In 25 years of brewing, I've got that twang 4 times.
I mostly do all grain - no twang ever with that.
Once a year I do extract beer with dry malt extract - no twang ever with that.
The only times I've ever got twang, it was when I used liquid malt extract - and not every time either.
The times I got it, twice was from an Australian Cooper's can and twice was from an American Briess plastic container.
I believe that DME is more stable than liquid.
My theory, for which I unfortunately have no proof, is that LME goes off in a short time, while DME doesn't. When LME goes off, what happens is that some nutrients essential to yeast degrade, and the yeast therefore cannot produce clean tasting beer. As a supporting observation, every time I've made an LME beer, it was horribly low in protein, as shown by no head at all. If it is low in protein, perhaps it's also low in amino acids and other nutrients.
I rest my ass. Case. I mean case. Yes. I rest that.
