Final Thoughts
When judging your own beer with an eye towards improving your brewing, it’s most important that you be objective and honest with yourself. It’s often difficult to judge your own work, but you have to set aside those feelings and put yourself in the shoes of a dispassionate judge in a competition or a consumer at a bar. Judge your beer as you judge other beers.

Practice is an important part of building and maintaining any skill. While self-study is helpful, you also need to periodically check your skills against others so you know that you have learned them well. In matters of perception, you need to know that you haven’t got a perceptual blind spot (for example, not detecting diacetyl) or other bias that may affect your judging. For your tasting notes to be useful to you, you have to be able to trust them.
   
When performing structured tastings of your beer, take good notes. You will want to record a full evaluation at least once, and then make notes on how the beer changes over time or how it tastes under different serving conditions. You are developing a profile of your beer that you will use as a reference. If you make changes, you will want to compare your current version against previous incarnations.
   
Use the feedback wisely, whether your own or from others. Be careful about making too many adjustments at once. You need to be able to gauge the impact of your changes. If you are fine-tuning a beer, you probably should only make one change at a time. If you are quite far away from your target or have multiple problems, feel free to make more changes.
   
Finally, know why you are brewing. Are you trying to brew better beer for yourself, are you trying to win competitions or do you simply want to have something you’re proud to share with others? Keep in mind that judging and tasting are subjective, and that you won’t always please everyone. As long as you are happy with yourself, you are getting the right enjoyment out of brewing.

Common Beer Faults

Here are some of the faults most frequently seen in beers submitted to homebrew contests, and their most likely causes: 

Acetaldehyde ?D Acetaldehyde is a precursor to ethanol in beer yeast’s fermentation pathway. It lends a green-apple-like aroma to beer. Running a healthy fermentation and letting beer condition sufficiently will eliminate any excess acetaldehyde.

Astringency ?D Astringency is a dry, puckering mouthfeel of the same type as found in many teas, some red wines and when eating tannic fruits (such as chokecherry). In very astringent beers, it can also have a rough or sandpaper-like feel on the tongue. It is often confused with bitterness. Astringency is caused by tannins, a large group of polyphenols found in plants. Problematic levels of tannins can be extracted from malted grains by excessive sparging, or sparging at temperatures over 170 °F (77 °C), especially near the end of lautering. Very dark malts and highly-roasted grains yield more tannins than lighter malts. Any other plant material added during the boil, including hops or other spices, can contribute astringency.

Contamination ?D Contaminating microorganisms can cause a variety of off flavors, aromas and mouthfeels. Common contamination-related faults include flavors and aromas that are tart/sour, plastic/Band-Aid-like, butterscotch/buttery (see diacetyl below), medicinal, vegetal and vinegar-like. Some of these faults can be caused by other things as well. Although there are a number of different bacteria and yeast that can contaminate wort or beer, the corrective measure is the same in all cases ?D clean your brewing equipment and environment thoroughly and sanitize any piece of brewing equipment that will come in contact with chilled wort or beer. Likewise, do not repitch yeast following a contaminated batch.

Diacetyl ?D At moderate levels, diacetyl tastes and smells like butterscotch. It also contributes a slick, coating mouthfeel. At high levels, it strongly resembles butter. (Diacetyl is the main ingredient in the butter flavoring used on popcorn.) At low levels, it can be confused with caramel flavors. Most beer drinkers describe beers without diacetyl as being “cleaner” than those containing diacetyl and it is considered a fault in most beers. Diacetyl is produced by yeast ?D and, if present, some contaminating microorganisms ?D during fermentation. In late fermentation, it is taken up by the yeast. Contamination or prematurely separating the beer from the yeast in the primary fermenter are two common causes of excess diacetyl. In some lagers, the fermentation temperature is raised to ale-like temperatures near the end of fermentation. It is held there until sampling indicates that the diacetyl level has fallen below the level of perception. This is called a diacetyl rest. Aerating your wort during primary fermentation will increase diacetyl production.

DMS ?D Dimethyl sulfide (DMS) lends a cooked corn flavor to beer. It is primarily caused by a weak boil or slow wort cooling, especially when a large amount of very pale malts are used.

Higher alcohols ?D Higher alcohols (also called fusel alcohols or fusel oils) are alcohol molecules with more carbon atoms than ethanol, a 2-carbon alcohol. Higher alcohols lend a “hot” alcoholic, solvent-like character to beer, which can be reminiscent of nail polish remover. The primary cause of excessive higher alcohols in a beer are high fermentation temperatures or yeast that struggles to complete a fermentation. Strong beers are more prone to developing this fault, especially when not adequately pitched with healthy yeast. Some yeast strains produce more higher alcohols under stress than others.

Oxidation ?D Oxidation causes stale flavors and aromas in beer that resemble paper or cardboard. It can also cause Sherry-like flavors, especially in strong beers. Any exposure to oxygen after the beer has been fermented sets the stage for oxidative flavors and aromas to develop. Splashing of beer when racking to bottles or kegs is a common cause. Eventually, all beers will show signs of oxidation. Storing beers cold will prolong the amount of time they remain fresh.
 
(from:byo.com)