Why Does Wort Boil Over (Plus How To Prevent It!)

Wort boilovers are a truly frustrating part of beer making and can ruin a promising batch by changing your ratios and making a mess of your kitchen. In order to make sure you never have to deal with a boil over again, it’s important to understand why we boil wort and what causes it to overflow.

One of the most important reasons brewers boil wort is to induce a ‘hot break,’ during which loose proteins and tannins coagulate into foamy clumps to be separated out. These proteins are the exact reason wort commonly boils over though, since this foam can expand exponentially as it gets filled with more and more water vapor. 

So now that we understand what causes boilovers, let’s take a look at some best practices to ensure a safe, clean boil to leave behind a great quality beer and a clean kitchen stove.

How vigorously should you boil wort?

Before your brew ferments, it’s wort – basically, the sweet and malty liquid that will eventually become your delicious home-brewed beer – and one of the formative processes of getting from one to the other is getting your wort boiling.

Keeping your wort at a rolling boil long enough for a good hot break will result in a clearer final product and more consistent mixing. The proteins that make up the hot break can still coagulate when kept at a low boil, so you should prioritize safety and avoid spillovers, even if that means a less vigorous boil. 

When you consider the reasons for boiling your wort, it becomes clear that maintaining a long, gentle boil is preferable to keeping your pot over a full flame.

Those reasons include:

  • Sterilization
  • Stopping enzymatic activity
  • Reducing wort volume
  • Coagulating proteins
  • Removal of dimethyl sulfide
  • Extracting hops

Sterilization

Sterilizing your wort will help kill any foreign bacteria that could negatively impact your beer during fermentation.

But since pasteurization occurs at temperatures well below a simmer (145°F, to be exact), you could sterilize your wort without ever even bringing it to a boil.

Stopping enzymatic activity

While living bacteria will be killed at pasteurization temperatures, many enzymes will continue to function under much hotter conditions.

Still, the most ubiquitous enzyme amylase begins to break down at around 175°F, still well below a boil.

Reducing wort volume

Reducing the amount of liquid in your wort can be an important step in preparing your beer at a proper initial gravity.

However, reducing your wort at a high, rolling boil can burn the wort and puts you in danger of experiencing a boilover.

Coagulating proteins (Hot Break)

While many of the proteins in beer coagulate due to both the heat and agitation, there’s no need for an extended rolling boil.

In fact, your hot break can occur without any agitation – although it can be helped along by stirring the wort while it boils gently. 

It is important to note that a long rolling boil will improve the rate of coagulation and result in a clearer beer. However, many of the proteins that you remove during a long, vigorous boil are the same proteins that help produce and retain a good head of foam in your finished beer, so you’ll have to make a decision based on your priorities for each brew.

Removal of dimethyl sulfide

Another common reason for a long boil is to remove dimethyl sulfide (DMS), a common off-flavor that gives off a sulfurous, eggy aroma.

Boiling the wort will produce dimethyl sulfide and release it from your wort for better flavor.

While a rolling boil will always release volatile compounds faster, research has shown that dimethyl sulfide is still released faster than it is produced at temperatures just below the boiling point. Rather, the majority of DMS is produced from long cooling times after the wort is finished boiling. 

So keeping your wort at a gentle boil or simmer is perfectly acceptable for DMS reduction, as long as you cool your beer quickly after the boil is over.

Extracting hops

While hops are commonly infused into boiling wort, there’s really no need to keep a high temperature to extract those bittering and aromatic flavors.

Just like with protein coagulation, stirring can replace vigorous boiling to get proper utilization of your hops.

Without ever reaching a boil, hops can produce the same flavor, aroma, and shelf stability you would expect from any other hopped beer. In fact, up to 90% of the aromatic compounds in hops can be lost during vigorous boils, one of the reasons why dry hopping is a popular method to reintroduce flavors lost from the wort.

When you take into account all of the discussion here, it makes sense that industry research has shown examples of perfectly acceptable beer brewed without ever boiling the wort. While this probably won’t work for every recipe or beer you make, it’s something to keep in mind next time you decide between turning down the flame or risking a boil over.

Why boil-overs are bad for homebrew

So your wort just overflowed all over your stovetop. Is the batch ruined? 

While it is certainly a frustrating experience, a boil-over will never completely ruin your project. However, since boil-overs can throw off your recipe, your final beer probably won’t turn out exactly as intended.

The main reasons that boil-overs are bad for homebrew are pretty straightforward:

  • They can mess up the ratios in your recipe
  • They can cause you to lose some of your beer
  • They’re messy and hard to clean off your stove

Because they are so annoying, the best way to deal with a boilover is to never have a boilover. Let’s take a look at some preventative measures to make sure you have a clean, safe brewing experience.

How to stop wort from boiling over

There are plenty of great methods you can use to help prevent a boilover, but no matter what it is essential that you keep an eye on your wort. 

Here are a few tips to help prevent any future batches from boiling over:

  • Use a larger pot
  • Don’t cover your pot
  • Break up the foam
  • Skim off the foam
  • Keep an eye on the temperature
  • Use a pot watcher
  • Use some fermcap

Use a larger pot

One of the easiest ways to reduce the chance of a boil-over is to use a bigger pot.

If your wort overflows because there’s not enough space for it to expand without overflowing, adding more space will directly fix that problem! 

The exact volume you’ll get from a hot break will depend on each batch, but using a pot with enough headroom is essential. Even if there’s not enough space to contain all the foam, it will at least give you a chance to catch it before it overflows all over your stovetop.

Don’t cover your pot

When boiling wort, it’s essential to never put a lid on your pot.

Covering your pot may seem like a great way to retain heat, but in practice, it can have catastrophic results. 

First, when you remove a cover, the difference in temperature will cause the steam trapped inside the pot to escape as quickly as possible. Since the foam is mostly filled with steam, this sudden temperature shift will cause a sudden expansion of foam that will almost always spill over. 

The second reason to leave your pot uncovered is that it allows the off-flavor, dimethyl sulfide, to escape. Covering a boiling pot of wort can let the DMS build up inside your beer leading to nasty aromas.

Break up the foam

When your beer foams up near the edge of your pot, it’s always a safe choice to break up the foam.

Since foam is mostly composed of water vapor, breaking up the foam will have no adverse effects on your beer and will leave the leftover proteins behind for you to skim off later. 

Some popular ways to break up foam include deflating it with a spray bottle or simply stirring it with a wooden spoon.

Skim off the foam

Since the goal is to remove the proteins and tannins that make up the foam, you might be wondering why you can’t just remove the foam as it forms.

Skimming of the foam is an acceptable way to avoid a boilover, but you need to take care not to make a bigger mess when doing so.

As long as you can do so in a clean and efficient way, skimming foam off the top of your wort is a great way to control a potential spill before it happens.

Keep an eye on the temperature

You don’t need to keep a rolling boil to get a good hot break. 

It’s always a good idea to keep a thermometer in your wort to keep it in the range of 195-212°F.

If it gets close to the top of that range, or if you start to see aggressive bubbles forming, don’t be afraid to turn down the heat.

Use a pot watcher

Traditionally used when boiling food high in protein or starch, like pasta, rice, or milk, a pot watcher is a tool to reduce foaming and prevent boilovers.

A pot watcher’s uneven surface prevents large bubbles and encourages smaller ones. This results in a finer foam that is less likely to boil over.

Many homebrewers like to use glass marbles in their wort for the exact same function.

Use some Fermcap

Fermcap is a foam inhibitor that lowers the surface tension of water, making it harder for bubbles to form in your wort. This likewise makes it harder for coagulated proteins to form foam. 

If you’ve tried some of the other suggestions here but still have trouble with boil-overs, Fermcap is an easy, potent solution for your problems with spills, even through vigorous boils.

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