Theoretical question here.

What happens when wort is boiled? Why do we do it? Why is the process not done at cooler temperatures?

My limited understanding, with some assistance from this post and this post, is:

  • Pasteurization
  • Maillard reactions, Melanoidin formation, possibly some caramelization
  • Isomerization of hop acids

But I'm not confident that's the whole story. I know why #1 is important, I can guess at why #2 is important, and I assume #3 is important because all homebrewing instructions tell you to do it.

The genesis of my question is the discovery of this little beastie: http://www.williamswarn.com/

(I have no intention of buying one. It's just an item of curiosity.)

The kits they sell include LME (presumably pre-hopped) and DME, with dry yeast. The instructional video on the home page show the procedure as putting hot-but-not-boiling water in the machine and then adding the extract and the yeast right on top of it before closing the lid. No boiling of the malts at all. That generates the question: why is wort boiled?

One would presume that these extract kits are pasteurized, removing that part of the boil equation. Digging further in the web site reveals recommendations for doing partial mash; steeping specialty grains and hops (in separate pots) before adding to the machine. There's even a reference to all-grain brewing, assumedly a mash conducted before adding wort to the machine, which led me to the conclusion that the WilliamsWarn is not a brewing machine per se, but rather an automated fermenter with integrated carbonation and serving. But I digress.

So what is the difference in procedure between a normal homebrew process and this machine which allows them to get away without a boil? What purpose does the boil serve us?

Hard facts, assumptions, and wild speculation welcome.

  • I too am interested in this question. I think boiling does obviously pasteurise VERY effectively but it can be argued that pasteurisation could be carried out at a lower temp that 100degreesC. For example milk is pasteurised at around 70 degrees. Maillard reactions and Melanoidin formation do happen one supposes but not to a great extent. If they did how would we make very pale lagers by boiling the wort? personally I think the majority of the colour comes from the roasted grains. Heating to 100 degrees is the main process for isomerisation of the hop bittering compounds but to be precise ther Jul 12, 2016 at 19:46
  • Converting SMM to DME then driving off DME, hope isomerisation and sanitizing the wort are the main reasons to boil. There are other side effects such as converting peptides to foam positive compounds such as LTP, hot break etc.. as mentioned above by tallie.
    – Mr_road
    Jul 12, 2016 at 22:41
  • There is no need to boil wort to isomerise the hope oils. One only has to boil the hops in water. I accept boiling will very effectively pasteurise the wort. But is boiling necessary to pasteurise - could it be done by the time spent at 66 degreesC in the mash tun? I imagine it might, but who knows. As for driving of DMS that could be done at 66 degrees as DMS is a volatile compound. For example DMS is produced during fermentation of lager and is driven off by simply allowing the temperature to rise to 20 degrees C. So it would seem there is still an argument for not boiling the wort... Jul 13, 2016 at 8:10
  • DMS exists largely as the precursor SMM (S-Methylmethionine) which degrades to DMS upon heating. Said DMS is indeed very volatile, but SMM only degrades to DMS appreciably above ~80°C and has a half-life at 100°C of about 40 minutes. Probably a good reason (at least in many situations) to boil. Jul 14, 2016 at 14:44
  • Does anyone know what SMM does to the taste? DMS is malodorous in any concentration but would be boiled off in mashing/mashing out. If SMM only decomposes appreciable above 80 degrees C. then not getting that hot should stop it decomposing. to any great degree - and what does decompose to DMS doesn't matter because it will be driven off at 66 degC anyway. Hence the question - what does SMM do for the taste (in small amounts). Jul 15, 2016 at 23:04

1 Answer 1


You've covered the main reasons that I could think of off the top of my head, ie, pasteurisation, flavour formation and isomerisation of hops. A quick search reminded me of the other two, which is that it drives off unwanted volatile compounds (eg, SMM - the precursor to DMS) and it causes proteins and polyphenols/tannins to bind and precipitate out in the form of hot break. Whilst not being a reason per se, but more of an added benefit is the fact that it also concentrates the wort, thereby gaining efficiency that is lost to sparging.

All of those additional benefits would also occur during the production of pre-hopped extracts. Those pre-hopped extract cans are pretty much the standard entry point in the Australian homebrewing scene, and presumably New Zealand too, which is where that bit of kit is made.

Edit: There's a fairly comprehensive article on the reasons for boiling on www.byo.com.

  • Phenomenal article. Thanks for the link!
    – Zac B
    Jun 17, 2012 at 17:21
  • The article does answer my questions. Thanks! Jun 18, 2012 at 18:42

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