A solid rolling boil is essential in the brewing of good beer. It is energy intensive and potentially dangerous but a brewer skimps on boil time or intensity at his/her own peril. The boil must be vigorous and rapid, generally not longer than an hour. The intensity of the boil can be judged by the amount of water evaporated, with a figure of at least 10% being considered the minimum required to achieve what needs to done during the boil. The chemistry involved in wort boiling is immensely complex but can be broken down into a number of relatively straight forward mechanisms that contribute to beer quality.
Alpha acid isomerisation
One of the most important roles of the boil brewers carry out is the production of bittering compounds from the isomerisation of hop alpha acids. Isomerisation is a chemical process which involves molecules being converted from one configuration to another. Alpha acids are dubbed iso-alpha acids once isomerised but they contain the same amount of atoms, merely in a different configuration. The isomerisation reaction is favoured by alkaline conditions with a pH of around 9 being optimal, but these conditions are never met during the boil and this explains the notoriously poor level of hop utilisation during the brewing process which rarely exceeds 40%. Wort becomes steadily more acidic during the boil due to the formation of break material so the extraction of bittering compounds becomes less efficient as the boil goes on. Along with specific pH conditions, magnesium or another divalent ion and a vigorous boil are required to carry out the isomerisation reaction. The gravity of the wort can further influence the isomerisation reaction with high gravity worts impeding the progress of the isomerisation step. The loss of precious bittering compounds is bad enough, but the brewer can expect to further lose what little bittering has been achieved through adsorption to yeast and filter material and also some will be scrubbed by CO2 production during fermentation.
We all love bright haze-free beer and this is one of the major reasons that wort boiling must be carried out properly. During the boil molecules called polyphenols stemming from malt and hops bind to the protein in the wort and form complexes that precipitate out of solution and comprise the hot break material. These processes can take up to 2 hours, but boils rarely exceed 90 minutes for reasons of economy. If the boil is not of sufficient duration to allow break formation, polyphenols and protein matter will persist into the beer and cause problems with clarity. After boiling, wort can contain up to 8000 mg/L of break material which is generally removed before the wort is fermented, though some brewers think that the break material provides additional nutrition for yeast and leave a degree of it in. Kettle finings such as carrageen moss is used to aid precipitation by binding the break material forming large flocs which fall from solution more efficiently than the break material would alone. It should be noted that specific doses of kettle finings are required to get maximum sedimentation and brewers carry out trials to determine the optimal dosage.
The crushed malt that brewers mash in with is awash with unwanted and deleterious microorganisms such as bacteria and wild strains of yeast. The boiling of wort provides the very important role of sterilising the wort before fermentation lest the unwanted microbes present in the freshly produced wort wreak havoc with the fermentation process.
The fresh wort drawn from the mash tun is a cocktail of active enzymes that have been hard at work during the mashing process. This is a cause of concern to the brewer who has no doubt carefully selected the grain bill and carried out the mash at a specific temperature with the intention of attaining desired characteristics mainly pertaining to body and residual extract. If mashing enzymes such as the amylases are permitted to continue what they do best in the wort, the residual sugars which the brewer fought to keep in the wort will be degraded and metabolised during fermentation. Beta galactosidase is an enzyme of concern because it breaks down dextrins and makes them accessible to the remaining amylases. This enzyme has been shown to survive mashing and is exploited by distillers - who do not boil their wort - to ensure maximum fermentables are available during fermentation. Distillery fermentations often drop down as low as .997 thanks to the activity of beta galactosidase, so brewers would be best served to stop this enzyme in its tracks before fermentation starts.
The large plumes of steam that pour from the kettle during the boil carry out the important action of removing with it unwanted volatile compounds that would negatively impact on the flavour of the beer should they be permitted to stay in the wort. These compounds stem from the action of heat on hop constituents and also compounds present in the malt. Unpleasant, harshly bitter hop molecules are driven out in the steam along with dimethyl sulphide (DMS) which is present in lightly kilned malt. DMS is not a major factor during ale brewing because the slightly higher kiln temperature during the production of pale malt drives off most of the DMS. Lager malt has higher levels, but a slight DMS character is considered to be part of the style of European lagers so total elimination during the boil rarely occurs. An important practical consideration with respect to the removal of these compounds is to ensure that the kettle is not covered during the boil to prevent the volatile containing steam condensing and flowing back into the wort. Commercial kettles often contain a trap in the flue to prevent this from occurring.
Colour and flavour addition
The intense heat generated during boiling promotes various chemical reactions that contribute to the colour and flavour of the beer. Most of these are reactions involving sugars that are caramelised or complexed with proteins in Maillard reactions to from dark compounds that influence beer colour. The degree of colour and flavour formation is influenced by the manner in which the heat is applied to the wort. Directly fired kettles can create large amounts of these compounds because of the high temperatures at the point of heat application. This scorching often adds specific character to the beer, but can be problematic because the scorched wort material may adhere to the base of the kettle and prevent effective heat transfer. For this reason kettles must be cleaned throughly to prevent build up of debris on heating elements.
Wort boiling is just one of the many complex mechanisms that goes into making our beloved pint. It is the most energy intensive of the processes that brewers undertake and for this reason much effort has gone into optimizing the time required to do it effectively. Thankfully, as home brewers we do not have to worry about such things, but we do have to ensure that our boil is long and vigorous enough the carry out the mechanisms that I outlined above. Most home brewing is well suited to do this, and we can turn our attentions to perfecting the other more precarious aspects of brewing such as mashing and fermentation.