The purpose of wort boiling is to stabilize the wort and extract the desirable components from the hops. The principal biochemical changes that occur during wort boiling are as follows:
Although the wort separation and sparging processes are normally conducted at elevated temperatures (76-80°C), wort entering the kettle contains numerous microorganisms-yeast, molds, and bacteria-which can result in off-flavors and numerous other problems.
Boiling fixes the carbohydrate composition of the wort by inactivating residual enzymes that are responsible for carbohydrate and protein degradation and that may have survived mash-off or sparging.
During the boiling process, it is necessary to decrease the level of high molecular weight nitrogen found in the malt. Protein precipitation occurs as the wort loses its turbidity during boiling, and material breaks out of suspension and precipitates as proteins coagulate.
During boiling, the color of the wort becomes darker. This is brought about by the formation of pigments (melanoidins), the oxidation of polyphenols, and the caramelization of sugars.
Production of Melanoidins
The production of melanoidins-the Maillard reaction-occurs when reducing sugars from carbohydrates react with amino acids that are derived from the proteins during mashing.
Oxidation of Polyphenols
The oxidation of polyphenols is another source of color formation. Polyphenols are sometimes referred to as "tannins" and may be derived from malt husks and hops.
Carmelization of Sugars
Caramelization is a chemical process that affects sugars subjected to temperatures of 200°C or greater.
The major flavor contribution of hops in beer is bitterness from iso-alpha acids. During the boil, the insoluble alpha acid extracted from hops is converted to a more soluble iso-alpha acid.
Dissipation of Volatile Constituents
Boiling of wort drives off volatile wort compounds, including dimethyl sulfide, aldehydes, and hydrocarbon components of the hop oils. The principal malt derived volatile lost during boiling is dimethyl sulfide or DMS which is rapidly lost through evaporation.
Concentration of Wort
The wort must be concentrated by evaporation since the water used in mashing and sparging has produced wort lower in specific gravity than the target gravity.
Reduction in Wort pH
As previously mentioned, the pH of the wort starts to decrease during mashing and continues to fall during wort boiling to reach a final pH of between 5.2 and 5.3 (25).
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