for (e.g. a 1 ounce Cascade addition at 15 minutes is boiled for the final 15 minutes of the boil).
Kettles and Burners
Boil kettles are mostly a function of batch size. Because foaming can be extreme in some cases, it is best give yourself plenty of headspace in a kettle. If using a smaller-sized kettle, it is best to perform a partial-boil in which a smaller boil is performed, and the volume is topped up to the batch size in the fermenter. This is commonly done with extract and partial-mash brewers, but cannot be performed by all-grain brewers. For a 5-gallon extract batch, the smallest kettle recommended is 5 gallons. With this sized kettle, you can perform a 3 to 3 ½ gallon boil, and top up to ~5 ¼ gallons in the fermenter. Additionally, this sized boil can easily be performed on a standard kitchen stove.
For a 5 gallon, full-volume boil batch, a 7 ½ gallon kettle is about the smallest allowable. Brewer's who do a full-volume boil batch usually prefer to use a larger, 10 gallon kettle, to allow plenty of room for foaming. However, most kitchen stoves will not be able to boil the full volume, thus a propane burner is commonly used, moving the brewday outside. These burners provide plenty of heat output, and can vastly speed up a brew day, as you will not have to wait as long for water or wort to heat to temperature.
The material the kettle is made from is also important. Stainless steel is the most commonly used, as it is very easy to clean and maintain over the long term. Aluminum kettles can also work, but the oxidative layer (the dull sheen) must be maintained, otherwise aluminum ions will find themselves in the beer and cause a metallic bite to the finished beer. Similarly, enameled kettles can also be used, but must be constantly checked for cracks or other weaknesses in the enamel, otherwise off flavors will develop in the finished beer.
Some kettles may advertise a tri-ply bottom. Here, the bottom of the kettle is composed of a "sandwich" of metals; stainless steel is on the outside, and a layer of aluminum composes the internal layer. While this will slow down the rate of heating of the kettle, it also allows the kettle to heat much more evenly, preventing scorching of malts. For kettles without a tri-ply bottom, stirring the wort while it heats to boiling will help prevent malt scorching.
Cooling the wort
After the boil is completed, the wort must be cooled down to fermentation temperature (~70°F). The wort is then aerated, transferred into the fermentation vessel, and yeast is added. There are several methods to do this.
The first method is to use an ice-bath. Here, the kettle is placed in a sink filled with cold water and ice. The wort is gently stirred to allow even contact with the cold edges of the kettle. This method will typically cool 5 gallons of wort from boiling to fermentation temperatures in around 30-45 minutes. This is most commonly done with 5 gallon kettles, as these can be more easily moved into the sink.
For larger kettles that can’t be moved as easily when filled with hot wort, an immersion wort chiller can be used. The chiller is place in the boil at 15 minutes to sanitize the coils. After the boil, cold water is pushed through the coils. The cold water absorbs the heat of the wort, and moves it out of the kettle. For this reason, it is best to have constant action of hot wort moving across the cold coils. This can be done either by moving the chiller through the wort (use a cloth to protect your hand from the hot side of the coil), or stirring the wort with a spoon. This method will typically cool the wort in ~10-15 minutes, depending on how cold the water going into the chiller is.
Counter-flow chillers and plate chillers both involve either pumping or gravity-flowing wort through the chiller while cold water is pushed the opposite way through the chiller. These chillers are extremely quick, but can be more difficult to clean and maintain. These chillers are much more efficient for larger (15+ gallon batches).
While the wort chills, cloudy looking pieces will form, making the wort look like egg-drop soup. The pieces are coagulated proteins, and is called cold break. Letting this form will aid in beer clarity. Adding irish moss or whirlfloc during the boil will promote the production of cold break.
Aerating the wort
After cooling, the wort must be aerated. Yeast require dissolved oxygen to help build their cell walls and bud; this is the first stage of fermentation. Higher gravity batches have more focus on a clean, quick fermentation, thus more dissolved oxygen is needed. Aeration has completed when a thick, frothy layer has built up on top of the wort. Aeration is performed in one of several ways. Keep in mind, because this process is done after the wort has cooled, any items that touch the wort must be sanitized thoroughly.
The simplest method is to simply stir the wort vigorously with a spoon. This is good for low gravity batches, but it can result in insufficient oxygen for moderate to high gravity batches. A more thorough way, without using extra equipment, is to toss the wort between two vessels. The vessels can include the boil kettle and a bucket fermenter, or two extra bucket fermenters.
An option that does not require manual labor involves an aeration or oxygenation system. Aeration systems use an air pump and air stone to push ambient air into the wort. This method requires extra time, usually 30 minutes to 1 hour, in order to properly aerate wort.
Oxygenation systems use a disposable oxygen tank, regulator, and air stone to dissolve pure oxygen into the wort. This method is much quicker, typically taking 30 or so seconds. However, there is a risk of over-oxygenating, and oxidizing the batch, leading to diminishing flavors over the lifespan of the beer.