Will adding eggshells to ground coffee before brewing tame an overly strong brew?
The tradition of adding eggshells to brew is linked with so-called cowboy (or campfire) coffee, which is made by boiling ground coffee in water. Since the resulting drink can taste acidic, bitter, or too strong due to the high heat (which releases tannic acids) and extended contact between the coffee and the water, some campfire cooks add a crumbled eggshell to the mix in hopes of tempering sharp flavors. The finished drink is then strained or ladled off, leaving the grinds and shell behind.
The practice makes a lot of sense: Eggshells are composed mostly of calcium carbonate, a fairly alkaline material that has the ability to absorb some of the acid in the coffee. In fact, when we made traditional and campfire brews with and without eggshell added to the ground coffee, the eggshell samples boasted a significantly mellower (but less complex) taste than the shell-less ones.
We wondered if eggshells could also be used to extract bitter compounds from coffee that accidentally turns out too strong, so we purposely brewed a batch for too long, lightly crushed an eggshell (rinsed in vinegar and then water to kill any bacteria), stirred it into the potent coffee, and strained out the shell. Indeed, this quick treatment produced a milder cup.