After separating and extracting carbohydrates, proteins, and yeast nutrients from the mash, the clear wort needs to be conditioned by boiling in the kettle. The purpose of boiling the wort is to stabilize it and extract desirable components from the hops.
The main biochemical changes that occur during wort boiling are as follows:
Although wort separation and sparging processes are usually carried out at high temperatures (76-80°C), the wort entering the kettle contains various microorganisms such as yeast, molds, and bacteria, which can cause off-flavors and other issues.
Boiling fixes the carbohydrate composition of the wort by deactivating residual enzymes responsible for carbohydrate and protein degradation that may have survived the mashing or sparging process.
During boiling, it is necessary to reduce the level of high molecular weight nitrogen present in the malt. Protein precipitation occurs as the wort becomes less turbid, causing suspended material to coagulate and precipitate as proteins.
The color of the wort becomes darker during boiling. This is due to the formation of pigments (melanoidins), oxidation of polyphenols, and caramelization of sugars.
Production of Melanoidins
Melanoidins, formed through the Maillard reaction, are produced when reducing sugars from carbohydrates react with amino acids derived from proteins during mashing.
Oxidation of Polyphenols
Oxidation of polyphenols, also known as “tannins,” contributes to color formation. These polyphenols can be derived from malt husks and hops.
Caramelization of Sugars
Caramelization is a chemical process that occurs when sugars are exposed to temperatures of 200°C or higher.
The primary flavor contribution of hops in beer is bitterness from iso-alpha acids. During boiling, the insoluble alpha acids extracted from hops convert into more soluble iso-alpha acids.
Dissipation of Volatile Constituents
Boiling the wort causes volatile compounds, including dimethyl sulfide, aldehydes, and hydrocarbon components of hop oils, to evaporate. The main malt-derived volatile compound lost during boiling is dimethyl sulfide (DMS).
Concentration of Wort
Evaporation is necessary to concentrate the wort since the water used in mashing and sparging results in wort with lower specific gravity than the target gravity.
Reduction in Wort pH
As mentioned earlier, the pH of the wort starts to decrease during mashing and continues to decrease during boiling, reaching a final pH between 5.2 and 5.3.