The other day I did an experiment on a leichtbier, the German equivalent of a Bud Light. I did a sensory evaluation and then a second evaluation after leaving a coaster on top of it for a minute. I repeated this a few times over the course of half an hour and kept coming up with the same results.

The experiment? Most off-flavours and aromas vary wildly depending on the concentration.

Once you get to a certain level as a homebrewer, you end up with the same problems as pro-brewers and there is an unholy trifecta issues that all pro-brewers face: Oxygen, aldehydes and DMS.

Of these three, DMS is the easiest to fix. There are definite causes and definite solutions; increase your evaporation if using basic, cheap 2-row or pilsner malt, or make sure you have a healthy fermentation if you’re using ale malt. I’ve written before about DMS, but it’s surprising how common it is still.

The other two are truly the bane of the conscientious brewer. Aldehydes, most commonly seen as acetaldehydes, which smell and taste like green apples, fresh pumpkin, or astringency, are a class of chemicals that are responsible for a whole host of aromas and flavours. Humans are very sensitive to them, which is an issue when it comes to beer, though I've met a few judges who are completely blind to acetaldehyde in particular.

Common wisdom is that aldehydes are overcome if your fermentation is the correct temperature and you let it ferment long enough. This is not even remotely true, but it will limit the amount of acetaldehyde that remains in your beer. This is because acetaldehyde is produced by yeast in the conversion of sugar to alcohol and then reabsorbed and metabolised once the sugar has run out. Coming out of the fermenter, brite tank or (fresh) bottle, this solves your problem, however, it’s a reaction that can be reversed and alcohol can convert back into acetaldehyde under some very common storage conditions and containers.

And that’s just one aldehyde. There are many, many others. Heat stress, for example will cause Amadori compounds and melanoidens, those lovely, malty flavour compounds to degrade into aldehydes over time and Amadori compounds are mostly pleasant, but some are terrifying. And by heat stress, I’m not talking about storage, I’m talking about boiling your wort! If the temperature differential between the heating surface and the wort is high, you’ll create a massive amount of these compounds that will then degrade into aldehydes. These are the weird ones – the ones that taste like green banana, almonds, honey, beans - the compounds that are recognised very differently depending on the concentration (hence, the experiment).

Oxygen is the hardest to control and seems to be the least well-accepted cause for off flavours. I’ve had homebrewers tell me that it’s impossible for oxygen to have crept into their system because they pushed it out of a fermentor directly into a keg. Then they go on to blame it on the CO2 manufacturer. While it’s possible to have oxygen contamination in a CO2 cylinder, it’s by far your least likely source of contamination. Thankfully pro-brewers tend to appreciate the havoc that oxygen causes a bit better.

To start, it is impossible to avoid oxygen. You will never, ever produce an oxygen-free beer. Even if you bottle condition and assume the yeast will consume all the oxygen (which they won’t), when the yeast cells lysate, they will release that oxygen back into the beer to be reused. It is entropy in action. Beer is simply a product with a time limit.

Now that that’s out of the way, What you’re aiming for is to limit exposure to oxygen. This will give you at least more time, and even that is relative. There’s always the Arrhenius factor which (without getting into specifics) explains how those oxidation reactions occur faster the hotter it gets (until you get to pasturisation temperatures, which has it’s own equations).

So now that you’re thoroughly depressed about these unsolvable problems with your beer, lets get into what you’re going to taste and what those causes might be, a sort of addendum to the BJCP off-flavour guide:

  • Aroma                                                                           Cause
  • Onion, garlic                                                       - Hops with high sulphur compounds (Nelson Sauvin, Simcoe)

                                                                                       - Thiophenes – heat stress + oxygen

  •  Apple, pear, peach (low concentration)        - Phenylaldehydes – heat stress + oxygen
  •  Honey, melon, almond (medium)
  • Waxy, peanut (high concentration)

  • Apple, pear, honey (low concentration)        -2-pentanol – alcohol degraded by oxygen
  • Green banana, musty (medium)
  • Beany, tofu (high concentration)

  • Rancid nuts                                                        - 3-pentanol – alcohol degraded by oxygen

  • Paper, cardboard                                              - Mash oxidation

  • Rancid butter, tobacco                                    - Amadori compounds – heat stress


Now for the good news, even though there are a multitude of terrifying flavours, the solution to all of this is simple: reduce oxygen and lower the temperature of your burner/element until you get to a boil. At any level of brewing, you can limit the temperature of your element fairly easily. Although oxygen is difficult to control and impossible to eliminate, you can prevent it from ruining your fresh beer through commonly known best practice techniques. Avoid splashing when transferring, reduce headspace and keep refrigerated; your beer should last you a couple of months before those effects start showing up.