Unlike with wine, there is not a lot you can do with it if a beer goes wrong. You have to try and identify what went wrong and avoid it in future. Even this is more easily said than done as some problems can have more than one cause, so you need to both keep a record of everything you do during the brewing process and observe and record where things did not quite go as expected. If you experience significant bottle variation in the same beer, it is likely that you used a dirty bottle! So, here are some of the more common faults and causes.
Taste and aroma of green apples, in some cases reminiscent of sherry. Caused by oxidation of the finished beer or possibly bacterial spoilage. Make sure you keep demijohns full and that the airlock is not allowed to dry out.. Most likely if fermentation progresses slowly, or does not complete.
Recognised by a sharp vinegary smell and taste. Caused by exposure to air. Prevent as for acetaldehyde.
Caused by worts deficient in nutrients. Exaggerated by fermentation at high temperature and common in kit beers.
Dryness and puckering in the mouth cause by tannins and related substances. Usually caused by over crushing malt, over sparging or sparging with water that is too hot. Black malt can be a cause.
Harsh dry after taste on the back of the tongue. Due to over hopping or high concentration of roast malts.
The feel of the beer on the tongue and palate. Light body may be due to low mash temperature, or by excessive use of adjuncts or sugar.
This is a wild yeast that imparts somewhat unusual flavours to a beer - often described as sweaty horse blanket, dirt, earth and barnyard. However, some Belgian beers are deliberately made using such yeast strains. However in most homebrews this is not a desirable property. Because these yeasts are quite slow fermenting, these off flavours may not become evident until a beer gets a bit more mature.
Butterscotch or toffee like aroma and taste. Can result from premature separation of the beer from the yeast or by exposure to oxygen during fermentation. You are unlikely to get this if you introduce fresh yeast at the bottling stage (krausening).
Dimethyl Sulphide is a sulphur compound beneficial to the taste and bouquet of lager up to a certain level. Above this it can give vegetable or sweet corn flavours. High levels may be due to a short or closed boil or to long slow fermentation.
Fresh-cut grass, green leaves on the aroma. Can be caused by dry hopping or use of old hops.
May be caused by starches, proteins or bacteria. Most common reasons for occurrence would be failure to cool the wort rapidly or the omission of Irish Moss at the end of the boil.
The time for the head to subside and eventually disappear. Clean dry glasses help. Recipes containing flaked wheat and barley tend to have better head retention. You can purchase head retention agents, but to me these seem to impart an odd flavour.
Caused by ultraviolet light. Bottle your beer in brown bottles and keep it in a dark place.
Stale aroma and taste caused by oxidation in the mash or boil or by exposure to air when racking or bottling. Also caused by use of old crushed malt which has absorbed moisture.
Reminiscent of dry sherry - caused by exposure to air or by chemical reactions in beer that is kept for a long time. Acceptable in small amounts in barley wines or old ales where it adds complexity.
Antiseptic taint that reminds me of ‘Germolene’. Results from the combination of chlorine in water or sterilising agents with phenols in the ingredients. Boil all added water and/or treat with metabisulphite. Avoid sterilising with chlorine based compounds. May also be caused by fermentation at too high a temperature.
Perceived on the sides of the mouth towards the rear of the tongue. Usually caused by lactic or acetic acid, due to poor sanitation, or by mashing for too long at lower temperatures.
Aroma and taste of rotten eggs, shrimp or rubber. Possible causes include yeast autolysis (poor quality yeast, beer left too long on the lees), bacterial spoilage and water contamination.
Cooked, canned or rotten vegetables often reminiscent of cabbage or asparagus. Caused by slow start to fermentation, poor ingredients (particularly malt extract) or oversparging.