I've just looked it up and Coke is the same as 17% phosphoric acid. For acidifying the mash on my last brew I had to use 0.15 ml of 60% lactic acid per litre - I could do the same thing with 0.5 ml / litre of coke. That's nothing - would you taste that in a beer? Probably not. Anyone wanna give it a go? As a caveat I haven't worked out if the carbonic acid from the fizz would play a factor but I'm sure that would be driven off at mash temperatures. Yeah, just whiling away the hours until I don't get scowled at if I start to drink.