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Posted by
Mike Benson
on 30/07/20

Mike is the sales manager for Wales and the West of England and is located in Wigan.

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Wort boiling is all about chemistry, physics and putting the keys back where they belong.

Whilst it isn’t the most engaging part of the brewing process. You get the temperature right, you add hops, you add kettle finings. That’s what you do. Clearly your input is limited – whereas the role of science definitely isn’t.

In the early days of my career, I was working in a lovely new brewhouse with a mash conversion vessel, lauter tun and an elegant stainless steel kettle.  Having spent some time following around the head brewer and learning the ropes, I was at last left in charge of the mashing and wort boiling. I was confident in my ability to follow processes, the kit was in pristine condition and it all felt pretty straight forward.

Until the pump to the external boiler refused to start up. It was a complete mystery: I couldn’t understand it. I called the engineers. This was serious: three of them arrived on the scene, each keen to solve the mystery as to why something had gone wrong with such pukka equipment. They conferred. They checked the valves. They conferred again.

All eyes turned to the new brewer. What the devil had I done to their new piece of kit? I began to lose confidence. What had I done to it?  I talked them through my paces. I retraced my steps.

And then I put my hand in my pocket. There was the key to the interlock system. Duh.

Wort boiling is a fundamental part of the brewing process, a stage unique to beer. This post looks at it from a chemistry and physics perspective.

What happens during wort boiling?


Once you get to wort boiling, it’s time to dig out the tun, grab a cuppa and take it easy for a moment or two. Meanwhile, inside the kettle, it’s a whole different ‘boil’ game (sorry). No rest for the chemistry. It is working to:-

  1. Halt enzyme activity
  2. Sterilise the wort
  3. Concentrate the wort by evaporation
  4. Reduce the pH and precipitate unwanted compounds
  5. Extract bitterness, flavour and aroma from the hops
  6. Remove unwanted volatiles
  7. Develop flavour and colour

Before digging deeper into what happens, let’s first heat the liquid.


Heat Transfer

There is a little bit of science to wort boiling.  The rate of heat transfer by the wort heater (Q) depends on three things:

  1. The heater’s surface area in contact with the wort (A)
  2. The temperature difference between the heating medium and the wort (𝚫T)
  3. The resistance created by the heating surface wall, including any fouling (U)

These factors can be expressed in the following formula:       Q=UAΔT

There are three methods of getting heat into the wort.

  1. Forced convection is a very efficient way to transfer heat.  Wort is pumped at high velocity and turbulent flow over the heating surface.  It picks up heat quickly, but the high velocity and turbulence also cause shear forces that can be detrimental to wort quality.  You often see this kind of set up on stainless steel kettles with external wort boilers. The more sophisticated systems have a way of bypassing the pump to allow a thermosyphon, where the wort flows by thermal and flow dynamics without any pumping needed.
  2. Nucleate boiling is ideal for wort. Materials with a ‘wetted surface’, such as copper, promote bubble formation on the heating surface. The bubbles then migrate quickly, taking the heat with them.  As they disperse, they are replaced with fresh liquid that heats and moves away, causing a thermosyphon.  It’s important the bubbles circulate in order to avoid film boiling.
  3. Film boiling is promoted by materials with a ‘non-wetted’ surface, such as stainless steel. When the differential temperature between the heating surface and the wort is high, the bubbles don’t move away.  Instead, they cling to the wall creating a vapour film that acts as an insulator.  This increases the wall temperature further and causes a poor boil and increased fouling.


As the wort is boiled, fouling on the heating surface builds up, reducing heat transfer.  Poor heat transfer leads to a poor boil, so we need to keep on top of it.  There are ways that fouling can be reduced, but this is all dependent on your process. The best thing you can do is regular cleaning. When cleaning kettles, detergent strengths are high, and you can add a detergent booster to give the caustic a kick.  The type of kettle and your heating method dictate how often you need to clean. Your chemical supplier should be able to provide you with good advice to help you get the right products and strengths for your particular kit.

In the brewery

Wort can be heated using different techniques which can be categorised under heating methods (heat source) and heat transfer surfaces.  We’ll start with the different methods. The table below shows the most common heat sources used in breweries.   You can see that each has a different effect on the wort.



Direct Heating




Hot Oil Generator


Brewery SizeUp to 15 brl


10 to 50 brl (16hl -82hl)20 brl up


20 brl up


Methods of heat transferElement in kettleHot flue gasses through jacket or coilThrough jacket, internal or external calandriaJacket, coil, internal or external calandria
Capital CostLowMediumHighMedium – High
Differential TemperatureHighHighLowLow
Colour Pick upHigherHigherLowerLower
Speed of heatingSlowHighMediumMedium
Use in other parts of the brewery?NoNoYes in all partsYes (but not steam sterilisation

The heat source is no use without having a way to transfer the heat.

Electric elements:

Electric elements give the desired conditions for wort boiling in a way that electrodes added to the kettle just wouldn’t.  The major advantage of electric elements is the low cost to buy and install. The major expense is installing the correct electricity supply – but of course, that is needed for all installations.

On the downside, the elements are expensive to run and have a high differential temperature (𝚫T) causing higher colour pick up and increased fouling. You can minimise this by using a power controller.  It’s tempting to spread out the arms of the elements thinking it will reduce local overheating, but in fact, it will have the opposite effect as the convection currents are disrupted. I learnt that one by experience!

Internal Coils

Stainless steel coils are wound around the inside of the kettle.  Steam, hot oil or hot flue gasses from a burner, usually attached to the side of the kettle, are forced through the coil.  While efficient, it’s difficult to control and high differential temperatures (𝚫T) cause caramelisation and burning onto the coil – which is difficult to remove.  Coils are often seen on HLT’s. While the high differential temperature doesn’t have any adverse effects on the liquor, the coils can quickly become scaled, making heat transfer slow.


Jackets on the inside of the vessel can be heated by steam, hot oil or hot flue gases.  They are thermally inefficient and difficult to control, causing caramelisation and burning.  Often an agitator is needed to create any vigour – and they are very hard to keep clean. Believe me.


 Put simply, calandrias are a shell and tube heat exchanger located inside or outside the vessel.

Internal Calandrias

Until the 1960’s, calandrias were always internal and the tubes made from copper.  They are still commonplace and are often the preferred heating method, although modern examples are stainless steel and have a pump fitted.  While thermally efficient, older designs require the calandria to be filed before heating can begin, slowing down the process time.  Wort circulation relies on thermal currents so turbulence over the heating surface can be low, causing caramelisation.  Pumps on modern calandrias overcome these issues.

External Calandrias

The majority of calandrias are now fitted externally. Location outside the vessel means they are generally longer, which increases the heating surface area and reduces fouling.  Wort is pumped from the bottom of the vessel through the external calandria and fed back into the vessel through a spreader plate or at a tangent so that the need for a whirlpool can be removed.  Wort can be heated once the kettle is around 15% full.  The mechanical mixing from the pump adds vigour but also shear forces. Some have a ‘pump bypass’, allowing mixing by thermosyphon which reduces the shear force.

We compare the different heat transfer methods below. Of course, unless you’re replacing the brewing equipment, you can’t choose the methods you use.  What you can control is how often you clean! Take a look at the bottom of the table for some guidance.

Electric ElementInternal JacketInternal CoilInternal CalandriaExternal Calandria
Brewery SizeUp to 15 brl


10 to 50 brl (16hl -82hl)20 brl up


50brl up


13 brl up


Methods of heatElectricityDirect fire, steam or HOGDirect fire, steam or HOGSteam or HOGSteam or HOG
Colour pick upHigherHigherHigherMediumLower
Speed of heatingSlowSlowHighSlow (due to volume needed)High
Cycles before CIP2-32-55-88 – 108-16



Wort Boiling Objectives

Wort Boiling in process | Crisp Malt

So that’s covered the effects that the heating medium and methods have on wort boiling. So let’s go through the crucial issue of Safety, then look at what happens to the wort.

Sweet wort is collected from the mash tun or lauter tun, and we begin to slowly heat it with the aim of reaching boiling just as the kettle is full.  It is boiled for between 1 and 2 hours.  The effectiveness of the boil is measured by the evaporation rate: as the volume reduces, the gravity increases. We add hops throughout the boil for bitterness, flavour and aroma. Around 39% of the total brewery energy is used for wort boiling.


Wort Boiling Safety


Anything hot can cause burns, but hot sticky liquids are particularly dangerous. You know that this isn’t an area to take short cuts or risks: it’s crucial to take the right precautions.

  1. Before sampling or making additions to the kettle, make sure you turn off the heat source and any agitation, such as pumps.  Modern installs may have an interlock system where this is done automatically (as mentioned in the intro!). If you are installing new equipment, it’s worth the expense.
  2. Add any additions slowly. It’s possible for heat pockets to be disturbed, with the additions acting as nucleation sites and causing over-foaming.
  3. Use hops or anti-foam early in the boil to avoid over-foaming
  4. Ensure there are written Standard Operating Procedures (SOP’s) and that staff are all trained
  5. Use PPE, make sure it fits and is in good condition
  6. .Make sure you follow any pressure regulations.
  7. If you use a ladder and are working at height, take the correct precautions. Always.

Why boil the wort?

Wort Boiling process | Crisp Malt

Enzyme Deactivation and Sterilisation

When using a mash conversion vessel, the mash is heated to between 76oC and 78oC deactivating any natural enzymes from the malt.  When using a mash tun, the enzymes are still active until the sparge raises the temperature of the mash above 76oC.  If enzymes such as fungal beta-glucanase or amylo-glucosidase (AMG) are added to the mash, these will be active throughout the runoff, but boiling will deactivate them and fix the sugar spectrum of the wort.

If you are making a zero attenuation beer (PG 1001 or lower) such as a brut IPA or low carb beer, further AMG will need to be added to the fermenter to fully convert all the sugar.  Boiling will also kill any bacteria, spores, yeast and fungi, allowing for a clean fermentation (providing the wort is kept clean).


If we held wort between 98oC and 100oC without boiling or agitation, it would remain cloudy. That is one of the reasons to boil.

While the temperature of the wort can be measured, it’s harder to measure and control the vigour, so we use the evaporation rate. As the wort is boiled, it becomes concentrated as the volume decreases. This increases the wort gravity.  To measure the evaporation rate, we can use the volume drop or the gravity increase.

Evaporation Rate % = ((post-boil volume or gravity/pre-boil volume or gravity)-1)*100

Testing the wort gravity can be slow, as it needs to be cooled down to 20oC. Hydrometers can be a little inaccurate, so watch out for that.  The same can be said for sight glasses, but a good way of measuring the volume is to use a dipstick.  Use this to take a measurement from the top of the vessel (the same point must be used), take a dip at the start of the boil and one at the end. You will see that as the volume drops, the measurement increases.  To turn this into volume, you need to calibrate the kettle. This can be done by:

  • Calculation: Use the vessel diameter to help calculate the volume each mm dip gives.  Any dish at the bottom must be taken into account.
  • Flowmeter: Measure every 10mm drop and make a table
  • Measurement: Fill the kettle with water and decant it into a container of known volume, take dips as you go and make a table.

Larger and more complex kettles may have mass flow meters. The volume of steam required to boil the wort is calculated, and the control system adds the correct amount of steam to achieve the desired evaporation rate over the required time.

The time of the boil is also very important, 6% evaporation in 1 hour has significantly more vigour than 6% evaporation over 2 hours.

Trub Formation and pH Reduction

During boiling, protein and polyphenol complexes form and precipitate out of the wort. This is crucial for beer haze stability. It’s collectively known as trub and comes in 2 forms, hot break and cold break.

Hot Break

Polyphenols from malt and hops are in oxidised form and complex with protein to form hot break.  It also contains insoluble salts, hop resin material and a significant proportion of lipids from sweet wort and hops.  If these lipids pass over into the wort, they can cause head retention issues.  They predominantly come from the last runnings, so it’s important to make sure you don’t run the PG below 1005.  Trub formation is helped by:

  • The vigour of the boil
  • The rate of energy added to the system
  • The duration of the process
  • Essentially the evaporation rate!

pH also plays a key part in trub formation. The optimum pH for trub formation is 5.2, but the pH of the wort at the start of the boil can be between 5.8 and 5.9.  The pH drops by around 0.2 units during the boil due to:

  • Melanoidin formation
  • Hop acids
  • Precipitation of phosphates and polyphenols by calcium
  • Release of H+ ions

To reduce the pH further, you can add phosphoric or lactic acid or calcium salts such as calcium chloride or calcium sulphate.  Remember, Sulphate and Chloride remain in the wort and will add to the total levels in the beer.  You will need to keep the ratios correct for the beer style you are producing.

Trub Removal

Methods of hot break removal depend on the type of hops you use:

  • If using whole hops, hop backs are used. The leaf hops form a filter bed to separate the sediment
  • If using pellets or extract, whirlpools are used. The wort enters tangentially, creating centrifugal forces.

Trub particles are relatively large. This allows them to be easily filtered in the hop back or quickly sedimented in the whirlpool.  We have mentioned shear forces several times, but this is where it becomes an issue.  The shear forces can break up the particle size making them smaller.  This reduces the effectiveness of the hop back or whirlpool, causing cloudy worts and longer boil times.

Cold Break

Polyphenols also complex with other protein degradation products.  These are small particles and remain in solution until the wort is cooled and they precipitate as cold break.  Cold break formation is aided by the addition of kettle finings which contain K-carrageenan, a negatively charged polymer which is soluble in hot water – but gels on cooling.  The negatively charged kettle finings interact with the positively charged proteins, as the wort cools. As the K-carrageenan gels, the cold break drops out of solution.

Kettle finings are usually added 5-10 minutes before the end of the boil – but do check with your suppliers on the correct use. They will advise on optimising the addition rates, so you get the best results.  An optimisation is usually done once a year on the new season’s malt. However, there are a host of factors that affect the addition, so it’s good practice to optimise several different beers and check the cold break clarity and sedimentation every brew.  Some of the factors are:

  • Hot wort clarity
  • Wort pH
  • Malt variety
  • Hot wort clarity
  • Level of cold break
  • Wort gravity
  • Wort polyphenol levels

Kettle finings improve wort clarity. Clearly (ha!), if you are making a beer that is naturally hazy, don’t add them.

Hops and Bitterness

Hops are added to the boil to extract bitterness, flavour and aromas.  They contain 𝛂-acids, which are isomerised when boiled, becoming iso-𝛂-acids and adding bitterness to the beer.  The isomerisation process is never 100% efficient and is dependent on:

  • Boil time – 45-60 minutes is usually enough to complete isomerisation. Additions early in the boil fully isomerise and add bitterness.  Additions later in the boil will not fully isomerise, resulting in more flavours and aromas.
  • Boil temperature – Isomerisation a chemical reaction. Chemical reactions are quicker with elevated temperatures.
  • Wort pH – Alkaline pH promotes hop isomerisation and solubility. As the wort pH drops during the boil so do the formation of iso 𝛂-acids.
  • Hop addition rate – The higher the 𝛂-acid content in the hops, the higher the losses. High 𝛂-acid content will give higher iso-𝛂-acid, but the percentage conversion is less.
  • Wort gravity – Hop utilisation is most efficient in lower gravity worts.
  • Type of hops added – Leaf hops have a lower (25-30%) utilisation than pellet hops (30-40%).

You can work out the hop utilisation by using the following formula:

Utilisation % = (Iso ɑ-acid in beer/ɑ-acid added to the kettle)*100

Bitterness is measured in IBU’s (international bitterness units) and 1 IBU = 1mg/ltr.  To calculate the hop grist, we use:

  • The target bitterness of the beer
  • The volume being brewed
  • The % 𝛂-acid of the hops being used
  • The % utilisation of the hops

To work out the weight of hops to add takes a few steps:

iso ɑ-acid in final beer = (Volume*Target IBU)/1000

Then take the utilisation into account:

g of iso ɑ-acid =(100/utilisation)*iso ɑ-acid in final beer

Finally, here’s how to get to the weight of hops you need to add:

g of hops to add = g of iso ɑ-acid*(100/hop alpha) 

 Adding one hop variety at the start of the boil is quite straightforward to calculate, but when adding several different hop varieties at different addition times, it can become quite complex.  The different hop varieties will have different 𝛂-acid, and the different addition times will have different utilisation times. Hops added towards the end of the boil will still contribute bitterness, but the utilisation is reduced to 5-20%

To improve hop aroma, it’s becoming commonplace to cool the wort to 80oC before adding the late hops.  This reduces the utilisation and improves the aroma.

Hops also help to control the foam created during the boil. If you are making a beer where the bitterness comes from late hop addition, it’s a good idea to add a very small amount to the start of the boil to avoid boil-overs and improve head retention.  Alternatively, anti-foam can be used.

Removal of Volatiles

As the wort is boiled, volatiles evaporate.  The main volatile we speak about is Di-Methyl-Sulphide (DMS) which tastes like sweetcorn, cooked veg or tinned tomatoes.  It is formed from the precursor S-Methyl Methionine (SMM) found in all malts but is present in higher levels in lightly kilned malts.  Standard levels are below 5mg/ltr, and it can be found on the certificate of analysis as SMM or DMSp.  The flavour threshold in beer is between 40 and 60ppb and can be acceptable in some lager styles up to 100ppb.

DMS is formed by the thermal decomposition of SMM during kilning and boiling, and it is rapidly driven off by evaporation.  It does, however, continue to break down during whirlpool stands and wort cooling.  Any formed at this stage will stay in the beer.

Controlling DMS levels in the brewery is straightforward:

To Reduce DMSTo Increase DMS
Boil for a minimum 60min 6% evaporationUse a malt with higher levels of SMM
Limit whirlpool stand to 15 min maximumReduce the boil time – 45 min
Cool the wort rapidly – 60 minIncrease the whirlpool stand time – 45 min

DMS can also be produced from micro contamination during fermentation. Slow and laggy fermentations are susceptible to this kind of infection.

DMS is not the only volatile to be concerned with. Grassy and grainy aromas from mashing can be present, but these are typically driven off with only 2% evaporation.  Some hops can have an unpleasant vegetable and grassy aroma, and this can take up to 60 mins before it is driven off.  These flavours can be added back accidentally at dry hopping if the hops are slurried in hot liquor before addition.

It’s important to make sure the volatiles don’t make their way back. As they rise up the stack, they can condense and fall back into the kettle.  Most chimneys are fitted with a channel to catch any condensate but make sure it’s not blocked and is cleaned regularly.

Colour and Flavour Development

Crisp Malt Webinar Sensory EvaluationDuring boiling, wort colour can increase by 2 to 4 EBC depending on the raw materials and boil conditions.

This colour and flavour development comes from:

  • Maillard reactions
  • Caramelisation of sugars
  • Oxidation of polyphenols.

Back in 1912, French chemist Louis-Camille Maillard noticed a reaction between amino acids and sugars at high temperatures.  The non-enzymatic reaction, now known as the Maillard reaction, is responsible for colour and flavour development during wort boiling.  The FAN levels in the malt are directly related to the Maillard reaction, and if you see high colours, it’s a good place to start looking.

Kettles with high differential temperatures between the heating surface and wort also promote sugar caramelisation, increasing colour and flavour.

Reducing oxygen content in the kettle will reduce the oxidation of polyphenols.  In practice, there is not much you can do about this, but it can be helped by:

  • Bottom filling
  • Slow filling until the inlet is covered
  • Covering the vessel
  • Using Clear Choice malt – it has no polyphenols to oxidise.

Final Words


So while you are wort boiling, check the evaporation rate and if you want someone to have a coffee with, give me a shout. We advise and support. We share our experience and expertise. We relish any opportunity to help with beer recipe creation or with solving issues around anything connected with brewing and beer.

Just give the sales team a shout: 01328 829391 or email

Posted by
Mike Benson
on 30/07/20

Mike is the sales manager for Wales and the West of England and is located in Wigan.

Read More
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