Brewing Processes - Boiling

Why we boil?

In our last article on wort separation we showed how the sugary wort is separated from spent grain. The next step is to boil the wort by simply heating it up and maintaining a rolling boil, typically for 60 minutes.

The importance of the boil should not be understated. It’s integral in developing hop bitterness and removing off-flavours, removing haze-causing proteins and killing potential microbiological contaminants. Let’s dive deeper into a few of the reasons we boil.

Developing hop bitterness

When hops are added to the boil we get the formation of bittering compounds in the wort. Alpha acids are the compounds found in hops that transform into bitter iso-alpha acids over time when heated in the boil - the process is called isomerisation. The chemical composition of the original compound remains the same but its structure changes. Its physical and chemical properties may also change. This can be seen when insoluble and non-bitter alpha acids are isomerised to soluble and bitter iso-alpha acids.

The isomerisation process in the boil is relatively inefficient. In a typical 60 minute boil, only around 30% of the alpha acids from hops added at the start are isomerised, and much less for later additions. Brewers can use the alpha acid percentage of different hops and their expected isomerisation efficiency to calculate their expected bitterness yield. These calculations help them determine how much hops to add, and at what stage of the boil.


Removing volatile compounds

The heat and agitation of the boil helps to volatilise and remove a number of compounds from the wort. The most important of which is Dimethyl sulfide (DMS).

Ever been unfortunate enough to get a huge whiff of canned corn or tomato sauce in a beer? It was most likely due to the common off-flavour DMS. The compound comes from the precursor s-methyl-methionine (SMM) which originates in malt. As the wort is heated, SMM converts to DMS, which is then rapidly boiled off. It is worth mentioning that a rolling boil (more than a simmer but not violently bubbling out of the pot) is important to agitate the wort and remove DMS.

A side-effect of boiling is the loss of aromatic hop oils. These oils lend a range of aromatics to beer and can be perceived most clearly in the big fruity character of many IPAs. Most hop oil compounds are very volatile and are rapidly lost in the boil. For this reason aromatic hops are added closer to the end of the boil and in the dry hop.

Protein coagulation


When the wort is heated to a boil you will notice the formation of a browny-white coloured foam on the surface of the wort. This is the coagulation of the proteins from malt. The proteins become insoluble and form protein solids in the wort known as ‘hot break’. At the end of the boil it’s important that the coagulated protein is separated from the wort to avoid it ‘coating’ the yeast and affecting yeast performance.

Perhaps even more importantly, the coagulation and removal of hot break proteins is an important control measure for minimising protein haze in the finished beer - particularly for those commercial brewers who produce brilliantly clear beers.

Killing microbiological contaminants

A very important aspect of heating wort to a boil is killing off any microbiological contaminants which may be in the wort. At boil temperatures most contaminating bacteria and wild yeasts are killed. When the wort is cooled we need to be meticulous in ensuring no contaminating organisms can get in. This is done by ensuring all equipment which touches the beer after the boil is clean and sanitised, as well as minimising exposure to the open air.

Deactivation of enzymes

In the article on mashing we discussed the amylase enzymes which break down starches into simpler sugars. Hot temperatures are required to deactivate these enzymes and lock in our degree of fermentability. Heating the wort to a boil rapidly achieves this. If the enzymes were to continue working for longer than expected, the beer may ferment to a higher degree and produce and drier beer.

Wort concentration

Another consequence of the boil worth mentioning is the concentration of the wort as water evaporates during the boil. A typical rate of evaporation is around 10% over a 60 minute boil, although some commercial breweries may have much lower rates. As a result of wort concentration the colour may darken slightly and the concentration of sugars will increase. 


Without a boil your beer will potentially end up smelling of canned corn, hazy, prone to infection, lacking bitterness, or any combination of these issues. The number of physical and chemical changes occurring over the boiling process go a long way in ensuring your beer comes out tasting delicious and as you expect it.