Beer astringency is an off flavor and is perceived as a dry grainy, mouth-puckering, tannic sensation (think of sucking on a wet tea-bag). Although bitter flavors may be caused by bacterial contamination, it is usually the result of processing.
There are many causes of process-related beer astringency: over-sparging, sparging with water above 168°F (76°C), steeping your grains too long, mash pH above the 5.2-5.6 range, over hopping, or boiling your grains can extract excessive tannins from the husks. Milling your grains too fine and poor hot-break removal resulting in too much trub may be sources of bitterness as well.
Removing the Braun Hefe
Traditional brewers in Germany have always removed the brown yeast layer floating on the surface during fermentation. They call this layer "braun hefe" or brown yeast. If you've ever tasted it, you will see why you might not want it in your beer. It is extremely astringent and bitter, consisting of yeast, trub, and hop resins. Their open fermenters made skimming this goo off the top relatively easy. For homebrewers using conicals or carboys, this becomes more problematic.
If you feel you must remove the braun hefe from your beer, one method used by many homebrewers is to utilize a blow-off tube and small head space. The majority of the krausen will be blown-off during the most vigorous phase of fermentation. The problem here is beer losses, which may be substantial.
There are many factors affecting the amount of blow-off you can expect, including original gravity, temperature, fill level, yeast selection, and wort aeration. The best thing you as a homebrewer can do is to experiment. Try the blow-off method and then brew the same beer without using the blow-off tube. If you notice a discernible difference between the two beers, then you may want to continue using the blow-off tube method.
Most homebrewers don't bother, and brew perfectly good beer. The fact that a lot of the brown goo sticks to the sides of the fermenter and is left there after siphoning the beer to secondary or to the keg, helps most of us too.
When Sparging, watch your temperature and don't sparge with water hotter than 168°F (76°C). Also, don't over-sparge with too much water (learn about the No-Chill Brewing method that does not require you to sparge at all). Try to stop the run-off when the specific gravity gets to 1.010. The mash pH is increasing during the sparge and beyond this SG, you run the chance of stripping too much astringency from the grain bed. If you are measuring the run-off pH, stop when the pH rises to about 5.8 to avoid beer astringency problems.
When you steep your grains, be sure you don't let the water come to a boil before you take them out.
If you are adding fruit to your wort, never boil them. Instead, add the fruit to the primary or secondary fermenter.
Don't add too much hops, or hops with too high of an alpha acid content for the style you are brewing.
Use a blow-off tube. If you feel that allowing the krausen to fall back into the beer will negatively affect the flavor, attach a large diameter tube into the mouth of the fermenter to allow the krausen to "blow off" into a "catch" container.
Watch your mash pH. If your mash pH tends to be greater than 5.6, try using a pH stabilizer like 5.2 from five-star. 5.2 is a proprietary blend of food-grade phosphate buffers that are similar to brewing salts. It will "Lock In" the mash pH at 5.2, no matter what pH your brewing water started at. This is especially helpful when brewing with hard water. There are several other benefits to using this pH stabilizer. Learn more and download the tech sheet in pdf here.
Make sure you don't allow the hot-break trub to get into the fermenter. Allow the cooled wort to settle prior to siphoning it to the primary fermenting vessel. Before I transfer my cooled wort into a fermenter, I will normally stir the wort vigorously, creating a whirlpool effect inside the kettle. The centripetal force will pull everything to the center of the kettle. Then I carefully drop the racking cane down the back edge of the kettle as the volume drops.
It is inevitable that the trub will gradually move towards the suction, and at some point you have to stop the transfer. Sometimes trub losses can be significant, depending on how much break material is in the kettle.
I have transfered the remaining mixture into a 3 gallon carboy to settle overnight if the losses are more than I want (I'm talking two gallons or more). But, beware of the off flavors that may develop by letting the wort steep overnight with all the trub including he hop residue. I've only done this a couple of times and the resulting beers were less than spectacular. I'll normally just accept the loss and expect less beer into the keg or bottles.
Information for this article on beer astringency was adapted in part from The Home Brewer's Answer Book by Ashton Lewis, How To Brew by John J. Palmer, and “Off” Flavors, Their Causes and How to Avoid Them compiled by Robin Wada from More Beer.