Consequences of contamination in a beer?

I had a batch of dunkelweizen that became contaminated, I think from siphoning with my mouth. The off flavour is obvious, it tastes sour. At first I thought it was completely ruined, but I held onto the batch to wait it out. Over time the flavours softened and it is actually pretty tasty now. Sort of like a dark berliner weisse.

Sanitization is definitely important, but just because you slipped up doesn't mean your batch has to be ruined. Hold onto it and try it every once and a while. You might be pleasantly surprised.

The biggest contamination risk in brewing is acetobacter, for the simple reason that acetobacter is everywhere and on everything. It's in your kitchen, on your hands, and frequently on the surface of fruit fresh from the orchard. It's airborne and settles on every available surface.

Why is this a problem? Two reasons:

1. Acetobacter produces acetic acid, also known as vinegar. Got a vinegary tang in your brew? Chances are it's from some sort of acetobacter infection.
2. Acetobacter metabolizes alcohol to produce acetic acid. This is different from yeast and most other bacteria - alcohol inhibits yeast growth, but it fuels acetobacter growth.

How do you turn perfectly good brew into vinegar? Just leave the brew in an open container (cover with gauze to keep bugs out) in a warm room. Within a few days, it will start to smell vinegary, and within two weeks you will have a vat of vinegar. (ps Never taste-test homebrew vinegar - high acid content can cause serious chemical burns to your mouth!)

Sanitation is also important to reduce the risk of pathogens taking up residence in your brew and causing extreme illness or death in anyone who drinks your product. Truly toxic bacteria such as Clostridium botulinum (botulism) may be killed by boiling for extended periods, but their spores may survive boiling and begin reproduction sometime later. Sanitation with chemicals and/or high heat (above 250F) is the only way to destroy the bacteria and their spores.

I can immediately think of three indicators.

1. Off flavours or strange aromas
2. Beer that ferments vigorously for longer than expected
3. Moulds or other growth on the wort.

2 and 3 can sometimes be normal, depending on conditions (temperature etc.) and the gravity and fermentability of your wort. Occasionally yeast might cause odd-looking growth on the beer.

As for off flavours, the most obvious is probably a vinegar-like taste. The beer may also taste of wet cardboard, rotten milk, etc. There are a vast range of off flavours caused by contamination. Read John Palmer's "How To Brew" for a comprehensive scary list.

However, I've had film moulds on the beer that don't affect flavour at all, but don't look too nice in the fermenter. Some wild yeast will cause symptom 2 and ferment all the sugars they can, so you end up with a very dry beer with no body, but without really bad flavour problems.

IMO you'll definitely get contamination sooner or later if you don't sanitise properly.