Thursday, January 8, 2009

The Boil

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 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 form 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.

Colloidal stability

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 carageenan 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.

Enzyme inactivation

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.

Volatile Removal

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 complex 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 problem 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.


David Curran said...

Great article Tom.

How much of this do you think is "onions in the varnish" manufacturing processes that had a purpose at the time but now are no longer needed? For example with mead old books have you boiling stuff for hours whereas some new books say not to bother boiling at all.

Brewing is such an old craft that vestigial practices have to have been picked up over the centuries.

Thom said...

That's a good point, but much of the info I have gathered comes from modern brewing texts written by brewers who have all worked in large commerical breweries where accountants have a large say in what goes on. If the length and cost of boiling could be reduced without effecting quality it would be done.

The boiling practise I described is for home brewers and micro brewers along with larger regional brewers. The really big guys can use fancy things like Merlin systems and post fermentation bittering with iso alpha extract which reduces boil time.

The Merlin system is amazing. It involves cascading the wort over a metal symbol shaped device that is heated to several hundred degrees centigrade. The wort briefly contacts the metal is rapidly boiled, alpha acids are isomerised and break material forms without the need to bring a vast volume of wort to the boil. So if you have the money, bringing a big ol' pot of wort to the boil is indeed an archaic means of producing beer.

Boak said...

This is very useful stuff. I've ranted before on how different home brewing guides tell you that a long boil is necessary for x, or the exact temperature is essential for y, but they never seem to back it up with any actual science.

I've often wondered why "an hour" is the default time for a boil. Or, for that matter, when decoction mashing, all the homebrewing books always talk about bringing the decocted mash to the boil and boiling it for 15 minutes. Why 15, and not 10 or 30?

All of that said, I do see brewing as a bit like making a nice curry. I'm sure one can go into lots of detail about the precise spice measurements, the exact time you caramelise the onions for, but when all's said and done, you add a lot of nice stuff together and get something tasty...

Thom said...


The one hour boil seems to be a compromise between the length of time to do what needs to be done and the cost of carrying out the boil. Ideally the boil could do with being longer for colloidal stability but it would be costly and a great deal of water would be evaporated. As for the hops, wort is not a suitable medium at all. In better conditions a far shorter boil could be carried out.

As for decoction; I have no experience of it but I do know that it is/was carried out on under modified malts with high protein content and the various heating steps increased extract from otherwise poor malt. The amount of time required for boiling would have been empirically established for specific malt types I imagine, depending on the extent of modification.

Bionic Laura said...

Great article.