From MoreBeer faults list.
Astringent
Tastes/Smells Like:
Tart, vinegary, tannin, drying, puckering sensation, may feel powdery or metallic in the
mouth, like sucking on a grape skin or a tea bag
Possible Causes:
Astringency can be caused by many different factors. Polyphenols or tannins are the
number one cause of such flavors. Tannins are found in the skins or husks of the
grain as well as in the skin of fruit. Steeping grain for too long or grain that has been
excessively milled or crushed can release tannins. When mashing, if the pH exceeds
5.2 – 5.6, astringent flavors can be produced. Over-hopping can also lend a hand in
creating astringent qualities.
How to Avoid:
Avoid grain that has been “over-milled”. Grain should be cracked open but not crushed
or shredded. When sparging, pay close attention to the temperature and the amount
of the water used. When steeping grains, be sure to take them out before the water
gets to a boil. Fruits should never be boiled in the wort; instead, they can be added to
the fermenter or to water that is hot but not boiling for 15-30 minutes. Make sure that
the amount and varieties of hops used are the correct types for the style of beer.
Metallic
Tastes/Smells Like:
Metal, mainly iron, also described as tasting like pennies or blood, Felt on the front
of the mouth and back of the throat
Possible Causes:
Wort being boiled in unprocessed metals, mainly iron, but also aluminum, and steel
(excluding stainless) is usually the source of metallic flavors. Metallic flavors can also
be extracted from metal brewing equipment, bottle caps and/or kegs. Using water
that has high levels of iron will impart iron flavors. Improperly stored grains can also
cause metallic off flavors.
How to Avoid:
Use stainless steel pots and brewing equipment (fittings, spoons, etc.) when possible.
Avoid using iron for anything that will be coming in contact with beer/wort. If using
a ceramic coated steel pot, always check for cracks or scratches before using. Stainless
steel will not give off any metallic flavors. Aluminum pots will generally only cause
metallic flavors when using alkaline water with a pH over 9. If using an aluminum
pot, you can “bake” the pot in an oven at 250ºF for 6 hours to increase the protective
oxides. Always use fresh, properly stored grain. Avoid using water with iron in it, such
as unfiltered well water.