Tweaking your Hefeweizen
Seeing as how summer has officially begun (at least as of the time of this writing) let's talk about everyone's favorite summer craft beer style - the hefeweizen. It's an old style, going back to before the Reinheitsgebot because it was the only one allowed to skirt the rules at the time*
A lot of home brewers get into brewing hefeweizens because they feel that it's an easy style to brew. Here, the rules don't really apply. You're allowed to have banana esters and clove phenols that other styles eschew and it's easier to finish a keg of hef than it is to polish off 20L of a 7% Belgian style. After awhile though, the brewer who continues to develop their taste for the hefeweizen will start to want to change things a little bit. Perhaps the balance is off. Maybe there's too much clove or too much banana. What can the brewer do to change these things?
There's a lot of terrible advice online, so here's some verified tips on brewing a hefeweizen along with reasons for why that advice is sound. As with all great estery beers, you start with the yeast.
Strain selection is the number one factor when it comes to ester production - both amount and type. Esters take energy to make and they're important in regulating certain coenzyme functions, which I won't get into here, but suffice to say that each strain has it's own ester signature. There are several commercial strains out there, but my preference is the Weihenstephaner strain produced by Wyeast. I find it quite balanced, but if you want more banana, choose a different strain.
Grists vary in hefeweizens with the minimum being 50% wheat and the upper range being somewhere around 70%, though higher isn't unheard of. What you're controlling with grist is phenol production, specifically 4-vinyl-guiacol. The precursor to this clove-flavoured phenol is ferulic acid, which, for our purposes, mostly resides in the husks of barley. You can see then that adding less wheat and more barley is going to significantly alter the amount of ferulic acid and therefore 4-vinyl-guiacol produced.
Much has been made about acid rests, sometimes called ferulic acid rests. They aren't necessary for producing ferulic acid and the pH range for the enzyme's optimal activity is on the high side, which is detrimental to the beer. They're really only performed by traditional brewers looking to avoid using acid in the mash and the prolonged rest in that temperature range will almost definitely oxidise the beer, shortening it's already short shelf life. There are other, better levers for controlling the amount of clove flavour than a ferulic acid rest.
Esters and gravity are positively correlated. Higher gravity means more room for yeast growth and yeast growth produces esters. Glucose and fructose especially are correlated with esters, though we're still not sure why on that one. The esters are going to be banana, but if you push it too far in that direction, other esters come through, particularly strawberry, which together, give a bubblegum flavour.
Oxygen and Pitch Rate
I'm putting these two together because they should always go together. Increasing the pitch rate without increasing the amount of oxygen you add will give you a bunch of starved yeast cells and, if you repitch, a crappy yeast crop. Increasing both will allow the cloves to shine as esters will be suppressed since less growth will be happening. As well, oxygen alone suppresses ester production since it directs pathways toward unsaturated fatty acids rather than esters. Be warned though, increasing oxygen without increasing pitch rate will result in a bunch of higher alcohols and lead to a solventy flavoured beer. Higher alcohols also oxidise into some of the weirder off flavours.
Also don't underpitch. I've seen that comment on BJCP sheets occasionally. That's a terrible idea that will also lead to an unacceptable amount of higher alcohols, as well as acetaldehyde and diacetyl. A proper pitch rate is still going to be 3 smack packs for a standard 5 gallon batch if you don't use a starter (you should always make a starter). As a side note, home brewers generally never pitch enough yeast.
Protein rests are usually part of any hefeweizen simply because of the sheer amount of protein in the beer. A shorter protein rest ~20 minutes, is going to favour esters, while a longer protein rest, up to 1 hour, will favour cloves. Those ferulic acid enzymes I told you not to worry about? Yeah they work at higher temperatures as well.
Generally I advocate for keeping as much trub out of your fermenter as possible. However, for full disclosure, trub will suppress ester production because the yeast can take some of the things they need in the pathways leading to esters directly from the trub. Keep the trub out if you want those esters.
The last one is temperature. Again, esters have a positive correlation with initial starting fermentation temperature. 18C is good. 20C will give you more. Suppressing esters by keeping the temperature around 17C and fermenting for longer will let those clove flavours shine.
*Although not called a "hefeweizen" at the time, (they didn't know what "hefe" was) wheat beer was allowed to be brewed only by the king's brewers, and likely had many of the same banana and clove characteristics.