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, 169–176°F), wort entering the kettle contains numerous microorganisms—yeast, molds, and bacteria—which can result in off-flavors and numerous other problems. Thus, it is important to eliminate these microorganisms by boiling the wort above 100 degrees C (212°F) for at least 45 minutes.
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. If left unchecked, these enzymes will continue to alter the carbohydrate composition in the wort.
Wort contains high levels of nitrogen in the form of proteins and polypeptides. If the protein-polyphenol complexes are allowed to persist it could affect the pH, colloidal stability (chill haze and permanent haze), fining and clarifying properties, fermentation, and taste of beer. Loss of proteinaceous material takes place throughout the brewing process with a significant amount being removed during boiling. Protein precipitation occurs during boiling, and material breaks out of suspension and precipitates as protein complexes.
Color and Flavor Development
Several other events take place during boiling. The most noticeable of these is the formation of color. Color increases during boiling due to three reasons: (1) the formation of pigments (melanoidins), (2) the oxidation of polyphenols, and (3) the caramelization of sugars.
Production of Melanoidins
Melanoidins are brown-colored compounds that are formed through the Maillard reaction of sugars and amino acids at high temperatures and low water activity. Melanoidin production is most active in the malting process, but it continues to some extent during mashing and to a somewhat lesser extent in boiling.
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. High-carbonate water can also hasten color formation by increasing the extraction of polyphenols from husks and hops during mashing and sparging and by increasing caramelization.
Caramelization of Sugars
Caramelization is a chemical process that affects sugars subjected to temperatures of 200 degrees C (392°F) or greater.
Hops are added to the wort at various points during boiling to provide flavor, aroma, and antimicrobial attributes to beer. 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 acids. One factor that greatly influences isomerization is a long, vigorous boil. On average, only about one-third of the alpha-acid added is recovered in the boiled wort in the form of iso-compounds (Knuze, 1996).
Dissipation of Volatile Compounds
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 (DMS) which is rapidly lost through evaporation. However, the breakdown of S-methyl methionine, its precursor, can continue during the period between the end of boiling and wort cooling and can survive in the packaged beer. DMS gives beer a strong “sweet corn” or “lagery” flavor.
All kettles have a condensate return in the throat of the stack to prevent condensed volatiles returning to the boiling wort. The condensate return should typically be about 0.3 meters (2ft) above the hip of the kettle or about 0.3 meters (2ft) above the first section of the straight wall before any 90-degree bends.
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. The amount of water removed during the boil is directly proportional to the rate of evaporation. Evaporation is usually measured as a percentage of the total volume at the start of boil. Evaporation rates vary between 4 and 10 percent and are a function of vessel design, vessel material, energy system, and air pressure. In many small brew pub systems, a 5 percent evaporation rate is common (Ryder et al., 2006).
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 (Leiper et al., 2006). The drop in pH is due to the reaction of calcium (Ca2+) compounds with phosphates and polypeptides to form insoluble compounds that release hydrogen ions.
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