The thing to remember when researching off flavors in beer is that many of these flavors are desirable (to some extent) in particular beer styles. Their presence in a beer is not entirely negative. For example, a Hefeweizen that tastes banana-and-clove-ish is right on style, but a crisp Lager that suddenly tastes like an entire bunch of bananas – not good.
In an effort to help out homebrewers who taste something not-quite-right with their beer and cannot put their finger on it we’ve collected the top 5 most common off flavors that brewers detect.
Acetaldehyde – green apple, cidery, grassy
Acetaldehyde is a compound that produces green apple flavor and aroma in homebrew, and is a byproduct of fermentation. However, one must remember that you need acetaldehyde in order to have beer! Why is this?
Acetaldehyde is formed during the conversion of sugar to ethanol by yeast. In simple terms: yeast creates acetaldehyde which is a precursor for alcohol – no acetaldehyde, no beer. The most common cause for acetaldehyde is removing your beer from the yeast to early, and not allowing the yeast to completely convert the sugar. If you can smell green apple and/or an acidic/vinegar smell, you need to keep fermenting.
Ways to avoid acetaldehyde being left in your beer after fermentation are: pitch more yeast and make sure you have good temperature control. If, after fermentation, you still taste or smell the green apple, you can still get rid of it with a good, long conditioning.
On the flip side, ethanol (created by acetaldehyde) can create additional acetaldehyde through an oxidation process. Therefore, the best thing to do is to avoid oxygen contact post-fermentation at all costs!
Diacetyl – buttery, butterscotch
Diacetyl is normally referred to as the buttery or butterscotch flavor found in homebrew. It is a volatile compound that formulates in early fermentation, but begins to decrease as beers mature.
The thing to know is happy, healthy yeast will reabsorb diacetyl. This means that conditioning and maturing your beer with some yeast leftover will greatly increase your odds of removing that buttery smell and flavor from your homebrew. Also keep in mind that fermenting at warmer temperatures increases diacetyl production in your beer.
The best things to do to avoid diacetyl are to pitch healthy, happy yeasts, ferment at proper temperatures, and make sure you condition at proper temperatures to allow leftover yeasts to reabsorb any diacetyl that was created during fermentation.
Esters – fruity, banana, clove
If you are a Belgian and German wheat beer fan, you’ve no doubt enjoyed some estery brews. It is on style for Ales like those to be slightly fruity. Esters are created by the yeast, and different yeast strains will provide different types and amounts of esters. Higher fermentation temperatures will produce more esters, which is why esters in a Lager would be peculiar. So, if your Lager is tasting estery, keep that fermentation temperature down.
Oxidation – winy, sherry-like, wet cardboard
Winy flavors usually result from the oxidation of your beer. Oxidation happens when your beer is exposed to oxygen, which degrades your beers flavors and aromas.
Now, this isn’t to say that anytime your beer comes in contact with oxygen during the brewing process, you can expect the flavors of cardboard and wine. It simply means that once your yeast is pitched in your fermenter and it has been sealed, you want to avoid oxygen contact at all costs. The same goes for bottling time: it is best to do so “quietly,” meaning you want to avoid any splashing, dumping, or all around agitation of your beer.
Simply bottle, seal and leave it alone. This minimizes the amount of oxygen being introduced to your beer, and lowering your chances of oxidization.
Soapy flavors in a beer can of course come from leaving soap remnants in the fermenter, but they can also be created by fermentation conditions. If your beer stays in the fermenter for longer than is suggested (we always say 4 weeks max. for initial fermentation), a soapy taste can occur because of the breakdown of fatty acids in the trub. Since soap is by definition the salt of a fatty acid – you are literally tasting soap.