

If you’ve ever wondered why your hefeweizen smells like bananas, here’s why: isoamyl acetate. Say that three times fast and then breathe easy because all you really need to know is that isoamyl acetate, along with ethyl caproate (apple), ethyl acetate (solvent), and others are common esters you will certainly encounter in your homebrewing adventures. Without delving too deep into biochemistry, there are a few practical things you should know about esters, namely what they are and how you create and control them.Esters are fermentation byproducts formed when organic acids and alcohols react, creating fruity aromas and flavors.



Esters take time to develop since the yeast first has to convert sugars into alcohols in order for the latter to combine with organic acids derived from malt and hops. While esters are a natural component of beers, their detectable appropriateness differs from style to style, meaning esters are essential in a hefeweizen, but rather undesirable in altbier. Once you determine the desirable ester character for your next homebrew, you can control ester production through your wort composition, your choice of yeast, and fermentation conditions.



Beer starts as wort and so does ester production. The higher your original gravity, the higher the zinc levels, and the more fermentable your wort, the more you will promote the production of esters. High original gravity and high fermentability are great attributes for many a Belgian ale in which esters are desirable, but not in a Strong Scotch Ale, which calls for low to moderate esters at best. Varying your wort composition depending on desired ester levels is certainly an option: a high gravity beer must not necessarily be highly fermentable; balance gravity and fermentability; dilute your brew water with RO water if your zinc levels are alarmingly high. Furthermore, promoting production does not mean you will actually end up with a very ester-y beer because your choice of yeast and your fermentation conditions are arguably much more important factors.



Some yeast strains will simply produce more esters than others. Your regular American ale strain (WLP001) is cleaner than many British (WLP002) and Belgian (WLP500) ale strains, but even among strains, regardless of national association, ester production will vary. Knowing what you want out of your beer will help you pick the right yeast and I've found descriptions of strains to be helpful and accurate. Although not directly related to your choice of yeast, pitching rate has a major influence on ester production. The lower your pitching rate, the more you encourage ester production, which could come in handy if you forget to make a starter for a hefeweizen. Just remember that deliberately under-pitching to stimulate ester levels may not be the best way to go in terms of attenuation and overall fermentation character of your beer. Study your strains carefully and select and pitch according to the beer you intend to brew.



Fermentation conditions are your last and crucial control point for ester production. It all starts with your fermentor selection: carboy or bottling bucket. The higher the head pressure, the more ester production will be inhibited, and head pressure is higher in taller, narrower vessels (read carboy) than in shorter, wider vessels (read bottling bucket). Aside from yeast selection, another reason why hefeweizens from Germany and Sierra Nevada’s Kellerweis exhibit bewitching notes of banana is because they utilize specially-constructed, wide, shallow basins for fermentation. The higher your fermentation temperature, the more you stimulate ester production. Even a few degrees can make a big difference. I fermented the last hefeweizen I brewed about 2 to 4 degrees lower (my temp control is not perfect) than previous versions and the difference in banana character was so evident that from now on I’d rather err on the warm side for hefes. Agitation of your fermentation vessel also increases ester production, which is something to consider if you’re thinking about swirling or stirring your beer in order to fix a stuck fermentation by re-suspending the yeast. Any other undue stress during fermentation will likely also increase ester production.



I love all kinds of ester-y beers, but not every beer must smell like fruit salad, so knowing how to control ester production is an essential part of brewing beer. Wort composition, yeast selection, and, above all, fermentation conditions impact ester production. Among these myriad ways, there is plenty of opportunity to find a happy solution that will complement the overall character of the beer you’re brewing—enjoy finding yours!