Ask Mr. Wizard

Questioning Boil Duration


Darrell Rose — Calgary, Alberta asks,

I just read your excellent article on potentially reducing mash times for modern malts. There seems to be the same argument for potentially reducing boil times, based on modern malt design, down from the traditional 60 minutes to 30 minutes. I understand that the shorter boil times may affect bittering hops requirements, given that they are not in contact with the wort for as long, but the additional amount needed is easily calculated by brewing software (or is it?). Is a shorter boil time legitimate when using “modern” malts? I apologize if you’ve addressed this, as I am still sifting through everything you have, having just picked up homebrewing again after a long hiatus. 


Thanks for the great question, Darrell! This is not a topic I recall addressing and is certainly a modern brewing topic of interest. Before digging into your specific question about the relation of malt to boiling times, I want to review the textbook goals of wort boiling. You can skip to the end if you already know all of this and just want to read the part about malt.

The primary reasons for boiling include wort sterilization and denaturing mash enzymes, removal of colloidally unstable proteins and polyphenols, removal of DMS (dimethyl sulfide) and other undesirable volatiles, extraction and isomerization of hop alpha acids, wort color and flavor development, and wort concentration. How long is long enough when it comes to boiling? Let’s run through this list in a bit more detail and address the time requirement.

Sterilization and Enzyme Denaturation: Wort boiling sterilizes wort, effectively killing off any unwanted microorganisms present in the wort, and denatures enzymes carried forward from mashing into the kettle. One special brewing challenge is cooling and transferring wort to the fermenter without contamination, but that’s a different question. Wort sterilization is essential for modern brewing where selected microbes are introduced during fermentation.

Although wort boiling is a handy method used to sterilize wort, boiling times and temperatures far exceed the requirement for producing “sterile wort.” I am using quotes here because the proper food science term used to describe this process is “commercially sterile” in contrast to aseptic. The bottom line is that wort sterilization can be accomplished at sub-boiling temperature in a matter of minutes.

Protein and Polyphenol Coagulation: During the boil, proteins from malt and other grain sources coagulate and form larger particles (colloids). Although proteins coagulate during boiling in the absence of polyphenols, the hot-break trub formed during wort boiling is a combination of proteins, polyphenols, and lipids. Vigorous wort boiling increases the formation of hot-break and improves finished beer clarity, especially when coupled with the use of kettle finings like Irish moss (a cold-water seaweed) or finings extracted from warm-water seaweeds. The crazy thing about wort boiling is that trub formation continues to increase as boil duration is increased, so it’s hard to know when enough is enough. We’ll get back to this in a bit.

The crazy thing about wort boiling is that trub formation continues to increase as boil duration is increased, so it’s hard to know when enoughis enough.

Elimination of Undesirable Compounds: Boiling helps volatilize and remove unwanted compounds in the wort, especially Dimethyl Sulfide (DMS). In the case of DMS, its precursor (S-Methyl Methionine, or SMM) converts to DMS when heated followed by DMS removal with water evaporation. This is arguably the biggest driver of wort boiling duration. More to come there too . . .

Color and Flavor Development: Wort color and flavor both increase during boiling. Maillard reactions between sugars and amino acids are the primary causes of the changes. Because different beer styles benefit from more or less of these reactions during boiling, brewers may select a shorter or longer boiling time that may not be optimal for other boiling goals. 

Hop Bitterness: Alpha acids from hops are extracted from lupulin glands during boiling before the heat of boiling causes the chemical rearrangement, or isomerization, of the alpha acid molecule (there are several different types all based on the same general structure) into a different form known as an iso-alpha acid. Iso-alpha acids are more soluble and bitter than alpha acids.

Although hop isomerization plateaus after approximately 60 minutes of boiling, there is little difference between boiling for 30 minutes versus 60 minutes. And as you point out, brewing software can be used to adjust hopping rates to account for the reduced rate of isomerization. It’s also noteworthy that boiling is not required for this reaction to occur and sub-boiling temperatures in the 205–212 °F (95–100 °C) range result in similar isomerization rates to boiling.

Wort Concentration: Boiling increases wort gravity by evaporating water. This is a critical step in the production of strong beers because wort concentration removes excess sparge water used to minimize extract losses. And with no-sparge brewing, there is a practical limit to wort gravity because extract production during mashing requires a minimum volume of mash water that makes the no-boil max gravity somewhere around 25 °P (1.106 specific gravity). Outside of strong beer production, it’s hard to argue that wort must be concentrated during wort boiling because slightly overshooting target gravity and dilution with water is a great way to produce consistent wort.

wort boiling in a brew pot
Boiling wort can serve many functions other than just sterilizing it for your brewing yeast. Photo by Charles A. Parker/Images Plus

This really leaves us to dive into DMS removal, colloidal stability, color, and flavor. Brewers have settled into some rules of thumb that can be an obstacle to improvements and the “required” 60- to 90-minute boiling time is a great example. DMS removal has been the benchmark for kettle performance during the last 30+ years of continual improvement in wort boiling technology. In general terms, advancements in wort boiling are aimed at reducing energy use while having no negative effect on the key objectives of boiling. Because DMS is undesirable in Pilsner-type beers, the most common style commercially brewed, DMS in straw-colored beers brewed with Pilsner malt is a great metric to monitor.

The typical brew kettle used by most craft brewers evaporates about 4% of the wort volume per hour and typical boil times are 60–90 minutes. If the boil time decreases, straw-colored beers brewed with Pilsner malts start to show DMS, thereby reinforcing the validity of the rule of thumb.

However, this is a big contrast to the 10–15% hourly evaporation rates common to homebrewing. Questioning the 60- to 90-minute rule of thumb is a very smart question because the commercial rule is misaligned with homebrew kettles. The next time you brew, consider choosing a lighter style you like and have brewed before and try reducing your boil time to 45 minutes. The proof is in the pint, and you can determine for yourself if the change makes a difference. But be aware that not all beer consumers have the same ability to detect DMS and we all don’t use the same malt, so what works for you may not work for someone else. That’s why rules of thumb are not absolute.

If 45 minutes works for you, try reducing to 30 minutes. Or if you really want to reduce the length of your brew day, consider no-boil brewing. The catch-22 with boiling and DMS is that boiling increases wort DMS before reducing it through evaporation. Remember, DMS reduction is only one of the goals of boiling. As you consider reducing boil time, be sure to review the topics discussed earlier to keep your eyes open to all changes that may accompany your trials.

For me, colloidal stability is important because we first drink and eat with our eyes. Colloidally stable beers are not always clear, but they don’t fall apart during storage. Haze particles that develop during storage, often causing beer to appear like a snow globe, are a common manifestation of colloidal instability. These beers may taste OK, but the appearance is simply off-putting. If you go too far and notice unwanted changes in clarity, consider increasing your kettle fining dosage rate and/or increasing boil time.

Malt selection is absolutely an important part of this topic. Maltsters produce malt to satisfy brewers and reducing SMM and DMS is one of the things modern maltsters monitor. SMM serves an important role in plant biochemistry and is not something that can be removed from barley, but fertilizer application and barley protein both influence SMM in green, or un-kilned, malt. During malt kilning, SMM is partially converted to DMS and is removed with the hot air used to dry malt. As malt color increases, SMM and DMS levels reduce.

Because Pilsner-style beers use very pale malt, these brewers are particularly concerned about SMM and DMS. If you primarily brew styles with more color, choosing a slightly darker base malt takes much of the load of SMM and DMS reduction off your shoulders. And the differences are not big; increasing your base malt color from about 1.5 °Lovibond (3 EBC) to 2 °Lovibond (4 EBC) makes a significant difference in malt SMM and DMS levels. Once you get into the 3–4 °Lovibond (6–8 EBC) color range of Vienna, pale ale, and Munich malts, DMS is really a non-issue.

This leaves us with color and flavor. Yes, wort boiling can be used to develop desirable colors and flavors. However, wort boiling also contributes to wort thermal stress, usually measured using the thiobarbituric acid index (TBI). As TBI increases with boiling, beer flavor stability is reduced. Reducing TBI is of major interest to commercial brewers always looking for ways to ward off the onset of beer staling. While this may not be a concern to homebrewers, we can look at how commercial brewers use specialty malts to develop wort color and flavor as a substitute for wort boiling and decoction mashing. Because extending boiling and extended kilning both lead to more Maillard reaction products, brewers can find a wide range of these flavors and colors in specialty malts. Reduce your boiling times by seeking biscuit, toast, vanilla, nutty, caramel, and dried fruit aromas from malt along with golden and deep amber color hues.

Thanks again for the great question and I hope this information allows you to reduce your boil times without sacrificing beer quality!

Response by Ashton Lewis.