You ever take a sip of beer and notice a bit of a metallic taste? Sometimes, yes, bordering on the unpleasant? Unless you accidentally poured a pale ale into your loose change jar, there are a few possible culprits behind that tinny, nasty taste.

The first makes a lot of sense: contact with metal.  Believe it or not, this has nothing to do with beer cans, since they’re lined with a polymer coating, the beer never actually comes into contact with the aluminum.  If you take a sip of beer and notice a metallic flavor, it might actually be coming from the proximity of your nose to the can itself; as you smell the aluminum can and sip the beer, the sensations can be intermingled. Which is why it’s always a good idea to pour your beer into a glass (plus you get a cool visual that way too).

But contact with metal might happen within the brewery itself. Iron and copper are used in the brewing process (the iron tends to come from brewing water). In fact, the iron in water used to make beer can actually increase the beer’s corrosive properties. And with a low pH, beer is already capable of eating away at nickel coating on brewing equipment, resulting in direct contact between the beer and metal.

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And whereas copper only increases the perceived “harshness” of a beer (still a tragedy), iron in the beer will actually taste a bit like your beer was steeped with a bunch of pennies. (And it doesn’t take a lot; according to the Beer Judge Certification Program, it’s perceptible at .05ppm.)

The Knights of the Mashing Fork—really their name, and it’s awesome—are a Connecticut-based homebrewing group, and their documents attribute the metallic tastes to “the ferrous ion (iron) and some organic compounds formed by hydrolysis of cereal lipids in grain, and oxidation of free fatty acids.” Terrifying chemistry words, sure, but hydrolysis just means the “breaking of a bond in a molecule,” in this case some grain lipids (or fats). The KOTMF also blame the brewing process itself: a “freshly-scrubbed stainless steel that has not been allowed to oxidize” prior to use, low quality grain, and—once more—a “high iron content in water.”

The basic fixes are using better grain—fresher grain—for the mash, water that’s lower in iron content, and oxidizing your stainless steel in advance so oxidation doesn’t take place in the brewing process, ensuring your beer won’t come out tasting like it’s been pre-digested by the Tin Man.