Boiling wort is one of the most crucial and complex aspects of the brewing process. In the days before people developed the ability to fashion metal vessels, wort was often run into wooden or stoneware vessels and heated to the boiling point by the addition of glowing-hot stones. Today there are very few types of beer where the wort is not boiled. Among the better-known types is the Finnish traditional beer called Sahti, for which the wort is merely heated; see sahti. Though they knew nothing of microbes, ancient brewers soon learned that beer made from boiled wort lasted longer and was healthier to drink.

The boiling of wort serves various functions:

• inactivation of residual enzymes from the mash

During an effective boil, all of these goals can be accomplished, usually in under 90 minutes. But achieving an effective boil is not as simple as it sounds. With the exception of the conversion of dimethyl sulphide (DMS) precursor (S-methyl methionine) to free DMS (a compound with an often undesirable aroma reminiscent of cooked corn; See dimethyl sulfide (DMS)) a reaction fundamentally impacted by time and the duration of the boil, the other chemical and physical events in boiling are generally quantified empirically by measuring evaporation rate. Water is usually boiled off at a rate of about 4% per hour.

Vigorous mixing is mandatory for the effective coagulation of proteins to form flocks, although shear forces need to be minimized if such flocks are not to be subsequently disrupted, making the unwanted protein difficult to remove. Polypeptides need to be denatured during boiling so as to manifest their optimum foam-stabilizing properties, although this simultaneously increases the extent to which they are lost owing to their elevated hydrophobicity.

When hops are boiled in wort, the bittering alpha acids are isomerized; it is only then that they become soluble and can be extracted into the wort. See alpha acids. At 212°F (100°C) in wort of pH 5.2, approximately 1% of the total alpha acid is isomerized every minute. Wort boiling, therefore, takes time.

The end result of boiling is impacted greatly by the vessel configuration and the heating regime, as well as by the composition of the sweet wort and of any other materials introduced to the vessel (sugars and syrups; clarification aids; and, of course, hops and hop products).

The rate of transfer of heat from condensing steam to wort is greatly impacted by the area of the heating surface and the temperature difference between the steam and the wort (ΔT). If unwanted cooking reactions are to be avoided, it is best to maintain a low ΔT. Establishing the appropriate surface area is important and influenced by any fouling and by refreshment of the boundary layer through wort flow. Pumping the wort will facilitate heat transfer but then hot break integrity is jeopardized.

Boiling is carried out in kettles, sometimes referred to as “coppers” because of the metal from which they were originally fabricated. These days they are usually made from stainless steel, but sometimes contain some “sacrificial copper” to bind sulfur compounds such as hydrogen sulfide. But the modern brewer must be careful; over-heating will “burn-on” and caramelize sugars, and lead to the formation of undesirable “cooked” flavors, darkening of the wort, and possibly to carbonyl substances that contribute to aged or stale character in beer. For centuries most wort boiling was done in large direct-fired pots with wood, coal, or gas flames roaring beneath them. Today, direct firing is largely avoided for the reasons above.

There are many shapes for kettles. The essential criterion is the extent to which a “rolling boil” can be achieved. It is only when the boil is vigorous that there will be efficient steam volatilization of off flavors, effective precipitation of proteins and polyphenols, etc.

The kettles used by most small breweries today employ low-pressure steam jackets within the walls of the vessel. Most larger modern kettles feature an internal (or external) bundle of heater tubes (calandria) through which the wort passes upwards before striking a spreader plate that returns the wort to the surface. In the spraying, volatiles escape. An example of such a system is the Stromboli, produced by Krones of Germany.

In dynamic low-pressure boiling systems that allow for a rapid evaporation of volatiles while reducing overall water evaporation, the pressure is raised and lowered between pressures of 1.0 bar and 1.2 bar (corresponding to temperatures of 100°C–102°C [12°F–215°F] and 104–105°C [220–221°F]); this occurs six times per hour. Every pressure release stage leads to an instant boiling of the kettle contents, with attendant sweeping away of volatiles.

The Merlin system, also produced by Krones, is claimed to allow quality beers from worts despite evaporation rates as low as 4%. It comprises a vessel containing a conical heating bottom for boiling and evaporating wort. The whirlpool, below the Merlin device, serves as a collection vessel for wort. A pump circulates wort between the vessels. Wort is lautered into the whirlpool, at the end of which the wort is pumped as a thin film over the conical heating surface of the Merlin. The thin film, high flow rate, and turbulent flow ensure that heat transfer is very good with a very low temperature differential between steam and wort, as well as very good volatile stripping. Over a 40- to 60-minute period the wort passes four to six times over the heating surface, with just 1.5%–2.5% evaporation and low thermal damage of the wort. The whirlpool is insulated, so breakdown of DMS precursor occurs therein. The contents of the whirlpool are in rotation throughout boiling, with good separation of hot break, so the “rest” can be as short as 10 minutes. Wort is pumped once more over the heating surface en route to the cooler, in which “stripping” phase the free DMS formed in the whirlpool is driven off virtually completely.

There is a physical limitation to the surface area that can be achieved with internal devices, which shifts the focus to heating systems outside of the kettle. One such example is Symphony™ from the Briggs company, with its very large heater surface area that is some four- to five-fold greater than the norm. This large area allows a much lower ΔT with the attendant benefits in terms of lessened “cooking” and also the facilitation of more brews between cleans. The substantial surface area and low ΔT make for huge quantities of small vapor bubbles, maximizing the liquid/vapor interface area and promoting trub formation and volatile stripping. See trub. Lower evaporation rates are possible with savings in energy costs.

External systems can be used over a wide range of brew lengths; they overcome the disadvantage of internal heaters in that the latter cannot be used until submerged below wort level.

The PDX Wort Heater uses direct steam at speeds of 3,000 feet per second to break wort into mist droplets with a huge surface area, thereby allowing for the efficient stripping of volatiles.

The Ecostripper comprises a sequence of wort kettle, whirlpool, wort stripping column, and cooler. In the stripping step, the unwanted volatile compounds are purged by clean steam injection. The stripping column is filled with packing to ensure a large surface area and the wort flows downwards counter to steam. The claims are that this saves energy through reduced boiling; heat recovery; less color development; and less heat abuse.

See also wort.