Besides talking about the general history of Bavarian white beer and historic brewing methods, my latest book also discusses the practical brewing side of modern Bavarian white beer, most often known as Hefeweizen, Weissbier or Weizenbier. This article should give everybody a bit of an insight into what’s involved in controlling the typical aroma and flavour profile of this style.
The Aroma and Flavour Profile
Two separate groups of compounds drive the typical aroma and flavour profile of Hefeweizen. There are esters that give it a fruity note that professional tasters often describe as banana, pear drops, sometimes even as bubblegum or generally fruity. And then there are phenols that impart a spicy aroma and taste, often similar to cloves. Some people prefer banana, others prefer the clove, and a lot of brewers try to find a find a balance between the two.
The formation of both the right esters and phenols depends on the yeast strain, so make sure you get a proper Hefeweizen strain. Most commercial producers, especially those catering to homebrewers, will have at least one, such as White Labs WLP300, Wyeast 3068 (both liquid yeasts), Lallemand Munich Classic, or Fermentis SafAle W-68 (both dry yeasts), just to name a few (other strains from other producers are available).
So this is how you can influence the formation of both of these aroma/flavour compounds:
Phenols
The main phenol is 4-vinyl guaiacol (4VG). It is formed mostly enzymatically from its precursor, ferulic acid, and to a small extent (10-15%), also thermally at temperatures greater than 80°C. This precursor is located in the malt (in barley malt more so than in wheat malt), and is brought into solution at an ideal temperature range between 40 and 45°C (104-114°F) and a pH range between 5.8 and 6.1.
So to promote the formation of phenols, mash in low, do an extended rest between 40 and 45°C, and use at least 40% barley malt in your mash.
If you want to minimise the formation of phenols, mash in high, and ideally use as little barley malt as possible. 100% wheat malt mashes are possible if you use rice hulls or straw as a lautering aid.
Esters
The formation of esters on the other hand depends on a large number of factors that a brewer can influence:
- fermentation temperature
- glucose content in the wort
- wort aeration
- yeast cell count
- yeast strain
- fermenter geometry
- choice in malts
Fermentation Temperature
The main ester responsible for the banana or pear drop flavour in Hefeweizen is isoamyl acetate, closely followed by ethyl acetate. They are formed from alcohols through an enzymatic process called esterification.
More esters tend to be formed with higher fermentation temperatures, ideally between 20 and 24°C (68-75°F).
But just cranking up the temperature is a double-edged sword, as increased fermentation temperatures will also “scrub” some of these esters through the CO2 produced during fermentation. At 20°C, about 40% of all fermentation-derived esters are “washed out”, while at 25°C, this increases to 50%.
As ester formation depends on the formation of alcohols, ester formation will only kick in once sufficient alcohols have been produced through fermentation (typically 24 hours), and will continue until the end of fermentation. So if fermentation finishes too quickly, not enough esters may have been formed for the aroma or flavour to really shine through.
These two effects of course can compound, and thus fermentation temperatures too high can leave behind a beer mostly devoid of the typical ester aromas.
Glucose Content
Research has shown that a higher ratio of glucose to maltose in the wort has a positive effect on the formation of esters during fermentation. Brewers not bound by German beer legislation can influence this by simply adding glucose syrup to the wort, while German brewers can use the Herrmann method during mashing to change the ratio of glucose to maltose.
Wort Aeration
Increased oxygen content in the wort promotes yeast growth, which in turn reduces the amount of acetyl coenzyme A (acetyl-CoA), a precursor to the esters produced. Oxygen also suppresses the activity of the AATase enzyme which is responsible for ester formation. Aeration therefore both reduces the available substrate and inhibits the enzymatic activity for ester formation.
So if you want to promote esters, reduce your wort aeration. On the other hand, if you do not want to have as many prominent banana esters, increase wort aeration.
Yeast Strain
Different strains produce different amounts of esters. There is no easy answer which yeast strain is the best for a banana-forward flavour, though: high levels of esters do not necessarily correlate with a clear sensory perception of them. The most prominent example is the TUM-175 strain which in trials produced the highest levels of both isoamyl acetate and 4VG compared to other strains, but a sensory panel only noticed a lightly fruity and clove aroma. The reason for that is that esters and phenols were overshadowed by other yeasty aromas. Instead, in the same trials, the TUM 68 strain performed the best in the sensory panel.
Ultimately, it’s up to you to select a good strain that suits you, but the TUM 68 strain, which is available commercially under various names by a multitude of producers and is really the most common Hefeweizen strain out there, is a good start.
Fermenter Geometry
Fermentation in cylindroconical tanks (CCT) tends to suppress ester formation. This is because tall, narrow fermenters have larger convention, which means that nutrients will be distributed more evenly, leading to stronger yeast growth, causing an imbalance between esters and higher alcohols. The increased convention would also potentially scrub more esters.
Shallow fermenters on the other hand tend to promote ester formation.
Choice in Malts
Caramel malts have a tendency to “smooth out” the flavour too much and cover up the typical aromas of banana and clove. Therefore, keep your recipe simple and mainly use pale base malts and pale or dark wheat malt. If you want to adjust the colour of your beer, use very small amounts of roasted wheat malt or a debittered roasted barley malt. This doesn’t mean that you shouldn’t use caramel malts at all, just use them sparingly and judiciously and be aware of the potential effects of them on the overall flavour profile.
Conclusions
There are a lot of factors that play into the formation of esters and phenols during Hefeweizen fermentation. But none of these mean that just because of one of these factors is off that it completely throws off the beer. Just start from a simple baseline and work from there.
Depending on your practical situation, you can use this information to dial in your beer to an optimum. You can even combine ester-suppressing techniques with ester-promoting ones if you’re constrained in some shape or form, e.g. a part of your equipment or your process inevitably suppresses esters but cannot be changed.
Also, “more is better” is not necessarily a good approach, as too many esters and phenols can actually muddle the flavour. The aromas and flavour of banana and clove actually are actually the most pleasant within certain concentration thresholds.
Of course, this is just a brief insight into the complex world of aroma and flavour formation in Hefeweizen. If you want to learn more about how to brew great Hefeweizen and also get educated about the amazing history and historic brewing methods of this beer style, go and get my book!