Published March 1, 2008. From Cook's Illustrated.
Why does soaking dried beans in salted water make them cook up with softer skins?
Why does soaking dried beans in salted water make them cook up with softer skins? It has to do with how the sodium ions in salt interact with the cells of the bean skins. As the beans soak, the sodium ions replace some of the calcium and magnesium ions in the skins. Because sodium ions are weaker than mineral ions, they allow more water to penetrate into the skins, leading to a softer texture. During soaking, the sodium ions will only filter partway into the beans, so their greatest effect is on the cells in the outermost part of the beans.