This is a guide to identifying some common off flavours with tips on how to eradicate them or prevent them the next time you brew. It’s also worth noting that some of these ‘off’ flavours are desirable in certain beers, it all depends on style. You’ll also find that the simplest solution to a problem is usually the best cause of action. So here it goes:
DMS (Dimethyl Sulfides): This gives off a cooked corn or vegetable flavour. It is produced at various points of the brewing process. The malt itself contains a chemical that is responsible for DMS. However it is a problem that is easily solved as most of it is evaporated during the boil. Make sure you leave the lid off when boiling to allow for evaporation and if you’re making a lager or pale ale boil the wort for 90 minutes as this gives more time for the DMS to fully evaporate. Darker beers and those with plenty of adjuncts like crystal, caramel and chocolate malt don’t usually have a problem with DMS as the flavour is masked by the variety of malts used.
Cooling the wort quickly after the boil also helps to reduce the amount of DMS. When making a lager or pale beer choose your yeast carefully and ferment at the correct temperature. Even if you have followed all of this and can still detect DMS don’t throw your brew away. DMS can fade over time especially during the lagering process.
DMS is caused by SMM (S-Methyl Methionine) found in all malt. It is formed during the malting process.
Sulphur: Smells like rotten eggs, lovely! It is most common in lagers. It is desirable to a certain extent in some brews but is generally viewed as an off flavour/smell. A stressed or an old yeast can produce sulphur so choosing the correct yeast and fermenting at the correct temperature will reduce it. Having said that some lager yeasts, even under good conditions, produce more sulphur than others so a little experimenting may be required. Generally sulphur is a natural by product of fermentation and will fade if a beer is given time to mature as it is a volatile compound.
Vinegar: Some brews will have a tart or vinegar like smell when fermenting and when the beer is still young, especially if fermenting from a beer kit. This can be a result of a bacterial infection however most of the time this flavour will fade if the beer is left to mature. If the vinegar taste and smell is very strong then the brew may have become infected. As long as you keep good hygiene practises when brewing bacterial infection shouldn’t be a problem.
Diacetyl: This is a compound that tastes of butter or butterscotch. In fact it is used to add a buttery flavour to food products such as popcorn. It occurs naturally in fermentation and again is most noticeable in light beers such as lager. High levels of diacetyl can be caused by numerous factors such as yeast strain, fermentation temperature, aeration and bacterial infection.
A diacetyl rest at the end of lager fermentation encourages the yeast to absorb the diacetyl therefore eliminating this off flavour. Diacetyl is more common in lagers because they are fermented at a low temperature. Raising the temperature to between 18 and 20 degrees for a few days before the end of fermentation helps the yeast to absorb diacetyl therefore eliminating it as an off flavour. Because ales are fermented at a higher temperature anyway the yeast will absorb the diacetyl therefore limiting it as an off flavour in ales.
Esters: This gives off a banana flavour and is a desired taste in some beers. For example Hefeweizen. Some yeast strains are used specifically because they give the beer this flavour. Other yeasts can give off this flavour as a by product of stress caused by the temperature going too high. To prevent this ferment at a lower temperature, aerate and lower the starting gravity so that the yeast doesn’t get too stressed. Alternatively ensure that you pitch the correct amount of yeast so that if you have a high SG there is enough yeast to digest the sugars.
Sour: This is a tart, sour and acidic flavour and again is required in some beers like Gose style or wild beers. Otherwise this is an indication that the brew has become contaminated with bacteria. Again this can be caused by stressing the yeast out so making sure you are using the correct strain, have a suitable pitch rate and ferment at the correct temperature will help to avoid this. Also cooling the wort quickly after the boil will help to prevent contamination. Good cleaning practises will also prevent any bacterial infection. Keep in mind that if you are adding any acidic fruit to your brew that this can also cause a sour taste that may not be desirable.
Oxidation: This gives the beer a musty and paper flavour. This happens during the aging process and can result from the introduction of oxygen during the brewing process. To minimise the risk of oxidation avoid splashing or vigorously stirring the wort once fermentation is complete. Leave only 1 to 2 inches head room in your bottles or if kegging purge with Co2 before adding the brew. As long as you keep good brewing practises you’ll be fine.
Phenolic: This gives off a clove like flavour and again it is desirable in some beers such as wheat beers. There are specific yeasts that will give a beer this flavour so make sure you choose the right yeast if you don’t want this flavour in your beer. If you do come across this flavour and have used the correct yeast it may be your water. Tap water can often lend a medicinal taste to beer. High levels of Chlorine in water react with phenols in beer to form chlorophenols which gives off the medicinal flavour. Get a water report to find out what’s in your water and then adjust accordingly. Here’s a link to a water calculator that will help you to alter your water to the correct balance for brewing: https://www.thehomebrewforum.co.uk/pages/water-calculator/ It is best practise when brewing from grain to alter your water profile, however if you do have a clove flavour in your beer it’s probably the yeast, changing this will more than likely solve the problem.
Astringent: This is an extremely dry mouth puckering feel, almost powdery. It’s caused by over sparging or sparging at a temperature that is too high which releases too many tannins into the wort. Any temperature higher than 76 degrees C and you risk extracting too much tannin from the grains. Keeping the mash pH between 5.2 and 5.6 also stops any astringent flavour. The pH of the mash is affected by over sparging or sparging at too high a temperature so keeping this in check will help to keep the mash pH in the correct range.
Yeasty: The most common cause for this is that the beer is green. Leaving it longer to mature means that the yeast will fall out of the liquid reducing the yeasty flavour. The other cause can sometimes be an unhealthy yeast however this is unlikely with a yeast bought from a home brew shop.
Some useful sources I used for this article: