Roasted and kilned malts are often used to lower the pH of the mash. Which chemicals are created by the roasting process that have such a low pH?
According the Brau Kaiser, it's acidic melanoidins. Melanoidins are composed of sugars and amino acids, and are created through the Maillard reaction.
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Hmm, after some searching, I found that it could also be some acetic acid in the malt, but acidic melanoidins seems the most likely candidate. Too bad I can't find any information about what acidic melanoidins are made of – nulvinge Sep 11 '14 at 10:46