6 Essential Things You Need to Know before the Boil


Sometimes, the boil can come as a relief.

It’s a time when you can kick back, crack a home brew and maybe, just maybe get some cleaning done.

After all, the boil isn’t as complicated as the mash. Hell, all you have to do is crank up the heat and let’er get on with things … right?

Not so fast!

Here are 6 essential things you need to know before the boil.

What is the hot break?

Throughout the boil, proteins increasingly bind with polyphenols (tannins) to form little clumps of nastiness. This is referred to as the hot break.

Ever had eggdrop soup? Yeah … the hot break looks like that.

However, the hot break is good news, and should be encouraged when you’re brewing a Kölsch. See, proteins can cause a hazy beer. By allowing them to clump together, they can be easily removed at the end of the boil.

One simple method of removing the hot break is by using a whirlpool. Here’s an easy example of how whirpooling works:

Take a bowl of water and toss in some sand. By stirring the water with a spoon and creating a whirlpool, the sand will come together, and rest in the centre of the boil.

The same thing happens when you whirpool the wort.

And by collecting all the hot break in the centre, it becomes possible to drain the wort off the side, leaving the hot break behind. The easiest method of whirpooling is by the use of a pump, some tubing and a bit of copper.

Check out this page if you want to learn more about building a whirlpool immersion chiller.

What contributes bitterness?

Bittering compounds are created throughout the boil by a process called isomerisation.

Through isomerisation, alpha-acids are isomerised (read: rearranged) into iso-alpha-acids. Unlike alpha-acids, the iso-alpha-acids are water soluble and dissolve into the wort, leaving a bitter taste in the beer.

Therefore, iso-alpha-acids – not alpha-acids – contribute bitterness.

And because alpha-acids are isomerised throughout the boil, the longer you boil the hops, the more bitterness you get. This is why early hop additions are often referred to as bittering additions.

Cool, eh?

Where does hop aroma come from?

Hop aroma is created by hop oils. 

Hop oils make up about .5 – 1.2% of the hops, and are incredibly difficult to classify without a gas chromotographic examination taking place (Kunze 2003). I’m guessing you don’t have access to the facilities needed to carry out a big, scary examination … But hey! It’s food for thought.

Plus, when it comes to hop aroma, your senses are key:

Rub some between your palms, take a sniff, mediate on them … whatever. But use your senses!

And last but not least, hop oils are highly volatile and evaporate during the boil. So, if you want hop aroma to break through into your beer, make sure you add some near the end of the boil.

Brewers often refer to this addition as the aroma addition.

Why does the wort change color?

Ever baked a cookie, or grilled a steak? Ok, I’ll forgive you for never baking a cookie (I leave this to my girlfriend), but you must have grilled a steak!

Anyways, when you grill a steak, the outside darkens as it cooks. This is due to something called the Maillard reaction. The Maillard reaction also occurs throughout the boil.

Take a look at this:

image (3)

As you can see, unboiled wort has a lighter color than the boiled wort. And as the boil progresses, the beer gets darker and darker. But why?

This happens when amino acids bind with sugars to form reddish-brown compounds called the melanoidins. Binding occurs at any temperature over about 90c, so melanoidin production increases throughout the boil (Kunze 2003).

And as melanoidin content increases, the wort becomes darker.

So take a look at the color of your wort before you begin your next boil, and compare it to its color at the end. You might be surprised at how much it changes.

Why does specific gravity change during the boil?

This is one of those phenomenons that, despite its simplicity, often stumps beginner brewers. Check this out:

image (6)

From this you can see that the percentage of extract changes throughout the boil.

Here’s how I came to understand why this happens.

During the boil, some of the wort’s water content evaporates. However, unlike the water, the sugar content remains the same throughout the boil.

Sugars don’t evaporate during the boil.

So by boiling off some of the water, the post-boil wort (also known as the cast) contains a higher percentage of extract (sugars) than it once did. And because the cast now has a higher percentage of extract, the SG is higher. Simples!

Why does my beer taste like sweet corn?

In short, dimethyl sulphide (DMS). Luckily, you can control its production.

Throughout the boil, the DMS Precursor, S-Methyl Methionine (SMM) converts into DMS through a process called thermal splitting.

Take a look at this graph:

image (5)

As you’ll see, the level of DMS swiftly decreases within the first 30 minutes of the boil, and continues to reduce until you turn off the burner. Great news for us Kölsch brewers, because we hate the stuff!

By ensuring that the wort boils with the lid off for at least 60 minutes, the risk of DMS making it through to the bottle is greatly reduced.

Curiously, the level of DMS pops back up after the boil ends. This is due to small amount of SMM left in the wort, which, so long as the temperature of the wort remains over 85c, will continue to convert into DMS.

In fact, if you leave your wort to sit at a temperature of above 85c for a period of 60 minutes, the DMS levels can increase by a whopping 30% (Beersmith.com)! Ouch!

This is why it’s so important to chill the wort to below 85c as quickly as possible.

Over to you

Sure, this list is by no means complete. But, it does summarise some of the key things that I wish I had known before I first boiled some wort.

How about you? What do you wish you had known before you first turned up the heat?

See you in the comments!


Beersmith.com,. ‘Dimethyl Sulfides (DMS) In Home Brewed Beer’. N.p., 2012. Web. 3 July 2015.

Kunze, Wolfgang, and Trevor Wainwright. Technology Brewing and Malting. 3rd Rev., International ed. Berlin: VLB, 2004.

Home brew beer fanatic and lover of all things Kölsch. Follow me on Twitter!

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  1. kg says

    Curious if you think that leaving the break material (both hot and cold) in the wort during fermentation makes a difference. It all seems to settle with the yeast into a nice compact layer (depending on flocculation of the yeast, cold crashing, etc) and can be avoided when transferring to bottling bucket or secondary.
    I know there are wildly varying opinions on whether to leave the “trub” when transferring to primary fermentation. I think brulosopher looked at it too.
    I always do this kind of “trub/no trub” experiment. I make 10 gallon batches and create a nice whirlpool. Then I carefully siphon 5 gallons to one fermenter and just dump the other 5 gallons (trub and all) to the other. I’ve tried but cannot detect a difference in the finished product.
    Anyhow nice blog. Cheers!

    • says

      Hi kg

      I always try and leave behind as much break as possible. But, if I can’t, I don’t stress out over it.

      Most brewing books that I have read stress the importance of break removal for clarity. I’m not convinced that it makes a HUGE difference; however, because Kölsch is such a delicate beer I am usually over cautious.

      Glad you like the site!


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