Chemistry - If caffeine is an alkaloid, why is coffee acidic?

Solution 1:

Coffee contains hundreds, if not thousands, of other compounds in addition to caffeine. Included among these other compounds are many acids. Many small, organic acids such as citric, malic, lactic, pyruvic and acetic acid are present, but both quinic acid and chlorogenic acid (and their derivatives) are usually present in even higher concentration. Phosphoric acid, an inorganic acid, is also present. The exact concentration of these various acids depends upon processing variables such as roast conditions and grind size. Here is a link to an interesting, one-page discussion of the subject.

quinic acid

chlorogenic acid

Edit: In Season 3, episode 6 of "Breaking Bad", we meet Walt's new lab assistant Gale, who is brewing coffee using a lab glassware set-up. Walt says that it is the best coffee he has ever tasted and Gale responds by mentioning "quinic acid" as something to be considered in brewing great coffee.

Solution 2:

Alkaloids are low molecular weight nitrogen-containing compounds, mostly from plants. The nitrogen is usually in the form of amines and this often results in a basic compound, but there are many alkaloids where this is not the case. In fact, caffeine is a pretty weak base. I calculated the pKa of the imine nitrogen with MarvinSketch as -.92, so caffeine is not going to affect the pH of coffee much.

Also, caffeine isn't even a major component of brewed coffee, only accounting for ~1% of the dry mass. If you look here you'll see that brewed and green coffee naturally contains a fair bit of soluble acids, including things like nicotinic acid (vitamin B3), which probably accounts for the pH of brewed coffee, rather than the caffeine or any processing.