Clarified sweet wort is delivered to the copper (kettle) where it is subjected to a heat treatment. In beers subject to the Reinheitsgebot laws, only hops are added to an all-malt sweet wort boiling. In other countries additional sources of fermentable sugar in the form of liquid sugar syrup adjuncts may be added with the sweet wort.
The copper boil serves many functions. It sterilises the wort and inactivates all malt enzymes before it is cooled and added to the fermenter. Hop alpha acids are isomerised during the copper boil. This aspect of copper management is dependent on the type of hops used. Varieties which require extensive heat treatment for isomerisation are added early on, whereas those which are added for aromatic hop oil content are added towards the end. Inevitably some compromise is necessary between the need for isomerisation and the potential for loss of aromatic components. In addition, prolonged boiling can result in conversion of alpha acids to humulinic acids, which lack bitterness. Removal of most of the essential oil component of hops in the steam exhaust from the copper is essential for balanced flavour in the final beer. Apart from these hop components other volatiles derived from malts, many of which impart undesirable ‘vegetable’ odours, are also lost. For example, S-methylmethionine derived from malt decomposes to dimethyl sulphide during mashing and in the copper boil. The latter is an essential contributor to lager character .
Wort boiling assists with clarification and removes substances which may cause problems in downstage processes. Thus, polyphenols from malt and hops react with some proteins to form insoluble precipitates. In addition, some of the other proteins coagulate. The resultant mixture is termed ‘hot break’ or ‘trub’. Most of this material is separated from the wort during transfer from copper to fermenter. Oxalate, which can form beer haze, is precipitated as the insoluble calcium salt. The high temperatures in the copper promote many other chemical reactions. Maillard reactions between reducing sugars and amines form melanoidins, which contribute to beer colour. These melanoidins may also displace aldehydes from sulphite adducts in packaged beers, thereby contributing to staling. The boiling process provides an opportunity to concentrate the wort. This allows correction for dilution in the mashing stage due to sparging operations.
Several copper designs are in use . The name suggests the metal originally used for their construction; however, modern vessels are invariably made from stainless steel. Early versions were simply open vessels with a flue for escape of steam and heat provided by an underbuilt fire. Modern coppers are usually heated by steam, either with internal heat exchangers or by circulating the wort through an external loop fitted with a tube and shell heat exchanger known as a calandria. Such systems use convective forces to drive the boiling wort around the loop.
After the boil is completed, solids in the form of ‘trub’ and any hop material have to be separated from the hot wort before it is cooled and delivered to the fermenting vessel. Various types of plant may be used to accomplish this. Where whole hops are used plant is required to separate the spent cones. Traditionally this is achieved in ale breweries using a hop-back, which is similar in design to a mash tun, in that it has a false-slotted base and sparge arms. Like the mash tun the principle of operation is that the spent hops form a bed through which the hot wort passes and in so doing filters out trub. Sparging provides a means of recovering some of the wort entrained in the hop bed. Traditional lager brewers employ a similar device called a hop-jack or montejus. This is a closed tank fitted with a mechanical agitator. Hot wort is fed in at the top of the vessel and is discharged from the base. Solid materials are retained by an internal mesh, which forms a cage inside the vessel.
Where pelleted hops or hop extracts are used there is insufficient material to make hop-backs or hop-jacks practicable. In this case, which applies to the majority of modern breweries, a whirlpool is used to clarify worts. Several designs are used which are claimed to offer various advantages but all use the same basic principle of operation. The whirlpool consists of a cylindrical insulated vessel into which the hot wort is pumped via a tangentially mounted entry main. This induces a hydrocylone effect in the vessel so that as the liquid circulates, the solid material drops out and forms a compact mound in the centre of the base. The clarified wort can then be run off leaving the solid matter in the whirlpool for subsequent removal.