- Sterilising the wort
- Coagulating proteins out of the wort (the "hot break")
- Converting hop acids to bittering compounds
- Extracting flavour and aroma compounds from the hops
- Creating flavour compounds (e.g. caramel, melanoidins) from complex sugars from the grain
Typically, the wort is boiled for 60 minutes: this is long enough to perform all the above functions. Longer boils (e.g. 90 minutes or even 120 minutes) will result in more caramel and melanoidin flavours, and will of course evaporate more water thus concentrating the wort.
Sulpher Compounds and a Rolling Boil
S-methyl methione (SMM) is present in very pale malts such as pilsener malts. During the boil, SMM is converted to Dimethyl Sulphide (DMS). If the boil is covered or not properly ventilated, the DMS can accumulate in the wort. DMS is detectable at quite low concentrations and is associated with generally undesirable cooked cabbage or corn-like vegetal flavours.
Other sulpher-containing chemicals with similar cabbage-like flavours are produced by browning reactions during the boil, becoming problematic if the boil is too vigorous. (This is surprising to many brewers who have been taught to value a vigorous "rolling boil", both to maximise protein break and drive off DMS). In "Principles of Brewing Science" (2nd ed, 1999), George Fix recommends a maximum evaporation rate of 12%/hour and suggests that evaporation rates as low as 2%/hour may be sufficient for most purposes, although he recommends a minimum 7%/hour as a general rule.