The boil process may seem simple but there are many things occurring that can affect the quality of the finished beer. Some of the things that happen during the boil are listed below.
Stabilize the Wort.
Boiling the wort kills most wild bacteria that are present in the water, air, and malt extract. A 20-minute boil is sufficient for killing most bacteria but is not sufficient to stabilize the wort. Once the boil is finished the wort must be cooled to pitch temperature as soon as possible. Care must be taken during cooling and transfer not to aerate or contaminate the wort. Most ‘wort spoiler’ bacteria and yeast thrive between 120ºF and 80ºF. When the wort is cooled quickly from boiling to pitch temperature many of the suspended proteins will clump together and fall out of solution, this is known as a Cold Break. Once cooled this provides a stable environment for the selected yeast to thrive while minimizing the potential for wild bacteria or yeast growth.
If you are brewing an all grain batch, boiling the wort stops the enzyme process that started during the mash. If allowed to continue, this process would further break down the long sugar chains, known as dextrins, into fermentable sugars resulting in a beer that gets thinner and higher in alcohol as it ages.
During the boil water molecules uncouple from large proteins and tannins that are present in the wort. This is known as ‘denaturing.’ These molecules become insoluble and cloud the wort. They must be agitated in order for flocculation to occur. This can be accomplished through a good rolling boil. This clumping action results in these particles falling out of solution. This is known as the Hot Break. A good hot break is essential to removing proteins, tannins, and other compounds that may affect the flavor of the beer or show up as Chill Haze, cloudiness when the beer is chilled.
Extraction of Hop Compounds
Hops contain aromatic hop oils and hop resins. Hop resins are only slightly soluble in room temperature water and require heat, time, and agitation to extract them. This process, known as isomerization, changes the hop resins into a soluble form that can be absorbed into the wort. These resins contain both alpha and beta acids. The alpha acid is the primary component responsible for bittering. The alpha acids present in a hop variety are listed as percentage by weight but can be greatly reduced due poor storage. The percentage of these resins that remain in the finished beer is known as Hop Utilization. As gravity of the wort increases hop utilization is reduced. Aromatic hop oils are very volatile and will outgas off with minimal heat. Aroma hops should be added late in the boil to retain as much of these hop oils as possible.
Removal of Unwanted Volatiles
Standard brewers malt contains compounds that are converted to dimethyl sulfide (DMS) when heated. Large amounts of DMS are driven off with the steam of a vigorous boil so pots should never be covered during the boil.
When heated, sugars and amino acids can combine resulting in the formation of melanoindins. These substances are formed during the malting process and give the malts complex flavors and dark colors. These same compounds are formed during the boil process. Time and specific gravity are the two things that have the greatest effect on caramelization. A longer boil will darken the wort and a high gravity wort will darken quicker than a lower gravity wort.
Gravity Adjustment, Evaporation, Shrinkage, and System Losses
A low gravity mash runoff may be boiled down to a higher gravity wort at the tradeoff of reduced volume of finished product. To hit a batch size/specific gravity target, the evaporation rates during the boil (~5%/hr), the shrinkage during cooling (~4%), and system losses must all be taken into account.