Brewing Boiling Tips – Kettle Reactions

Today, I have some brewing boiling tips to share. I recently brewed with a novice brewer on his kit. He was a nice, guy wanted to learn, but just thought the boil was the boil.

So, we got into some discussions about the science of the boil. He found it useful. So, after that, I thought I’d write and article and share here what I shared with him and more.

Brewing Boiling Tips
Wort Coming Up To The Boil

Knowing the reactions in the kettle during the boil and how it affects beer quality will help a new brewer be a more successful.

You must know that the chemical processes that happen during the boil are known as kettle reactions. One of the first terms I learned in brewing was the maillard reaction, which has been one of my favorite brewing terms since.

So, let’s take a look at which kettle reactions take place during the boil.

Kettle Reaction – The Science of the Boil

So, the common effects of boiling the wort is:

  • Stops all enzymic activity
  • Increases the wort gravity – due to reduction in wort volume due to boil off
  • Sterilization of the wort
  • Beer will become bitter – due to isomerization of alpha acid from hop additions
  • Precipitation and coagulation of protein matter (which we call trub)
  • A drop in wort pH by around 0.2 to 0.3
  • Dissipation of volatiles in the wort (mainly s-methyl methionine)

Brewing Boiling Tips – Stopping Enzyme Activity

Did you know that some fungal and bacterial enzyme activity that’s present in the mash can survive in temperatures of up to 93°C (200°F)? This is the exception rather than the rule but, boiling your wort will make sure that all enzyme activity is deactivated.

Increases Wort Gravity

With any liquid that’s boiled, there will be a reduction in overall volume. It’s the same with boiling wort.

As the volume goes down the viscosity of the liquid goes up as the sugar concentration increases. The length of your boil will directly impact the volume and hence the final gravity too.

Brewing Boiling Tips - Vigorous Boiling

Sterilizing the Wort – Brewing Boiling Tips

When it comes to brewing cleanliness is extremely important. When of my first head brewers told me:

“cleanliness is next t godliness Neilly” …that has stuck with me 25 years later!

When you boil the wort in the kettle it will sterilize it. Boiling of wort is required even just to ensure that the liquid is containment free.

Increased Bitterness

Using hops to add bitterness to your beer is a key element of boiling. This bitterness off-sets the sweetness coming from the malt and other sugar sources.

Whole books have been written on the subject but a simplified version is thus:

Please note, hops contains alpha acid, which are the bittering compound. By boiling hops in the wort these alpha acids are isomerized into iso-alpha acids. Iso-alpha acids are much more soluble in wort providing bitterness.


The amount of bitterness you will get from your hops depends on how much hops you use and when they go in the boil.

If you use hop pellets in a vigorous boil for 60 minutes you will get 28-40% utilization. Although 35% and below is more realistic (closer to 30%). If you use whole leaf hops you can expect 20-25% utilization.

Decreasing Wort pH

The pH of the wort will decrease by about 0.2 to 0.3 during the boil as a result of the calcium and magnesium ions. As well as residual alkalinity, the formation of Maillard products and by the adding of bittering substances.

Precipitation and Coagulation of Proteins and Boiling Off Volatiles

Lastly, boiling the wort to precipitate and coagulate proteins helps produce a clearer more stable beer. Furthermore, boiling wort drives off volatile compounds.

With one of the key undesired volatiles being SMM, the precursor to DMS (Dimethyl Sulfide). If you have DMS in beer it’ll taste of cooked corn and is undesirable at high levels in beer.

Brewing Boiling Tips – Secondary Effects

There are also some secondary effects due to boiling wort which are:

  • The homogenous mixing and sanitation of kettle adjuncts
  • The darkening of wort colour arising from melanoidin production
  • Improved stability in the finished beer
  • Dissolution (as a result of essential oils in the hops) of some hop flavour and aroma compounds.

So, How Can We Improve Beer Quality?

Now we know what happens during the boil; we can look at how to improve beer quality.

Let’s get started…

Ensuring a Vigorous Boil

In general brewers like a good vigorous “rolling” boil. Now the stronger the boil the more wort is evaporated per hour. Brewers look to reduce the wort volume by 3-10% per hour boil.

Brewing Boiling Tips - Brew Kettle

Knowing your boil off rate will allow you to calculate your gravity after boil. Although you will measure it anyway. Let’s take an example:

Gravity at the start of the boil is 15°P

Evaporation rate is 6% per hour

You boiled for 90 minutes

Your calculation would be: 15°P * 0.06 * (90/60) = 1.35°P Increase

Therefor your gravity after boil would be 16.35°P

This calculation is good to know if you’ve added other sugar during the boil and you want to know gravity increase from the boil only.

Please note: When brewing the more data, you collect the better. Why well click this link.

Length of the Boil

The boil period for most regular beers should be a minimum of 60 minutes. Depending on beer style 90 minutes could be appropriate, such for lagers for example.

Now, with today’s modified malts 90 minutes boil might not be needed. However, as this post explains not all modern malts are processed in the same way.

Pilsner malt and certain specialist malt for making lager can still contain more DMS pre-cursors than some ale malts. However, longer boil allows for these volatiles to be dissipated, so you don’t end with corn aroma and flavour in your final beer.

Bottled Beer China

Longer boils over 2 hours or more, are for specialty beers such as Imperial stouts and Scottish Wee Heavy’s. Then there are crazy long boils for new styles like pasty stouts where boils can be in excess of 24 hours.

I even saw someone who made a barley wine by brewing with an all pilsner malt grain bill for 18 hours, the caramelization from the longer boil made the beer a beautiful colour.

Longer boil times do increase hop bitterness. However, after 60 minutes there will not be much significant increase in bitterness. Many brewers wait 10-30 minutes depending on beer style before adding the first hops to allow for some of the hot break to from.

With my lager I like to wait 30 minutes before adding my first hops. As I will boil for 90-minutes they still get a 60-minute boil for full isomerization of the bittering addition.

The Trub – Brewing Boiling Tips

When you boil the wort; proteins that’ve been carried over from the mash and lautering process with coagulate forming larger clumps. This is called the hot break.

In the hot break are proteins and nitrogen constituents which interact with carbohydrates and polyphenols.  

This break will form naturally in the boil, although can be assisted with the addition of Irish moss, which is a form of red seaweed.

Early in the boil some of the proteins will rise to the surface and can be skimmed off at this time. Some brewers believe that “scum” contains some of the harsher character of the wort and need to be taken out.

Most modern brewer don’t “skim the scum”. Even if you do skim the protein, most of the hot break will remain in suspension in the kettle.

Stop the Trub

It’s better for your beer if you don’t let the hot break carryover into your fermentation vessel (FV). Modern brewing vessels are designed to make sure the carryover doesn’t happen.

The separation is either done in by whirlpooling or via a hop back with other straining methods also available depending on the system you’re brewing on.

When using hop pellets most modern breweries use a whirlpool. A whirlpool creates a strong circular motion is your vessel (after flameout). This strong motion forces the hop pellet material and hot break to the centre of the vessel forming a cone.

This now “clear” wort is taken from the side of the vessel away from the cone. It is also good to have a damn next to the whirlpool outlet to help ensure trub or hop material isn’t carried to your FV.

Bear Pond Brewing Wuxi
Bear Pond Brewing Wuxi, China

If you’re a home brewer, you can from your own whirlpool using a spoon or paddle and then drawing off the clear wort to your FV leaving the hops and break behind.

Also note that hop back were originally used when brewing with whole leaf hops. They can work with pellets too, as along as a bed of whole leaf hops is used to line the hop back (to act a filter).

Although some modern brewing kit designs no-longer need the whole leaf bed.

This hop bed in the hop back filters out the hops and hot break leaving it behind, and allowing clear wort to go to the FV. The added benefit of the hop back is an increased hop aroma and flavour.

Whirlpool Hop Additions

You can get the same benefit using a whirlpool, with whirlpool hops. After flameout you can chill the wort down to 80-82°C and then add the hops.

This will allow for more of hops oils to stay in the beer, rather than be boiled off for greater aroma.

Other brewers go lower in temp, 71-76°C, whilst some recommend 75-80°C. What’s the right answer for temperature of whirlpool hops?

It’s brewers preference, but this post by Cloudwater, one of the best hop forward brewery’s in the UK has lot of good information to work off (they use 80°C by the way).

Beer Artsakh - Svaneke Brewery

Here is a good paper on hop additions and tracking IBU’s that covers many things discussed above that is worth checking out too.

Maillard Reaction

The Maillard reaction that I spoke of earlier, is a term to describe the chemical changes that happens when sugars are heated.

The change happens when amino acids (liberated in the mashing process) and the reducing sugars combine under heat to produce melanoidins.

Melanoidins add colour and body to the beer. This reaction also happens in the malting of some grains as well as in the brew kettle.

For some beer styles, such as Bock beers, this melanoidins production can be very favourable. Toffee, nutty, biscuit and malty are some flavours associated with melanoidins.

Brewing Boiling Tips – Conclusions

So’ lets break this down shall we. Our brewing boiling tips are (although some styles are different):

  • Have a vigorous boil to ensure volatiles are driven off
  • Make sure your boil for 60 minutes or more to avoid DMS in your final beer
  • Ensure the trub and hot break are left behind and not carried to the FV.
  • After flameout, cool the wort to 80C for your whirlpool hops for hop forward beers

There you have it. You might have been following these brewing protocols anyway. However, I hope this article might have filled in a few blanks as to why you’re doing so.

Also, some of the links of this article make for great further reading to increase your brewing knowledge base.

If you have any questions or have feedback please feel free to email me at:

Or you can simply leave a comment below. If you feel this might be useful to a friend or colleague then please feel free to share this article too.

Thanks for stopping by and reading my article. You can download a PDF version for offline viewing by clicking here (will want to save as).

Have a good day and happy brewing!



Brewing Boiling Tips Graphics

Neil Playfoot

Neil is a brewer with 25 years international brewing experience. Based out of China (first came in 2010) he works as a brewing consultant helping brewers with their projects and brewing processes. To find out what services Neil can provide your brewery please click here. If you'd like to contact Neil you can email at
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Good insight to boiling wort.?