Acetic or sour flavors in beer are usually only appropriate for beers such as lambics, berliner weisse, and when the brewer intends to "sour" their beer. They are perceived as vinegar-like, and tart on the sides of your tongue.
The primary causes of unwanted sourness in beers are bacterial infections. These bacteria produce lactic acid, which, can give a smooth sourness to your beers, and acetic acid, which will give a sharper, tangier bitterness. The production of these acids by Enteric, Lactobacillus, or Pediococcus bacteria is enhanced by using too much refined sugar, citric acid or ascorbic acid in your beer.
Bacterial growth may occur from:
Basically anything that allows bacteria to get a head-start in your wort.
Remember, you must get a healthy batch of yeast pitched as quickly as possible so they can out-compete all the bacteria and other wild creatures present.
Be especially careful with your sanitation procedures if you notice these flavors in your beer. Thoroughly inspect all equipment. Clean and disinfect everything. Be wary of using plastic fermenters which have been in use for more than several batches due to the scratches which can occur and harbor these bacteria.
If using StarSan sanitizer, discard it when it becomes very cloudy as this is a sign that the pH has changed and it no longer is an effective sanitizer. Make sure to give all your equipment the proper amount of contact-time with the sanitizer before using or rinsing to insure full sanitization.
Check the manufacturer's label for each product. Most will require at least 2 to 3 full minutes of contact time for sanitization. If it's a no-rinse sanitizer, don't rinse. Sometimes the act of rinsing introduces contaminants which will spoil your beer. This may come from improper handling or from the water supply itself.
There is a reason the sanitizer is "no-rinse" and it's not always because it makes the job easier. It's also one less activity through which bacteria and other contaminants may be introduced into your system.
If you are not having any contamination problems, great. But if you
are, try some of the things mentioned above to see if it corrects the
problems. Check out the BJCP Beer Faults page for more information.
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