By Harold McGee
The simplest method for preserving eggs is to treat them with salt, which draws the water out of bacteria and molds and inhibits their growth. The eggs are immersed in a 35% salt solution, or coated individually with a paste of salt, water, and clay or mud. After 20 or 30 days, the egg stops absorbing salt and reaches chemical equilibrium. Strangely, the white remains liquid, but the yolk at its center becomes solid. The high levels of positive sodium and negative chloride ions actually shield the albumen proteins from each other, but cause the yolk particles to agglomerate into a grainy mass. Salted eggs, which are variously called hulidan and xiandan, are boiled before they’re eaten.