Why Do We Boil Wort? The Processes Taking Place

I’ve already written some articles on boiling but, just realized I’d not written an article on “why do we boil wort”? So, let’s rectify this now; starting with the boiling off of volatiles.

Boiling off Volatiles

There are volatiles in raw material like barley which need to be boiled off. If they aren’t, you can end up with off-flavors in your final beer. With the main one being DMS (Dimethyl Sulphide).

Dimethyl Sulphide – If you’ve ever smelt corn aroma in a beer then you’ve come across DMS. In light beer and lager, it’s detectable at low levels by the average drinker. The precursor to DMS is SMM (S-methyl-methionine), which is produced during germination of barley.

Brewery Heating Options - Corn DMS
DMS present as corn on the nose

When you heat SMM in the mash, it breaks down into DMS. However, with a vigorous boil it drives a most of it away. When it comes to driving off DMS, boil like you mean it (see evaporation below).

Boiling also drives off other unwanted malt and hop volatiles, which can negatively affect the final beer as well. Furthermore, the additions late hops (near the end of boil or after flameout), keep the aroma hydrocarbon components of hop oils in the wort (so not boiled off). Allowing for improved hop notes on the nose, in the finished beer.


When you lauter your wort, sending it to the brew kettle. You add more water to make sure you extract as much of the sugars from the mash as you can.

Please note though, never over sparge, keep final running above 2.5 Plato (1.010 SG), to be on the safe side. Otherwise, you might extract tannins leading to unpleasant bitterness in your beer.

You need to concentrate the wort. We can do this by boiling off water in the brew kettle. Typical boil off rates are 5 to 10% an hour, with most brewers pitching for around 8% an hour.

Brewing Boiling Tips
Look for around 8% an hour boil off rate

Why Do We Boil Wort? Isomerization of Hops and More

When worst is boiled, the alpha and beta acid in the hops (hop resins and bittering substances) are solubilized in the liquid.

Boiling wort changes the molecular arrangement of the α-acids (give the bitterness) to “iso-alpha acids”. These now, iso-alpha acids are much more soluble so, more easily impart bitterness to the wort.

As mentioned in the “boiling off volatiles” section, hop oils can potentially contribute harsh bitterness (especially if oxidized) are driven off in the boil. Furthermore, polyphenols are dissolved, helping with break formation as well (more on this later too).

Coagulation and Precipitation (Break Formation)

During the boil the coagulation and precipitation of proteins is vital, if you want to produce clean, stable and consistent beer. Therefore, controlling the hot break formation is key.

Observing the hot break is a good gauge of how effective the boil has been in reducing the number of high molecular proteins. If you see “big fluffy bits” break out of your wort at the start of your boil you know your own the right track.

These high molecular proteins can negatively affect colloidal and foam stability in your final beer. During the boil the proteins are denatured through heat, pH change, oxidation/reduction (of sulfhydryl groups) and hydrogen bonding with polyphenols.

The proteins change from hydrophilic (attracted and dissolved by water) to hydrophobic (not attracted) plus take on a positive charge. They then combine with the negative charged polyphenols aided by some carbohydrates which help with their aggregation and precipitation.

TDLR: The proteins drop out of the wort, form part of the trub and not carried to the FV.

Why Do We Boil Wort - Precipitation of Proteins

Why Do We Boil Wort? Sterilizing Wort and Enzymic Destruction

When you mash and lauter, the temperature usually doesn’t get above 78°C (for mash out). This temperature isn’t high enough to “inactivate” some of the microbes in the grains, hops and adjuncts used in brewing.

These microbes are though “dealt with” when you boil wort. Most of the bacteria in the wort will be inactive after the first 15 minute of the boil. When you add hops to the wort it lowers pH which also aids the sterilization process.

Mallard Reaction and Colour Pickup

Earlier in the piece we linked to an outside site about the Maillard reaction for further reading. However, let’s take a short look at here too…

Maillard reaction is a non-enzymic browning which adds colour and flavour to beer during the boil. It’s the chemical reaction between the carbonyls of reactive sugar and the amino groups of amino acid.

There’s a lot of science to explain the reaction. However, what need to know is the final production of the Maillard reaction is melanoidins (brown nitrogenous polymers). When you boil wort, it can develop deeper colours as seen in the picture below.

Why Do We Boil Wort - The Maillard Reaction
Wort gaining color through Maillard reaction during a 12 hours boil taken from Reddit/TheBrewery

Furthermore, in decoction mashing where you may “boil” some of the mash, Maillard reaction occurs there too. There are some brewers who claim this process gives a greater depth of malt flavour.

Why Do We Boil Wort? pH Drop When Boiling

There’s a natural drop in pH when you boil the wort. Calcium continues to precipitate, as it does in the mash. If enough calcium is present in the wort, it’ll continue to drop in pH.  Ideally for most beer styles you want a pH of 5.0 to 5.2 after the boil.

The reasons are:

  • Improved character extraction from the hop additions
  • Improves hot break
  • Lowers overall colour pickup

Hop isomerization is also influenced by pH as well. The higher the pH the better the conversion to iso-alpha acid. If the wort was at a pH of between 8 and 10, you can get up to 90% conversion.

What Are Hops?
Cross Section of a Hop Leaf

However, wort in the kettle is usually at 5.2 to 5.4 so, the conversion is less, with usual upper limit around 35%. Even with a lower conversion rate, it’s best to have lower pH for the reasons above, plus character and bitterness are “coarser” at higher pH levels

Also, the hot break formation is better at lower pH with 5.2 being the optimal figure. You’ll know if you’re in range at the beginning of the boil, if you see “big fluffy bits” breakout in your wort.

Additions in the Boil

When your boil wort in the brew kettle, other ingredients can be added too. In the boil when wort is hot, it’s easier to add, mix, dissolve and a sterilize these ingredients.

What Can You Add to the Boil?

Well, the list is long and we can’t go through them all here. However, there’s one main one we need to talk about first Irish Moss.

Irish Moss

Irish moss is made from a blend of processed seaweeds. When added to the boil it helps your finished beer be “brite”. The three main sources which make your beer cloudy are:

  • Tannins
  • Excess Proteins
  • Yeast

Now for Irish moss, we’ll concentrate on tannins and excess proteins which are related to the mash and boil.

Why Do We Boil Wort - Irish Moss and Pellets Hops
Irish Moss and Pellet Hops

Tannins naturally occur in barley husks which are released during the mashing/lautering process. Proteins are also found in grains, particularly so in wheat, oats and flaked barley.

Well Irish moss helps remove these when added in the last 15 minutes of a boil. In summary it speeds up the coagulation of proteins at the end of the boil, which we discussed earlier and prevents chill haze.

Other Kettle Additions

There are more kettle additions one can add to the boil, which helps brewers:


Some brewers like to add antifoam to their boil to guard against boilovers, making brewing safer. The addition of these specially designed products doesn’t affect head retention or beer foam in the final product.

Yeast Nutrients

There are many different versions of yeast nutrient, some are added during boil and some after flameout. They can be used to help high gravity beers ferment or aid fermentation of other products, like hard seltzer.

Coolship Yeast – Allagash Brewery

An old brewer’s tip is: If you run out of yeast nutrient, use some yeast from a fermentation vessel added at towards then end of the boil. It’ll help add nutrients for the yeast pitched later in FV.

Other Clarifiers

There are other products when add to the boil, help improve colloidal stability (covered earlier). These can be particularly useful in heavy hopped beers. There’s even PVPP products, you can add to counter chill haze as well.

Lactose and other sugars

Lactose is the processed sugar from milk. With normal beer yeasts it can’t be fermented however, with other yeasts like Brettanomyces claussenii fermentation is possible. It’s used by brewers in beers like sweet stouts, to have more residual sugars in the final beer.

Other sugars you can be add to the boil include corn syrup. Which some brewers use to increase original gravity and overall %ABV (alcohol by volume) of the end product.

Especially handy if you’ve limited mash and/or lauter tun space, and need a big abv beer. Adding the sugars in the boil, makes it easier to mix with the wort as well.

Other Flavours and Aromatics

There are many other ingredients as a brewer, you can add to the boil. Usually, they’re added towards the end of the boil or after flameout. Here are just a few:

Orange peel – Used in some Belgium wit beers

Heather and ferns – Used in Scottish brewing before hops were discovered, some newer breweries are reviving the use of these ingredients.

Coffee – Most brewers add coffee later in the process, but I’ve heard of some brewers using coffee in the boil.

Tea – A lot of teas can be added to the boil. However, be careful of tannins, you might get unpleasant astringency in your beer from the tea if you’re not careful!

Spices – The other ingredient often used in Belgian Wit is coriander seeds. They give a spicy note and complement the orange peel. Many different spices can be used such as clove, which is often use in “Christmas” beers.

Honestly, there are so many flavour and aroma additions you can add to the boil. Just do your due diligence, making sure whatever you add, is safe for consumption.

Why Do We Boil Wort - Coriander Seeds Used in Brewing Belgium Wit
Coriander Seeds Used in Brewing Belgium Wit

Conclusions to Why Do We Boil Wort

So, there you have it, there’s lots of layers to “Why Do We Boil Wort”? I hope this article was informative plus given you a greater understanding of the processes happening during the boil.

As you can see many important reactions and physical changes happening. From making the beer more “shelf stable”, concentrating the wort to adding bitterness and aroma through hop additions.

The best way to improve your beer over time is, to record as much data before, during and after the boil. These can include:

  • Preboil pH & gravity
  • Post boil pH & gravity
  • Hot break can be measured in a graduated glass cylinder, even keeping a photograph on file
  • Evaporation rate – volume before and after boil
  • If you’ve control of the steam then percentage open of the steam valve during boil, can be recorded as well.

You keep these files, recording information from brew to brew. Overtime you’ll have some great data to work from. They’ll show patterns, or make it easier to understand where a brew has gone wrong, thus allowing you to address and rectify any issues more easily.

Who Am I?

Asian Brewing Consultant - Neil Playfoot Mashing In

Hello, my name is Neil, I’m a British brewer based in China. I first came to China in 2010. I’ve been lucky enough to have brew on a number of systems, in various parts of the world in my 25-year brewing career.

I now assist people with their brewing projects, helping people in a number of ways from recipe formulation to sourcing brewing equipment.

Need Help With Your Brewing Project?

If you’ve an upcoming brewery project and would like some help, or simply want to talk with someone, then please feel free to email me


However, if you’d like to reach out me to me on your preferred social network instead, then please scan the QR code below to send me a connection request:

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I’m currently advising on projects from the UK to New Zealand. I’m happy to answer some initial questions you might have and assist in any way I can.

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Thanks for taking the time to read my article today, I hope you found it useful. Have a great day and happy brewing.



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 neil@asianbeernetwork.com.
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