Why does raspberry syrup become "globby" when added to coffee?

Dear Mr. Alice:

I am a barista at a local coffeehouse. Among the common items one finds on our menu are flavored syrups used to enhance the various coffee drinks: vanilla, hazelnut, and so forth. One of these syrups, raspberry, when added to hot coffee drinks, frequently congeals into a globby mass resembling pink cottage cheese. This doesn't happen with Irish cream, caramel, or any other flavors. Why raspberry?

-- Chris Griffin, Kensington

Anyone who adds raspberry syrup to a perfectly good cuppa coffee gets what he deserves. Pink cottage cheese? Oh well. Nature's way of telling you to switch to something you can actually tolerate drinking straight. Besides, it will speed up the waiting line for us "large house to go, no cream, no sugar" types. We usually have to bring a novel to read while the barista builds those vanilla-caramel espresso mocha latte with whipped cream, nutmeg, and jimmies things for everybody ahead of us. It's a myth that people are crazy about coffee. If they were, they wouldn't put raspberry syrup in it.

But when they do put raspberry syrup in it, or any of that Italian fruit goop, the syrup's citric acid plus the heat of the coffee congeals the milk. Wanna make your own cottage cheese? Add lemon juice to moo juice and warm it up. Sooner or later, the protein in the milk will seize up into little globs.