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Time to Boil

A vigorous boil is a vital step in the brewing process. Jester Goldman explains what goes on during the boil and offers guidelines on how long the boil should be.

Jester Goldman May 19, 2017

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Old School beer kits consisted of a can of malt extract syrup with a pack of dry yeast under the plastic lid. The directions were simple: stir the contents of the can in with 4.5 gallons (17 liters) of water, add a couple of pounds of sugar, then add the yeast, and wait. Having tasted some of those brews, I’m amazed that people stayed with the hobby. Aside from being too dependent on simple sugars for fermentation, these beers skipped out on a vital step in the brewing process: a vigorous boil.

Later, Charlie Papazian’s The Complete Joy of Homebrewing would address both of these issues for a new generation of homebrewers. In particular, he recommended boiling the wort for at least 45 minutes. Today, a 60-minute boil has become the accepted practice, but you’ll come across recipes that range from 30 minutes to 2 hours (e.g., this recipe for a Russian imperial stout). There are even some outliers that run for several hours.

Understanding what goes on during the boil can make all of this a little bit clearer.

The Heat is On

Sterilization

The biggest problem with those no-boil kits is that you typically created a war zone between the bacteria all around you and the yeast you add. Without some kind of pasteurization, the chances of an infected batch are high. A boil solves this problem handily. But if sterilization were the only goal, it wouldn’t take long to accomplish this, and a lower temperature, such as 150–165°F (66–74°C), would be sufficient.

Hops and Malt

But consider brewing as a form of cooking; much of the magic arises from chemical reactions supported by the heat. Boiling the wort affects both the hops and the malt.

For hops, the key idea is isomerization. We think of alpha acids in the hops as the source of bitterness, but these compounds are not particularly soluble in their raw state. Between the wort pH and the energy of the boil, though, the alpha acids are converted into iso-alpha acids that will dissolve into the wort. This process is time-dependent. This should make sense, because it underlies the distinction between bittering-, flavoring-, and aroma-hops additions. The later the addition, the less bitterness is extracted.

Malt is affected in numerous ways. During a vigorous boil, proteins from the grain can bind with tannins and precipitate out. This is what forms the hot break, the kind of fluffy scum that appears during the course of the boil. A good hot break positively impacts the appearance of the beer—because it reduces protein haze—and the flavor—by removing astringent tannins.

Malt also contains precursors for dimethyl sulfide (DMS), which are transformed into DMS during the mash and boil. A strong boil will volatilize DMS and its precursors away; otherwise, they can contribute an unpleasant cooked corn/cooked cabbage character.

Finally, a longer boil will foster Maillard reactions. The same chemical changes that create a bread loaf’s golden brown crust can produce melanoidins in the wort. These add distinctive flavors such as toffee and nuts, they contribute a darker color, and they can bolster the mouthfeel and head retention of a beer.

Concentration

As a natural result of evaporation, the length and intensity of the boil will affect the gravity of the wort. You should be aware of how much water you typically lose to evaporation, so you can predict your beer’s original gravity based on the gravity at the start of the boil.

So, How Long is Enough?

The idea of a 60-minute boil is most likely rooted in optimizing hops utilization. After an hour, the alpha acids in the hops should all be isomerized and additional hops utilization drops off. A shorter boil leaves unconverted alpha acids, while a longer one doesn’t pick up any more hops bitterness. As a side benefit, that provides plenty of time for a strong hot break and sterilization.

If you’re willing to toss in some extra hops to account for utilization, there is some experimental data to indicate that a 30-minute boil is sufficient. There are plenty of brewing calculators that can help you work out the utilization impact. If you’re in a hurry, you might give it a try.

On the other hand, there are good reasons to consider a longer boil of 90–120 minutes. Boiling for 15–30 minutes before the first hops addition can reduce the chance that the hot break will glom onto hops particles. Also, if your recipe has a large proportion of Pilsner malt, you may need the extra time to drive off more DMS. Finally, some styles call for the richer malt depth that comes with more extensive Maillard reactions. That wouldn’t be appropriate for a pale ale, but bigger beers such as a Scottish Wee Heavy or an Old Ale will benefit from the extra time.

There’s another reason to take more time for a higher-gravity beer: it lets you start with a more manageable initial gravity. A lower gravity allows for greater hops utilization before evaporation concentrates the wort, and with all-grain brewing, a long boil may be the easiest way to hit a target OG greater than 1.100.

The accepted standard of an hour long boil serves us well most of the time, but now that you know a little more, you can pick the right time for your beer.

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Have you brewed this recipe? What did you think?