By Harold McGee
The real transforming agent in pidan is the alkaline material, which gradually raises the already alkaline egg from a pH of around 9 to 12 or more. This chemical stress causes what might be thought of as an inorganic version of fermentation: that is, it denatures the egg proteins, and breaks down some of the complex, flavorless proteins and fats into simpler, highly flavorful components. The disruptively high pH forces the egg proteins to unfold, and at the same time confers on them a strongly repelling negative charge. The dissolved salt, with its positive and negative ions, moderates the repulsion enough that the fine strands of widely dispersed albumen proteins are able to bond into a solid yet transparent gel. In the yolk, the same extreme conditions destroy the organized structure of the yolk spheres, and with it the usual graininess; the yolk proteins coagulate into a creamy mass. The extreme alkalinity also browns the albumen by accelerating the reaction between the proteins and the trace of glucose, and it greens the yolk by encouraging the formation of ferrous sulfide throughout the yolk, not just at its surface (as in hard-cooked eggs). Finally, the alkalinity intensifies the egg’s flavor by breaking down both proteins and phospholipids into hydrogen sulfide, distinctly animal fatty acids, and pungent ammonia (the fumes from a freshly opened egg will turn litmus paper blue).