Why does brining dried beans in salt water (instead of just soaking them in plain water) ward off tough, unpleasant skins? It has to do with how the sodium in salt interacts with the cells of the beans’ skins. The pectin molecules in bean skins are tightly bound by calcium and magnesium ions. As the beans brine in the salt water, the sodium ions replace some of the calcium and magnesium ions in the skins, causing the pectin to weaken.
Because sodium ions are more weakly charged than calcium and magnesium ions, they allow more water to penetrate the skins, leading to a softer texture. During brining, the sodium ions will filter only partway into the beans, so their greatest effect is on the cells in the outermost part of the beans.