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Battling the Boil

I think that the thing that I enjoy the most about the brewing process is the whole sensory experience. The sweet, malty smell of the grain as it is stirred into the mash. The ever growing number of tropical fruit and pine aromas as you crack open the bag of hops. Even the fruity ester smell of the yeast starter as you prepare it for pitching. It may come as no surprise, then, to hear that I hate boiling wort during brewing. It ruins all of this! Not only that but it also renders my brew untouchable with the naked hand and I have to spend half an hour afterwards cooling it down. So this begs the question:

If I get all of my gravity during the mash, do I need to bother boiling my wort at all?

The answer is, unfortunately, yes. Whilst the boil maybe time consuming, laborious and just plain boring; it actually serves quite a few purposes. Mainly, the simple act of boiling actually removes things that you don’t want in your beer and helps to add things that you most definitely do want in your beer. I’ll explain.


The first thing you may notice as you boil your soon-to-be beer is that a skin forms on the surface and then quickly dissipates. This is known as ‘hot-break’. The heat from the boil causes semi-soluble proteins to stick together. This causes them to become insoluble and drop out of solution. This means that they can then be removed by filtration or syphoning later. The advantage of this is that these semi-soluble proteins won’t then drop out of solution later and make the beer in your glass cloudy (no matter how cool that may be at the time of drinking).

The second, not so obvious, thing that boiling helps to remove is a compound called DMS (dimethyl sulfide). Wort is a mixture of thousands of chemicals, some of them helpful, some of them delicious and some of them not so desirable. DMS is one of the lesser desirable tag-alongs when grain is mashed and is said to taste like corn or cabbage depending on the concentration.

Something that seems counter-intuitive is that DMS is actually made during the boil from its precursor SMM (S-methyl methionine) which is actually fairly innocuous. The heat from the boil degrades the harmless SMM into the undesirable DMS. However, DMS has a much lower boiling point than SMM and, whilst the conversion may take a while, DMS is boiled-off with a half-life of around 4 mins (i.e. quite quickly) as long as the wort is boiling*. Again, there is method in the madness of producing a so-called off-flavour and that is that some yeast are also able to catalyse the conversion of SMM to DMS but when this occurs at low temperatures, the DMS is not removed by boiling. Removing DMS in the boil means your yeast can’t produce it in the fermentor.


The boil is the part of the brew day when hops are added! And with hops comes bitterness! Bitterness in this case comes in the form of alpha acids. However! Nothing in the boil is as simple as adding something you actually want easily! The alpha acids in hops aren’t very water soluble. In fact, they are amongst some of the least water soluble compounds present in beer.

Therefore, we need to boil the crap out of them to actually extract them from the hops in the first place (thing of making a cup of tea: the longer the tea bag is in the hot water, the stronger the tea). Boiling the elusive alpha acids however does something magical (read: chemical) and actually changes the structure of the alpha acids into iso-alpha acids. These compounds are much more soluble and are actually a little more bitter than their un-isomerised counterparts. Two ways in which boiling can help bitterness.


Unfortunately, chemistry is never a completely free ride. As you boil hops to extract bitterness, you also risk losing some of the delicate flavour compounds that are also locked inside everyone’s favourite flower either by destroying them or by evaporating them. Therefore, it is best to do a staggered hop addition in which bittering hops are added at the start of the boil and flavour hops are added at the end.

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