From time to time my kitchen sink needs to be unplugged, and each time I invariably find accumulations of a crusty, crumbly white material in the pipes. It looks a lot like old cheese, has no particular odor of note, is often in ring shapes (so I assume it is accumulating against the pipe walls), and can be quite hard at times requiring a fair bit of manual force to snap out of place.

These accumulations don't occur at our bathroom sink and bathtub drain, both of which have corrugated plastic hosing, so I suspect it is biological (feeding off sink detritus) rather than mineralogical?

Just curious!