Some aromas and flavors in beer are caused by brewing problems, infections during fermentation by troublesome microbes, and/or improper handling. Here is some information to help you distinguish these "off-flavors." Please note that none of these problems can make you ill - they just make the beer less enjoyable.

- Acetaldehyde: This is a green apple-like aroma or flavour that can be caused by unwanted bacteria.

- Diacetyl tastes and smells like butter or butterscotch does. This is acceptable in small levels in some ales, but should never overpower a beer.

- DMS (dimethyl sulfide): A corn-like or vegetable-like aroma and flavour produced by certain problems in the brewing process. A good English ale should be crisp and have the aroma and flavour of malt and hops - not creamed corn.

- Skunky beer happens when light converts a chemical in the hops into the smelly compound found in the musk gland of a skunk. Be sure the beer you buy is in brown bottles (clear and green don't offer much protection) and has never been exposed to excessive light.

- Paper and cardboard flavours/aromas happen when beer gets old and oxidized. That microbrew that was fantastic when it left the brewery is not very appealing after sitting on a shelf for a year.

- Phenolic flavours are reminiscent of plastic, clove, or band-aids. Wild, airborne yeasts can produce these strange compounds that are appealing in a German wheat beer, but horrible in a pale ale.

- Sour beer is caused by spoilage bacteria that produce lactic or vinegar-like acids.