It's likely that because you're cooking them in a basic (ph, acid vs base, etc) solution that the egg is being changed chemically. Using a harsher base might shed more light on that (though would possibly make it inedible).

> though would possibly make it inedible

Nope: http://en.wikipedia.org/wiki/Thousand_year_egg#Modern that's with alkaline, and you can use acid to make a pickled egg.

Looking at it, i'd consider it inedible :) But good to know that stronger bases have been used to do that kind of thing.

Baking soda is alkaline and breaks down proteins especially together with heat.