This post is part of a series on German wheat beers.
As I mentioned in the post that began this series, in many ways brewing a German hefe-weizen is similar to brewing any other ale. And conducting the boil is one part of the brewday where this is mostly true.
As with the vast majority of beers, you want a vigorous boil to sanitize the wort, coagulate the break material and isomerize the alpha acids from the hops. (There are also a host a minor goals that the boil accomplishes.) There is one minor twist when boiling a hefe-weizen wort.
Two differences between a wheat beer wort and a “regular” ale wort are that a hefeweizen wort contains more protein and fewer tannins. Wheat malt has a higher protein content and the low hopping rate (typically 20 IBUs or less) contributes fewer hop-derived tannins to the wort. Because protein-tannin complexes are part of what you want to precipitate in the boil, you should boil your wort vigorously — at home for at least 90 minutes (and perhaps up to 120 minutes). Commercial brewers, if they employ an external calandria, can cut the time to 70 or 80 minutes. Other than extending the boil longer than some ales, the hefe-weizen boil can be handled as you normally would.
With 20 IBUs or fewer, and all the hops typically added at the beginning of the boil, your hop choice is not critical. Any German noble hop will do. (And in reality, almost any fairly neutral hop could be used without anyone knowing any different.)
A hefe-weizen fermentation should yield a beer that exhibits the typical hefe-weizen aroma. This aroma is complex, but dominated by a spicy, phenolic note and a banana-like aroma. The clove-like phenolic character is conferred by the molecule 4-vinyl guaiacol (4VG) and the banana aroma by iso-amyl acetate, an ester that can appear in “regular” ales fermented at higher temperatures.
The most important variable in getting the appropriate hefe-weizen aroma is choosing the right yeast strain, and I’ll cover that separately in an upcoming post. However, how you conduct the fermentation also effects the expression of these characters.
Fermentation, 4VG and Iso-Amyl Acetate
Iso-amyl acetate (the “banana ester”) production is increased by all the factors that increase esters in general in beer fermentations. Higher gravity worts, lower pitching rates, lower aeration rates and higher fermentation temperatures all promote ester production.
The factors that stimulate 4VG are largely the same as it is a yeast by-product just as iso-amyl acetate is — higher gravity worts, lower pitching rates, lower aeration rates and higher fermentation temperatures.
In addition, open fermentations have been found to stimulate both ester production and 4VG production in hefe-weizens fermented with a hefe-weizen specific yeast strain. Open fermentation also allows the brewer to “skim the scum” and top-crop the yeast, both of which are traditional practices. (Many commercial breweries have switched to cylindro-conical fermenters, however.) At home, fermenting in a bucket and opening it during the most active part of fermentation can be done. The bucket is not a shallow fermenter, as a traditional open fermenter would be. And you should really only open the bucket in a room that was clean and when the wort is (at least partially) protected by kräusen. However, this is worth a try for homebrewers who want their hefe-weizens to be brewed as authentically as possible.
In practice, hefe-weizens are usually pitched with 10–15 million cells per mL, right in line with the usual million cells per mL per degree Plato rule of thumb. Fermentation temperatures can range from 54 °F (12 °C) to 77 °F (25 °C). In his book, “German Wheat Beers” (1992, Brewers Publications), Eric Warner states that an old German rule of thumb was that the pitching temperature plus the fermentation temperature should equal 30 °C (convert to Celsius first, then add temperatures). For example, if a beer was pitched at 54 °F (12 °C), it should be fermented at 64 °F (18 °C), and that combination is common Most homebrewers pitch and ferment closer to “regular” ale fermentation temperatures, 68–72 °F (20–22 °C). (And remember that the yeast character is influenced by many factors, so that’s not necessarily bad)
Striking a balance between the clove and banana in a hefe-weizen is important. And again, your yeast strain choice is going to be the most important variable. However, other things also play a role. The most obvious example is the ferulic acid rest, which boosts 4VG levels in the final beer, but doesn’t effect ester levels. In his textbook, “Technology Brewing and Malting: 3rd International Edition” (2004, VLB Berlin), Wolfgang Kunze also gives a mash pH of 5.7 to 58, at least 40% malted barley, one or two cycles of repitching the yeast and early harvesting of the yeast among the variable that increase 4VG levels. Some of these have more subtle effects than others (esp. the percent of wheat vs. barley malt) and other don’t mesh with typical homebrewing practices (cropping yeast and repitching for multiple batches.)
Other than those factors, the things that drive 4VG and iso-amyl acetate production are the same, and the factor with the biggest impact among them (after yeast strain) is fermentation temperature. However, how to you favor one molecule over another when the “volume knob” is the same for both? Let me use an analogy to explain.
Let’s say you had a stereo and you labelled the two speakers 4VG and iso-amyl acetate. Let’s further say that, as very quiet levels, the 4VG speaker was slightly louder than the other. However, for every “tick” you turned the volume knob up, the 4VG speaker got louder by 2 dB and the iso-amyl acetate speaker got louder by 3 dB. As you turned the stereo up, the iso-amyl acetate speaker would gradually drown out the 4VG speaker even though both were getting louder. And that, by analogy, is how your fermentation temperature works to control the balance of clove to banana.
If you want a banana dominated beer, strive to stimulate ester production. This will mask the 4VG in your beer. Your most effective controller of this — beyond the yeast strain selection — is a higher fermentation temperature. If you want a clove-dominated beer, strive to keep ester production in check, which will allow the 4VG to come through. Lower fermentation temperatures and secondarily higher pitching rates are what you want in that case. The exact temperatures at which banana starts to exceed clove depend on your yeast strain and pitching rate — and given all the variables is something you would need to fine-tune in your brewery.