Boiling the wort is an integral part of the brewing process. All recipes call for a strong, rolling boil, for 60-90 minutes, depending on requirements.
But what are these requirements? Most home brewers know that the boil is used to extract hop characters. There are, however, many other reasons for giving your brew a good, long boil.
Sanitizing the Beer
This is the most important reason for boiling. The pre-boil wort can contain a myriad of different microflora, capable of souring a beer very quickly. Some beer recipes call for the addition of certain yeasts or bacteria to impart sour or “funky” characteristics, but this is done in controlled circumstances using laboratory-prepared ingredients. Regardless, the brew must be boiled for at least an hour.
Removing the “Hot Break” from the Beer
Excess proteins result in a hazy beer. Boiling the brew will denature and coagulate these proteins, which precipitate to the bottom of the boiler. This is known as “trub” and includes other solids such as hop particles. These will be left behind when the brew is transferred to the fermentor.
Imparting Hop Bitterness and Aroma
Boiling hops allows bitterness, flavor, and aroma characteristics to infuse the beer. The isomerization of bittering compounds requires at least an hour of rolling boil. More details of using hops can be found in the article Using Hops in Home Brewing.
Reducing the Beer Volume
The alcohol volume in beer is, for the most part, determined by the proportion of available fermentables in the wort prior to fermentation. This proportion, or Original Gravity (OG), represents the density of these fermentables in the liquid. The greater the OG, the greater the potential for a stronger beer. The reduction of fluid volume by boiling will increase the OG of the beer.
During the boil, malt sticking to the base or sides of the boiler may be scorched. This may or may not be desired, and can be avoided by continuously stirring the wort during the boil, ensuring no malt sticks to the inside surface of the boiler. If it is desired, however, caramelization will convert some of the malt into non-fermentable components, imparting greater sweetness and body to the beer, at the cost of some alcohol.
Darkening the Wort
The caramelisation process will also result in a darker beer. Other chemical processes take place during the boil that darken the beer as well. These include the production of melanoidin compounds and oxidised polyphenols.
The mashing process involves grain enzymes converting starches into sugars. Boiling will halt this process, thereby stabilising the wort. As enzymes are complex proteins, these will denature and precipitate in the hot break along with other proteins.
Reducing the pH of the Brew
The hot break is greatly assisted by a pH level of 5.0 – 5.5. Boiling the wort will bring the pH down, but the addition of acid or calcium carbonate will assist further. Most home brewers, however, will not consider this a necessary step, and simply use the boil only.
Adding other Flavors
Other than hops, some recipes call for the addition of herbs or spices to the brew. While many can be added directly to the fermenter, the boiling process is necessary for hardier spices such as ginger root, cinnamon, cardamom and others.